Design Mold

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W I T H E N G I N E E R I N G

T H E R M O P L A S T I C S

DESIGNING FORPERFORMANCE AND VALUE

Our Mission is to

satisfy customers

with engineering resins

and specialty compounds

supported by leading-edge

technologies and

services resulting in

cost-effective solutions.

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Contents

Introduction 2

Nature of thermoplasticmaterials 3

Dimensional stability 5

General design guidelines 9

Assembly techniques 15Snap-fits 16Welding 18Adhesive bonding 21Mechanical fastening 24

Mold design 28Mold construction 28Runners 32Gate design 33Ejection systems 35Mold cooling 36Tool steel 38Surface finish 39Hotrunners 42

DSM thermoplasticsproduct range 44

DSM product portfolio 45

Contact information back cover

DSM

Founded in 1902, DSM is a highlyintegrated international chemicalsand materials company. With annualsales of more than $7 billion and awork force in excess of 23,000 peo-ple the company operates more than200 sites around the world. DSM’sactivities are grouped into three clus-ters: performance materials, poly-mers and industrial chemicals, andlife sciences. The company’s princi-pal products are plastics, syntheticrubbers, fiber intermediates, and finechemicals for pharmaceuticals.

DSM has a strong technological baseand good market positions. For sev-eral products, including caprolactam,melamine, EPDM Rubber, and anti-infectives, DSM is a global marketleader. The company has an estab-lished and growing position in perfor-mance materials and life sciences.

DSM continuously develops newproducts and processes through aresearch and development activityfocused on innovation and cost effec-tive solutions. We have developedand patented numerous breakthroughmaterials for use in industry includingStanyl® 46 nylon, the first high tem-perature nylon, and Dyneema®, theworld’s strongest fiber.

With its products and services DSMmakes a meaningful contribution tosociety providing directly and indi-rectly for human needs such as food,clothing, housing, health care, trans-portation, and recreation.

DSM Engineering Plastics

With a global organization of over1,000 dedicated employees DSMhas a strong position in theEngineering Plastics field.

The company operates in all threemajor regions of the world: TheAmericas, Europe, and Asia. InEngineering Plastics, DSM is one ofthe fastest growing competitors witha strong emphasis on providing costeffective solutions for complex needs.

The focus of DSM EngineeringPlastics is on the production andcompounding of:

- Nylons (6, 6/6, 4/6)- Polyesters (PBT, PET)- Polyester Elastomer (TPE-E)- Polycarbonate.

In addition, we also produce a wideassortment of compounds andblends, including:

- Conductive Thermoplastics- Lubricated Thermoplastics- Reinforced Polypropylenes.

DSM Engineering Plastics is taking along and distinguished history in thecompounding market and enhancingit with an integrated position as apolymer producer. This results in afast, flexible, and customer focusedbusiness which provides the marketwith a portfolio of materials that arewell suited to meet the needs of awide variety of industries.

Commitment to the development ofnew applications for EngineeringPlastics drives our research anddevelopment efforts. The companyis a leader in the use of CAE(Computer Aided Engineering) tomodel and analyze potential oppor-tunities. Ongoing assistance fromthe Customer Service departmentand the QS 9000 recognized manu-facturing organization insures cus-tomer satisfaction.

Click on a topic in the table of contents to link directly to that page

2Table of Contents

Designing in thermoplastics requiresa good understanding of the behav-ior of materials processing, molddesign and assembly techniques.

DSM is a supplier of a broad rangeof thermoplastics with a portfolio thatincludes:

- Akulon® nylons

- Stanyl® 46 nylon

- Arnitel® copolyester elastomers

- Xantar® polycarbonate

- Arnite® thermoplastic polyester

- Electrafil® conductive thermoplastics

- Plaslube® lubricated thermoplastics

- Nylatron® lubricated thermoplastics

- Fiberfil® reinforced & filled

thermoplastics

DSM is able to add value for our cus-tomers by assisting in design andprocessing. Our experience with ourproducts can eliminate problems thatcould otherwise slow the productdevelopment process, and addinsight to help accomplish designand performance objectives.

The first section of this brochureaddresses general design guide-lines; an explanation about the char-acteristics of thermoplastics and theimpact of the material properties onthe part design. The second sectionis concerned with assembly tech-niques; what techniques are avail-able and what is the effect of thetechnique on the design.

The last section discusses molddesign. Again, the emphasis is onthe interaction between mold designand thermoplastic, general principlesabout mold design and issues likehotrunners and tool-steel.

For further assistance please contactour technical support help desk at800-333-4237, extension 7785, oryour local sales engineer.

Introduction

Processing

PRODUCT DESIGN

Tolerances MoldConstruction

EconomicFactors

FunctionalRequirements

AestheticRequirements

MaterialProperties

Figure 1.1 Design considerations.

chemical resistance + warpage –fatigue resistance + –wear resistance +flow properties +corrosion resistance +

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The process of developing thermo-plastic parts requires a full under-standing of typical material proper-ties under various conditions. Thischapter will discuss material charac-teristics in relation to their physicalstructure.

Thermoplastics can be categorizedby their molecular structure as eitheramorphous or semi-crystalline plas-tics. Amorphous polymers have astructure that shows no regularity.Semi-crystalline plastics, in theirsolid state, show local regular crys-talline structures dispersed in anamorphous phase.

These crystalline structures areformed when semi-crystalline plas-tics cool down from melt to solidstate. The polymer chains are partlyable to create a compacted structurewith a relatively high density.

The degree of crystallization dependson the length of the polymer chains,the viscosity, the melt temperatureand the mold temperature.

Examples of DSM semi-crystallinematerials are Akulon PA6 and PA66,Stanyl PA46, Arnite PBT, and Fiberfilpolypropylene. Examples of DSMamorphous polymers include XantarPC and Stapron® C PC/ABS blends.

Figure 1.2 Semi-crystalline and amorphous polymer structures.

Nature of thermoplastic materials

Table 1.1 Different properties due to different molecular structures.

amorphous thermoplastics

dimensional stability + notch sensitive –creep resistance + chemical resistance –low shrinkage +transparency +

semi-crystalline thermoplastics

4 Table of Contents

Molecular structure may causeremarkable differences in properties.The shear modulus curve illustratesthe temperature limits of a thermo-plastic.

Typical properties are reviewed inTable 1.1. Various properties are timeor temperature dependent. Theshear modulus, for instance,decreases at elevated temperatures.Figure 1.3 shows that the shape ofthe curve is different for amorphousand semi-crystalline thermoplastics.Glass transition temperature (Tg) andmelt temperature (Tm) are indicated.

Figure 1.4 shows time dependentcreep moduli. Resistance againstcreep is often higher for amorphouspolymers.

Due to higher densification of semi-crystalline plastics, a considerablyhigher shrinkage should be allowed(see Figure 1.2).

Figure 1.3 Different loss of elasticity.

Figure 1.4 Differences in creep modulus.

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Dimensional stability

Shrinkage. During injection mold-ing the polymer melt is injected intothe mold. Once the mold is complete-ly filled the dimensions of the moldingare the same as the dimensions of themold cavity at its service temperature(see Figure 1.5-B). While coolingdown the polymer starts to shrink (seeFigure 1.5-C). During the holdingstage of the injection molding cycle,shrinkage is compensated by post-fill-ing/packing. Both the design of thepart as well as the runner/gate shouldallow for sufficient filling and packing.

The process of shrinkage continueseven after the part has been ejected.Shrinkage should be measured longenough after injection molding totake into account post-shrinkage(see Figure 1.5-D).

Thermal expansion. An importantcondition for the dimensions of a partis the use of temperature. Thermo-plastics show a relatively high thermalexpansion (10-4/ °C) compared to met-als (10-5/ °C). Thermal expansion can-not be ignored for large parts whichare used at elevated temperatures(see Figure 1.5-F).

Isotropic versus anisotropic shrinkage. Both unfilled and miner-al-reinforced thermoplastics are large-ly isotropic with respect to shrinkage;shrinkage in flow direction is aboutequal to the shrinkage across flow.The glass fiber reinforced grades, onthe other hand, show anisotropicproperties. Due to fiber orientation inthe direction of the melt flow, shrink-age values in flow direction often aresubstantially smaller than across flowdirection (Figure 1.6).

Figure 1.6 Relation between the shrink-age of glass fiber reinforced plasticsand the orientation of the glass fibers(in thickness direction).

Figure 1.5 Dimensional stability through time.

Part dimensions Sum of Dimensional Deviations

A

Mold dimension at 23°C

B

Thermal expansions of metal

due to mold temperature

C

Part mold shrinkage

D

Post shrinkage through volume relaxation

E

Water absorption (polyamide)

F

Thermal expansion due to use temperature

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Moisture absorption inpolyamides. Akulon and Stanylparts, like all polyamide moldings,show dimensional changes (increase)after molding due to moisture absorp-tion (see Figure 1.7). Moisture absorp-tion is a time dependent, reversibleprocess which continues until an equi-librium is reached. This equilibriumdepends on temperature, relativehumidity of the environment and thewall thickness of the molding.

