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A Pneumatic Conveying Powder Delivery System For
Continuously Heterogeneous Material Deposition
In Solid Freeform Fabrication
by
Shawn Fitzgerald
Thesis submitted to the Faculty of the
Virginia Polytechnic Institute and State University
in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCEIN
MECHANICAL ENGINEERING
APPROVED:
_________________________________Dr. Jan Helge Bøhn, Chairman
______________________________ ______________________________Dr. Arvid Myklebust Dr. Ronald Kander
July 1996Blacksburg, Virginia
Keywords: Rapid Prototyping, Powder Deposition, Selective Laser Sintering,
3D Printing, Freeform Powder Molding
A Pneumatic Conveying Powder Delivery System For
Continuously Heterogeneous Material Deposition
In Solid Freeform Fabrication
by
Shawn Fitzgerald
Jan Helge Bøhn, Chairman
Department of Mechanical Engineering
Great improvements are continuously being made in the solid freeform fabrication (SFF)
industry in terms of processes and materials. Fully functional parts are being created
directly with little, if any, finishing. Parts are being directly fabricated with engineering
materials such as ceramics and metals. This thesis aims to facilitate a substantial advance
in rapid prototyping capabilities, namely that of fabricating parts with continuously
heterogeneous material compositions. Because SFF is an additive building process,
building parts layer-by-layer or even point-by-point, adjusting material composition
throughout the entire part, in all three dimensions, is feasible. The use of fine powders as
its build material provides the potential for the Selective Laser Sintering (SLS), Three-
Dimensional Printing (3DP), and Freeform Powder Molding (FPM) processes to be
altered to create continuously heterogeneous material composition. The current roller
distribution system needs to be replaced with a new means of delivering the powder that
facilitates selective heterogeneous material compositions. This thesis explores a dense-
phase pneumatic conveying system that has the potential to deliver the powder in a
controlled manner and allow for adjustment of material composition throughout the layer.
iii
Acknowledgments
I would like to thank:
• My family for all of their love, understanding, and monetary support.
• My friends for always being there when I wasn’t doing work.
• Dr. Bøhn for introducing me to rapid prototyping and all of his help.
• My committee members, Dr. Myklebust and Dr. Kander, for taking the and effort to
provide assistance during the academic year and summer sessions.
• Scott Houser, for providing help whenever needed and being there to hear my
complaints.
• The Virginia Tech CAD laboratory for providing the hardware and software needed in
the design of the experimental apparatus.
• To everyone in the ME machine shops for helping me build the apparatus.
This research was supported in part by the Naval Surface Warfare Center, Dahlgren
Division under contract N60921-89-D-A239, Order 0045. Any opinions, findings,
conclusions, or recommendations expressed in this thesis are those of the author and do
not necessarily reflect the view of the Naval Surface Warfare Center, Dahlgren Division.
iv
CONTENTS
ABSTRACT ........................................................................................................................i
ACKNOWLEDGMENTS .................................................................................................iii
CONTENTS ......................................................................................................................iv
LIST OF FIGURES...........................................................................................................vii
LIST OF TABLES.............................................................................................................xii
CHAPTER 1 INTRODUCTION.......................................................................................1
1.1 PROBLEM STATEMENT..............................................................................6
1.2 SOLUTION OVERVIEW.................................................................................6
1.3 THESIS OVERVIEW........................................................................................8
CHAPTER 2 LITERATURE REVIEW...........................................................................10
2.1 SECONDARY SUPPORT MATERIALS.....................................................10
2.2 SECONDARY MICROSTRUCTURE MATERIALS..................................14
2.3 MATERIAL DEPOSITION..........................................................................17
2.4 PNEUMATIC CONVEYING........................................................................21
2.5 OBSERVATIONS .........................................................................................28
CHAPTER 3 METHODS.................................................................................................29
3.1 EXPERIMENTAL APPARATUS.................................................................29
3.1.1 POWDER PREPARATION & LOADING.....................................33
3.1.2 AIR HANDLING SYSTEM............................................................35
v
3.1.3 MECHANIZED SLIDER COLLECTION SYSTEM......................41
3.2 EXPERIMENTAL SETUP.............................................................................45
3.2.1 ESTIMATING THE VOLUMETRIC FLOW RATE.....................46
3.2.2 EVALUATING THE POWDER PATH CHARACTERISTICS....48
3.2.3 EXPLORING MULTIPLE POWDER DEPOSITION...................50
CHAPTER 4 RESULTS...................................................................................................52
4.1 LINEAR VELOCITY EXPERIMENTS........................................................52
4.2 POWDER VOLUMETRIC FLOW RATE....................................................57
4.3 POWDER PATH CHARACTERISTICS......................................................63
4.4 MULTIPLE POWDER MIXING..................................................................71
4.5 MULTIPLE MATERIAL TRANSITIONS..................................................75
4.6 OBSERVATIONS .........................................................................................76
CHAPTER 5 MULTIPLE POWDER DEPOSITION SYSTEM...................................78
5.1: INTRODUCTION........................................................................................78
5.2: NOZZLE ARRAY AND POWDER DEPOSITION HEAD.......................80
5.3: MOTION SYSTEM......................................................................................83
5.4: POWDER PLUG CREATION & DELIVERY.............................................86
5.5: SOFTWARE & CONTROL..........................................................................89
5.6: POWDER PROPERTY EFFECTS...............................................................90
5.7: PERFORMANCE ANALYSIS.....................................................................92
vi
5.8: DESIGN OVERVIEW....................................................................................93
CHAPTER 6 CONCLUSION ........................................................................................96
6.1: CONCLUDING REMARKS........................................................................96
6.2: CONTRIBUTIONS.......................................................................................97
6.3: RECOMMENDATIONS FOR FUTURE WORK......................................98
REFERENCES ...............................................................................................................101
APPENDIX A BILL OF MATERIALS FOR EXPERIMENTAL
APPARATUS .............................................................................106
APPENDIX B PARTS LIST FOR EXPERIMENTAL
APPARATUS .............................................................................107
APPENDIX C SUPPLEMENTAL DRAWINGS OF EXPERIMENTAL
APPARATUS .............................................................................108
APPENDIX D SUPPLEMENTAL DRAWINGS OF PROPOSED
SYSTEM ....................................................................................135
VITA ...............................................................................................................................140
vii
LIST OF FIGURES
1.1: Typical Selective Laser Sintering system.................................................................3
1.2: Typical Three Dimensional Printing system...........................................................3
1.3: Freeform Powder Molding process..........................................................................4
1.4: Powder leveling system............................................................................................5
1.4: Schematic of experimental apparatus.......................................................................8
2.1: The Solid Ground Curing (SGC) process...............................................................11
2.2: FDM head dual-material nozzles...........................................................................12
2.3: Model Maker 3D Plotter.......................................................................................13
2.4: Possible methods for powder deposition...............................................................18
2.5: Internal secondary air pipe system........................................................................23
2.6: Plug formation using timer operated air knife........................................................24
2.7: Plug formation using alternating air valves.............................................................24
2.8: Flow pattern of plug phase conveying...................................................................26
3.1: Complete experimental apparatus..........................................................................30
3.2: Air handling and powder deposition components.................................................31
3.3: Mechanized slider collection components.............................................................31
3.4: Multiple plugs passing through parallel tubing system.........................................32
3.5: Loading of powder into polyurethane tubing........................................................35
viii
3.6: Powder plug moving through tubing......................................................................35
3.7: Main air line division by three port manifold........................................................36
3.8: Screw clamp operating as flow rate controller.......................................................37
3.9: Nozzle with supply tubing and connections.........................................................38
3.10: Nozzle and support stand......................................................................................39
3.11: Severe stair stepping in adjacent layers..................................................................40
3.12: Motor control circuit diagram................................................................................42
3.13: Motor and thread winding system.........................................................................43
3.14: Exploded slider collection system..........................................................................44
3.15: Potentiometer and calibration template..................................................................45
3.16: Parameters tested for deposition characteristics....................................................49
3.17: Deposited powder path characteristics evaluated..................................................49
4.1: Output voltage vs. angular displacement of potentiometer...................................54
4.2: Velocity of slider system vs. motor input voltage.................................................56
4.3: Volumetric flow rate vs. powder plug lengths for a pressure of 34.5 kPa.............59
4.4: Volumetric flow rate vs. powder plug lengths for a pressure of 25.9 kPa.............60
4.5: Volumetric flow rate vs. powder plug lengths for a pressure of 17.2 kPa.............61
4.6: Actual powder path showing width and spread....................................................64
4.7: Powder deposition width vs. nozzle lengths.........................................................65
4.8: Powder deposition width vs. nozzle diameter.......................................................66
ix
4.9: Powder deposition width vs. nozzle height...........................................................67
4.10: Powder deposition width vs. slider collection system speed................................68
4.11: Powder deposition width vs. air pressure..............................................................69
4.12: Default parameter setup, original image (a), annotated image (b)..........................72
4.13: Nozzle height of 7.0 mm, original image (a), annotated image (b)..........................73
4.14: Nozzle height of 9.0 mm, original image (a), annotated image (b)..........................73
4.15: Nozzle length of 10.0 mm, original image (a), annotated image (b)........................74
4.16: Nozzle diameter of 3.0 mm, original image (a), annotated image (b)...................74
4.17: Transition from red to green powder, original image (a), annotated image (b).....76
5.1: Multiple Powder Deposition system....................................................................79
5.2: Proposed deposition nozzle...................................................................................81
5.3: Nozzle array positioning........................................................................................82
5.4: Powder deposition head.........................................................................................83
5.5: Proposed motion system for the multiple powder deposition system.................85
5.6: Portion of motion pattern for powder deposition.................................................86
5.7: Powder plugs passing through tubing....................................................................87
5.8: Plug loading system, 1 of 54 required....................................................................87
5.9: Operation of proportioning valve for powder composition adjustment................89
A.1: Base board............................................................................................................109
A.2: Track groove........................................................................................................110
x
A.3: Main support stand.............................................................................................111
A.4: Main air line 1......................................................................................................112
A.5: Main air line 2......................................................................................................113
A.6: Luer fitting............................................................................................................114
A.7: Luer lock...............................................................................................................115
A.8: Middle tubing.......................................................................................................116
A.9: Screw clamp.........................................................................................................117
A.10: Reducer fitting......................................................................................................118
A.11: Teflon tubing........................................................................................................119
A.12: Straight micro-fitting............................................................................................120
A.13: Nozzle .................................................................................................................121
A.14: Nozzle support....................................................................................................122
A.15: Motor spool.........................................................................................................123
A.16: Circuit support.....................................................................................................124
A.17: Slider top ............................................................................................................125
A.18: Slider bottom........................................................................................................126
A.19: Adhesive surface..................................................................................................127
A.20: Eye hook..............................................................................................................128
A.21: Funnel ..................................................................................................................129
A.22: Layout of fixtures attached to base board...........................................................130
xi
A.23: Layout of fixtures attached to main support.......................................................131
A.24: Exploded manifold assembly................................................................................132
A.25: Exploded nozzle assembly...................................................................................133
A.26: Exploded motor assembly....................................................................................134
A.27: Powder print head................................................................................................136
A.28: Proposed nozzle...................................................................................................137
A.29: Motion system.....................................................................................................138
A.30: Powder loading system........................................................................................139
xii
LIST OF TABLES
3.1: Laserlite™ LPC3000 polycarbonate properties....................................................34
4.1: Output voltage for potentiometer angular displacement increments.....................54
4.2: Time measurements for calibrated motor input voltage.........................................55
4.3: Linear velocity of slider collection for calibrated motor input voltages.................56
4.4: Deposited powder length for air pressure of 34.5 kPa and a linear velocity
of 192 mm/s............................................................................................................59
4.5: Volumetric flow rate for an air pressure of 34.5 kPa.............................................59
4.6: Deposited powder length for air pressure of 25.9 kPa and a linear velocity
of 192 mm/s............................................................................................................60
4.7: Volumetric flow rate for an air pressure of 25.9 kPa.............................................60
4.8: Deposited powder length for air pressure of 17.2 kPa and a linear velocity
of 192 mm/s............................................................................................................61
4.9: Volumetric flow rate for an air pressure of 17.2 kPa.............................................61
.10: Default parameter values........................................................................................64
4.11: Powder path width and spread for various nozzle lengths....................................65
4.12: Powder path width and spread for various nozzle diameters................................66
4.13: Powder path width and spread for various nozzle heights....................................67
4.14: Powder path width and spread for various slider collection system speeds.........68
xiii
4.15: Powder path width and spread for various air pressures......................................69
5.1: Laserlite™ LN4010 nylon compound properties.................................................91
5.2: Laserlite™ LNF5000 nylon compound properties...............................................91
5.3: Laserlite™ LWX2010 wax compound properties.................................................92
1
CHAPTER 1
INTRODUCTION
Solid freeform fabrication (SFF) is a set of modern technologies in which three-
dimensional solid objects are built directly from computer-aided design models. They are
additive processes which build objects by successively adding raw material in particles or
layers to create a solid volume of the desired shape [Burns93]. The major applications
for these objects include quick production prototypes for testing and design verification
and as patterns for casting and tooling. SFF can dramatically reduce the time and costs
required to bring a product to market.
Several technologies have been developed that accomplish these prototyping
objectives. They include photopolymer resin curing by ultraviolet light
(Stereolithograthy and Solid Ground Curing), thermal fusion of powders by laser scanning
(Selective Laser Sintering), joining of powders by ink-jet spreading of a binder (Three-
Dimensional Printing), compacting and sintering of powders (Freeform Powder Molding),
extruding heated thermoplastics (Fused Deposition Modeling), and laser cutting of
laminated sheets of paper (Laminated Object Manufacturing). SFF technologies are
progressing tremendously in terms of processes and materials used for the creation of
conceptual and functional parts, yet versatility in the selection of materials for each
system is still relatively limited and varies substantially for each specific system.
2
Current SFF technologies offer multiple material choices. However, they only
permit a single material for each region of a part. Changes among material composition are
discrete. These additive build processes have the potential to fabricate parts with
selective heterogeneous material compositions. Realizing this potential, and, in particular,
developing the capability to fabricate parts with continuously variable material
composition will revolutionize existing SFF and manufacturing capabilities.
The ability to fabricate parts with continuously changing material composition
throughout the part would give an endless control over final part characteristics.
Engineers may then begin to design at the microscopic level, optimizing material
composition to provide the final part properties desired. Not all SFF systems can readily
adapt to a heterogeneous material blend. Some are best suited for building with a
premixed uniform material type, at least within each layer. Selective Laser Sintering
(SLS), Three-Dimensional printing (3DP), and Freeform Powder Molding (FPM) on the
other hand, fabricate parts out of powder and hence have the greatest potential for
adapting to heterogeneous material compositions.
