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Experimental Determination of Fused Deposition Modelling Parts

Compressive Strength

Nectarios Vidakis1, Petousis Markos1, Savvakis Konstantinos1, Achilles Vairis1, Maniadi Athina1, Arapis Manolis1

1 Mechanical Engineering Department

Technological Educational Institute of Crete Heraklion, Crete 71500, Greece

{vidakis@emttu.org, petousis@emttu.org, savakis@staff.teicrete.gr, vairis@staff.teicrete.gr,

athinamaniadi5100@yahoo.com, arapis.man@hotmail.com}

Abstract: Acrylonitrile–butadiene–styrene (ABS) is a popular engineering thermoplastic for its unique properties, like an excellent mechanical response, chemical resistance, fine finished surface appearance, and good processing characteristics. ABS is the most commonly used material with the Fused Deposition Modelling (FDM) technology, for the production of both prototype as well as functional parts. For this reason, it is critical to know the mechanical properties of these parts, which, as expected is different from the nominal mechanical properties of this material. In this work the compressive strength of parts build with the Fused Deposition Modelling process is measured experimentally. ABS and ABS plus parts were built with the Stratasys Dimension BST758 and Stratasys Dimension Elite additive manufacturing machines respectively. Parts were built with different building parameters and they were tested according to the ASTM D695 standard on a Schenk Trebel tensile testing machine. It was found that ABS parts build with larger layer thickness had lower compressive strength, while the ABS plus parts showed similar compressive strength for all cases. ABS specimens on average had about half the compressive strength of the ABS plus specimens, while the ABS plus specimens had a lower compressive strength than the stock ABS material. Keywords: Fused deposition modelling, 3d printing, compressive strength, Acrylonitrile-

butadiene-styrene (ABS) 1. INTRODUCTION

Acrylonitrile–butadiene–styrene (ABS) is widely used in a number of industries for its unique properties, like its excellent mechanical response, chemical resistance, fine surface finish, and good processing characteristics [1]. For this reason there are several studies [2-7] on the mechanical properties of this material and its composites under different conditions as well as on their improvement. ABS is the most commonly used material in the Fused Deposition Modelling (FDM) of Additive Manufacturing (AM). The FDM technology, developed by Stratasys, begins with the development of the three dimensional geometry using a Computer Aided Design (CAD) software tool, which is exported to an STL formatted file. In this file format the outer shape of the part is represented by triangles with the process of tessellation. This geometry is imported into the FDM machine software tool, were it is put in the orientation in space and mathematically sliced into horizontal layers with the thickness that the machine prints with. A support structure is created where needed depending on the position of the part and its geometry. After reviewing the path data by the user and generating the appropriate toolpaths, data are downloaded to the FDM machine for printing.

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FDM process is used for both prototyping as well as for production purposes [8]. For this reason, it is necessary for the design engineer to know the mechanical properties of these parts, not in their bulk unprocessed state, but after manufacturing them with AM. The nominal mechanical properties of ABS [9-10] do change, depending on the process parameters selected among other factors, let alone the fact the FDM process introduces anisotropy in the final parts. Mechanical properties of FDM parts such as the tensile strength [9, 10] have been experimentally studied in literature. Few studies in literature focus on the experimental determination of the compressive strength of FDM parts [8, 10-13] and even fewer employ prismatic specimens for the experiment [8, 12]. In this work the compressive strength of prismatic specimens build with the FDM process is presented. Due to the lack of nominal ABS filament compressive strength values by the material supplier Stratasys Ltd, results were compared with equivalent results from literature and stock ABS material properties. 2. METHODOLOGY

A compressive test specimen was created as a 3D CAD geometric model (fig. 1a), with dimensions as specified in the ASTM D695 standard (prismatic 12.7mm X 12.7mm X 50.8mm). The geometric model was exported as an STL file for the FDM machine software tool. In this study two different FDM machines were used, a Dimension Elite and a Dimension BST768. The Dimension Elite uses ABS plus material (Table 1) and soluble supports. Dimension Elite uses the ABS material only (Table 2) and breakaway supports. Both machines use the Catalyst software tool and in this study parts were processed with version 4.0. For this study all specimens were built as solids. Each slice of the parts is built in the same way throughout the run, with the outline of the slice being built first and then filled with parallel diagonal beads, at a 45 degrees angle (fig. 1b).

Table 1. ABS plus mechanical properties (courtesy of Stratasys Ltd.).

Test Method Metric Tensile strength, type 1, 2"/min, (51mm/min) ASTM D638 36 MPa Tensile modulus, type 1, 2"/min, (51mm/min) ASTM D638 2272 MPa Tensile elongation, type 1, 2"/min, (51mm/min) ASTM D638 4% Flexural strength ASTM D790 52 MPa Flexural modulus ASTM D790 2204 MPa Izod impact strength, Notched (73oF, 23°C) ASTM D256 96 J/m

Table 2. ABS mechanical properties (courtesy of Stratasys Ltd.).