A change in moisture content willresult in different product dimensions.The designer should anticipate varyinghumidity conditions during use of theproduct (see Figure 1.5-E). The mois-ture absorption of reinforced gradesdiffers from those of the unfilledgrades.

The moisture content not only affectsthe dimensions but also various impor-tant properties. Yield stress, modulusof elasticity and hardness decreasewith increasing moisture absorption,while toughness shows a considerableincrease.

Although polyamide moldings arealready comparatively tough in thedry state, the high toughness, whichis characteristic of Akulon and Stanyl,is not reached until the material hasabsorbed 0.5-1 moisture. UnreinforcedStanyl already shows a dry as mold-ed impact resistance twice as highas other polyamides, so conditioningis less critical.

Shrinkage values. Many factorsmay influence shrinkage. It is not pos-sible to predict exact shrinkage valuesfor a specific polymer grade. Therefore,the maximum and minimum values forthe various DSM thermoplastics aregiven in Figure 1.8.

Figure 1.7 Effect of time and humidity on moisture absorption.

Figure 1.8 Dimensional effect of moisture absorption.

Akulon® PA6

PA6 + 30% GF

PA66

PA66 + 30% GF

Stanyl® PA46

PA46 + 30% GF

Arnite® PET + 35% GF

PBT

PBT + 20% GF

Xantar® PC

PC + 20% GF

Stapron® C ABS/PC

Fiberfil® PP UF & GF

Shrinkage of DSM Polymers (%)Both DirectionsIn Flow DirectionAcross Flow Direction

0 0.5 1.0 1.5 2.0

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Example of dimensional stabil-ity. Examples of the dimensional sta-bility of unfilled and reinforced Akulonand Stanyl are shown in Figure 1.9.For polyamide grades in general, theswelling of the thickness is substan-tial, especially when compared to theswelling in the two other directions.This should be taken into accountwhen designing parts with thick walls.

Dimensional deviations/toler-ances. All factors discussed influ-ence the final dimensions of the part.The maximum dimensional deviationof the part is the sum of the individualcontributing factors (see Figure 1.5).

Tolerances and product costs.Establishing the correct toleranceswith respect to the product function isof economic importance. The design-er should be aware that dimensionswith tight tolerances have a big influ-ence on the costs of both productand mold.

Akulon PA6

Akulon PA66

Stanyl PA46

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% S

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A

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C

direction of flow

perpendicular todirection of flow

wall thickness 1)

Figure 1.9 Dimensional stability of Akulon PA6/PA66/ Stanyl PA4/6.

A % swelling at equilibrium moisture content at 23°C

(73°F) and 50% RH

B % shrinkage (dry)

C % shrinkage - % dimensional increase (Important

value for the calculation of mold cavity dimensions)

mineral reinforced (PA6 30%, PA66 40%)

glass fiber reinforced (PA6 30%, PA66 35% & PA46 30%)

unreinforced

1) wall thickness in this example 4 mm (0.16 in)

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Even slightly over specifying toler-ances may adversely influence toolcosts, injection molding conditionsand cycle time. It is recommended toindicate only critical dimensions withtolerances on a drawing.

Depending on the application, a divi-sion into three tolerance classes canbe made:

- normal; price index 100- accurate; technical injection mold-

ing; price index 170- precise; precision injection mold-

ing; price index 300.

The most important characteristics ofthe tolerance classes are given inTable 1.2.

Mold design, mold cavity dimensions,product shape, injection-molding con-ditions and material properties deter-mine the tolerances that can beobtained.

Table 1.3 gives a summary of the fac-tors that play a major role in establish-ing dimensional accuracy.

Table 1.2 Characteristics of the tolerance classes.

Table 1.3 Factors affecting parts tolerance.

Normal Accurate PreciseStandard tool making techniques Accurately dimensions mold cavities High-precision molds

Multiple cavity molds Multiple cavity molds occasionally Only single-cavity molds

Conditions adapted for Molding conditions more critical Molding conditions carefullylow-cost manufacturing controlled

Scrap can be reused Reuse of injection-molding scrap Processing scrap not allowedpossible to a limited extent

Random inspection Statistical quality control Statistical process control

Part Design Material Properties Processing Mold Design

Product use Shrinkage (isotropic or anisotropic) Machine capacity Mold cavity tolerances

Wall thickness(es) Dimensional stability Injection pressure/speed Number of cavities

Draft Viscosity Holding pressure/time Runner system

Symmetry Reinforcements Melt and mold temperature Ejector system

Surface finish Clamping force Cooling system

Dimensions; length Reproducibility Design/Layout

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The characteristic behavior of differentpolymers has to be taken into account.A number of general design principleswill be discussed in this chapter.

Wall thickness. The shrinkagebehavior of thermoplastics dependson cooling of the thermoplastic fromprocessing temperatures to ambienttemperature. It is important to achieveproper cooling, and thus predictableshrinkage, over the part.

While observing functional require-ments, keep wall thicknesses as thinand uniform as possible. In this wayeven filling of the mold and anticipat-ed shrinkage throughout the moldingcan be obtained in the best way.Internal stresses can be reduced.

Wall thickness should be minimized toshorten the molding cycle, obtain lowpart weight and optimize materialusage. The minimum wall thicknessthat can be used in injection moldingdepends on size and geometry of themolding and on the flow behavior ofthe material. Where varying wall thick-nesses are unavoidable for reasons ofdesign, there should be a gradualtransition as indicated in Figure 2.1.

Generally, the maximum wall thick-ness used should not exceed 4 mm(0.16 in). Thicker walls increase mate-rial consumption, lengthen cycle timeconsiderably, and cause high internalstresses, sink marks and voids (seeFigure 2.3a and b).

Corners. An important principle isto avoid sharp internal corners. Due tothe difference in area/volume-ratio ofthe polymer at the outside and theinside of the corner, the cooling at theoutside is better than the cooling atthe inside. As a result the material atthe inside shows more shrinkage andso the corner tends to deflect (seeFigure 2.2). In addition, a sharp inter-nal corner introduces stress concen-tration.

A rounded corner has:

- uniform cooling- little warpage- less flow resistance- easier filling- lower stress concentration- less notch sensitivity.

General design guidelines

Figure 2.2 Sharp corners.

Figure 2.1 Wall thickness transition.

10 Table of Contents

Figure 2.3a Sink marks due to largewall thickness.

Figure 2.3b Voids due to large wallthickness.

Figure 2.4 Example of wall thickness reduction.

Figure 2.5 Example of a profile structure.

Ribs and profiled structures.If the load on a structural part requiressections exceeding 4 mm (0.16 in)thickness, reinforcement by means ofribs or box sections is advisable inorder to obtain the required strength at an acceptable wall thickness.

Ribs and box sections (Figure 2.4 and2.5) increase stiffness, thus improvingthe load bearing capability of themolding. These reinforcing methodspermit a decrease in wall thicknessbut impart the same strength to thesection as a greater wall thickness.

The use of ribs substantially reducesinternal stresses that normally occurduring shrinkage in thick sections.From an economical point of view, theuse of ribs results in savings of mater-ial and shorter molding cycles.

Ribs are preferably designed parallelto the melt flow and should be thin-ner than the wall to be reinforced.The thickness of a rib must notexceed half the thickness of the wallas indicated in Figure 2.6.

Ribs with a thickness larger than halfthe wall thickness will cause clearlyvisible sink marks on the surface ofthe wall opposite the ribs. In addition,

thick ribs may act as flow leaderscausing preferential flows duringinjection. This results in weld linesand air entrapment as shown inFigure 2.8.

Due to the same phenomena the ori-entation of glass fibers will be affect-ed. The flow patterns are clearly visi-ble and spoil the appearance of themolding.

Examples of how to avoid large wallthicknesses at the rib connectionsare shown in Figure 2.7.

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Figure 2.9 Influence of gating on glassfiber orientation and shrinkage of theproduct.

Figure 2.8 Influence of rib design on flow behavior of the melt.

.

Figure 2.6 Example of rib structure. Figure 2.7 Rib structures.


Glass fiber reinforced thermo-plastics. The degree of glass fiberorientation depends on several fac-tors such as:

- the wall thickness of the molding- the position and type of gating- the gate size- the injection speed.

In general, there will be a higher glassfiber orientation in thinner wall sec-tions, e.g. less than 2 mm (0.08 in)and as injection speed increases. Ahigh injection speed is required toobtain a smooth surface.

The direction of orientation is influ-enced by gate type and location and, of course, by the shape of the prod-uct (see Figure 2.9).