SLS and 3DP use similar processes to prototype parts. First, a computer solid
model of the prototype is generated and sliced appropriately to generate the layerwise
data. Next, a powder-leveling mechanism lays a thin (75 µm to 250 µm) uniform layer of
powder. The material is then bonded, by laser thermal fusion (SLS) or by liquid adhesion
(3DP), in the image of the corresponding two-dimensional slice. The unbound powder
3
Figure 1.1: Typical Selective Laser Sintering system [SFF Group95].
Figure 1.2: Typical Three-Dimensional Printing system [Michaels92].
remains for support. Finally, another layer of loose powder is deposited and the process
is repeated until the three-dimensional solid object is complete. Figures 1.1 and 1.2
illustrate the two fabrication systems.
4
The FPM operates with a slightly different process to prototype parts. Rock and
Gilman [Rock95] describe the fabrication process. First, the layerwise cross-sectional
data is generated from the CAD model. Next, a thin layer of powder is deposited. One
powder type is deposited in the locations corresponding to the model’s cross-sectional
data, and a second powder is deposited elsewhere for support. No details of the
deposition method for the FPM are provided. After each layer is deposited, it is leveled
and compacted by a vertical press. After the deposition of all of the layers, the entire
powder volume is sintered. The two powders have different thermal responses, thus
only the powder deposited in the model’s location will be sintered. The secondary
powder is then removed. Figure 1.3 illustrates this process.
Figure 1.3: Freeform Powder Molding process [Rock95].
Machines in both the SLS and 3DP processes currently use a powder leveling
drum to distribute uniform layers of powder. First a piston raises a small increment
exposing an amount of powder to the roller mechanism. The roller advances spreading the
5
powder over the buildspace. The roller returns in the opposite direction compacting the
powder, ensuring a near uniform layer. Figure 1.4 illustrates this process. This
application method allows for the use of multiple materials, but they must be mixed prior
to their distribution and therefore may be varied layer-by-layer at best. Development of
an alternate method of powder distribution is necessary in order to reach the potential of
building parts of heterogeneous material compositions, continuously varying in three
dimensions.
Figure 1.4: Powder leveling system [Lee93].
6
1.1 PROBLEM STATEMENT
There is a need to develop continuously heterogeneous materials for high-performance
fabrication and physical rapid prototyping. Because SFF is an additive building process,
built layer-by-layer, adjusting material composition throughout the entire part, or even
layer, is theoretically feasible. The use of fine powders as their build material makes SLS,
3DP, and FPM potentially adaptable to a heterogeneous material composition. However,
a new method of powder deposition is required that will allow for the selective placement
of multiple combinations of powder composition to make possible the fabrication of
continuously heterogeneous parts. The objective of this thesis is to explore a new
powder deposition approach and to design, fabricate, and test a low cost experimental
apparatus to illustrate its fundamental operating principles.
1.2 SOLUTION OVERVIEW
A viable replacement to the roller distribution system currently used in SLS is a
pneumatic conveying system. Pneumatic conveying is the movement of dry material
through an enclosed pipeline by the motion of air. It has been used successfully for many
years in the chemical and processing industries for the transportation of materials such as
flour, granular chemicals, lime, soda ash, plastic chips, and coal [Sadler49]. Several
advantages can be gained by pneumatic conveying. They include dust and contamination
free transportation with a great flexibility in routing. It allows for build materials to be
7
stored outside a prototyping device. It is easily automated and controlled, thus the
proportional mixing of several powders can be performed without manual interference.
A delivery system of this kind provides the potential for depositing continuously
heterogeneous powder layers for use in SLS, 3DP, and FPM. The feasibility of
employing such a system is shown by the design and fabrication of a low cost
experimental apparatus that explores the operating principles and performance
capabilities of a larger scale system. It demonstrates the ability to control the proportion
of three materials deposited through a single nozzle and the results are extrapolated to
estimate the performance of a large scale deposition system of this manner.
Figure 1.5 illustrates the fundamental components of the experimental apparatus
used for testing. A standard connection to a compressed air line supplies the pressurized
air. A filter and pressure regulator cleanse the compressed air of any impurities or surges.
A manifold distributes the pressurized air into three parallel lines. The air then travels
through a flow rate control which adjusts the air flow rate to a desired condition. Each of
three materials is transported through the three parallel lines, respectively. By adjusting
the airflow used in each individual pipe, different rates of mass flow are achieved for each
material. Thus, the proportion of each material deposited by the nozzle can be
controlled. The pipes merge to allow mixing before exiting through the same orifice in a
nozzle. The material is deposited ont a collection system for visual inspection and
8
Figure 1.5: Schematic of experimental apparatus.
analysis. The linear motion is provided by a motor and slider system capable of
adjustable and constant linear velocity.
1.3 THESIS OVERVIEW
Chapter Two reviews existing SFF material systems, methods of powder deposition, and,
in particular, pneumatic conveying.
Chapter Three provides a detailed description of the design and fabrication of the
experimental apparatus. Also discussed are methods of data collection and how they are
related to the performance of the experimental apparatus and a proposed larger scale
system.
Chapter Four includes the results and data analysis of the deposition achieved by
the experimental apparatus.
9
Chapter Five develops a conceptual design of a scaled up version and estimates
the performance that can be expected based on the performance results of the
experimental system.
Chapter Six remarks on the performance capabilities of the experimental system
and that expected for a larger scale model. It also analyzes the deficiencies expected in
delivering powder in a pneumatic conveying system and provides recommendations for
future work.
Finally, the Appendix includes a Bill of Materials, a parts list, and dimensioned
component drawings along with assembled and exploded drawings of the experimental
apparatus.
10
CHAPTER 2
LITERATURE REVIEW
Solid freeform fabrication (SFF) is a relatively new technology, yet tremendous progress
has been made in terms of the systems and materials. This chapter examines several
methods in which SFF systems are incorporating multiple materials and several methods
of material deposition, in particular pneumatic conveying. Section 2.1 reviews a few
processes which have added a secondary material for support generation. Section 2.2
examines methods of controlling the microstructure of fabricated parts. These methods
include dual material blends used for reducing the difficulty associated with fabricating
with ceramics and metals, selective material deposition within each layer, and depositing
material of controlled composition within each layer. Section 2.3 reviews the current
roller distribution system, alternative methods of depositing an entire layer of powder,
and ink-jet-style deposition. Section 2.4 examines the deposition method chosen for this
research, pneumatic conveying. Section 2.5 provides some general observations based
upon the literature reviewed.
2.1 SECONDARY SUPPORT MATERIALS
Layered manufacturing requires supports during fabrication to provide stability for
overhang structures. Some SFF systems have solved this problem by building support
11
structures using a temporary secondary support material. Examples are Solid Ground
Curing (SGC), Fused Deposition Modeling (FDM), Freeform Powder Molding (FPM),
and the Model Maker 3D Plotting System.
In the SGC process (Fig. 2.1), the part is fabricated using ultraviolet sensitive
photopolymer resin. During the processing of each layer, the uncured resin is removed
and wax is added to fill in the voids. Once solidified, the wax serves as a solid filler
providing complete support throughout fabrication [Burns93]. This petroleum soluble
wax is washed away during post-process finishing.
Figure 2.1: The Solid Ground Curing (SGC) process [Burns93].
12
The FDM system extrudes two materials using a dual nozzle system (Fig. 2.2).
One nozzle is used for dispensing the model material, and the second nozzle is used for
dispensing material for use as a base and support for the model material. The support
material is designed not to adhere well with the model material and is very brittle, thus it
can easily be removed during post-processing. Tests have also been performed using an
ammonium soluble ABS plastic as a support material [FDM96].
FDM head
Modeler tip Support tip
Model materialsource
Support materialsource
Figure 2.2: FDM head dual-material nozzles.
The FPM system described by Rock and Gilman [Rock95] uses a secondary
powder to contain the model material and provide three dimensional support (Fig. 1.3).
For each cross-section of the fabricated part, a primary powder type is deposited in the
locations corresponding to the part’s cross-sectional data and a secondary powder is
deposited elsewhere for total support. The two powders have different thermal
13
responses, thus during sintering only the primary powder will be sintered. The unbound
secondary powder is poured out during post-processing.
The Model Maker 3D Plotter (Fig. 2.3) distributes liquid thermoplastics and
waxes through a dual inkjet system [Sanders96]. The dispensed thermoplastic solidifies
to form the model and the dispensed wax solidifies to form the support structure. Similar
to SGC, the petroleum soluble wax is washed away during post-processing.
Figure 2.3: Model Maker 3D Plotter [Sanders96].
The dual material systems previously described have distinct areas of model and
support material. The support material serves no purpose in the final application of the
part and is used only as a means of building a better quality part. It is removed during
finishing and cleaning. However, these systems illustrate the potential of fabricating with
selectively placed materials throughout each layer.
14
2.2 SECONDARY MICROSTRUCTURE MATERIALS
Secondary materials have also been used in SFF to facilitate the fabrication of parts with
control over their microstructure. This section discusses several methods of gaining this
microstructure control. These include dual material blends used for reducing the difficulty
associated with fabricating with ceramics and metals, selective material deposition within
each layer, and depositing material of controlled composition within each layer.
Using a dual material blend for fabrication in SFF is not a recent advancement. In
this method, a secondary material is used to contain the primary material, usually ceramic
or metal, during fabrication. 3DP utilizes this approach to create ceramic [Yoo93] and
metal [Michaels92] preforms. The binder is selectively spread onto the powder bed,
either ceramic or metal, through ink-jet deposition and captures the powder in the three-
dimensional shape desired. After the preform is fabricated, a firing process removes the
binder and densifies the part.
SLS uses a similar dual material approach. In its indirect processing method, a low
melting temperature binder is uniformly premixed with the material of choice prior to the
sintering process [Bourell92] [Badrinarayan92]. After sintering, the binder is burnt off,
leaving behind the desired material. The sintering and debinding processes result in a
lower density “green” part. Post-processing is necessary to increase the strength and
utility of the part. Several methods have been studied, including infiltration [Deckard93]
15
[Vail92] [Tobin93] [Sindel94], hot isostatic pressing [Carter93], and heat treatments
[Prabhu93].
FDM and similar systems have also used a similar method. Agarwala et al.
[Agarwala95] have worked with fabricating with a ceramic powder blended within
polymer/wax based binder systems for use with FDM. Greul et al. [Greul95] have
developed a process similar to FDM called Multiphase Jet Solidification (MJS). They
deposit a blend of 50% binder (polymers and waxes) and 50% metal powders. After a
deposition process similar to FDM, the binder is removed in solvent and the porous
metal part is sintered to reach the final part density.
Other research has focused on adjustment of material content layer-by-layer.
Pegna [Pegna95] has been performing research using multiple reactant bulk materials to
form parts. In his experiments, alternate layers of sand and cement were applied. After
the preparation of each layer, they were placed into a pressurized steam chamber of
3 atmospheres and 300ºC. Successful bonding was achieved between the two reactant
materials. Applications of this sort were directed into the construction industry. Pegna
successfully combined multiple materials but they were done layer-by-layer and much
automation must be added to make this system feasible for rapid prototyping for each
layer of material was hand deposited and leveled.
Beck et al. [Beck92] report on a method of altering material content within each
layer. They developed the recursive mask and deposit, or MD*, process to fulfill this
16
need. The MD* process is a thermal spray shape deposition system. Thermal spray
methods are used to deposit thin, heated planar layers of material, which then solidify.
The part is built by spraying a succession of cross-sectional layers.
The MD* system has the potential for selective material deposition within each
layer, enabling multi-material parts to be produced during a single build process. The
process could be adapted to manufacture complete, integrated electromechanical
assemblies, including mechatronics. Mechatronics are parts that perform both mechanical
and electrical functions.
Selective control of the composition of particle clusters within each layer can also
be achieved by 3DP in its current configuration. The original powder composition
remains uniform, but variance in the particle clustering can be obtained by manipulating
the application of the binder. Multiple jets of different binder composition or
concentration could be used to build components with composition and density variations
on a fine scale [Cima92].
Cima and Sachs have been using a process similar to the original 3D Printing
device to build objects by controlling their microstructure and composition [Ashley95].
Rather than ejecting a binder, the nozzles eject a ceramic slurry. Alteration of the slurry
composition allows for changes in the material composition throughout the building cycle.
One unique application for this is products such as pills that release measured drug doses
at specified times during the day.
17
SFF devices often build parts on a point-by-point basis, and some devices allow
for the possibility of varying the composition and structure of particle clusters from
position to position with complete freedom. Several potential applications of this
building process include components with anisotropic thermal, electrical, or mechanical
properties or microengineered porosity [Cima92].
2.3 MATERIAL DEPOSITION
SLS and 3DP can benefit from an alternate powder deposition system. Not only could it
potentially allow for heterogeneous material composition, but it could also provide better
packed powder layers. There have been several alternative approaches of depositing
powder layers. This section reviews the current roller distribution system and several
alternative powder delivery methods including simultaneous deposition of entire layers
and deposition jet-style scanning.
The current powder deposition method, shown in Fig. 1.3, uses a powder leveling
drum to distribute a layer of powder. Yoo et al. [Yoo93] describe this deposition
process. First, the build area is lowered and a thin layer of loosely packed agglomerates is
created by spreading the powder over the build area with a counter-rotating spreader rod.
Next, the build area is raised slightly to expose the upper portion of the loosely packed
layer. The spreader rod is then rotated back across the build area to compact the powder
18
into a uniform layer rather than to sweep away the excess powder. This produces a
dense uniform layer upon which the sintering or binding is performed.
Van der Schueren and Kruth [Van der Schueren95] explored three methods of
powder deposition for a fabrication system based on the principles of SLS. The first
approach used a scraper blade to sweep powder over a build container (Fig. 2.4.a).
Several problems were reported. To ensure that the build plane was completely covered
with each new layer, a surplus of powder was needed at the beginning of each sweep.
The excess powder increased the weight of the material to be pushed, thus, there was an
increase in friction between the transported powder and the underlying layer. Another
problem arose from in the fixed intersection edge between scraper blade and the powder
surface. Any irregularity in the powder would be swept over the build surface and cause
unacceptable grooves in the surface. The final problem was the low powder bed density.
This would result in structurally weak parts with high porosity.
Figure 2.4: Possible methods for powder deposition [Van der Schueren95].
19
The second approach explored was the use of a counter rolling cylinder to sweep
the powder into place (Fig. 2.4.b). The rotary movement of the cylinder caused any
irregularity in the powder appearing in the intersecting edge between the cylinder and the
powder surface to be dissipated because the irregularity will only remain in the path of
the roller momentarily. This method also allows for compacting the powder while its
distributed by simultaneously applying a vertical vibration during deposition. This
vibratory counter rotating method is similar to the approach taken by Yoo et al. [Yoo93].