Test Method Metric Tensile strength, type 1, 2"/min, (51mm/min) ASTM D638 22 MPa Tensile modulus, type 1, 2"/min, (51mm/min) ASTM D638 1627 MPa Tensile elongation, type 1, 2"/min, (51mm/min) ASTM D638 6% Flexural strength ASTM D790 41 Mpa Flexural modulus ASTM D790 1834 Mpa Izod impact strength, Notched (73oF, 23°C) ASTM D256 106,78 J/m

The specimen was built in one direction (horizontal) on the machine’s platform (fig. 1b). Specimens were built at every layer thickness each machine supports, in each build direction, i.e. 0.1778 and 0.2540 mm for the Dimension Elite and 0.254 and 0.3302 mm for the Dimension BST 768 machine, so in total there are six test cases for each machine. For each

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case, 5 specimens were built to the ASTM D695 standard, which requires at least 5 specimens for each setting.

(a) Three dimensional geometric model

(a) 3d printing build trajectory

Fig. 1. ASTM D95 compressive strength test specimen.

All specimens were measured to record their maximum width, height, length and thus determine the deviation in the specimens’ dimensions and the process accuracy. The maximum deviation in the specimens’ dimensions was less than 1%, which is an acceptable according to ASTM D695. The compressive tests were performed using a Schenk Trebel Co. tensile test machine (fig. 2a) according to the ASTM D695 standard. CLG-2B load cells with 2 ton capacity, 1 Kp sensitivity and 0.5% accuracy of the applied load were used to measure forces during the experiments, while sensors (Sokki Kenkyujo Co. Ltd. Tokyo) were online with digital indicators. A SDP-100c strain gauge (extensometer, capacity 100mm, sensitivity 0.01mm and nonlinearity 0.2% RO) was also used to record strain. The micro sensors (Kyowa Co. Ltd.) for strain measurement were connected through a bridge circuit. Data sensors data were logged with Labview.

(a) Schenk Trebel test

machine

(b) Testing a 3D printed

compressive specimen

Fig. 2. Compressive test arrangement

The test machine chuck was set at the standard speed of testing according to the ASTM D695 standard, i.e. 5mm/min and all specimens were tested at room temperature. 3. RESULTS

Figure 3 shows the stress (MPa) – strain (%) graphs for different specimens, as calculated from compressive tests. As it is shown, in most cases the maximum stress and strain values

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recorded were close for all equivalent specimens. The plots produced during each experiment have similar patterns in most cases.

Fig. 3: Compressive strength.

Average compressive experiments results are shown in Table 3 and results are presented and analysed in Figures 4 - 6.

Table 3. Compressive test results (average values).

Material ABS plus Layer Thickness (mm) 0.17 0.25 Maximum Compressive Stress (MPa)

41.85 40.12

Strain (%) 0.21 0.28 Compressive Young Modulus (Gpa)

1.83 1.82

Material ABS Layer Thickness (mm) 0.25 0.33 Maximum Compressive Stress (MPa)

22.12 20.03

Strain (%) 0.12 0.27 Compressive Young Modulus (Gpa)

0.96 0.78

Fig. 4. Νominal and experimental

Fig. 5. Νominal and experimental

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maximum compressive stress values for

different materials and layer thickness.

compressive Young modulus values for

different materials and layer thickness.

Fig. 6. Compressive specimens.

4. DISCUSSION

As it was expected, the ABS plus specimens break under higher loads than the ABS specimens. The graphs produced for both materials are similar for all specimens build with the same material and layer thickness, which was the only parameter consider in the study. The machines employed automate the 3d printing of parts, providing an easy-to-set-up process, but without an ability to adjust build parameters, such as the layers fill pattern, which does affect the strength of the build parts [10], while build orientation does not significantly affects the compressive strength of specimens, as it was found in literature, with the results reported for differently oriented specimens being close [8], [13]. In all experiments performed no significant difference was shown between specimens with the same building parameters. All specimens broke during testing in the middle of the specimen and several also fail in the vertical direction, with a vertical tear starting from their top end and ending in the breakage area around their middle (figure 6). Calculated strain values are similar in all cases for both materials, except for the case of the ABS material with 0.25mm build layer thickness, in which strain values are significantly lower, showing a more brittle material behavior. The difference in the compressive strength between specimens build with the same material and layer thickness reached 29% for the ABS material and 9% for the ABS plus material, while the equivalent deviation for the compressive Young modulus was 8% for the ABS material and 12% for the ABS plus material. Specimens build with larger layer thickness showed lower compressive strength for both materials, with the ABS plus specimens showing smaller differences in their compressive strength than the ABS specimens. ABS specimens on average developed about half (49%) the compressive strength and half (52%) the Young modulus the ABS plus specimens developed, but still ABS plus specimens developed lower compressive strength than stock ABS material (52.7 MPa) [14]. ABS specimens developed in average about 40% the compressive strength of the stock ABS material, while ABS plus specimens developed on average about 80% of the stock ABS material compressive strength. The calculated compressive Young Modulus for the ABS specimens was on average about 55% of that of the ABS stock material (1.59 GPa) [14], while the calculated compressive Young Modulus for the ABS plus material was on average about 15% higher than the compressive Young Modulus of the ABS stock material.

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5. CONCLUSIONS

In this work the compressive strength of parts build with the FDM process was measured. Parts were tested in compression and results were compared to the nominal ABS stock material compressive strength. The aim of this work was to determine the actual mechanical strength of 3d printed parts in compression and provide this valuable information to designers. As expected, it was found that the compressive strength of the 3d printed specimens is lower than the nominal stock material strength. In order to accurately design load bearing parts with the 3d printing process, these differences between the maximum compressive strength for the 3d printed specimens and the nominal stock material should be taken into account. 6. REFERENCES

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