Warpage. An incorrectly dimen-sioned or located gate may alsoresult in undesirable flow patterns inthe mold cavity. This can lead tomoldings with visible weld line (seeFigure 2.10) or deformation by warp-ing or bending (see Figure 2.11).

Figure 2.10 Influence of gate location on flow behavior of the melt.

Figure 2.11 Warpage due to unfavorable gate location.

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Another factor related to the gatelocation is the pressure build-up inthe cavity. If the pressure build-up isnot uniform, partial over-packing maycause warpage. Uniform mold cool-ing is especially important. A differ-ence in temperature between moldhalves or inadequate temperaturecontrol will give uneven cooling andthus warpage.

In order to avoid or minimize warpagethe following guidelines should be considered:

Product

- design for structural integrity- use adequate radii- design with uniform wall thicknesses- allow for sufficient draft angle.

See also Figures 2.12, 2.13, and 2.14.

Mold

- optimize gate location- select the best gating system- ensure that gate and runners are

adequately dimensioned- design effective cooling lay out- use sufficient ejection surface.

Figure 2.12 Design of a timing belt pulley.

Figure 2.13 Example of a design study of a multiconnector.


Figure 2.15 Example of a design study for an electric motor shield based onpolyester to replace on aluminum shield.

Figure 2.14 Alternative B may result in mold construction cost savings of 60 % compared with design A.

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Molded or machined components ofDSM thermoplastics can be assem-bled using various joining techniques.Reliable and efficient joints in engi-neering applications need to bespecifically designed on a case bycase basis. Note that joints alwayscreate weaker spots in a producttherefore, it is best to design a joint ina non-critical area.

Optimal joints can be obtained whensome quantitative criteria have beendefined, e.g. mechanical strength.Relevant tests should be developed.Process optimization and control is ofprimary importance for applicationengineering. Standard solutions donot exist.

The choice of a technique dependson the following considerations:

- functional requirements of assembly

- material of the components (thermoplastic, thermoset, metal)

- dimensions of components- disassembly/recycling- production volume- costs.

For disassembly during product service life, detachable joints areadvised like snap fits and screws.Note that screwing may be time consuming and a variety of screwthreads and heads exist.

For recycling purposes it may beinconvenient to remove incompatibleparts like metal inserts and screws.Pre-determined breaking points mightbe incorporated in areas that experi-ence low stress levels under serviceload. Figure 3.1 gives a summary ofconventional assembly techniques.

Figure 3.1 Assembly techniques for Akulon and Arnite.

Assembly techniques

Permanent assembles

Snap fitsWelding_ vibration welding_ ultrasonic welding_ hot plateAdhesive bonding

Detachable assembles

Snap fitsScrewing_ self-tapping screws_ inserts: molded-in inserts or

inserts installed by ultrasonic insertion


Snap-fits

The snap-fit method is an assemblytechnique that fully utilizes themechanical properties of plastics.Thermoplastic parts can be fittedrapidly and economically to othercomponents made of metal, glass,plastics, etc. A snap-fit offers twoadvantages in disassembly. If theassembly is accessible it can be easi-ly disassembled. If it is not accessi-ble, it acts as a pre-determined break-ing point.

Snap-fits can be found in a wide vari-ety of shapes. Two examples of typi-cal snap-fit geometries are the can-tilever beam type (see Figure 3.2) andthe cylindrical type (see Figure 3.3).

Designing a snap-fit is rather complexdue to a combination of factors:

- the functional requirements of the product

- the requirements for the assembly- the mechanical properties of the

thermoplastic- the design of the mold and

notably part ejection.

Figure 3.2 Snap-fit cantilever beam type.

Figure 3.3 Snap-fit cylindrical type.

Figure 3.4 Factors for calculating the cantilever beam for a snap-fit.

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Example: the cantilever beam.A simple type of snap-fit, the cantileverbeam, is demonstrated in Figure 3.4which shows the major geometricalparameters of this type of snap-fit.

If a snap-fit fails during assembly, themaximum deflection of the cantileverbeam most likely exceeded the deflec-tion limit of the thermoplastic used. Theequation in Figure 3.5 gives the relationbetween the cantilever beam geometryand the deflection limit.

The four parameters that can bechanged by the designer are:

- the height of the snap-fit lip (h) is

directly related to the performance

of the lip. Changing the height

might reduce the ability of the

snap-fit to ensure a proper con-

nection.

- the thickness of the beam (t) is

uniform over the length of the

beam in this example. A more

effective method is to use a

tapered beam. The deflection of

the beam will reduce and the

stresses are more evenly spread

over the length of the beam.

- increasing the length of beam (L)

is the best way to reduce strain as

it (L) is represented squared in the

equation.

- deflection limits (ε) of DSM ther-

moplastics are indicated in Figure

3.5. Since the snap-fit is only a

small part of a product, it is better

to design snap-fit dimensions

based on a thermoplastic chosen

than to choose the thermoplastic

to make a specific snap-fit work.

A common factor causing failure of asnap-fit is the inside radius (r) or lackthereof. An inside radius which is toosmall will induce stress-concentra-tions. These sections with high stress-es are often weak because the deflec-tion limit is reached sooner.

The mating force required to assem-ble and the separation force requiredto disassemble the snap-fit is deter-mined by a different set of parame-ters. In addition to the previouslydiscussed parameters, the supportangle (θ1) and the guide angle (θ2)as well as the stiffness of the material

and the friction between the snap-fitand the mating material are relevantproperties.

In most cases the number of snap-fitscan be changed. Bending loads onthe cantilever beam after assemblyshould be avoided due to possiblecreep. The designer should be awarethat both the possibility of breakageand the required force to (dis)assem-ble can be handled more or less inde-pendently.

Figure 3.5 Permissible deflection for snap-fits.

GF

GF

GF

GF

GF

The calculations used in the exampleare a simplification. In general, thestiffness of the part the snap-fit isconnected to is important. The formu-las mentioned only roughly describethe behavior of both the part geome-try and the material. On the otherhand, the approach can be used as afirst indication if a snap-fit design andmaterial choice are feasible. Examplesof the cantilever beam are shown inFigure 3.6.

Welding

Welding of thermoplastic parts isbased on interdiffusion of molecularchains. This requires elevated temper-

ature, pressure and time to achievegood mechanical bond. In addition,this helps to clean the weld surfaces.There exist a variety of welding tech-niques. Most popular are vibration/spin, ultrasonic and hot plate welding.

Vibration/spin welding. Vibrationwelding is a resonant process at 100-400 Hz and a linear amplitude of0.5-2.5 mm (0.02-0.10 in). The weldsurface is heated by solid coulombicsurface friction. After melting, viscousforces take over and the meltedregion starts to flow. When vibrationstops, the welds cools down andsolidifies. Typical cycle time is 10 sec-onds; weld pressure 0.5-5 MPa (73-725 psi). Product size is not limited; in

general products larger than 200 mm(8 in) are joined with this technique.During welding a special mold isrequired for fixturing the components.

Spin welding is a similar process, butit is restricted to cylindrical parts witha maximum diameter of 250 mm (10 in). Surface friction at rotationalspeeds of 30,000-60,000 rpm is usedto create the weld.

To prevent part deformations duringwelding it is common practice todesign a flange at the weld surface.Proper welds will always show flash.For aesthetic purposes the part canbe designed to hide the weld. Due tothe weld a loss in the overall length of0.2-0.4 mm (0.01-0.02 in) should betaken into account. Correct alignmentof the components is important.

Figure 3.7 shows some typical welddesigns, which are self-centering. Thestiffness of the tapered sectionsshould be high enough to avoid defor-mation during welding.


Figure 3.6 Example of proper snap-fit design.Alternative B results in mold construction cost savings compared with design A.

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Figure 3.7 Typical weld designs for spin welding.

Figure 3.8 Typical weld designs for ultrasonic welding.

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Ultrasonic welding. Ultrasonicwelding is a fast and effective weldingtechnique for parts with a weld jointsmaller than 200 mm (8 in) in length.The welding equipment consists of anultrasonic generator running at a fre-quency of 20-40 kHz and an amplitudeof 10-50 mm, a booster for amplifica-tion and a horn to transfer energy tothe component.

The combination of the booster andhorn is unique for each design. Specificattention of the supplier is required.When using glass fiber reinforced ther-moplastics the horn needs a specialsurface treatment to prevent abrasion.

The weld zone is melted instanta-neously by internal friction. Typicalcycle time is 1 second; welding pres-sure 1-10 MPa (145-1450 psi). Themechanical strength of an ultrasonicweld may reach a value of 70 to 80%of the original strength of the material,

however the actual strength is verymuch dependent on the specificgeometry and materials being welded.

Drying before welding is not alwaysnecessary to obtain high quality. DSMdrying guidelines for injection moldingcan be followed. If the effect of mois-ture is unclear, it is advisable to test itsinfluence on welding strength.Components may be conditioned fortesting by submerging overnight inwater in advance of welding.