The third and final approach explored was the one chosen for implementation into
Van der Schueren and Kruth’s fabrication system. It delivers powder in a slot feed
mechanism (Fig. 2.4.c). The powder flows vertically out of the feeder when there is a gap
between the slot and the substrate. This method significantly reduces the friction created
in the other two methods. However, like the scraper blade method, it minimally
compacts the powder bed. Van der Schueren and Kruth therefore employ a vibratory
counter rolling cylinder in combination with the slot feeder in order to deposit the powder
with minimal friction while compacting the powder. An alternative compaction method
would be to apply a vertical press, as is done in the FPM process [Rock95]. This
method removes the contact edge and sliding friction problems. Any irregularity in the
powder bed will not be propagated throughout the rest of the build plane.
Melvin and Beaman [Melvin91] at the University of Texas at Austin developed
an electrostatic powder application system. It delivers the powder from a storage
20
container using compressed air, passes the powder particles through an electric field, and
sprays them onto a grounded plate. The experimental apparatus consisted of a powder
canister which holds and delivers powder similar to an aerosol can, flow control devices
that regulate compressed air flow into the canister, and air flow that carries the air from
the powder canister to the feed head. The powder leaves the canister and enters the feed
head which forces it through a spray nozzle and electromagnetically charges it. Two 13.5
kV, 0.31 mA power sources charge the powder in the nozzle. One source delivers the
positive charge, while the other source delivers the negative charge. The layer thickness
ranged from 0.012” (0.30 mm) to 0.016” (0.41 mm) with a variance of 0.002” (0.05 mm)
to 0.003” (0.08 mm). The new application method showed a reduction in the number of
air pockets as compared to the roller deposition method. Clogging was observed each
time the powder flow was cut off. These clogs were easily cleared using a pressurized air
purge.
Rather than depositing the powder by an entire layer, the method of powder
deposition developed in the work of this thesis experiment explores the use of ink-jet
style deposition using an array of nozzles for deposition. Multiple nozzles help to
reduce the time required to deposit a layer of powder over a given area. Their use is not
uncommon in either SFF technologies or in the printing industry. 3D Printing has used an
eight-nozzle print head [Ashley95] and a 32 nozzle print head [Michaels92] to distribute
the binder. Recent printheads have grown in size to 1280 jets [Ashley95]. These nozzle
21
clusters are moved at linear speeds reported between 1.65 [Sachs92] and 2.5 m/s
[Cima92].
In the printing industry, high resolutions must be maintained without any sacrifice
in speed. Arrays of 128 bubble jet nozzles have been produced on one chip with a nozzle
density of 12 nozzles/mm [Sachs92]. An example of a high speed application is the
“Djit” printer from Diconix, Inc. It uses a line printing bar containing 1500 jets to print at
linear speeds up to 5 m/s [Heinzel85].
The deposition requirements proposed for this thesis are unique. The deposition
systems previously discussed are only capable of depositing single materials, at least
within each layer. Adapting to an ink-jet style of powder deposition would provide the
potential for selective powder placement. Current ink-jet systems cannot accommodate
the deposition of powder due to the larger particle sizes and higher viscosity of the
powder. Alternative powder transportation methods need to be explored.
2.4 PNEUMATIC CONVEYING
The flow of powders represents a greater challenge than that of the low viscosity binder
and printing ink. Geldart [Geldart90] states that powders are not solids, although they
can withstand some deformation. Powders are not liquid, although they can be made to
flow. Powders are not gases, although they can be compressed. The space between
22
particles is filled with gases and therefore the solid/gas interaction, the interparticle
contact area, and adhesion between particles must be considered during flow analysis.
In the past, solids were commonly transported in suspension form via lean-phase
conveying. Lean-phase conveying is defined by the low volumetric concentration of
solids, typically less than ten percent. Conveying in this form may cause a few
problems. There is a high rate of pipe wear and particle attrition due to the high
velocities. Depositing powder in this manner for use in SLS, 3DP, or FPM would also
have several other undesirous effects. For instance, it is likely to deposit the powder
with too large a force and disturb the underlying powder bed, and the control over the
accuracy of desired powder compositions may be lost due to the local sandstorm effect.
An alternate transportation approach is dense-phase conveying. It is defined as
the conveying of particles by air along a pipe that is filled with particles at one or more
cross-sections [Konrad86]. It offers improvements in slower air speeds and lower
volumes of gas required for transporting the same amount of material, which is of
particular importance if an inert gas is needed to reduce the risk of explosions [Konrad86].
The work of Albright et al. [Albright51] was one of the first to study dense-phase
conveying. The purpose was to minimize the amount of gas required to feed solids into a
coal gasification reactor. Since then, its use has grown substantially.
Dense-phase conveying appears more appropriate for powder deposition in SLS,
3DP, and FPM. The slower air speeds are less likely to disturb the previously deposited
23
powder bed and greater control over powder placement should be gained. The lower
volumes of air should also benefit the SLS process which uses an inert gas, usually
nitrogen, in the process chamber to prevent oxidation or explosions [Behrendt95].
Dense-phase conveying is not without problems. Since a section of the pipeline is
completely filled with material, clogging can be a common occurrence and the flow
somewhat unstable. One approach to reduce this clogging has been to artificially create
distinct plugs of material. Conveying in this form is known as plug-phase conveying.
Konrad [Konrad86] reports of three commercial systems developed to deal
exclusively with plug-phase conveying. In the first, a bypass system was developed by
Lippert [Lippert66] to stabilize pressure fluctuations in plug-phase conveying (Fig. 2.5).
The conveying pipeline has an additional bypass pipeline in which there are holes at set
intervals. No additional air is blown into this bypass pipe. It merely serves as an
alternative route for the conveying air to break up lengthy plugs.
Inner TubeFluted Nozzles
Figure 2.5: Internal secondary air pipe system [Mainwaring93].
The other two systems artificially induce plugs of solid material separated by the
addition of a secondary air source. There are distinct plugs of material separated by plugs
24
of air. One such system uses an air knife to provide regular pulses of air that will chop
up the moving solids fed into the pipeline (Fig. 2.6). In the other system, the plugs are
created by using alternating air valves (Fig. 2.7).
Figure 2.6: Plug formation using timer operated air knife [Geldart90].
Figure 2.7: Plug formation using alternating air valves [Geldart90].
25
SLS, 3DP, and FPM could use a powder delivery system similar to the
commercial plug-phase conveying systems utilizing an additional air source with some
modifications. These systems do not provide a continuos flow of powder, but this can be
overcome by running two parallel pipelines operating at opposite phases. They would
also require the addition of a dispensing nozzle to mix and to accurately deposit the
powder onto the underlying powder bed. Various powders must be delivered to the
dispensing nozzle through different tubing systems for continuously heterogeneous
material deposition. By controlling the velocity of plugs throughout each of the material
supply systems, different proportions of each material could be combined and deposited.
Unfortunately, experimental evidence available in literature regarding plug-phase
conveying is contradictory [Konrad81]. Lippert [Lippert66] measured the pressure drop
across individual moving plugs in a horizontal pipeline. The pressure drop was found to
be proportional to the plug length squared divided by the pipe diameter. Flain [Flain72]
used a fine cohesive material to construct plugs of a given length in a horizontal pipeline.
He then measured the pressure drop required to move these plugs and concluded that this
pressure drop was proportional to the square of the plug length. While they agree that
the pressure drop is proportional to the square of the plug length for fine cohesive
material, Dickson et al. [Dickson78], however, suggest a linear relationship for both
coarse and fine materials. Fine powders are defined as those with particle sizes between 1
26
and 100 µm [Brown70] which are also commonly used in SLS [Lakshminarayan92] and
3DP [Sachs92].
Konrad [Konrad86] has done extensive work on plug-phase pneumatic conveying.
He found that solid materials flow in discrete plugs that fill the tube cross-section at
approximately maximum packing density. Between the plugs, the upper part of the pipe
contains moving air with some dispersed particles. The lower half is filled with a
stationary bed of particles (Fig. 2.8). Each plug picks up the particle bed in front of it,
while leaving behind a stationary layer of nearly equal thickness behind it.
Air
Stationary particles
Moving particles
Figure 2.8: Flow pattern of plug-phase conveying.
Konrad et al. [Konrad80] developed a theoretical model to predict the pipeline
pressure drop in horizontal dense-phase plug conveying. The theory is based on the
following premises. The material is conveyed only in the plugs and in the regions just in
front of and behind them. There is a layer of stationary material between the plugs. The
27
flow pattern resembles that of a gas-liquid system. The pressure drop required to move a
single horizontal plug is therefore given by the expression:
( ) ( )∆P
H
F
D
c
D DB w
w w w w w wk k c= + ++ ⋅ +
+24 4 1 4cos cos
(2.1)
where,
H is the plug length. ρB is the bulk density.
g is the acceleration due to gravity. µw is tan φw.
φw is the angle of wall friction. φ is the angle of internal friction.
kw is the Jansen coefficient at the wall. D is the pipe diameter.
F is the stress on the front end of the plug. C is the interparticle cohesion.
ω = sin-1 (sin φw / sin φ). Cw is the particle wall cohesion.
This equation can only be used to predict general flow trends in the experimental
apparatus since it uses multiple tubing systems with several of the above parameters
being unknown. There is therefore a need to collect experimental data for each of these
tubing systems in order to determine these parameters and thereby enable the use of this
equation.
28
2.5 OBSERVATIONS
The literature has shown that incorporating multiple material systems into SFF
technologies is beneficial in fabricating for a variety of utilities. The use of multiple
materials is evolving from a temporary fabrication aid to a tool for producing parts with
“spatially tailored material properties” [Rock95] for a variety of applications. SLS, 3DP,
and FPM could benefit from an alternative deposition mechanism that will provide
potential for fabrication of three-dimensional heterogeneous parts. For this research, the
deposition mechanism combines principles from ink-jet printing and plug-phase
pneumatic conveying for potentially delivering powder with continuously heterogeneous
material composition.
29
CHAPTER 3
METHODS
This thesis explores the feasibility of implementing a plug-phase pneumatic conveying
powder delivery system for three-dimensional continuously heterogeneous material
deposition for potential use in SLS, 3DP, and FPM. This chapter describes the
experimental apparatus developed to explore deposition of this kind. Section 3.1
describes the design, fabrication and operation of the experimental apparatus. This
includes the powder preparation, air handling system, and mechanized slider collection
system. Section 3.2 describes the experimental setup and testing procedure for each
evaluation of the performance of the experimental apparatus.
3.1 EXPERIMENTAL APPARATUS
An experimental apparatus (Fig. 3.1) was designed and built to demonstrate the
deposition characteristics achievable by plug-phase pneumatic conveying powder
delivery. The apparatus is capable of depositing a narrow path of powder in a thin layer
with selectively variable powder composition.
30
Figure 3.1: Complete experimental apparatus.
The experimental apparatus consists of two major functional components. One is
the pressurized air handling system, which is responsible for transporting the multiple
powders through the tubing system, delivering them to the nozzle in appropriate
quantities, and discharging them through the nozzle orifice in the composition desired
(Fig. 3.2). The other major functional component is the mechanized slider collection
system, which is responsible for providing linear motion of constant, yet adjustable,
velocity for the collection of an even distribution of deposited powder (Fig. 3.3).
31
Figure 3.2: Air handling and powder deposition components.
Figure 3.3: Mechanized slider collection components.
32
This experimental apparatus demonstrates the feasibility of depositing powder
with selectively continuous heterogeneous composition. It simulates the operation of
commercial plug-phase conveying systems, such as those discussed in Section 2.4, by
manually creating and transporting a single plug. In order to deposit multiple powders
simultaneously, three plug-conveying systems are run in parallel, with each transporting a
different powder. They join at a nozzle prior to being deposited (Fig. 3.4). Pressurized
air is used to propel the plugs of powder through the tubing. Control of the air flow rate
driving each plug may be used to control the rate at which each of the parallel plugs reach
the nozzle, thus controlling the contribution each plug makes to the final deposited
powder composition.
Parallel powder plugs
Intersection of parallel tubing systems at nozzle
Discharge point (nozzle)
Mixed powder
Air
Air
Air
Figure 3.4: Multiple plugs passing through parallel tubing system.
33
The following sections discuss the material preparation and the major components
and functions of the experimental apparatus used for continuously heterogeneous powder
deposition by plug-phase pneumatic conveying.
3.1.1 POWDER PREPARATION & LOADING
The material used for deposition in the experiments was Laserlite LPC3000
polycarbonate which was supplied by the DTM Corporation. It is one of the materials
available for use in DTM’s commercial Sinterstation 2000 systems (SLS processes). It
was chosen for this experimental work for several reasons. Many deposition methods
require the powder transported to be able to flow with a minimal amount of shear applied
to it [Sachs92]. The polycarbonate’s spherical particle shape allows for easy flow.
Guidelines suggest that the inside diameter of the pipe should be at least three times that
of the largest particle size to avoid potential pipe blockage [Marcus90]. Even with the
miniature tubing inner diameters used, 1.5 mm, the particle sizes, 30 - 175 µm, are well
within this limit. Polycarbonate is not easily affected by environmental conditions, and its
material properties should therefore remain consistent throughout testing. The general
properties of Laserlite LPC3000 polycarbonate are shown in Tbl. 3.1.
34
Table 3.1: Laserlite™ LPC3000 polycarbonate properties [DTM94c].
General Properties Value Test MethodSpecific Gravity, 20ºC 1.20 g/cm3 ASTM D792Moisture Absorption, 20ºC, 0.35% ASTM D570 65% relative humidityPowder Tap Density 0.62 g/cm3 ASTM D4164Volume Average Particle Size 90 microns laser diffractionParticle Size Range, 90% 30-175 microns laser diffraction
The polycarbonate was also chosen because it is amorphous and hence easily
dyed. This was an important consideration because the deposited powder composition
was to be visually inspected. Quantities of powder were therefore dyed red, green, and
blue. The dying procedure was performed by diluting standard food coloring with water
and mixing it with a quantity of polycarbonate powder. This resulted in a thick slurry
with much conglomeration (i.e., groups of particles adhered together). The mixture was
dried at 70°C for several hours with periodic mixing. During mixing, all conglomerates
were broken to prevent an increase in particle size. The drying and mixing processes
returned the powder to characteristics similar to the original with exception of its color.
The powder was sealed in a bag with desiccant to eliminate the potential effects of
atmospheric humidity during long-term storage.
To simulate the operation principles of the plug-phase pneumatic conveying
systems commercially available, the plugs were manually created. Prior to pressurizing
the tubing system, a single plug of loosely compacted powder was formed in each of the
three polyurethane tubing lines. They were created by bending a section of tubing and
filling it with powder via a funnel (Fig. 3.5). Once pressurized, the plugs were pushed
35
into the smaller diameter Teflon tubing and their density increased significantly. The
powder would then be ready to flow through the tubing in plug form until discharging out
the nozzle (Fig. 3.6).
