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ScienceDirect Procedia Manufacturing 10 (2017) 887 – 897

45th SME North American Manufacturing Research Conference, NAMRC 45, LA, USA

Experimental Study on Mechanical Properties of Single- and Dual-Material 3D Printed Products Heechang Kim, Eunju Park, Suhyun Kim, Bumsoo Park, Namhun Kim, and Seungchul Lee* Department of System Design and Control Ulsan National Institude of Science and Technology, Ulsan, Korea

Abstract The recent increase in application of Additive Manufacturing (AM) products has resulted in new demands throughout the industry. Although FDMbased products are used in various fields, the mechanical properties of such products still tend to be weaker than that of the products manufactured through conventional manufacturing processes. Therefore, improving the mechanical properties of FDM-printed products is a key factor that can greatly contribute to the manufacturing industry. In this study, tensile tests are conducted on a single material specimen to analyze the influence of various experiment variables that may add up to the enhancement of the mechanical properties of 3D printed products. Additional experiments are conducted with respect to the structural arrangement and material ratio of dual material 3D printing in order to investigate the effectiveness of dual material printed products. Studies on improving such mechanical properties are expected to contribute to the enhancement of the strength for single material printed products, and provide some guidance when manufacturing dual material printed products by considering the optimum efficiency of each material.

© 2017 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2017 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Scientific Committee of NAMRI/SME. Peer-review under responsibility of the organizing committee of the 45th SME North American Manufacturing Research Conference Keywords: dual material 3D printing, FDM, tensile strength, orientation angle, infill rate, material type

1. Introduction Additive manufacturing (AM), often referred to 3D printing among the public, is regarded as a promising technology which has a range of potential in the manufacturing industry [1]. The main principle of this technology is printing objects layer-by-layer using appropriate materials (liquid, polymer, metal, etc.) in accordance with 3D computer aided designs (CAD). AM technologies were initially developed mainly to serve the purpose of developing prototypes for functional testing. However, recent applications of AM technology are found in components from actual machines or medical appliances, and thus, the mechanical properties of fabricated parts have become an important factor within the manufacturing industry.

* Corresponding author. Tel.: +82-52-217-2726. E-mail address: [email protected]

2351-9789 © 2017 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 45th SME North American Manufacturing Research Conference doi:10.1016/j.promfg.2017.07.076

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Fused deposition modelling (FDM), first developed by Stratasys Inc., is one of the most commonly used techniques among AM technologies [2]. It is usually used to print plastic objects by extruding a thermoplastic filament through a nozzle. Acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA) are materials that are primarily used for FDM filaments. Due to the rapid growth of FDM technologies, innovative methods such as multiple-material printing techniques are developed [3]. Multiple-material printing techniques refer to the technology where the 3D printer has multiple nozzles that can control each linked filament. Although this may seem to be a simple improvement which enables product production in a variety of colors, such improvements can contribute to the enhancement of mechanical properties for the fabricated products in many ways. Among reported literatures, much research regarding the enhancement of the mechanical properties for FDM processes by adjusting the printing orientation angle or material composition has been actively conducted. Zhong et al. [4] studied the processability of glass fiber-reinforced ABS matrix composites with three different glass fiber contents, which were used as the feedstock filaments in the FDM process. The results showed that glass fiber could significantly improve the tensile strength and surface rigidity of the ABS filament. Ning et al. [5] presented the FDM of thermoplastic matrix CFRP composites and conducted the experiment to test the effects of carbon fiber (different content and length) on the mechanical properties of FDM-printed parts. Carbon fiber-reinforced FDM parts also exhibited enhanced mechanical properties for the specimens, and this was verified by observing the fracture interface using a SEM micrograph. Letcher et al. [6] showed that the mechanical properties such as the ultimate tensile strength can be advanced by altering the raster orientations in PLA on an entry-level 3D printer. Furthermore, multiple-material printing can be used to enhance specific parts of the printed-product with low mechanical properties due to geometric structure issues [7]. In this research, we aim to observe the possibility for improving the mechanical properties of FDM-printed products with multiple materials. The optimum parameters were analyzed through the investigation of their influences on the mechanical properties by using the analysis of variance (ANOVA) [8]. Considering previous researches, most experiments regarding the mechanical properties of 3D printed materials were conducted based on the orientation and infill rate. The specimens were printed with respect to not only the orientation angles and infill rate, but also the structural arrangement printed with two different materials. Even if an object is printed with similar proportions of two materials, we demonstrate that the structural arrangement has an influence on the tensile strength of the object. Therefore, through proper utilization of such structural arrangement, the maximized efficiency of manufactured parts such as partial tensile strength and manufacturing cost can be obtained. 2. Dual FDM 3D printer Two types of materials, PLA and ABS filaments were used to print the specimens in this paper. The diameter of the filaments was 1.75 mm. The FDM 3D printer used for the experiment was an entry-level 3D printer with two (dual) nozzles in Figure 1. The diameter of both nozzles was 0.4 mm and the temperature was configured as 210ć and 230ć for both the PLA and ABS filaments. The temperature of the heating bed was set as 80ć. The layer thickness was 0.2 mm. Each specimen was printed differently in terms of the infill rate, orientation, and material. For the multiple-material printing experiment, each material was printed in a sequence, rather than being printed simultaneously. Both specimens were designed in accordance with the ASTM D638 Type 1 Standard using CATIA (Dassault Systems, France) for the tensile test. In this paper, the tensile tests for the mechanical properties were performed using a universal testing machine (Instron 5982) in Figure 2 with a constant strain rate of 2 mm/min. Figure 2 shows that a strain gage was attached to the surface of the specimen to measure its strain. Tensile tests were performed five times for the single-material printed experiment and three times for the multiplematerial printed experiment in Figure 3. Then, tensile stress versus tensile strain curves were acquired from load and strain data through the tensile test. Among the various mechanical properties, the ultimate tensile strength ( V UTS ) was chosen as the criterion for the comparison