Melting takes place at the weakestspot of the part. Therefore, it is oftenadvised to use a line contact at thewelding surface. Two standard shapesdepicted in Figure 3.8A, 3.8B and 3.8Care used frequently for amorphousthermoplastics: the energy directorprinciple and the shear joint. In gener-al, a shear joint is advised for semi-crystalline thermoplastics because oftheir short melting range. Note that the

weld can be hidden either on theinside or on the outside corner toimprove appearance. The efficiency ofenergy transfer to the weld surfacedepends largely on the type of ther-moplastic. Stiff parts with low mechan-ical damping properties can be easilywelded. The distance between hornand weld surface may be larger than6 mm (0.24 in) (distant welding).Polyolefins should always be weldedunder near field conditions (less than 6 mm (0.24 in)), because of theirsemi-crystalline structure and their rel-atively low elastic modulus.

Internal sharp corners cause stressconcentrations. The use of fillet radiiis strongly advised when using ultra-sonic assembly. Proper welds alwaysgive flash.

Hot plate welding. Hot plate weld-ing uses thermal energy to melt thewelding zone through heat conduction.It is a time consuming process; typicalcycle time 60 seconds.

Welding pressure is relatively low, 0.1-0.5 MPa (15-73 psi). Part size is unlim-ited. The recommended plate tempera-ture depends largely on the specificthermoplastic. Amorphous plasticsrequire a temperature of 80-160°C(175-320°F) above the glass transitiontemperature (Tg). Semi-crystallinematerials are best welded at 40-100°C(100-210°F) above melting temperature(Tm). A PTFE coating is used to pre-vent parts from sticking.

Recommended hot-plate temperaturesfor DSM polymers are listed in thetable found in Figure 3.9. The platedirection should be between 60° and90°. Proper welds will give flash. Toimprove appearance the flash may betrapped as indicated in Figure 3.9. Aloss in the overall length should beaccounted for, due to the weld.


Figure 3.9 Indication of hot-plate temperature for DSM materials.

˚C ˚F

Adhesive bonding

The significant criteria for adhesivebonding are surface wetting and cur-ing of the adhesive to join compo-nents of various materials, e.g. ther-moplastics and metals. Important vari-ables for the application of adhesiveand distribution on a substrate aresurface contact angle, adhesive vis-cosity and chemical resistance ofsubstrate to adhesive.

In general, adhesion is based on vari-ous mechanisms as shown in Figure3.10. Interdiffusion is limited by crys-tallites, therefore, it tends to be morecomplicated to accomplish goodadhesion on semi-crystalline com-pared to amorphous thermoplastics.Adhesion on nonpolar thermoplastics,e.g. polyolefins, will improve consider-ably when the surface is pretreatedusing corona, UV-plasma or flame.

Poor adhesion takes place when theadhesive layer does not stick properlyto the substrate. Pretreatment, e.g.sanding, may be helpful.

Specific advantages of adhesives are:

- application on various substrates like thermoplastics, thermosets, elastomers and metals

- homogeneous distribution of mechanical loads

- differences in thermal expansionof components may be compen-sated in thick adhesive layers

- good aesthetics - no specialrequirements to hide the bond.

Potential limitations are:

- long term behavior- reproducibility/process control- curing time- disassembly.

Adhesives. A wide variety of adhe-sives are commercially available. Theperformance on some DSM thermo-plastics and the influence of pretreat-ment is shown in Table 3.1. The val-ues indicated are based on lap shearstrength (in MPa).

Epoxy. Various epoxy adhesives areavailable with different characteristicsand properties. Based on curingmechanism a division can be madeas follows:

- 2 component hot or cold curing- 1 component hot curing- UV-curing.

Standard epoxy adhesives are brittleand show a low peel strength. Toimprove toughness modified epoxyadhesives have been developed. Theuse temperature varies between - 40and 80°C (-40-180°F) for cold curingsystems. Hot curing epoxies can nor-mally be used up to 150°C (300°F).

In general, large deviations in lapshear bonding strength show updepending on the particular combina-tion of adhesive and material.

With some plastics, pretreatment cangive considerable improvements. Theadhesion of epoxies is susceptible tooils and grease.

Polyurethane. Polyurethane adhe-sives are relatively inexpensive andshow good adhesion. Varieties existfrom elastomeric to rigid. According tothe curing mechanism several typesare distinguished:

- 1 component thermosetting- 2 component catallized- reactive hot melts.

Polyurethane adhesives are tough andshow a high peel strength. They canbe used at temperatures between -80 and 100°C (-110-210°F).

Adhesion on engineering plastics isgood. Degreasing is often sufficient toobtain the required bonding strength.Polyurethanes are not suitable for usewith polyolefins.

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Figure 3.10 Impression of different adhesive mechanisms.


Acrylic. Acrylics are flexible andtough. Fast curing takes place atroom temperature. Care should betaken joining amorphous thermoplas-tics as environmental stress crackingmay occur. Several systems are available:

- 1 component UV-curing used for transparent plastics

- 2 component premix- 2 component no-mix.

Use temperature is between -55 and 120°C (-70-250°F). Acrylics showexcellent peel strength and are tough.

Good adhesion is obtained on amor-phous thermoplastics. Pretreatmentmay improve the lap shear bondingstrength considerably.

Cyanoacrylic. Cyanoacrylics arefast curing systems but rather brittle,which results in low peel strength andimpact properties in the joint area ofthe component. Rubber modifiedcyanoacrylics have been developedto improve toughness.

A very high lap shear bondingstrength can be obtained with mostengineering thermoplastics. Unfilledpolyesters and polyolefins show mod-erate results. Effective primers areavailable to improve the bondingstrength of polyolefins.

Table 3.1 Performance of DSM thermoplastics and the influence of pretreatment.

PA 6 uf/gf 3/10 6 7 10* 10*PA 66 uf 4 8 3 10 5

gf 4 8 4 - 10

PP gf 1/7 /2 1/6 4 /5

PC uf/gf 10* - 7* 5 7

* depending on specific combination plastic/adhesive indication value of lap shear strength in MPa; 10 indicates > 10 MPa.

2 comp 1 comp

Epoxy Polyurethane Acrylic Cyanoacrylic

PBT uf 1 6 3 - 1gf 2 9 4 - 5

PET uf 2/6 /10 5/7 /8 2\gf 4/10 /10 7/10* 1/10* 8*

Arnite

FIberfil

Xantar

Akulon

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Design for adhesive bonding.The load on the assembly can beapplied in several ways as indicatedin Figure 3.11. Thin layers are advisedin case of lap shear. Peel and splitloads are best taken up by a thicklayer of adhesive.

General design guidelines are:- design for lap shear loads- maximize the bonding surface

for instance, use a scarfed or a dovetail joint

- avoid stress concentrations at thick-thin sections

- take care of sufficient venting on substrate.

Recommended joint designs aregiven in Figure 3.12. Hermetic sealsrequired for containers and bottles areaccomplished with the designs shownin Figure 3.12A and B. Joint C is moreuniversal.

Figure 3.11 Lapshear, peel, split, tension and compression.

Figure 3.12 Joint designs for adhesive-bonded assemblies.

.

.

.

.


To ensure successful joining withadhesives it is important to know thefunctional requirements of the assem-bly and possibilities/limitations of theadhesive in combination with the sub-strate. The following checklist mightprove useful:

- product: design joints specific for adhesives

- mechanical load: lap shear, peel, split or tensile

- life of joint: use temperature, environment, relative humidity

- thermoplastic substrate: mechani-cal properties, wetting, moistureabsorption

- adhesive: temperature and chemi-cal resistance

- pretreatment: cleaning, etching, sanding, oxidation, primer

- safety: MSDS (Material SafetyData Sheet) chart.

The moisture content of polyamidesdoes not show a remarkable influenceon the bonding strength. It is advis-able to do some bonding tests withconditioned parts prior to production.

Mechanical fastening

Inserts. Insertion is a way to createa connection that can be assembledand disassembled repeatedly withoutproblems. A metal part is inserted inthe thermoplastic. The most common-ly used insertion techniques, molded-in and ultrasonic, will be discussed inmore detail.

Molded-in inserts. The insert is putinto the mold (cavity) during the injec-tion molding cycle. It is important toheat the inserts to near the mold tem-perature before molding. Due to differ-

ences in thermal expansion, stressescould be built up at the metal/plasticinterface. It is also essential that theinserts be clean and free of anyprocess lubricants.

Ultrasonic insertion. The insert ispressed in a hole in the plastic. Theultrasonic energy melts the plasticaround the insert. Once the insert ispressed in the plastic freezes offevenly around the insert. Molded-ininserts may cause failures becausethe metal part might induce sink-marks, internal stresses and warpage.

Inserts as shown in Figure 3.13, espe-cially developed for ultrasonic inser-tion, are commercially available in var-ious types and sizes.