Funnel
Polyurethanetubing
Figure 3.5: Loading of powder into polyurethane tubing.
Tubing
Powder plug
Figure 3.6: Powder plug moving through tubing.
3.1.2 AIR HANDLING SYSTEM
Pressurized air is used to propel the transported material within a pneumatic conveying
system. For this experiment, the air was provided through connections to high-pressure
air lines. These connections include a pressure regulator to eliminate pressure surges from
36
the supplied air and a controller and gage to adjust the pressure of the dispensed air. Two
connections are used to provide two independently controlled pressure sources. An end
of each main air line of the experimental apparatus slips over barbed fitting to connect to
the pressurized air source. These two main air lines are constructed of polyurethane
tubing that are 1.8 m and 3.4 m long, respectively, and each with an inner diameter of 3.2
mm and an outer diameter of 6.4 mm. The other end of each tube is connected to a three
port manifold. The manifold receives pressurized air from the two independently
controlled pressure sources and distributes it among three parallel tubing lines in
proportions determined by desired powder compositions (Fig. 3.7).
Main air lines
Three port manifold
Figure 3.7: Main air line division by three port manifold.
Each of these three parallel tubing lines is loaded with powder prior to
pressurizing the lines (see Section 3.1.1). Screw clamps are used to constrict the diameter
of each of the parallel tubing lines and thereby allowing for variance of the air flow rates in
each tube (Fig. 3.8). They are located prior to the powder plugs. They therefore affect
37
the air flow rate that reaches the powder plugs rather than adjusting the diameter of tubing
that the plugs pass through. Each tube has a length of 210 mm, with an inner diameter of
3.2 mm and an outer diameter of 6.4 mm, and is constructed of a flexible material
(polyurethane) to allow for easy routing and compression by the screw clamp. Each of
these parallel tubes is connected to smaller tubing line for delivery to the nozzle by a
reducing compression fitting. Each of the three smaller tubes have a 1.6 mm inner
diameter, a 3.2 mm outer diameter, and a length of 120 mm. Each of these smaller tubes is
coated in Teflon since its extremely low coefficient of friction facilitates the easy passage
of the powder plugs.
Screw clamp
Flexibletubing
Figure 3.8: Screw clamp operating as flow rate controller.
Each Teflon tube is connected to the deposition nozzle by straight compression
fittings (Fig. 3.9). The nozzle was fabricated out of ABS plastic with a FDM1600 rapid
prototyping system. The nozzle has three equally spaced tubing extensions, each
38
extending 30 degrees from the vertical with an inner diameter of 1.5 mm, an outer diameter
of 3.5 mm, and length of 15.0 mm. These tubing extensions attach to the material supply
lines. These extensions join and merge together through a 1.5 mm diameter passage of
length 6.0 mm before reaching the nozzle’s exit orifice. Alternative nozzles were
fabricated for testing of different lengths and diameters. The nozzle is held stationary in a
support bracket (Fig. 3.10). This support bracket provides three positions of various
vertical heights for nozzle placement and can be positioned along the track groove in a
variety of positions using Velcro adhesion.
Nozzle
Straight compressionfittings
Powder supplytubing
Figure 3.9: Nozzle with supply tubing and connections.
39
Figure 3.10: Nozzle and support stand.
Proper design and fabrication of the nozzle is fundamental to the success of the
powder deposition. The nozzle has three equally-spaced identical extensions to accept
powder from three sources, respectively. They are aligned 30 degrees from the vertical to
provide sufficient space for the fittings connecting it to the Teflon tubing. Too large an
angle could attribute to too severe a deviation in the flow pattern and too small an angle
would require tremendous lengthening of the extensions to provide sufficient space for the
fittings. The extensions join and merge together to allow for mixing of the powders
reaching the nozzle before discharging them through the nozzle orifice.
The fabrication process characteristics inherent to a FDM1600 system were a
significant factor in the design of the nozzle. It is important not to create areas where
support material is required and would be embedded inside the nozzle, and where it
would be difficult, if not impossible, to remove. Also, there is a great concern for the
surface quality of the interior passages of the nozzle. Severe stair-stepping patterns
between adjacent horizontal layers (Fig. 3.11) should be avoided or they will disrupt
40
Stair-step pattern observed betweenadjoining layers
Figure 3.11: Severe stair stepping between adjacent layers.
powder flow. Stairsteps become more pronounced as the part surface becomes more
horizontal.
After nozzle fabrication, finishing was required to minimize any stray ABS plastic
strands or fabrication inconsistencies from the interior of the nozzles. This was
performed by washing the nozzle, alternatingly with acetone and water. Acetone etched
away the ABS plastic and water cleansed the plastic of the solvent to eliminate any
residual effects. These fluids were flushed through the nozzle interior passages with eye
droppers. Next, the interior passages of the nozzle were bored out to ensure that each
passageway was clear of obtrusions. A final acetone and water wash was performed on
the nozzle interior. Finally, a light sanding was performed to enhance exterior surface
quality.
41
3.1.3 MECHANIZED SLIDER COLLECTION SYSTEM
A mechanized slider collection system was developed to provide a relative linear motion
of the powder delivery head. The system provides constant, adjustable head velocity and
collects the discharged powder such that it may be saved for subsequent evaluation.
The slider collection system is propelled by a Delco gear-reduced ball-bearing
motor which operates at 200 rpm with no load and a power supply of 12 VDC and
1 amp. A DC power supply plugged into a 115 VAC outlet supplies the requisite power
to run the motor. The velocity of the slider collection system can be controlled so that
the powder deposition may be analyzed at several linear speeds. This is provided
through an adjustable speed control circuit which controls the motor shaft’s angular
velocity and thus the velocity of the slider collection system. The motor control circuit
uses pulse-width modulation through an integrated circuit (IC) chip to provide a control
range of 5% to 98% by simple adjustment of the potentiometer knob. The motor
controller circuit diagram is shown in Fig. 3.12.
42
Figure 3.12: Motor control circuit diagram.
To translate the angular displacement of the motor shaft into linear motion of the
slider system, a reel system was developed. Attached to the motor shaft is a spool used
for the winding of a thin thread and thereby pulling the slider system (Fig. 3.13). The
spool has an outer diameter of 30 mm and was fabricated out of ABS plastic with a
FDM1600 system. Its diameter was made sufficiently large so that changes in the
diameter caused by the wrapping of the nylon thread would produce only negligible error.
The thread passes through an eye-hook to properly guide it before attaching to the slider
system. Extra thread can be let out to allow the motor to operate for a few seconds
before pulling the slider system. This allows the motor to ramp up and reach its steady
state angular velocity.
43
MotorSpool
Thread
Eye hook
Figure 3.13: Motor and thread winding system.
The slider system consists of several components. A track is used to guide the
slider platform in a straight line travel path. It has a travel surface area 500 mm wide and
700 mm long, with 10 mm high flanges on the sides acting as retaining walls. The slider
platform slides freely within the track flanges; loose enough to not cause too severe
friction, but securely enough to not deviate from straight line motion. It is attached to the
thread and is pulled with a constant velocity by the motor. The slider platform also is
used to transport the powder collection system.
The powder collection system consists of two parts (Fig. 3.14). One part is the
collection tray. It fits securely on top of the slider platform. The other part is a thin
adhesive surface. This fits underneath the tray and the adhesive surface is exposed to the
discharged powder through an opening in the collection tray. During the linear translation
of the slider system, powder is collected on the adhesive surface for subsequent analysis.
44
Slider platform
Collection tray
Adhesive surface
Thread
Figure 3.14: Exploded slider collection system.
Calibration of the motor and slider collection system is necessary for powder
deposition data analysis. This was performed by measuring the voltage supplied to the
motor and the corresponding average velocity of the slider system for various increments
of the potentiometer. A template is used to mark positions of the potentiometer knob in
18 degree increments (Fig. 3.15). Voltages are read for each of these positions along with
corresponding average linear velocities of the slider collection system for several of the
positions.
45
Figure 3.15: Potentiometer and calibration template.
3.2 EXPERIMENTAL SETUP
Several characteristics of the experimental apparatus dictate the performance capabilities
of deposition systems based on similar principles. One such is the volumetric flow rate
of the discharged powder for a variety of pressure and plug length combinations. The rate
at which the powder can be discharged through the nozzle is essential in estimating the
time required for the deposition of an entire layer. Also of importance is the
characteristics of the powder being deposited. The path shape width, height, and spread
of the powder is essential to providing consistent layers of powder. Finally, the ability
to deposit multiple powders needs to be observed. Tests were designed to observe the
mixing capabilities of the system and the ability of changing material composition on the
fly. The following sections describe the purpose of each experiment and the experimental
methods and details. These experiments include estimating the volumetric flow rate,
evaluating the powder path characteristics, and exploring multiple powder deposition.
46
3.2.1 ESTIMATING THE VOLUMETRIC FLOW RATE
One of the beneficial characteristics of plug-phase pneumatic conveying is its ability to
deliver large volumes of powder of consistent density with a minimal air pressure.
Minimizing the pressure required is necessary to reduce the potential for disturbing the
underlying powder bed. However, minimizing the air pressure also decreases the
volumetric flow rate and increases the time required for depositing a single layer of
powder. Tests were therefore designed to estimate the average volumetric flow rate for
several plug length and pressure combinations.
A single plug of powder is created in one of the supply tubes. Air flow at a set
pressure is provided to this one supply tube only. The reason for testing with a single
tube is that air is delivered to the manifold at a set pressure. The manifold distributes the
air to each of the three supply tubes. The air will take a preferential route in which the
pressure drop is least [Mainwaring93]. This may cause the pressure among the parallel
tubes to fluctuate and be disproportional.
After creating a plug in one of the tubing systems and an input voltage is selected
to be sent to the motor, the volumetric flow rate can be determined for various plug length
and pressure combinations. By measuring the length of the deposited powder, DL, with a
constant linear velocity for the slider and collection system, the average volumetric flow
rate ,Q, can be determined by performing the following calculation:
47
Q = PL · Ø2· π V (3.1) 4 DL
where PL is the powder plug length, Ø is the tubing inner diameter, and V is the average
linear velocity of the slider collection system.
One of the major performance criteria of implementing a new powder delivery
system is the rate at which each layer of powder is deposited. SFF technologies are
continuously trying to minimize fabrication times, so creating a large delay in the powder
deposition stage would be undesirable. For comparison, Sachs et al. [Sachs92] report of
times on the order of 0.1 - 1.0 second per layer. In the case of the experimental
apparatus, the deposition time, t, for a single layer given a buildspace of width, w, length,
l, and height, h, can be estimated by:
t = w · l · h (3.2) Q
Note that these times are not exact because of the acceleration and deceleration required at
the start and finish of each pass.
This time can be further reduced to:
t = w · l · h (3.3) Q ⋅ n
48
by using an array of n nozzles. Ideally, the number of nozzles in the powder distribution
head would equal the workspace width divided by the deposition width of the powder
from each nozzle. This would provide for a single-pass, single-direction deposition.
Development of a high speed deposition system is not the focus of this research.
However, selective placement of a controlled powder composition is of greater
importance. Further tests were performed to evaluate the deposition characteristics while
changing several parameters of the experimental apparatus setup.
3.2.2 EVALUATING THE POWDER PATH CHARACTERISTICS
Another area of importance in evaluating the performance of the experimental apparatus
is the characteristics of the path of powder deposited. This is necessary for evaluating
the tolerance to which multiple materials may be placed and in developing the spatial
positioning required for an array of nozzles. Control of the deposited powder is
essential. It should not be randomly sprayed, but be placed in the locations desired.
A test was designed to evaluate the effects that several parameters had on the path
of the deposited powder. For each test run, a single plug of powder is created in one of
the supply tubes. Then the pressure and voltage are set accordingly, and a single pass
deposition test is run to evaluate the effect of changing a single design parameter on the
deposition characteristics of the powder path.
49
The parameters to be examined include the nozzle length (L), nozzle diameter (Ø),
the deposition height (H), the velocity of the slider collection system (V), and the
volumetric flow rate (Q). Figure 3.16 shows these parameters. The characteristics of the
powder path of interest are the height (h), width (w) of the main proportion of powder,
and the total spread (s) of powder. Figure 3.17 illustrates the observed characteristics.
H
L
Ø
Q
V
Nozzle
Slider/Collection System
Figure 3.16: Parameters tested for deposition characteristics.
h
sw
Figure 3.17: Deposited powder path characteristics evaluated.
50
3.2.3 EXPLORING MULTIPLE POWDER DEPOSITION
The experiments previously described show the capabilities of depositing a single
material. In order to make a significant improvement over deposition methods previously
done, the simultaneous deposition of multiple materials needs to be achieved. A series of
experiments were designed to evaluate the mixing performance of the experimental
apparatus using a dual powder delivery: Two powder plugs of different color are loaded
in two supply tubes, respectively. The air from the two pressure sources are set equal
and directed through the manifold to each polyurethane tube. Single pass deposition runs
are performed to evaluate the effects that the nozzle height and nozzle diameter each have
on the mixing of the deposited powder. These experiments, however, are not data
intensive because of the limited control the experimental apparatus has over the
pressurized air. Instead they qualitatively show the feasibility of multiple material
deposition in such a manner.
The path of powder is inspected visually for the major mixing characteristics
evident for each of the parameters. Characteristics to be observed are segregation among
the different colored powder and the spatial pattern and thoroughness of the mixing that
occurs.
Another series of trials is attempted to demonstrate transitions among powder
composition. These are performed similar to the mixing experiments with one exception.
Instead of setting the plugs in motion simultaneously, one plug is sent slightly ahead of
51
the other. The air supplied to the leading plug is stopped at approximately the time air is
supplied to the trailing plug. A transition between the powder composition should be
observed. However, exact measurements are not possible due to the simplicity of the
controls of the experimental apparatus.
52
CHAPTER 4
RESULTS
An experimental apparatus has been designed and built to explore the feasibility of
employing plug-phase pneumatic conveying to deposit powder in SLS, 3DP, and FPM in
a manner that extends their fabrication capabilities to include continuous heterogeneous
material composition. Several tests were performed to evaluate the experimental
apparatus’s performance both quantitatively and qualitatively. These included
determining the linear velocity of the slider system, the volumetric flow rate of
corresponding air pressure and plug lengths, the powder path characteristics for several
design parameters and its ability for depositing heterogeneous compositions. The
following sections describe these experiments in more detail.
4.1 LINEAR VELOCITY EXPERIMENTS
The mechanized slider collection system was designed to travel at a constant, yet
adjustable, linear velocity. A series of experiments were performed to calibrate the motor
controller with the various linear velocities within the slider collection system’s operating
range. First, the motor input voltages were found in relation to the corresponding angular
increments of the potentiometer knob. Next, the average linear velocity of the slider
collection system was tested for various motor input voltages.
53
A template was created to divide the angular displacement of the potentiometer
knob into 18 degree increments (Fig. 3.15). For each of these positions, the output
voltage from the control circuit was measured. Table 4.1 shows the average output
voltage for corresponding potentiometer angular displacements from its origin. Figure 4.1
is a plot of the average output voltage with the addition of a maximum and minimum
voltage measured for each data set.
A series of tests were performed to determine the average linear velocity of the
slider collection system for several of the calibrated voltages. A pair of marks were made
within the track groove at an increment of 300 mm. The front of the slider collection tray
was positioned 35 mm behind the starting point to allow the slider collection system to
begin motion before measurements were taken, and thus, diminish the initial acceleration
from the linear velocity measurement. Also, a significant extra amount length of thread
was unwound from the spool to allow the motor to reach steady-state angular velocity
before pulling the slider tray. For each incremented potentiometer position, ten tests
were performed to measure the time required for the slider collection system to travel
300 mm. Note that lower voltages are not included in the data for they were not
sufficient to turn the motor shaft and pull the slider system. Also, higher voltages are not
included for they produced too extreme a velocity and measurement became severely
uncertain. The results are shown in Tbl. 4.2.
54
Table 4.1: Output voltage for potentiometer angular displacement increments.
Potentiometer Angular
Displacement (degrees)
Average Output
Voltage (Volts)
0 0.03418 0.03836 0.05254 0.5772 1.3790 2.3108 3.2126 4.2144 5.3162 6.4180 7.2198 8.1216 9.2234 9.7252 10.5270 11.2288 11.3306 11.4
Output Voltage vs. Angular Displacement of Potentiometer
0
2
4
6
8
10
12
0 18 36 54 72 90 108 126 144 162 180 198 216 234 252 270 288 306
Angular Displacement of Potentiometer (degrees)
Figure 4.1: Output voltage vs. angular displacement of potentiometer.
55
Table 4.2: Time measurements for calibrated motor input voltage.
VOLTAGE 4.3 V 5.3 V 6.3 V 7.5 V 8.5 V
time # 1 (s) 4.03 1.57 1.03 0.85 0.78
time # 2 (s) 3.97 1.50 1.00 0.84 0.82
time # 3 (s) 3.97 1.53 1.09 0.88 0.75
time # 4 (s) 3.87 1.50 1.06 0.85 0.72
time # 5 (s) 3.90 1.59 1.03 0.84 0.80
time # 6 (s) 3.94 1.57 1.07 0.88 0.84
time # 7 (s) 3.91 1.59 1.10 0.87 0.72
time # 8 (s) 3.94 1.62 0.99 0.89 0.79
time # 9 (s) 3.91 1.57 1.07 0.88 0.81
time # 10 (s) 3.95 1.60 1.05 0.90 0.81
mean (s) 3.94 1.56 1.05 0.87 0.78
standard deviation 0.0451 0.0412 0.0363 0.0215 0.0414
The average linear velocity of the slider collection system for each calibrated
voltage was then calculated by dividing the distance traveled (300 mm) by the time
measured for the linear translation to occur. The corresponding average linear velocities
are shown in Tbl. 4.3. These values along with maximum and minimum velocities
calculated for each data set are plotted in Fig. 4.2.
56
Table 4.3: Linear velocity of slider collection for calibrated motor input voltages.
Average Input Voltage (volts)Average Linear Velocity (mm/s)4.2 765.3 1926.4 2867.2 3458.1 385
Velocity of Slider System vs. Average Motor Input Voltage
0
50
100
150
200
250
300
350
400
450
4 5 6 7 8 9
Input Voltage (V)
Figure 4.2: Velocity of slider system vs. motor input voltage.
As seen by the low standard deviations in the time measurements, the linear
velocity calculated for the slider collection system is fairly accurate. This calibration is
not intended for use solely as an evaluation of the performance of the mechanized slider
collection system; rather it is to be used as a means for calculating other data.
57
4.2 POWDER VOLUMETRIC FLOW RATES
The volumetric flow rate achievable by the experimental apparatus will influence
deposition by plug-phase pneumatic conveying acceptance into the SFF industry. SFF
technologies are steadily trying to improve upon their build times in all stages of the
fabrication process. The time required for the deposition of an entire layer is directly
related to the volumetric flow rate of the powder. A series of measurements were taken
to calculate the volumetric flow rate of the discharged powder for a variety of pressure
and plug length combinations. Air pressure was kept at a minimum to reduce the force
that the powder would have on the underlying powder bed and to ensure that a transition
to lean-phase conveying did not occur.
A variety of plug length and pressure combinations were tested to determine the
volumetric flow rate of the polycarbonate powder. The plug lengths ranged from 40 mm
to 80 mm and the air pressures ranged from 17.2 kPa (2.5 psi) to 34.5 kPa (5.0 psi). The
longer plug lengths were not used in combination with the lower air pressures since there
was not enough pressure to transport the powder.
First, a single plug of powder is created in one of the supply tubes. Air at a set
pressure is provided to this tube only. After creating a plug in one of the tubing systems
and an input voltage is selected to be sent to the motor, the motor is activated and the
tubing pressurized. By measuring the length of the deposited powder path, the average
58
volumetric flow rate can be determined. This deposited length and the other parameter
inputs are substituted into equation 3.1 to determine the average volumetric flow rate.
This testing method relies on the results of the slider collection system velocity
calibration to determine the value for V at the set potentiometer position. For each of
these experiments, a linear velocity of 192 mm/s was used. The volume of powder
deposited was calculated by multiplying the cross-sectional area of the interior of the
polyurethane tubing by the powder plug length. The cross-sectional area of the interior
of the polyurethane tubing is equal to 8.04 mm2.
The nozzle used was set at a height of 5.0 mm, and it had a length of 6.0 mm and
diameter of 1.5 mm. The deposited powder lengths for each trial for each air pressure and
plug length are shown in Tbl. 4.4, 4.6, and 4.8. The average volumetric flow rate was
calculated for each of these data points using equation 3.1. Tables 4.5, 4.7, and 4.9 show
the average volumetric flow rate for each data set and maximum and minimum calculated
for each data set. The volumetric flow rate values are plotted in Fig. 4.3, 4.4, and 4.5.
59
Table 4.4: DEPOSITED POWDER LENGTH for air pressure of 34.5 kPa and linear velocity of 192 mm/s.
PLUG LENGTH 40 mm 50 mm 60 mm 70 mm 80 mm
trial # 1 (mm) 35 46 45 66 71
trial # 2 (mm) 38 45 47 64 78
trial # 3 (mm) 32 38 51 62 80
trial # 4 (mm) 42 42 47 63 85
trial # 5 (mm) 40 44 52 66 81
trial # 6 (mm) 38 43 54 68 87
trial # 7 (mm) 36 46 46 72 79
trial # 8 (mm) 41 42 48 70 88
trial # 9 (mm) 38 45 53 74 83
trial # 10 (mm) 40 44 47 67 79
mean (mm) 38 44 49 67 81
std. dev. 3 2 3 4 5
Table 4.5: VOLUMETRIC FLOW RATE for air pressure of 34.5 kPa.
PLUG LENGTH 40 mm 50 mm 60 mm 70 mm 80 mm
mean (mm3/s) 1628 1778 1874 1611 1526
maximum (mm3/s) 1933 2035 2062 1463 1743
minimum (mm3/s) 1473 1681 1718 1746 1406
Volumetric Flow Rate vs. Plug Lengths for a Pressure of 34.5 kPa
1000
1200
1400
1600
1800
2000
2200
40 50 60 70 80
Powder Plug Length (mm)
3/s)
Figure 4.3: Volumetric flow rate vs. powder plug length for an air pressure of 34.5 kPa.
60
Table 4.6: DEPOSITED POWDER LENGTH for air pressure of 25.9 kPa and linear velocity of 192 mm/s.
PLUG LENGTH 40 mm 50 mm 60 mm
trial # 1 (mm) 43 48 50
trial # 2 (mm) 39 45 55
trial # 3 (mm) 37 46 50
trial # 4 (mm) 42 47 61
trial # 5 (mm) 36 49 53
trial # 6 (mm) 40 47 58
trial # 7 (mm) 41 51 52
trial # 8 (mm) 38 48 62
trial # 9 (mm) 42 44 64
trial # 10 (mm) 37 49 65
mean (mm) 39.5 47.4 57
std. dev. 2.46 2.07 5.75
Table 4.7: VOLUMETRIC FLOW RATE for air pressure of 25.9 kPa.
PLUG LENGTH 40 mm 50 mm 60 mm
mean (mm3/s) 1566 1631 1628
maximum (mm3/s) 1718 1757 1856
minimum (mm3/s) 1439 1516 1428
Volumetric Flow Rate vs. Plug Lengths for a Pressure of 25.9 kPa
1000
1200
1400
1600
1800
2000
2200
40 50 60
Powder Plug Length (mm)
Volumetric Flow Rate (mm
3/s)
Figure 4.4: Volumetric flow rate vs. powder plug length for an air pressure of 25.9 kPa.
61
Table 4.8: DEPOSITED POWDER LENGTH for air pressure of 17.2 kPa and linear velocity of 192 mm/s.
PLUG LENGTH 40 mm 50 mm
trial # 1 (mm) 48 53
trial # 2 (mm) 45 58
trial # 3 (mm) 44 54
trial # 4 (mm) 43 52
trial # 5 (mm) 41 60
trial # 6 (mm) 46 58
trial # 7 (mm) 47 56
trial # 8 (mm) 49 61
trial # 9 (mm) 45 59
trial # 10 (mm) 42 55
mean (mm) 45 56.6
std. dev. 2.36 2.79
Table 4.9: VOLUMETRIC FLOW RATE for air pressure of 17.2 kPa.
PLUG LENGTH 40 mm 50 mm
mean (mm3/s) 1375 1366
maximum (mm3/s) 1509 1487
minimum (mm3/s) 1263 1268
Volumetric Flow Rate vs. Plug Lengths for a
Pressure of 17.2 kPa
1000
1200
1400
1600
1800
2000
2200
40 50
Powder Plug Length (mm)
3/s)
Figure 4.5: Volumetric flow rate vs. powder plug length for an air pressure of 17.2 kPa.
62
SFF technologies are continuously trying to improve upon build time without
sacrificing build quality. The additional complexity of depositing heterogeneous powder
layers incur a time penalty but it should not be too significant. For a deposition system
using plug-phase pneumatic conveying, the deposition time hinges upon the volumetric
flow rate reliably achieved within the operating parameters of the system. For each of the
tests performed, the average volumetric flow rate was on the order of 1.5 × 103 mm3/s. In
order to achieve continuous flow, conveying lines will have to be run in parallel 180
degrees out of phase to supply a steady flow of powder plugs. Assuming that the nozzle
has the mechanics to traverse the entire layer area in a raster fashion, the deposition of a
single layer can be estimated by equation 3.2.
Assuming that a 10” by 10” (254 mm by 254 mm) build area with a layer
thickness of 0.008” (0.2 mm) is required, the deposition time for a single nozzle system
with continuous flow would be on the order of nine seconds. Deposition in this manner
may also cause additional time penalties. In order to deliver the powder in the density
necessary for sintering or liquid binder adhesion, a thicker layer may need to be deposited
and then further compacted by a mechanical force to increase its density. Also,
acceleration and deceleration of the powder head during each pass of the raster motion
will cause an additional time loss.
The deposition time, however, can be reduced significantly. For instance, using an
array of n nozzles would reduce the deposition time an order of n. Including a rough
63
estimate of the time penalties discussed previously, an array of ten nozzles all capable of
simultaneous continuous deposition would result in a deposition time on the order of two
seconds per layer. This is slightly higher than the 0.1 to 1.0 second times reported by
Sachs et al. [Sachs92], but not too high for the greater fabrication capabilities gained.
4.3 POWDER PATH CHARACTERISTICS
The rapid deposition of large quantities of powder can be achieved through several
alternative methods, yet none match a plug-phase pneumatic conveying systems control
of the powder throughout its transportation. This is because when it reaches the nozzle
orifice, it remains intact as a densely packed plug with a known flow rate and can be
deposited with control. Experiments were performed with the experimental apparatus to
observe the effects of the design parameters on the deposited powder path characteristics.
These parameters include the nozzle length, diameter, and height; slider collection system
speed; and air pressure (Fig. 3.16). All parameters were kept at their default values while
one parameter was modified at a time to observe its effect on the powder path
characteristics. The default values used in these experiments are shown in Tbl. 4.10. The
powder characteristics measured were the main width, total spread, and the height
(Fig. 3.17).
64
Table 4.10: Default parameter values.
PARAMETER DEFAULT VALUEnozzle length 6.0 mm
nozzle diameter 1.5 mmnozzle height 5.0 mm
slider collection velocity 192 mm/sair pressure 17.2 kPa
powder plug length 50 mm
The height of the powder path was measured at its peak (Fig. 3.17). The heights
were on the order of 0.1 mm to 0.3 mm for each test. Heights remained consistent for
each sample taken and their values varied indirectly with the main powder width. Exact
values are not reported because it was not possible to make accurate measurements. For
each test, also measured were the main width and the total spread of the powder path
(Fig. 4.6). Measurements were taken along areas of consistent width and spread. These
reported values varied roughly 10% among each data sample. These results are shown in
Tbl. 4.11-15. The average width and spread along with maximum and minimum values
measured for each data set are plotted in Fig. 4.7-11.
widthspread
mm
Figure 4.6: Actual powder path showing width and spread.
65
Table 4.11: Powder path width and spread for various nozzle lengths.
NOZZLE LENGTH (mm)2.0 6.0 10.0
width (mm) spread (mm) width (mm) spread (mm) width (mm) spread (mm)trial # 1 3.0 5.0 3.5 9.0 3.0 5.0
trial # 2 3.0 6.0 4.0 7.0 2.0 4.0
trial # 3 4.0 7.0 3.0 8.0 4.0 6.0
trial # 4 4.0 7.0 3.5 7.5 3.0 5.0
trial # 5 3.5 8.0 3.5 8.0 3.5 5.0
trial # 6 3.5 5.0 2.0 8.0 3.5 6.0
trial # 7 4.0 7.0 3.5 6.0 4.0 7.0
trial # 8 3.5 7.0 3.0 7.0 3.0 6.0
trial # 9 4.0 6.0 4.0 8.0 4.0 6.0
trial # 10 3.0 7.0 3.5 7.0 3.0 5.0
mean 3.55 6.50 3.35 7.55 3.30 5.50
std. dev. 0.44 0.97 0.58 0.83 0.63 0.85
Note: Measurements are taken to the nearest 0.5 mm.