Recommendations about hole diame-ters (see Figure 3.13 A1 and B1) andinsertion conditions are available frommanufacturers of inserts and ultrason-ic equipment.

Figure 3.13 Inserts for ultrasonic insertion.

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Ultrasonic insertion gives a shorter molding cycle than molded-in inser-tion. However, it also represents anadditional manufacturing process.Care should always be taken toensure the insert is solidly embeddedin the substrate.

Recommendations:

- design simple inserts with under-cuts for pull-out retention andgrooves or knurls for torque reten-tion (see Figure 3.14)

- avoid sharp corners- use brass, stainless steel or plat-

ed steel inserts; raw steel insertsmay rust

- use clean inserts to safeguardoptimal interfacing between themetal and the thermoplastic (freefrom oil, grease, etc.)

- ensure that adjacent walls have sufficient thickness to prevent the insert from being pulled out during assembly

- keep knurls away from part edges for notch sensitivity.

Figure 3.14 Molded-in inserts undercut with grooves and knurls.


Screw assembly. Self tappingscrews for the assembly of plasticparts can be distinguished into threadcutting screws and thread formingscrews.

Thread cutting screws cut the threadduring assembly. That means thatevery time the screw is assembledsome material will be cut away. Forthat reason this type of screw is notrecommended for repeated assemblyand disassembly. In general, self tap-ping screws are used for thermosetswith a low elongation at break and lowplastic deformation.

Thread forming screws do not cut butdeform the thermoplastic. Close to thescrew the stresses can be high. If thistype of screw matches the screwgeometry as described in this chap-ter, they can be used for the range ofDSM thermoplastics. Thread formingscrews can be used for repeatedassembly and disassembly. Figures3.15 and 3.16 give an example ofsuch screws.

With regards to screw geometry thefollowing requirements should beobserved:

- thread flank angle: as small aspossible (30°) in order to obtainsmall radial tensions in the boss

- thread core design: possibly pro-filed in order to allow a trouble-free material flow during thethread-forming process

- thread pitch: possibly below 8°in order to obtain dynamicallysecure joints.

Figure 3.15 Thread design system PTfor plastics up to 40% GF.

Figure 3.16 Thread design system for plastics up to 40% GF.

Figure 3.17 Boss design.Figure 3.18 Example of boss designs (provided adequate fillet radius).

dc

Boss Ø

Hole Ø

d = Nominal Ø of screwdc = d + 0.2 mm

(0.3

÷ 0

.5)

x d

di

2/3 S (REF)

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Akulon PA 6 and PA 66 0.75 x d* 1.85 x d 1.70 x d

PA 6 + 30% gf 0.80 x d 2.00 x d 1.90 x d

PA 66 + 30% gf 0.82 x d 2.00 x d 1.80 x d

Stanyl PA 46 0.73 x d 1.85 x d 1.80 x d

PA 46 + 30% gf 0.78 x d 1.85 x d 1.80 x d

Arnite PET & PBT 0.75 x d 1.85 x d 1.70 x d

PET & PBT + 30% gf 0.80 x d 1.80 x d 1.70 x d

Xantar PC 0.85 x d 2.50 x d 2.20 x d

PC + 30% gf 0.85 x d 2.20 x d 2.00 x d

Fiberfil PP 0.70 x d 2.00 x d 2.00 x d

PC + 30% gf 0.72 x d 2.00 x d 2.00 x d

*d = nominal screw diameter

In special cases the usefulnessshould be established by means ofcomponent tests. For the optimumconstruction the recommendations inTable 3.2 apply. In order to find outthe recommended hole-boss diameterof insertion depth, the given factorsshould be multiplied with the normaldiameter of the corresponding screw.

A cylindrical lead-in counterbore,according to Figure 3.17, should beconsidered in the design in order toreduce edge stress.

When using screw assembly, a distinctdifference can be noticed betweenthe torque required to assemble andthat required to overturn the screw.

This allows for automatic assemblywithout damaging the screw or thethermoplastic part.

During the assembly process a maxi-mum speed of about 500 rpm shouldbe observed. Higher speed and theresulting friction may melt the material.

Table 3.2 Recommended boss design (source Ejot).

Material PT Screw System Insertion DepthHole Ø Boss Ø di


Mold design and constructionrequires special attention for optimalproduct quality and reliable molding.A detailed specification is required inadvance:- product shape and tolerances- mold in relation to molding

equipment- parting lines; venting- number of cavities- runner lay-out and gating system- ejection system- cooling system lay-out- type of tool steel- surface finish.

Mold-machine combination.The mold should be tuned to theinjection molding equipment withrespect to mold mounting, injectionunit and clamping force. Relevantmolding machine data can be foundin Table 4.1. The maximum shotweight of the injection unit is the

amount of plastic that can be injectedper shot. The weight of the moldingshould not exceed 80% of the maxi-mum shot weight.

Mold construction

A standard injection mold is made ofa stationary or injection side contain-ing one or more cavities and a moving

or ejection side. Both sides togetherenclose one or more cavities. Relevantdetails are shown in Figure 4.1. Highquality molds are expensive becauselabor and numerous high- precisionmachining operations are time-con-suming. Product development andmanufacturing costs often can be sig-nificantly reduced if sufficient attentionis paid to product and mold design.

Table 4.1 Mold mounting dimensions.

Figure 4.1 Impression of a standard injection mold.

Molding Machine Mold

minimum/maximum mold height mold closed heightopening stroke ejection stroketie bars spacing mounting plate dimensions mounting holes or grooves knockout patternknockout pattern locating ring diameter nozzle alignment length of sprue bushinginsertion depth of nozzle sprue bushing radiusnozzle radius sprue orifice diameternozzle orifice

Mold design

The minimum plasticizing capacitydepends on the relationship betweenshot weight and cooling time. Forexample, molding 300 g (0.66 lbs) in30 seconds requires a minimum plasti-cating capacity of 36 kg/hr (79 lbs/hr).

The required clamping force of amolding machine is determined by thecavity pressure during the injection/holding stage and the projected areaof the part in the clamp direction.Various factors affect the moldingpressure, e.g. length over thicknessratio of the molded part, injectionspeed and melt viscosity. Typicalinjection pressures are 40-50 N/mm2

(5000-8000 psi), resulting in arequired clamp force of 0.4-0.5tons/cm2 (3-4 tons/square inch).

The way in which the mold is con-structed is determined by:

- shape of the part- number of cavities- position and system of gating- material viscosity- mold venting.

A simple mold with a single partingline is shown in Figure 4.1. Morecomplex molds for parts with under-cuts or side cores may use severalparting lines or sliding cores.Thesecores may be operated manually,mechanically, hydraulically, pneumati-cally or electro-mechanically.

Figure 4.2 shows an example of asliding cam. The cam pins whichoperate the cams are mounted undera maximum angle of 20° - 25° in theinjection side. The angle is limitedbecause of the enormous force whichis exerted on these pins during moldopening and closing.

The method of construction is deter-mined not only by the part shape andmaterial properties (flexibility, rigidity,shrinkage) but also by the partrequirements. Cams, and rotatingcores for example, may be used forexternal screw threads, but consider-able increase in costs will result.

Three-plate molds, as shown in Figure4.3, have two parting lines that areused in multi-cavity molds or multiplegated parts. During the first openingstage automatic degating takes placewhen the parts are pulled away fromthe runners.

The opening stroke is limited byadjusting bolts, which also operatestripper plate A.

Runners are stripped from slightlyundercut cores at the injection side.Then, the mold is opened at the mainparting line. Stripper plate B ejects the parts.

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Figure 4.3 Three-plate mold with two stripper plates for ejection.

Figure 4.2 Cammed mold for part with undercut cams move in vertical directionwhen mold is opened.


To vent the air in the mold cavity itmust be able to escape during moldfilling. If there are insufficient ventscompression of air may take place.The pressure and local temperaturerise quickly, potentially causingincomplete filling or even burning ofthe thermoplastic.

Venting should be taken into accountin the design stage and positioned atthe last points to fill.

If the air is trapped with no way out tothe mold parting line, it is advisable toplace a venting pin/ejector pin to per-mit the air escape through the clear-ance between pin and hole.Dimensions of venting channels canbe read from Figure 4.4; the dimen-sions are chosen in such a way thatair can escape without flash.

Inadequate venting may result in vari-ous molding failures:

- burnt spot- weak and visible weld lines- poor surface finish- poor mechanical properties- incomplete filling, especially

in thin sections- irregular dimensions- local corrosion of the mold

cavity surface.

Multi-cavity molds. The number ofcavities and mold constructiondepend both on economical and tech-nical factors. Important is the numberof parts to be molded, the requiredtime, and price in relation to moldmanufacturing costs. Figure 4.5shows the relation between the totalpart costs and the number of cavities.