Powder Deposition Width vs. Nozzle Length
0
2
4
6
8
10
2 6 10
Nozzle Length (mm)
Width
Spread
Figure 4.7: Powder deposition width vs. nozzle lengths.
66
Table 4.12: Powder path width and spread for various nozzle diameters.
NOZZLE DIAMETER (mm)1.5 3.0
width (mm) spread (mm) width (mm) spread (mm)trial # 1 3.5 9.0 5.0 8.0
trial # 2 4.0 7.0 4.5 7.0
trial # 3 3.0 8.0 5.0 7.0
trial # 4 3.5 7.5 5.0 7.0
trial # 5 3.5 8.0 4.5 6.0
trial # 6 2.0 8.0 5.5 7.0
trial # 7 3.5 6.0 5.5 7.0
trial # 8 3.0 7.0 5.0 8.0
trial # 9 4.0 8.0 5.0 7.0
trial # 10 3.5 7.0 5.5 8.0
mean 3.35 7.55 5.05 7.20
std. dev. 0.58 0.83 0.37 0.63
Note: Measurements are taken to the nearest 0.5 mm.
Powder Deposition Width vs. Nozzle
Diameter
0
2
4
6
8
10
1.5 3.0
Nozzle Diameter (mm)
Width
Spread
Figure 4.8: Powder deposition width vs. nozzle diameter.
67
Table 4.13: Powder path width and spread for various nozzle heights.
NOZZLE HEIGHT (mm)5.0 7.0 9.0
width (mm) spread (mm) width (mm) spread (mm) width (mm) spread (mm)trial # 1 3.5 9.0 3.5 11.0 4.5 8.0
trial # 2 4.0 7.0 4.0 9.0 5.0 9.0
trial # 3 3.0 8.0 3.5 12.0 4.0 11.0
trial # 4 3.5 7.5 3.0 13.0 4.0 12.0
trial # 5 3.5 8.0 3.5 13.0 4.5 13.0
trial # 6 2.0 8.0 4.0 10.0 4.0 12.0
trial # 7 3.5 6.0 3.0 12.0 5.0 11.0
trial # 8 3.0 7.0 3.5 13.0 5.0 12.0
trial # 9 4.0 8.0 3.5 10.0 4.5 12.0
trial # 10 3.5 7.0 4.0 14.0 4.5 13.0
mean 3.35 7.55 3.55 11.70 4.50 11.30
std. dev. 0.58 0.83 0.37 1.64 0.41 1.64
Note: Measurements are taken to the nearest 0.5 mm.
Powder Deposition Width vs. Nozzle Height
0
2
4
6
8
10
12
14
5 7 9
Nozzle Height (mm)
Width
Spread
Figure 4.9: Powder deposition width vs. nozzle height.
68
Table 4.14: Powder path width and spread for various slider collection system speeds.
Slider Collection System Speed (mm/s)192.0 286.0 385.0
width (mm) spread (mm) width (mm) spread (mm) width (mm) spread (mm)trial # 1 2.5 13.0 3.5 9.0 2.5 6.0
trial # 2 2.5 10.0 4.0 7.0 3.0 7.0
trial # 3 3.0 11.0 3.0 8.0 2.5 6.0
trial # 4 3.0 12.0 3.5 7.5 2.5 8.0
trial # 5 2.5 11.0 3.5 8.0 2.0 6.0
trial # 6 2.5 11.0 2.0 8.0 3.0 8.0
trial # 7 3.0 11.0 3.5 6.0 2.5 7.0
trial # 8 3.0 12.5 3.0 7.0 2.5 6.0
trial # 9 2.5 12.0 4.0 8.0 3.0 7.0
trial # 10 3.0 11.0 3.5 7.0 2.5 8.0
mean 2.75 11.45 3.35 7.55 2.60 6.90
std. dev. 0.26 0.90 0.58 0.83 0.32 0.88
Note: Measurements are taken to the nearest 0.5 mm.
Powder Deposition Width vs. Slider
Collection System Speed
0
2
4
6
8
10
12
14
192 289 385
Velocity (mm/s)
Width
Spread
Figure 4.10: Powder deposition width vs. slider collection system speed.
69
Table 4.15: Powder path width and spread for various air pressures.
Air Pressure (KPa)17.2 25.9 34.5
width (mm) spread (mm) width (mm) spread (mm) width (mm) spread (mm)trial # 1 3.5 9.0 2.5 15.0 4.0 20.0
trial # 2 4.0 7.0 2.0 14.0 4.5 20.0
trial # 3 3.0 8.0 2.5 16.0 4.0 20.0
trial # 4 3.5 7.5 2.0 15.0 4.5 20.0
trial # 5 3.5 8.0 2.0 13.0 5.0 20.0
trial # 6 2.0 8.0 2.5 13.0 4.5 20.0
trial # 7 3.5 6.0 2.5 16.0 4.0 20.0
trial # 8 3.0 7.0 2.0 15.0 5.0 20.0
trial # 9 4.0 8.0 2.5 14.0 5.5 20.0
trial # 10 3.5 7.0 2.5 17.0 4.0 20.0
mean 3.35 7.55 2.30 14.80 4.50 20.00
std. dev. 0.58 0.83 0.26 1.32 0.53 0.00
Note: Measurements are taken to the nearest 0.5 mm and the maximum width of the deposition areais 20.0 mm which was equaled or exceeded each case by the 34.5 KPa air pressure.
Powder Deposition Width vs. Air Pressure
02468
1012141618
20
17.2 25.9 34.5
Air Pressure (KPa)
Width
Spread
Figure 4.11: Powder deposition width vs. air pressure.
70
The results show that a powder path of 3 to 4 mm can be consistently deposited.
The average width for all values not including the wider diameter nozzle is 3.4 mm with a
standard deviation of 0.84. The total spread of the powder changed dramatically with the
various parameters. The average value was 10.63 mm with a standard deviation of 4.55.
Changing the nozzle length had little effect on the width and spread of the powder
path other than slightly reducing the width as the length increased. Enlarging the nozzle
diameter caused the main width to increase but had little effect on the overall spread of
the powder. In future systems, a wider nozzle could be used in areas where the tolerance
of the position of the powder is relaxed. Increasing the height of the nozzle from the
deposition surface had minimal effects on the main width of powder but caused
significant widening of the overall spread. Increasing the speed of the slider collection
system had minimal effect on the main width but caused the overall spread to reduce.
This is because the longer time the nozzle spends over a particular area, the more force is
applied to the underlying powder which causes the powder to spread further.
Varying the air pressure effected the deposited powder path more than any other
of the parameters. Changing the air pressure causes a change in the volumetric flow rate
and hence the force in which the powders impact the collection surface. No linear trend
was observed on the main powder width but the higher air pressures caused a significant
increase in the total spread of the powder. This shows that higher air pressures should be
71
avoided to reduce the force the powder has on the underlying bed and to make sure that a
transition to lean-phase conveying does not occur.
4.4 MULTIPLE POWDER MIXING
The main benefit of powder deposition with the experimental apparatus is not its ability
to accurately and rapidly deposit a powder layer, but rather its ability to deposit
selectively heterogeneous material compositions throughout each layer. A series of
experiments were therefore performed to analyze the effects that system parameter
changes had on the mixing of two powders being deposited simultaneously. The
parameters tested were the nozzle’s vertical height, diameter, and length. Observations
were made on the general mixing capabilities and any patterns observed were recorded.
As before, the default experimental parameter settings were set as shown in Tbl. 4.10.
The experiments were performed by creating two equal powder plugs, red and
blue, in two of the parallel polyurethane tubes. The air pressure from the two air sources
were set equal and directed through the manifold to each polyurethane tube. The powder
plugs were then set in motion and reached the nozzle simultaneously, where upon they
were deposited on the traversing slider collection system.
With the original nozzle setup (length equal to 6.0 mm, height equal to 5.0 mm,
and diameter equal to 1.5 mm), the mixing was poor and somewhat segregated (Fig. 4.12).
Mixing occurred in the main width, but there were distinct areas of red and blue rather
72
than a uniform blend. The different powders seemed to be divided by the centerline of
the cross-section. The spread was completely divided by color on each side of the
centerline.
blue side
red side
(mm)
(a) (b)
Figure 4.12: Default parameter setup with a height of 5.0 mm;
(a) original image, (b) annotated image.
Elevating the nozzle height to 7.0 mm produced greater mixing in the main powder
width but the powder types were still somewhat segregated by the centerline (Fig 4.13).
The mixing was not thorough for there are distinct areas of bi-modal colors rather than a
uniform blend. Raising the nozzle height to 9.0 mm produced a greater mixing (Fig. 4.14).
The main width appeared thoroughly mixed with no spatial patterns evident. However,
the mixing remained macroscopic rather than microscopic. In particular, there were areas
of red and blue powder rather than blended powder particles. Increasing the length of the
nozzle produced greater mixing at the center of the main powder width but the powders
were still somewhat segregated by the centerline (Fig 4.15). Increasing the nozzle
73
diameter improved mixing in the main width, but uniform mixing was still not achieved
(Fig. 4.16).
blue side
red side
(mm)
(a) (b)
Figure 4.13: Nozzle height of 7.0 mm; (a) original image, (b) annotated image.
blue side
red side
(mm)
(a) (b)
Figure 4.14: Nozzle height of 9.0 mm; original image (a), annotated image (b).
74
blue side
red side
(mm)
(a) (b)
Figure 4.15: Default parameter setup with nozzle length of 10.0 mm;
(a) original image , (b) annotated image.
(mm)
(a) (b)
Figure 4.16: Default parameter setup with nozzle diameter of 3.0 mm;
(a) original image, (b) annotated image.
Uniform mixing was not achieved for any setting. Future systems should
therefore consider developing a flow analysis of the powder passing through the nozzle to
better stimulate thorough mixing of the multiple powders. Also, it was not possible to
analyze the proportions of powder deposited with various volumetric flow rates because
the differences of volumetric flow rates achieved by this experimental device are on the
order of ten percent. Future systems should therefore also consider maintaining a
75
constant volumetric flow rate for each powder supply line and control the deposited
material composition by proportioning the amount of powder which each nozzle receives.
4.5 MULTIPLE MATERIAL TRANSITIONS
In order to facilitate truly continuous heterogeneous material composition, it is important
that the deposition system is capable of changing the material composition on the fly.
The experimental apparatus provides this capability. In particular, it can currently mix
two powders on the fly, and if automated, it would also be able to change this
composition on the fly to enable a point-by-point layer design.
Experiments were therefore conducted with the experimental apparatus to examine
its transitional powder mixing capabilities. In these experiments it was not possible to
measure quantitative data to accurately illustrate the transitions because of the lack of
automated control within the experimental apparatus. However, transition from one
powder color to the next was tested. The experiment was set up identically to that
described in Section 4.4 with one exception: Instead of setting the plugs in motion
simultaneously, one was sent slightly ahead of the other. The air supplied to the leading
powder plug was ceased as the trailing powder plug approached the nozzle. This resulted
in a transition from one material type to the next. Figure 4.17 illustrates a transition from
red to green powder colors.
76
red green
(mm)
(a) (b)
Figure 4.17: Transition from red to green powder, (a) original image, (b) annotated image.
These transitions are obviously severely error prone with the current experimental
setup. A more precise transition would require timing to the fraction of a second to
provide the transitions desired. A future system would require automation to control the
timing and would need to be calibrated to determine the transient stages required for
initiating and stopping powder flow.
4.6 OBSERVATIONS
The experimental results and analysis demonstrate that depositing continuously
heterogeneous powder composition with the experimental apparatus is feasible. Many
beneficial deposition qualities were illustrated by the experimental apparatus. However,
several deficiencies were also apparent. These deficiencies need to be overcome before
deposition by plug-phase pneumatic conveying can be integrated into SLS, 3DP, or FPM.
The results illustrate that powder can be transported through the tubing system
and deposited with control. Conveying at lower air pressures resulted in lower particle
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speeds which lend to this control. The results also show the potential for selectively
depositing continuously heterogeneous material compositions. Multiple powders were
directed to the tubing and deposited simultaneously through a single nozzle. Other
results demonstrate the capabilities of selectively altering the material composition of the
deposited powder on the fly.
Several deficiencies of the experimental apparatus also became apparent. First,
the range of operational volumetric flow rates is limited, varying only 10% among the
various setups. This is unfortunate because until this problem is overcome, it will not be
possible to achieve precise heterogeneous material deposition varying under computer
control. Also, three powders were unable to be simultaneously deposited. This was
because the pressure drops in the parallel tubing lines were not equal and the air bypassed
one of the lines. A third controlled pressure input would remedy this problem.
However, the basic feasibility of deposition of continuously heterogeneous
material composition by plug-phase pneumatic conveying has been demonstrated. The
next step, in addition to developing a more precise powder mixing control, is therefore to
explore how this concept can be integrated with SLS, 3DP, or FPM. Hence, Chapter 5
will present a conceptual design of a proposed larger scale system based on the results
and analysis of the performance of the experimental apparatus.
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CHAPTER 5
MULTIPLE POWDER DEPOSITION SYSTEM
5.1 INTRODUCTION
The results and analysis of the performance of the experimental apparatus illustrate that
depositing continuously heterogeneous powder layers is feasible for Selective Laser
Sintering (SLS), Three Dimensional Printing (3DP), and Freeform Powder Molding
(FPM) with a plug-phase pneumatic conveying system. In its present form, the
experimental apparatus is not suited for implementation into SLS, 3DP, or FPM. It lacks
the automation and the speed necessary for satisfactory operation. However, its
operating principles can be used as the basis for the design of a larger scale system to
provide selectively heterogeneous material fabrication potential.
The results from testing of the experimental apparatus are used for developing a
conceptual design of the multiple powder deposition (MPD) system (Fig. 5.1). Similar to
the experimental apparatus, the MPD system conveys three powders, each through its
own tubing system, to a nozzle where they selectively merge prior to deposition. Drastic
improvements are made by automating the powder plug creation and by depositing with
an array of nozzles to help reduce the deposition time. The array of nozzles scans the
build area in raster fashion rather than having the build area move under a stationary
nozzle.
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Powderprint head
Powder storageand loadingchambers
Supplytubing
MotionsystemDischarge
tubing
Figure 5.1: Multiple Powder Deposition System.
Included in this chapter are overviews of the major functional components
contained in the conceptual design of the MPD system, a review of the software and
control requirements, and the powder properties that most effect the conveying of
various powders. An analysis of the MPD system’s expected performance in depositing
a single layer of powder is estimated based on the results discussed in Chapter 4.
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5.2 NOZZLE ARRAY AND POWDER DEPOSITION HEAD
Depositing each powder layer with a single nozzle system would take the ‘rapid’ out of
‘rapid prototyping’. To aid in minimizing the deposition time required for each layer, an
array of independently operating nozzles is proposed. These nozzles are similar in
design to that used in the experimental apparatus with some minor modifications. The
nozzle array is contained within the powder deposition head and scans the build area in
unison during the powder deposition cycle.