The gating system and gate locationcan limit the design freedom for multi-cavity molds. Dimensional accuracyand quality requirements should beaccounted for.

The runner lay-out of multiple-cavitymolds should be designed for simul-taneous and even cavity filling.Unbalanced runner systems lead tounequal filling, post-filling and cool-ing of individual cavities which maycause failures like:

- incomplete filling- differences in product properties - shrinkage differences/warpage- sink marks- flash- poor mold release- inconsistency.

Figure 4.5 Total part costs in relation to number of cavities.

.

.

cavity

vent

relief

all dimensions mm

Figure 4.4 Construction of a ventingchannel.

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Examples of unbalanced runners areshown in Figure 4.6. With computeraided flow simulation it is possible toadjust primary and secondary runnerdimensions to obtain equal filling pat-terns. Adjusting runner dimensions toachieve equal filling may not be suffi-cient in critical parts to preventpotential failures. Special attention isrequired for:

- very small components

- parts with thin sections

- parts that permit no sink marks

- parts with a primary runner length

much larger than secondary

runner length.

It is preferred to design naturally bal-anced runners as shown in Figure 4.7.

When high quality and tight toler-ances are required the cavities mustbe uniform. Family molds are notconsidered suitable. Nevertheless, itmight be necessary for economicalreasons to mold different parts in onemold. The cavity with the largestcomponent should be placed nearestto the sprue.

The maximum number of cavities in amold depends on the total cavity vol-ume including runners in relation tothe maximum barrel capacity andclamping force of the injection mold-ing machine.

Number of cavities. A givenmolding machine has a maximumbarrel capacity of 254 cm3 (7 in3), aplasticizing capacity of 25 g/s (1 oz/second), 45 mm screw (1.77 in screw) and a clamping forceof 1300 kN (150 tons). A PC part of

30 cm3 (1.83 in3), shot weight 36 g (1.3 oz) and a projected area of 20cm2 (3.1 in2) including runnersrequires about 5 kN/cm2 (36 tons)clamping force.

The maximum number of cavitiesbased on the clamping force wouldbe 12. It is advisable to use only 80% of the barrel capacity, thus thenumber of cavities in this example islimited to 6.

When very short cycle times areexpected the total number of cavitiesmay be further reduced. A 6-cavitymold in this example requires a shotweight of 216 g (8 oz). The coolingtime must be at least 8.7 seconds.

Figure 4.6 Unbalanced runner systems.

Figure 4.7 Naturally balanced runnersystems.


Runners

The runner system is a manifold fordistribution of thermoplastic melt fromthe machine nozzle to the cavities. Thesprue bushing and runners should beas short as possible to ensure limitedpressure losses in the mold.

The sprue is provided with a cold slugwell with a reversed taper and will beextracted from the cavity side when themold opens. Runners should be pro-vided with cold slug wells at the end ofprimary and secondary runners.

Streamlining of runners prevents irreg-ular melt flow with air entrapments asa potential consequence.

The flow resistance can be decreasedby rounding off all corners in the run-ner system.

Runners with circular cross-sectionaffect favorable melt flow and coolingfor their optimal surface area to vol-ume ratio (see Figure 4.8). However, ittakes more effort to build circular run-ners because one half must bemachined in the fixed mold part andthe other half in the moving mold part.

This option is expensive as the twohalves must match each other withhigh accuracy. Semi-circular runnersare not recommended because oflarge heat losses.

Full trapezoidal channels in one of thetwo mold halves provide a cheaperalternative (see Figure 4.8).

The rounded off trapezoidal crosssection combines ease of machiningin one mold half with a cross sectionthat approaches the desired circularshape. The height of a trapezoidalrunner must be at least 80% of thelargest width.

The diameter of a runner highlydepends on its length but must neverbe smaller than the largest wall thick-ness of the product. Recommendedrunner dimensions can be taken fromTable 4.2.

D

D x 1.192

10˚

D x 0.839

D D

D x 1.192

10˚

Figure 4.8 Cross sectional area of various runner profiles.

(in) (mm)1/8 3.23/16 4.81/4 6.45/16 7.93/8 9.57/16 11.11/2 12.75/8 15.9

(in2) (mm2)0.012 8.00.028 18.10.049 32.20.077 49.00.110 70.90.150 96.80.196 126.70.307 198.6

(in2) (mm2)0.016 10.40.036 23.40.063 41.60.099 63.40.143 91.60.194 125.10.254 163.80.397 256.7

(in2) (mm2)0.015 9.70.033 21.80.059 38.70.092 59.00.133 85.30.181 116.40.236 152.40.369 238.9

Modified TrapezoidTrapezoidFull RoundD

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Gate design

The location of gates is of greatimportance for the properties andappearance of the finished part. Themelt should fill the entire cavity quicklyand evenly. For gate design the fol-lowing points should be considered:

- locate the gate at the thickest section- note gate marks for aesthetic reasons- avoid jetting by modifying gate

dimensions or position- balance flow paths to ensure

uniform filling and packing- prevent weld lines or direct

to less critical sections- minimize entrapped air to

eliminate burn marks- avoid areas subject to impact or

mechanical stress- place for ease of degating.

A distinction can be made betweencenter and edge gating of a part.Center gated parts show a radial flowof the melt. This type of gate is particu-larly good for symmetrical parts, suchas cup shaped products or gears,because the cavity can fill evenly andgive very predictable results. On theother hand, linear flow and cross flowproperties often differ. In flat parts, thiscan induce additional stress and resultin warpage or uneven shrinkage.

Because of their simplicity and ease ofmanufacture, edge gates are the mostcommonly used. These work well for awide variety of parts which are injec-tion molded. Long narrow parts typi-cally use edge gates at or near oneend in order to reduce warpage. But itis very difficult to mold round partsusing this type gate as they tend towarp into an oval shape.

Sprue gate. Direct gating with thesprue provides simplicity for symmet-ric, center gated single cavity molds.

This type of gating is particularly suit-able for thick moldings because hold-ing pressure is more effective. A shortsprue is favored, enabling rapid moldfilling and low pressure losses.

In general, the diameter at the begin-ning of the sprue should be approxi-mately 0.5 mm (0.02 in) larger thanthe orifice of the nozzle. A minimumtotal taper of 3° is required. The junc-tion of sprue and part should beradiused to prevent stress cracking.After demolding, the sprue is mechan-ically removed from the part.

Pin points. Pin points are popular foraesthetics reasons and ease of degat-ing. They offer an inexpensive solutionin standard two plate multiple cavity Gate diameters for unreinforced ther-moplastics range from 0.8 up to 6 mm

(0.03-0.25 in). Smaller gates mayinduce high shear and thus thermaldegradation. Reinforced thermoplas-tics require slightly larger gates ∆ 1mm (0.04 in). The maximal land lengthshould be 1 mm. Advised gate dimen-sions can be found in Table 4.3.

A special version of pin point for cylin-drical parts is the multiple point gate.Three plate molds and multiple pointgates are often employed in the mold-ing of critical tolerance parts such asgears.

Tabs can be regarded as unrestrictedgates. Tabs offer an alternative for pinpoint side gating to eliminate jettingand reduce local strains.

Table 4.3 Dimensions of gates.

0.7 - 1.2 mm (0.02 - 0.05) 0.7 - 1.0 / 0.8 - 1 (0.02 - 0.04 / 0.03 - 0.04)

1.2 - 3.0 mm (0.05 - 0.12) 0.8 - 2.0 / 0.8 - 1 (0.03 - 0.08 / 0.03 - 0.04)

3.0 - 5.0 mm (0.12 - 0.20) 1.5 - 3.5 / 0.9 - 1 (0.06 - 0.14 / 0.04 - 0.04)

> 5.0* mm (0.20) 3.5 - 6.0 / 0.8 - 1 (0.14 - 0.24 / 0.03 - 0.04)

* wall thickness larger than 5 mm (0.20 in) should be avoided

Table 4.2 Maximum runner lengths for specific diameters.

1/8 3 4 100 2 50

1/4 6 8 200 4 100

3/8 9 11 280 6 150

1/2 13 13 330 7 175

Runner Diameter Maximum Runner Length

Low Viscosity High Viscosity

(in) (mm) (in) (mm) (in) (mm)

Wall Thickness mm (in) Gate Diameter / Length mm (in)


. .

..

Figure 4.9 Gate designs.

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Tunnel gates. Tunnel or sub-surfacegates enable automatic degating afterejection. The tunnel can be locatedeither in the moving mold half or in thefixed half. A sub-gate is often locatedinto the side of an ejector pin on thenon-visible side of the part whenappearance is important. To degate,the tunnel requires a good taper andmust be free to bend.

Line/edge gates. Line gates areused for distribution of melt alongedges of parts. A land length of 0.5-1 mm (0.02-0.04 in) is required.Degating takes place in a post-mold-ing operation.

A flash gate is used for long flat thinwalled parts and provides even filling.Shrinkage will be more uniform whichis important especially for fiber rein-forced thermoplastics.

A fan gate uniformly spreads the flowfrom the sprue or runner to the partedge. This type is often used for thicksectioned moldings and enables slowinjection without freeze-off, which isfavored for low stress moldings.

Cylindrical parts requiring good con-centricity and strength can be moldedin single cavity molds using adiaphragm gate or internal ring.Uniform radial mold filling is achievedwithout weld lines. External rings areapplied for multi-cavity concentricmoldings where diaphragms can notbe used.

Ejection systems

The method of ejection has to beadapted to the shape of the moldingto prevent damage. In general, moldrelease is hindered by shrinkage of

the part on the mold cores. Largeejection areas uniformly distributedover the molding are advised to avoiddeformations.

Mold release is largely simplifiedwhen the part is sufficiently tapered inthe mold opening direction. Therequired draft depends on:

- height of the molding- rigidity of thermoplastic during

ejection- shrinkage- material flexibility - complexity of shape - ejection system- surface texture.

In general, rigid thermoplastics needless draft. Side walls with a rough sur-face generally require more draft thansmooth walls. It is recommended toapply a draft of approximately 1° perside. For small moldings a draft of0°30’ may be sufficient depending on

shape and wall thickness. Largermoldings require a draft of 2° or 3°.Sometimes a draft angle is notallowed, then the injection moldingconditions will have to be adapted toensure removal of the product.

Figure 4.10 shows the draft in mm forvarious draft angles as a function ofheight of the molding.

Several ejector systems can be used:

- ejector pin or sleeve- air valve- stripper plate.

When no special ejection problemsare expected, the standard ejector pinwill perform well. In case of cylindricalparts like bosses a sleeve ejector isused to provide uniform ejectionaround the core pin.

Figure 4.10 Draft (A) in mm for various draft angles (B) asa function of molding depth (C).

A central valve ejector is frequentlyused in combination with air ejectionon cup or bucket shaped partswhere vacuum might exist.

A high-gloss surface can have anadverse effect on mold releasebecause a vacuum may arise

between cavity wall and the molding. Release can be improved by break-ing the vacuum with an ejectionmechanism.

A stripper plate or ring is used whenejector pins or valves would not oper-ate effectively. The stripper plate is

often operated by means of a draw bar or chain.

Mold cooling

Mold cooling serves to dissipate theheat of the molding quickly and uni-formly. Fast cooling is necessary toobtain economical production anduniform cooling is required for productquality. Adequate mold temperaturecontrol is essential for consistentmolding. The lay-out of the coolingcircuit warrants close attention.

Optimal properties of engineeringplastics can be achieved only whenthe right mold temperature is set andmaintained during processing. Themold temperature has a substantialeffect on:

- mechanical properties- shrinkage behavior- warpage- surface quality- cycle time.

In particular semi-crystalline thermo-plastics need to cool down at optimalcrystallization rate. Parts with widelyvarying wall thicknesses are likely todeform because of local differences inthe degree of crystallization.

In general, the cooling system will beroughly drilled or milled. Rough innersurfaces enhance turbulent flow ofcoolant, thus providing better heatexchange. Cooling channels shouldbe placed close to the mold cavitysurface with equal center distances inbetween (see Figures 4.11 and 4.12).The mechanical strength of the moldsteel should be considered whendesigning the cooling system.

wall thickness of diameter of the center distance center distances

the product cooling channels with respect to between cooling

mm (in) mm (in) mold cavity channels

2 (0.08) 8 - 10 (0.31 - 0.40)

2 - 4 (0.08 - 0.16) 10 - 12 (0.40 - 0.47) 1.5 - 2 d 2 - 3 d

4 - 6 (0.16 - 0.24) 12 - 14 ( 0.47 - 0.55)


Figure 4.11 Basic principle of cooling channels.

Figure 4.12 Position of cooling channels.

“w” “d” “a” “b”

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Some thermoplastics, like Arnite and Xantar, may require mold temperatures of 100°C (210°F) orhigher for optimal processing and properties. Effective mold insulation is advised to minimize heat lossbetween the mold and the machinemounting platens. Insulation boardswith low thermal conductivity and relatively high compressive strengthare commercially available.

Care is required in the correct plac-ing of seals; they may be damagedby the sharp edges of the pocketwhen the mold insert is mounted

(Figure 4.13). Seals or O-rings shouldbe resistant to elevated temperaturesand oils.

Guidelines for optimal mold tempera-ture control:

- independent symmetrical coolingcircuits around the mold cavities

- cores need effective cooling (see Figure 4.14 and 4.15)

- short cooling channels to ensuretemperature differences betweenin- and outlet do not exceed 5°C(10°F). Parallel circuits are pre-

ferred over serial cooling asshown in Figure 4.16

- avoid dead spots and/or air bub-bles in cooling circuits

- heat exchange between mold andmachine should be minimized

- differences in flow resistance ofcooling channels, caused bydiameter changes, should beavoided.

Figure 4.13 Sealing and cooling channel lay-out.

Figure 4.14 Examples of core cooling.


Mold parts that are excessively heat-ed, like sprue bushings and areasnear the gates, must be cooled inten-sively. Rapid and even cooling isenhanced by the use of highly con-ductive metals, such as beryllium-copper. These metals are used to fulladvantage in places where it isimpossible to place sufficient coolingchannels.

Tool steel

For injection molds there are severalsteel types available. For long produc-tion runs a durable mold is required.The cost of tool steel is often not morethan 10% of total mold cost. Importantsteel properties are:

- ease of machining- dimensional stability after

heat treatment- wear resistance- surface finish- corrosion resistance.

Use of specific alloying elements likecarbon may increase single proper-ties, however, often at the cost ofother properties. Table 4.5 showssome popular grades of mold toolsteel.

Corrosion resistant hardened steelsshould be selected when conventionalflame retardants are used. In the caseof halogen free flame retardant DSMthermoplastics, standard steel typescan be selected.

Beryllium copper inserts may be usedfor improved cooling near hot spots.High heat conductivity is also requiredfor gate drops in hotrunner molds.

Standardization of mold parts is grow-ing, not only for ejector pins, leaderpins and bushings, but also for moldplates and even complete moldbases. These standard mold basesrequire only machining of the cores,cavities and cooling channels and fit-ting of an ejection system.Advantages are:

- cost savings (30-50%)- short delivery times- interchangeability- easy and rapid repair.

Figure 4.15 Examples of separate cool-ing of core top.

Figure 4.16 Cooling of the mold.

Surface finish

A high-gloss surface finish may beachieved with proper molding condi-tions and polished mold cavities.High-gloss polished cavities requirecareful handling and protection duringprocessing. Mold maintenance needsmore frequent attention.

Great care should be exercised whenremoving high-gloss parts from themold to avoid scratches.

Table 4.4 gives an indication of theprice index for the commonly usedsurface finish classes according ISO1302.

For low gloss, semi-matt or matt sur-face finishes, the tool cavity needstreatment to obtain fine to very fine tex-tured structures. A matt surface isobtained by vapor blasting techniques.Basic steel roughness should be N3 orbetter ra < 0.1 mm (0.004 in).

Textured part surfaces have a specialvisual and haptic appearance, e.g.soft touch. Compared to other surfacetreatments, textures are relativelycheap.

Their popularity is based on:

- appearance (wood grain orleather)

- functionality, e.g. anti-slip- masking of molding defects.

Main texturing techniques are:

- photochemical etching- EDM- engraving- brushing- laser engraving.

When high quality of textures areexpected use a low alloy tool steelwith a limited carbon content (< 0.45%). If nitriding is necessary, it should be preceded by texturing.

After long periods of use the moldsurface deteriorates due to wear. Useof glass fibers will increase abrasion.Frequent checks of the surface condi-tion are recommended.

Semi-crystalline thermoplastics areoften less scratch resistant when veryfine textures are used. Because oftheir good flow properties, the moldreproduction is better than that ofamorphous thermoplastics. Micro-scopic ridges at the part surface maybe easily damaged with a finger nail.

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Table 4.4 Price index for various surface finishing classes.

surf

ac

e f

inis

h r

eq

uir

em

en

ts

surf

ac

e f

inis

h r

eq

uir

em

en

ts

≤0.05 N0-N2 high gloss, no visiblescratches or flow lines 1000

0.1 N3 glossy, small, visible 500scratches acceptable

0.2 N4 “technical” finish 200

0.8 N5 no aesthetical requirements 100

Roughness Description Priceµm Index


Table 4.5 Steel types for injection molds.