The general form of the nozzle remains unchanged from that discussed in
Section 3.1.2 except for a few minor design modifications (Fig. 5.2). In order to better
accommodate the numerous tubes and fittings into a small area, vertical nozzle extensions
should be used. They should taper into the nozzle without severe bending. Based upon
results discussed in Chapter 4, a nozzle length of 10.0 mm is recommended. This length
minimized the deviation of the powder path and produced greater mixing of multiple
powders over the shorter lengths. This mixing was far from uniform though. There were
still significant areas of segregated powder types. A more accurate analysis of the flow
patterns observed within the nozzle will be needed. This flow analysis should be applied
to the design of a new nozzle which will allow for the optimization of multiple powder
mixing. Areas that should be addressed are the optimal angle for merging the nozzle
extensions and alternative shapes and sizes of the nozzle mixing passageway.
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Figure 5.2: Proposed deposition nozzle.
Future nozzles will furthermore have to be fabricated by a process other than
FDM. In particular, a process is needed that produces precise, smooth surfaces inside
the nozzle to minimize its friction with the powder. The surface quality of the nozzle is
currently the weak point in the tubing system. A glass blown nozzle is recommended for
its improved surface quality, ability to fabricate a variety of shapes, and its transparency
for visualizing internal flow patterns.
A space of 18.0 mm is required between the center points of adjacent nozzles
because of the robust size of the fittings required for connecting the nozzle extensions to
the tubing. The powder deposition width of each nozzle is on the order of 3 mm, so there
will be a significant void between each nozzle. To help minimize this void, a second row
of nozzles is used. Each nozzle is placed in the center of the pair of nozzles in the row in
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front at distance of 18.0 mm behind. A pair of rows each containing five and four,
respectively, nozzles is used.
As discussed previously, plug-phase conveying does not supply a continuous
deposition of powder. To obtain a continuous deposition, a second pair of nozzle rows
will be used. The second row pair will operate 180 degrees out of phase with the other
row pair and they will be aligned directly behind the first row pair at a distance of 18.0
mm (Fig. 5.3).
18.0
18.0
9.0
Primary nozzlerow pair
Secondary nozzlerow pair
18.0 18.0 18.0
18.0
18.0
9.0 9.0 9.0
dimensions in mm 100.0 mm
100.0 mm Fastaxis
Slowaxis
Figure 5.3: Nozzle array positioning.
Finally, a powder deposition head (Fig. 5.4) will be needed to house the array of
nozzles and transport them in unison with the motion system. In the future system, a
100.0 mm by 100.0 mm area head would contain the array of nozzles shown in Fig. 5.3.
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The open top accommodates the tubing lines which supply powder to the nozzles. The
circular openings permit it to slide on the motion system poles, and bearings are used to
reduce sliding friction during motion. The large opening shown is used for the exit of the
discharge tubing.
Opening fordischarge tubing
exit
Openings for slidingon the motionsystem poles
Figure 5.4: Powder deposition head.
5.3 MOTION SYSTEM
To deposit a complete layer of powder, the nozzle array must scan the entire build area.
Unlike the motion system used for the experimental apparatus, the MPD system moves
the nozzles over the build area rather than moving the build surface underneath a
stationary nozzle. This is the more feasible approach for there is greater difficulty in
moving the larger inertia powder bed with the same speed and accuracy.
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The motion requirement for the MPD system is linear motion with constant
velocity in two dimensions, namely x and y. Changes in the vertical height will be
provided by the lowering and raising of the powder bed, as is presently done with both
SLS and 3DP. Numerous mechanical systems can easily accommodate these design
requirements. The motion system designed for the MPD system operates with
principles similar to that of an ink jet printer. One axis of travel is fast. During travel on
this axis, the nozzles deposit powder onto the powder bed surface. The second axis of
travel is slower. Travel along this axis provides a small increment in position for the
nozzles to make a new pass along the fast axis over the powder bed.
The motion system designed for the MPD system is a simple series of pulleys,
sliders, and stepper motors (Fig. 5.5). Along the fast axis, a pair of slider poles is used to
guide the powder deposition head and maintain its stability. Connected to each side of
the deposition head is a V-belt. The deposition head is propelled along the axis in the
requisite direction by a stepper motor attached to one of the pulleys. The velocity
requirements are to be determined based upon the flow analysis, but should be on the
order of 250 mm/s. The slow axis operates with similar principles. A pair of stepper
motor driven pulley systems is attached to each side of the fast axis pulley system. Each
motor is controlled through a position feedback to aid in positioning control. After the
completion of each pass along the fast axis, the slower axis increments an appropriate
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Fast Axis
Slow axis
Figure 5.5: Proposed motion system for the Multiple Powder Deposition System.
distance. After deposition of each powder layer, the deposition head and slider poles can
be positioned as to not interfere with either the laser scanning or binder spreading.
The motion pattern required is similar to a simple raster pattern with some
modifications required due to the spacing of the nozzles. After a single pass along the
fast axis, 6 mm voids will be left in between the paths of powder. The slow axis will
increment the deposition head over 3 mm (the width of each powder path) and another
deposition pass will be made. This results in 3 mm voids between the powder paths.
After another 3 mm increment along the slow axis and deposition pass, a complete area of
86
powder is deposited. This area is 81 mm wide. Before making the next deposition pass,
the deposition head needs to be moved 82.5 mm along the slow axis and the pattern is
repeated until a complete layer is deposited. A portion of this motion sequence is shown
in figure 5.6. The motion system modeled can deposit a powder bed 300 mm by 250 mm.
Slow axis
Fast axis
Figure 5.6: Portion of motion pattern for powder deposition.
5.4 POWDER PLUG CREATION & DELIVERY
An automated plug creation system similar to the commercial systems illustrated in
Fig. 2.7 is used to feed powder to the nozzles. A timer operated valve switches the air
flow between the loading bin and the tubing. When air enters the bin, powder is forced
out the bottom and enters the tubing. After the desired powder plug length of is created,
the air flow is switched to the tubing system. This air propels the powder plug through
the tubing system. The use of alternating air valves creates equally spaced powder plugs
spaced by equal lengths of air within the tubing (Fig. 5.7).
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Figure 5.7: Powder plugs passing through tubing.
In the MPD system, each nozzle requires independent delivery of 3 powders.
There are a total of 18 nozzles, thus 54 plug loading systems are needed. Figure 5.1 of
the complete MPD system only shows a portion of these. Each plug loading system
(Fig. 5.8) operates as described for the commercial system. These containers are
relatively large and are therefore stored “remotely” while connected by tubing to the
system.
Pressurized air input
Timer operatedsolenoid valve
Powder loading bin
Figure 5.8: Plug loading system, 1 of 54 required.
The tubing system connecting the plug loading system to each nozzle needs to
have a low coefficient of friction and be flexible. During the motion of the deposition
head, the distance from the plug loading systems to the nozzle array will fluctuate
88
tremendously. Great care needs to be made in the routing of the tubing as to not cause
bends which will disrupt the flow of powder.
Calibration of the flow rates for various plug length and air pressure combinations
is required for each material and each of the 54 delivery tubes. Proportioning the powder
composition by adjusting the flow rate of powder through the tubing system is not
recommended for several reasons. As shown in testing of the experimental apparatus,
flow rates within the operating range of the system may only be different on the order of
10%. This would severely restrict the material compositions available. Also, changing
flow rates within each tube may not be achievable on the fly. For instance, the gradual
increasing of the volumetric flow rate of an individual plug may not be achieved because
the plugs traveling behind it will prevent an air flow rate adjustment from reaching the
leading plug.
A simple solution to the problem would be to convey the powder in each tubing
system at identical flow rates and to control the flow of powder entering the nozzle
through the nozzle extensions. A valve system needs to be designed and tested to
accomplish this need. The valve system should be able to direct the proportion of
powder desired to enter the nozzle through the tubing extensions and to divert the
remainder of the powder or conveying air into the discharge tubing (Fig. 5.9). Controlling
the powder flow in this manner will prevent stalling of the powder plugs within the
tubing and spraying of the powder bed with pressurized air.
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Discharge tubing
Powder delivery tube Nozzle extension
Unwanted powder portionand conveying air
Powder portion used fordesired material contribution
Proportioningvalve system
Figure 5.9: Proposed operation of proportioning valve for powder composition adjustment.
5.5 SOFTWARE & CONTROL
Current CAD software allows for specifying only discrete material regions. In order to
design for heterogeneous material blends, the current state of software needs to be
improved to include continuously heterogeneous material composition in three
dimensions. Through this software the material composition at each point of the three-
dimensional part needs to be determined to develop the control of the MPD system.
The software might break the three-dimensional part into a finite element grid with
the material composition specified for each node. The spacing of the grid will be
determined by the tolerance achievable by the MPD system. From this node data, the
necessary control parameters for the MPD system can be generated.
90
The most difficult area of control is the array of proportioning valves. 162 valve
systems need to be independently controlled based on the nozzle position over the build
area and the proportion of air or powder to pass into the nozzle extension. A
programmable logic controller (PLC) can handle the control of both the valve system and
motion system. The loading and conveying of plugs of powder can be done continuously
or only when the powder from the individual tube is to be deposited. The latter method
would save tremendously on material costs.
5.6 POWDER PROPERTY EFFECTS
The deposition experiments performed thus far have only used one powder,
polycarbonate, dyed different colors to simulate the deposition of multiple materials.
The ability to fabricate parts of multiple colors would only produce cosmetic
improvements. To capture all of the benefits of continuously heterogeneous material
deposition, a variety of materials with diverse properties should be able to be deposited.
Mainwaring [Mainwaring93] summarizes the effects that material properties have
on conveying. No one property dictates a material’s suitability for conveying. The
properties that influence material’s flow the most are its particle size distribution and
particle shape. A wide particle size distribution is more problematic than fine powders,
specifically those with an average size below 100 µm, as is used in SLS and 3DP.
Spherical particle shapes are also recommended. Particle shapes that naturally do not
91
flow well have difficulty being conveyed. These include cylindrical and disk shaped
particles.
The use of plug-phase conveying helps to initiate the flow of particles that
normally will not flow. Even if a product will not flow naturally in plug form, short
plugs can be artificially created to permit reliable conveying [Mainwaring93].
Guidelines suggest that the particle sizes be at least three times smaller than the
inner diameter of the tubing transporting them. Thus for the MPD system, particle sizes
should be maintained under 500 µm. All of the materials commercially available for use in
SLS satisfy these requirements and should be able to be deposited in the MPD system
with no modifications. Sample SLS material properties are shown in Tbl. 5.1, 5.2, and
5.3.
Table 5.1: Laserlite™ LN4010 Nylon Compound Properties [DTM94a].
General Properties Value Test MethodSpecific Gravity, 20ºC 1.04 g/cm3 ASTM D792Moisture Absorption, 20ºC, 1.0% ASTM D570 65% relative humidityPowder Tap Density 0.58 g/cm3 ASTM D4164Volume Average Particle Size 120 microns laser diffractionParticle Size Range, 90% 60-250 microns laser diffraction
Table 5.2: Laserlite™ LNF5000 Nylon Compound Properties [DTM94b].
General Properties Value Test MethodSpecific Gravity, 20ºC 1.04 g/cm3 ASTM D792Moisture Absorption, 20ºC, 1.0% ASTM D570 65% relative humidityPowder Tap Density 0.55 g/cm3 ASTM D4164Volume Average Particle Size 50 microns laser diffractionParticle Size Range, 90% 15-90 microns laser diffraction
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Table 5.3: Laserlite™ LWX2010 Wax Compound Properties [DTM94d].
General Properties Value Test MethodSpecific Gravity, 20ºC 1.04 g/cm3 ASTM D792Powder Tap Density 0.55 g/cm3 ASTM D4164Volume Average Particle Size 105 microns laser diffractionParticle Size Range, 90% 25-235 microns laser diffraction
5.7 PERFORMANCE ANALYSIS
While the complexity of the MPD system is much greater than that of the experimental
apparatus, both deposit powders with similar operating principles. Thus, the
performance expected for the MPD system should be accurately predicted from the
testing and analysis of the experimental apparatus. The most important parameters that
will affect the MPD system’s acceptance into the solid freeform fabrication industry are
the time required for depositing each layer and the spatial increment at which the material
composition may be specified and met by the deposition system.
The results of the experimental apparatus predict that a volumetric flow rate on
the order of 1.5×103 mm3/s can be expected for each nozzle. One might therefore expect
to completely deposit a powder layer of 10” by 10” (254 mm by 254 mm) with a
thickness of 0.008” (0.2 mm) with the MPD system in approximately one second. The
actual time may be slightly higher due to acceleration and deceleration of the motion
system. On par, the total deposition time, however, should remain below five seconds.
This is slower than the deposition time of the conventional roller distribution method, due
93
mainly to the added complexity of depositing continuously heterogeneous material
composition.
The spatial interval achievable for the specified powder composition may vary in
all three dimensions. The 3 mm width of the powder path dictates a spacing of roughly
3.0 mm in the direction of the slow axis. The composition of the material deposited may
be accurately controlled at a spacing of roughly 3 mm. The composition of the material in
between the 3 mm interval will be determined by the nearest node.
The spatial interval in the vertical direction will be on the order of the powder
layer thickness assuming that the powder does not “bleed” into the underlying powder
bed and a compaction method is used to level the surface. Material composition in
between the specified positions will be equal to the nearest node.
Lastly, the spatial interval relies on the transient capabilities of the control
system. Calibration is necessary to accurately predict the transient patterns of the
control system relative to the powder deposition. The control points in the deposition
system will be the proposed proportioning valves. Having these control points located
near the deposition points (nozzles) should help to minimize residual effects of
commencing and ceasing flow.
The limiting factor in minimizing the nodal spacing is the width in which the
powder may be deposited. Deposition in the direction of the head motion can be
accurately controlled to a much tighter interval.
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5.8 DESIGN SUMMARY
The proposed conceptual design of the MPD system applies the principles developed
and tested with the experimental device into a larger scale operation. This system should
be capable of depositing powder layers with continuously heterogeneous material
composition. It may selectively deposit three powder types through an array of nozzles
that as a whole are capable of providing continuous powder deposition.
An automated plug loading system is proposed. The powder plugs are created
and transported through the supply tubing similar to the commercial systems previously
discussed (Section 2.4). Identical volumetric flow rates should be used for each supply
tubing and the control of the deposited powder composition may be met through a
proportioning valve system.