Ch

rom

ium

Co

nte

nt

Abra

sion R

esi

stance

Corr

osi

on R

esi

stance

Po

lish

ing

Ab

ilit

y

We

lda

bil

ity

Ro

ck

we

ll C

Ha

rdn

ess

Ma

ch

ina

bil

ity

Designation Type Usage

P-20 Prehardened 30-36 1.4% F F VG G F High grade mold base plates, hot runner

manifolds, large cavities & cores, gibs

420 Prehardened 30-35 13.6% F G E F F Best grade base plates (no plating

required); large cores, cavities, & inserts

420 Stainless 50-52 13.6% G VG E VG G Best all-around cavity, core, and

insert steel; best polishability

440C Stainless 56-58 17.5% VG VG E VG G Small to medium size cavities, cores

inserts, and stripper rings

H-13 Air Hardening 50-52 5.3% G F VG E G Cavities, cores, inserts, ejector pins,

and sleeves (nitrided)

S-7 Air Hardening 54-56 3.25% E F G E G Cavities, cores, inserts, & stripper rings

D-2 Air Hardening 56-58 12.0% E F G F P Cavities, cores, & runner gate

inserts for abrasive plastics

P = Poor F = Fair G = Good VG = Very Good E = Excellent

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Figure 4.17 Examples of textured structures.


Hotrunners

Historically, hotrunners were mainlyused in molds for thermally stableplastics such as PP, PE and ABS.Today these systems are becomingmore important when processingamorphous and semi-crystalline engi-neering plastics like PC, PA and PBT.

Figure 4.18 shows a schematiccross-section of a hotrunner system.It is often cost effective to producelarge volumes with hotrunner molds,in spite of high investments. Thesesystems are used for a wide range ofapplications.

The electrical/electronic industry usessmall components, like connectorsand bobbins, that are molded in multi-cavity molds. On the other hand, largemulti-gated parts are used in the auto-motive industry, e.g. bumpers anddashboards. Yet both can benefit fromthe cost and technical advantages ofhot runners.

Cycle time reduction is possible whencooling of a cold runner would deter-mine the cycle time. Table 4.6 showstypical advantages and disadvan-tages of hotrunner systems.

In selecting a hotrunner system, sev-eral factors have to be taken intoaccount (see Table 4.7).

Taking all these factors into considera-tion, there is still a choice betweenmany types and variations of hotrun-ner manifolds and nozzles. Generalrecommendations can not be given.The best option depends on the ther-moplastic and the requirements of thespecific application.

Figure 4.18 Cross-section of a basic hotrunner section.

Table 4.7 Factors influencing selection of hotrunner systems.

Table 4.6 Advantages and disadvantages of hotrunner systems.

Advantages Disadvantages

production increase (cycle) higher investments

material saving critical molding conditions

quality improvement critical temperature control

less scrap start-up problems

automatic degating color change problems

energy savings abrasion (reinforced plastics)

flexible gating position

Economy Process

_ investments _ start-up_ number of parts _ total flow-path_ cycle time _ pressure-distribution_ material waste _ color-change_ energy _ melt homogeneity_ regrind _ residence time

Product Material

_ dimensions _ flow-behavior_ shot-weight _ melting temperature/range_ gate/sink marks _ process window_ reproducibility _ thermal stability_ required tolerances/warpage _ reinforcement_ fiber-orientation _ additives

The following guidelinesshould be respected:

- natural runner balancing- minimal pressure-losses- sufficient heating capacity for

manifold and each single nozzle- accurate, separate temperature

controls for manifold and nozzle- effective insulation between

manifold and mold

- optimal mold temperature control- no dead spots and flow restric-

tions in manifold and nozzles;- limited residence time of melt in

the hotrunner- adequate sealing of runners.

Figure 4.19 shows various basic typesof nozzle configurations with their typi-cal advantages and disadvantages.

With respect to externally and internal-ly heated manifolds the same conclu-sions are applicable as for nozzles.

A relatively cheap and robust alter-native for hotrunners is the hotrun-ner/ cold sprue. The hotrunner mani-fold is followed by a short cold spruethat eliminates the use of expensivenozzles.

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Figure 4.19 Advantages and disadvantages of basic nozzle configuration.

Advantage_ simple and cheap_ solid construction_ lower energy costs_ no leakage

Advantage_ homogeneous melt temperature_ small melt-volume_ applicable for broad range of materials_ low pressure-loss_ easy colour change

Advantage_ homogeneous melt temperature_ small melt-volume_ large gate diameter possible_ applicable for high viscosity and

thermally instable materials_ process control

Disadvantage_ temperature homogeneity_ mainly for polyolefines_ higher pressure losses_ difficult color changes_ high shear

Disadvantage_ high energy input_ possible leakage_ insulation from cold mold parts

necessary_ costs

Disadvantage_ high energy input_ insulation from cold mold necessary_ costs_ high maintenance_ high shear

Internally heatednozzle/manifold.

Externally heatednozzle/manifold.

Externally heated nozzlewith needle valve.


DSM thermoplastics product range

DSM Engineering Plastics has devel-oped a wide product portfolio in a

variety of resins, each with a uniqueprofile of properties that meet increas-

ingly demanding requirements in newapplications.

injection molding

injection molding and extrusion

injection molding





excellent resistance to chemicals; high notched impactstrength copolymer PP; high flow, available in mineralfilled or glass reinforced grades for increased strengthand stiffness

high impact strength, ductile down to -80°C (-112°F);temperature resistance, dimensional stability; self extin-guishing; transparent or opaque; available in unrein-forced clear grades and glass fiber reinforced gradesfor improved stiffness and dimensional stability

EMI shielding; ESD protection

impact resistance; very high rigidity (GF) 5-12 GPa(725,00-1,740,000 psi); good thermal stability; excel-lent flow; high arc tracking resistance; good resistanceto chemicals and aging; available either in unrein-forced or reinforced with glass fibers or mineral fillers

continuous use temperature 150-170°C (302-338°F);good property retention at high temperatures; high stiff-ness (GF) 5-12 GPa (725,000-1,7440,000 psi); creep,wear and fatigue resistance; compatible with most sol-dering processes; available in unreinforced, glass fiberor mineral reinforced grades

high temperature resistance; notched impact strengthdown to -40˚C (-40˚F); excellent flexural fatigue: goodresistance to chemicals and weathering; hardnessrange from Shore D 40 to 75

high gloss surface finish; good dielectric properties;reinforced: very high stiffness 7-19 GPa (1,000,000- 2,755,000 psi); high resistance to wear; low/con-stant friction; resistant to chemicals; unreinforced;impact strength; low creep; superior resistance tolong term wear

reinforced homopolymerand copolymer polypropylenes

polycarbonate (PC)

carbon black or carbon fiber reinforcedpolycarbonate

nylon 6 and 66

nylon 4/6

thermoplastic copolyesterelastomers

thermoplastic polyesterbased on polyethyleneterephthalate(PET) or polybutylene terephthalate (PBT)

Fiberfil®

Xantar®

Electrafil®

Akulon®

Stanyl®

Arnitel®

Arnite®

Amorphous Thermoplastics

Semi Crystalline Engineering Plastics

Type Properties Processing

Polyolefins

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Akulon® Nylon 6 and 6/6 in both unreinforced and reinforced nylons grades, including flame retardant products.

Stanyl® High temperature nylon which bridges the price-performance gap46 nylon between traditional nylons and high-performance materials.

Arnitel® High performance elastomers based on polyester.copolyester elastomers

Xantar® Unreinforced, reinforced, and flame retardant grades with outstanding polycarbonate impact resistance, dimensional stability, and high heat deflection

temperature.

Arnite® Unreinforced, reinforced, and flame retardant grades offeringthermoplastic polyester dimensional stability and low moisture absorption with good

chemical resistance.

Electrafil® Electrically conductive thermoplastic materials providing ESD and conductive thermoplastics EMI shielding.

Plaslube® Internally lubricated nylons to enhance wear and friction properties.lubricated thermoplastics

Nylatron® Internally lubricated nylons to enhance wear and friction properties.lubricated thermoplastics

Fiberfil® Reinforced and filled polypropylenes.reinforced & filled thermoplastics

DSM product portfolio

Table of Contents

1-800-333-4237www.dsmep.com

© 2000 DSM Engineering Plastics Printed in the USA DBD•020052 04/00 2500

Stanyl®, Akulon®, Arnitel®, Arnite®, Xantar®, Electrafil®, Plaslube®, Nylatron® and Fiberfil® are registered trademarks of DSM Engineering Plastics.

North American HeadquartersDSM Engineering Plastics

P.O. Box 3333

2267 West Mill Road

Evansville, IN 47732-3333

Tel. 812 435 7500

Fax 812 435 7702

www.dsmep.com

EuropeDSM Engineering Plastics

Poststraat 1, 6135 KR

Sittard

The Netherlands

Tel. 31 46 47 70077

Fax 31 46 47 73535

AsiaDSM Engineering Plastics

Asia Pacific

152 Beach Road

#10-01-04 Gateway East

Singapore 189721

Tel. 65 299 6080

Fax 65 294 3808

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