An array of 18 nozzles, with 9 operating simultaneously, will be used to accept
the powder from the supply tubing, uniformly mix each of three powder components,
and deposit the final powder composition onto the build area. The nozzle array is
necessary to help minimize the deposition time for each layer. The powder print head,
which contains the nozzle array, will scan the build area in an altered raster pattern
(Fig. 5.6). The motion is provided by a system similar to an ink-jet pulley system.
The MPD system’s acceptance into the SFF industry will rely on the deposition
time for each layer and the tolerance at which the powder can be deposited on a point-to-
point basis. Based on the results of the experimental apparatus, deposition times can be
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expected to be on the order of two seconds. Deposition tolerances will vary significantly
in each dimension. The use of plug-phase conveying helps to minimize deposition error
by providing deposition at low particle speeds and with a minimal amount of air
combined in the flow.
Further development of this conceptual deposition system requires much testing,
analysis, and calibration. However, based on the performance of the experimental
apparatus and that estimated for the proposed MPD system, plug-phase pneumatic
conveying combined with a raster motion is clearly a feasible method for the deposition of
continuously heterogeneous material composition for use in SLS, 3DP, or FPM.
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CHAPTER 6
CONCLUSION
This thesis has explored the fabrication of continuously heterogeneous parts in physical
rapid prototyping. It was achieved by developing a new method for depositing powder
layers for use in Selective Laser Sintering, Three-Dimensional Printing, and Freeform
Powder Molding. It presents a foundation to which other research should be built upon.
This chapter concludes the thesis by outlining the contributions made by this thesis and
suggestions for future work.
6.1 CONCLUDING REMARKS
There is a definite need for fabricating parts of continuously heterogeneous material
composition. When achieved, engineers may then begin to design at the microscopic level
with endless control over final part properties. Because they use additive build
processes, some solid freeform technologies, in particular SLS, 3DP, and FPM, can
readily adapt to selectively heterogeneous material blends.
Many SFF systems have fabricated using dual material build processes. These are
used primarily for support generation, but have expanded to include methods of
controlling the microstructure of fabricated parts. In order to adapt SLS, 3DP, and FPM
to the capability of continuously heterogeneous material composition, a new method of
97
powder deposition needs to be devised. Plug-phase pneumatic conveying is a viable
choice.
This thesis explored the feasibility of implementing a plug-phase pneumatic
conveying powder delivery system for three-dimensional continuously heterogeneous
material composition for potential use in SLS, 3DP, or FPM. It was done through the
design, fabrication, and testing of an experimental apparatus. The experimental apparatus
used plug-phase conveying to transport three materials through a tubing system to a
deposition nozzle where they were selectively mixed prior to deposition.
Results show that continuously heterogeneous material composition is feasible
through the deposition methods of the experimental apparatus. These results are used to
model a conceptual design of a larger scale deposition system. Further development of
this larger scale system should be the next stage in future research.
6.2 CONTRIBUTIONS
This thesis addresses the need for fabricating continuously heterogeneous material parts
in SFF by developing a method of powder deposition that can accommodate changing
material composition on the fly. The final results were limited by the simplicity of the
experimental apparatus, but they demonstrate that plug-phase pneumatic conveying is a
feasible approach for depositing continuously heterogeneous powder layers.
98
6.3 RECOMMENDATIONS FOR FUTURE WORK
This thesis builds a foundation for further research in fabricating parts of continuously
heterogeneous material composition. A number of issues remain before a fully functional
prototype system can be built:
• Development of a flow analysis and redesign of the nozzle. The nozzle used in the
experimental apparatus definitely had its limitations. Further research should focus
on analyzing the flow conditions that take place within the nozzle to achieve uniform
mixing. This will include exploring other nozzle shapes, sizes and adding static mixing
in the nozzle passageway. This analysis should be applied to a redesign of the nozzle
which optimizes its mixing and deposition potential.
• Method of proportioning powder composition. Because of the limited range of
operational flow rates achievable, an alternative method of varying composition needs
to be developed. This method needs to be able to selectively allow certain quantities
of each powder into the nozzle and be able to adjust the composition rapidly.
• Transient effects in the switching of powder composition. Once the methods of
proportioning are developed the transient effects of switching powder composition
needs to be further analyzed. These results need to be incorporated into the control
of the proportioning system.
• Deposition onto underlying powder bed. Testing needs to be performed to determine
the force at which the underlying powder bed becomes disturbed and adjust the flow
99
correspondingly. The underlying powder bed should be compacted and heated to
simulate the conditions present in SLS, 3DP, and FPM.
• Exploring other powder alternatives. This research focused on only one powder,
polycarbonate. Assumptions concerning the flow of other powders are made. These
assumptions need to be validated and the range of material and their relevant
properties need to be explored for potential implementation into this deposition
system.
• Height measurements. The height of the deposited powder needs to be accurately
measured. This could be done by shadowing as is used in measuring contact angles.
The path height could be projected onto a screen by a light source. The scaling factor
can be determined by calibration through the projection of an object of known height.
• Compaction methods. The powder layers deposited may be of lower density than
desired. Methods of compaction and leveling each layer need to be explored.
• Adjusting other parameters within SLS, 3DP, and FPM to adapt to heterogeneous
material composition. These fabrication systems currently use only homogeneous
material blends. Changing material properties throughout the part may also result in
changing fabrication techniques. For example, in SLS the time and intensity at which
the laser must pass over each point may become variable along with the material
composition.
100
• Development of software capable of heterogeneous material composition. Current
CAD software allows for specifying only discrete material regions. In order to design
for heterogeneous material blend, the current state of software needs to be improved.
The heterogeneous geometry needs to be converted into a form for controlling the
parameters of the deposition system.
101
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[Lippert66] A. Lippert, “Pneumatic Conveyance of Solids at HighConcentrations,” Chemie-Ingenieur-Technik, vol. 38, no. 3, 1966,pp. 350-355.
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[Mainwaring93] N. Mainwaring, “Characterization of Materials for PneumaticConveying,” American Ceramic Society Bulletin, vol. 72, no. 8,August 1993, pp. 63-71.
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[Melvin91] L. Melvin and J. Beaman, “The Electrostatic Application ofPowder for Selective Laser Sintering,” Proceedings SolidFreeform Fabrication Symposium, 1991, University of Texas,Austin Texas, pp. 171- 177.
[Michaels92] S. Michaels, E. Sachs, and M. Cima, “Metal Parts Generation ByThree Dimensional Printing,” Proceedings Solid FreeformFabrication Symposium, August 3-5 1992, University of Texas,Austin Texas, pp. 244- 250.
[Pegna95] J. Pegna, “Exploratory Investigation of Layered FabricationApplied to Construction Automation,” Design EngineeringTechnical Conferences, vol. 1, ASME 1995, pp. 219-226.
[Prabhu93] G. Prabhu and D. Bourell, “Supersolidus Liquid Phase SelectiveLaser Sintering of Prealloyed Bronze Powder,” Proceedings SolidFreeform Fabrication Symposium, August 9-11 1993, Universityof Texas, Austin Texas, pp. 317- 324.
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106
APPENDIX A
BILL OF MATERIALSFOR
EXPERIMENTAL APPARATUS
PART SOURCE CATALOG # COST QUANTITY TOTAL COST
manifold Cole Parmer H-06464-82 $6.00/each 1 $6.00luer adapter Cole Parmer H-06359-17 $8.00/25 4 $8.00
luer lock Cole Parmer H-06359-67 $8.00/25 4 $8.00screw clamp Cole Parmer H-06833-10 $10.25/3 3 $10.25
str. micro-fitting Cole Parmer H-06173-00 $41.00/3 3 $41.00reducer fitting Cole Parmer H-06391-70 $19.43/each 6 $58.29
Teflon PFA tubing Cole Parmer H-06375-01 $27.00/25 ft. $27.00Polyurethane tubing Cole Parmer H-06423-01 $20.75/50 ft. $20.75
instrument cart Cole Parmer MO02-K31 $309.50 1 $309.50power receptacle Cole Parmer AO-09 $52.00 1 $52.00
motor C and H DCGM7505 $29.95 1 $29.95motor control kit Marlin P. Jones 4057-MD $15.95 1 $15.95AC/DC converter Radio Shack 273-1653B $21.99 1 $21.99
board True Value plywood $5.00 1 $5.00tape Walmart Scotch Mailing
Tape$1.99 1 $1.99
nozzle self-fabricated 1nozzle support self-fabricated 1
spool self-fabricated 1slider platform self-fabricated 1
slider tray self-fabricated 1funnel self-fabricated 4
circuit support self-fabricated 1main support stand scrap metal 1
track groove scrap metal 1eye hook spare 1
thread spare 1powder DTM Laserlite™
LPC3000Polycarbonate
TOTAL $615.67
107
APPENDIX B
PARTS LISTFOR
EXPERIMENTAL APPARATUS
PART QUANTITY REFERENCE CODEbase board 1 A
track groove 1 Bmain support stand 1 C
main air line 1 1 D1main air line 2 1 D2
luer fitting 5 Eluer lock 5 Fmanifold 1 G
middle tubing 3 Hscrew clamps 3 Ireducer fitting 3 JTeflon tubing 3 K
straight micro-fitting 3 Lnozzle 1 M
nozzle support 1 Nmotor 1 O
motor spool 1 Pspeed control circuit 1 Q
circuit support 1 Rslider top multiple S
slider bottom 1 Tadhesive surface multiple U
eye hook 1 Vfunnel 4 W
AC/DC converter 1 X
108
APPENDIX C
SUPPLEMENTAL DRAWINGSOF
EXPERIMENTAL APPARATUS
109
Quantity : 1
Reference Code : A
Catlog Number :
Material : Plywood
Part : Base Board Source : True Value
Note: All dimensions in mm.
Figure A.1: Base board.
110
Quantity : 1
Reference Code : B
Catlog Number :
Material : Sheet Metal
Part : Track Groove Source :
Note: All dimensions in mm.
Figure A.2: Track Groove
111
Quantity : 1
Reference Code : C
Catlog Number :
Material : Sheet Metal
Part : Main Support Stand Source :
Note: All dimensions in mm.
Figure A.3: Main support stand.
112
Quantity : 1
Reference Code : D1
Catlog Number : H-06375-01
Material : Polyurethane
Part : Main Air Line 1 Source : Cole Parmer
Note: All dimensions in mm.
Figure A.4: Main air line 1.
113
Quantity : 1
Reference Code : D2
Catlog Number : H-06375-01
Material : Polyurethane
Part : Main Air Line 1 Source : Cole Parmer
Note: All dimensions in mm.
Figure A.5: Main air line 2.
114
Quantity : 5
Reference Code : E
Catlog Number : H-06359-17
Material : Plastic
Part : Luer Fitting Source : Cole Parmer
Note: All dimensions in mm.
Figure A.6: Luer fitting.
115
Quantity : 5
Reference Code : F
Catlog Number : H-06359-67
Material : Plastic
Part : Luer Lock Source : Cole Parmer
Note: All dimensions in mm.
Figure A.7: Luer lock.
116
Quantity : 3
Reference Code : H
Catlog Number : H-06423-01
Material : Polyurethane
Part : Middle Tubing Source : Cole Parmer
Note: All dimensions in mm.
Figure A.8: Middle tubing.
117
Quantity : 3
Reference Code : I
Catlog Number : H-06833-10
Material : Plastic
Part : Screw Clamps Source : Cole Parmer
Note: All dimensions in mm.
Figure A.9: Screw Clamp.
118
Quantity : 3
Reference Code : J
Catlog Number : H-06391-70
Material : PTFE
Part : Reucer Fitting Source : Cole Parmer
Note: All dimensions in mm.
Figure A.10: Reducer fitting.
119
Quantity : 3
Reference Code : K
Catlog Number : H-006375-01
Material : Teflon PFA
Part : Teflon Tubing Source : Cole Parmer
Note: All dimensions in mm.
Figure A.11: Teflon tubing.
120
Quantity : 3
Reference Code : L
Catlog Number : H-06173-00
Material : Plastic
Part : Straight Micro-Fitting Source : Cole Parmer
Note: All dimensions in mm.
Figure A.12: Straight micro-fitting.
121
Quantity : 1
Reference Code : M
Catlog Number :
Material : ABS plastic
Part : Nozzle Source :
Note: All dimensions in mm.
Figure A.13: Nozzle.
122
Quantity : 1
Reference Code : N
Catlog Number :
Material : ABS plastic
Part : Nozzle Support Source :
Note: All dimensions in mm.
Figure A.14: Nozzle support.
123
Quantity : 1
Reference Code : P
Catlog Number :
Material : ABS plastic
Part : Motor Spool Source :
Note: All dimensions in mm.
Figure A.15: Motor spool.
124
Quantity : 1
Reference Code : R
Catlog Number :
Material : ABS plastic
Part : Circuit Support Source :
Note: All dimensions in mm.
Figure A.16: Circuit Support.
125
Quantity : 6
Reference Code : S
Catlog Number :
Material : ABS plastic
Part : Slider Top Source :
Note: All dimensions in mm.
Figure A.17: Slider Top.
126
Quantity : 1
Reference Code : T
Catlog Number :
Material : ABS plastic
Part : Slider Bottom Source :
Note: All dimensions in mm.
Figure A.18: Slider bottom.
127
Quantity : 1
Reference Code : U
Catlog Number : Scotch Mailing Tape
Material :
Part : Adhesive Surface Source : Walmart
Note: All dimensions in mm.
Figure A.19: Adhesive surface.
128
Quantity : 1
Reference Code : V
Catlog Number :
Material : Steel
Part : Eye Hook Source :
Note: All dimensions in mm.
Figure A.20: Eye Hook.
129
Quantity : 4
Reference Code : W
Catlog Number :
Material : ABS plastic
Part : Funnel Source :
Note: All dimensions in mm.
Figure A.21: Funnel.
130
Figure A.22: Layout of fixtures attached to base board.
131
Figure A.23: Layout of fixtures attached to main support stand.
132
Figure A.24: Exploded manifold assembly.
133
Figure A.25: Exploded nozzle assembly.
134
Figure A.26: Exploded motor assembly.
135
APPENDIX D
SUPPLEMENTAL DRAWINGSOF
MULTIPLE POWDER DEPOSITION SYSTEM
136
Figure A.27: Powder print head.
137
Figure A.28: Proposed nozzle.
138
Figure A.29: Motion system.
139
Figure A.30: Powder loading system.
140
VITA
Shawn Fitzgerald was born in Pompton Plains, New Jersey on October 2, 1972. He
attended St. James High School in Carney’s Point, New Jersey. He obtained his Bachelor
of Science degree in Mechanical Engineering from Virginia Tech in May of 1994. After an
eight month internship with the Delaware River and Bay Authority in New Castle,
Delaware, Shawn returned to Virginia Tech to receive his Master of Science degree in
Mechanical Engineering in July 1996. He is working for the Defense Systems and
Electronics group of Texas Instruments located in the greater Dallas area.