Fig. 1. Dual extruder

Fig. 2. Tensile test machine Instron 5982

Fig. 3. Tensile tested-specimen (ABS)

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3. Key Manufacturing Factors for Single Material 3D Printing 3.1. Single material It is commonly known that many parameters such as the nozzle speed, nozzle temperature, bed temperature, and infill rate affect the tensile strength of the specimen [9]. Among these parameters, the infill rate, orientation angle, and type of materials were set as the most important factors for the experiment in this paper. The infill rates for the specimens were 50% and 100%, where the two materials selected for the specimens were ABS and PLA. The orientation directions were initially set to the x-direction, y-direction, and 45°-direction. Figure 4 shows the initial orientation for each specimen. However, due to the characteristics of the FDM printing process, the product is printed as consecutive layers of diagonal deposition, as shown in Figure 5. Therefore, the specimens with the x and y-direction exhibit similar results in terms of the printed output. To minimize the parameters, only the x-direction and 45°direction were selected for the orientation angles in this research.

Fig. 4. Specimen orientation for printing on the bed

Fig. 5. Schematic drawing of extruded filament

3.2. Critical process parameters for single-material 3D printing Table 1 shows the parameters or control factors used for the experiment. There are three parameters, and each has two levels. As a result, it produces eight types of specimens in total. To detect statistically significant effects, we designed an L8 23 table of orthogonal arrays as shown in Table 2. The first column of the table indicates the order of experiment. The experiment is conducted at 2 levels 23 and repeated five times. Figure 6 shows the average ultimate tensile stresses of each extruded specimen with respect to a single material. A total of eight specimens with different parameters were used for the tensile test. The test results of the average stresses per strain were expressed as a strain versus stress graph in Figure 6. Table 1. Experimental design for the single material printing experiment Control factors Level

Orientation

Materials

Infill rate

1

x

ABS

100%

2

45°

PLA

50%

Table 2. Two-level orthogonal array (full factorial DOE) No.

Orientation

Materials

Infill rate

1

1

1

1

2

1

2

1

3

1

1

2

4

1

2

2

5

2

1

1

6

2

2

1

7

2

1

2

8

2

2

2

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Fig. 6. The result of strain-stress experiment for single material

The results of analysis of variance (ANOVA) are given in Table 3. ANOVA for tensile strength shows a statistical significant effect of orientation F(1, 59.09), p = 0.00, materials, F(1, 860.64), p = 0.00 and infill rate, F(1, 257.81), p = 0.00. ANOVA also shows a statistical significant interaction effect of each factor. Table 3. ANOVA results Control factors

Sum of square

Degree of freedom

F

Prob > F

Significance

Orientation

93.54

Material

1362.7

1

59.09

9.2048e-09

***

1

860.64

7.2177e-17

Infill rate

408.11

***

1

257.81

1.0554e-24

***

Orientation × Infill rate

70.63

1

44.62

1.5414e-07

***

Orientation × Materials

16.47

1

10.41

0.0027

***

Infill rate × Materials

33.04

1

20.87

6.9328e-05

***

Orientation × Infill rate × Material

9.74

1

6.15

0.0186

**

Error

50.66

32

Total

2044.89

39

We first tested the average difference between the groups from ANOVA, followed by a test on the average difference between the levels in the significant parameters.

Fig. 7. The tensile strength for (a) orientation; (b) infill rate; (c) material; (d) all specimens

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Figure 7 shows the results of multiple comparisons. The average difference of tensile strength depending on the orientation angle, infill rate, and material are shown in Figures 7 (a), (b), and (c). The specimen with the strongest tensile strength is exhibited in Figure 7 (d). Table 4 shows which groups of the factors are significantly different, and it can be noticed that all factors have a significant difference. The specimen with respect to the x-direction shows a stronger tensile strength compared to that of the 45°-direction. For the infill rate, the specimen with the 100% infill rate exhibits a stronger tensile strength than the specimen with the 50% infill rate. Considering the material property of the specimen, PLA demonstrated better properties in terms of the tensile strength than ABS. Table 4. Strength difference between levels Factor

Difference

Material

6.3883

95% Confidence interval 5.5779

7.1988

Orientation

3.0585

2.248

3.8689

Infill rate

11.6735

10.863

12.4839

Orientation Angle An optical microscope view was used in this paper to comprehend those results of why the specimen with x-direction has a stronger ultimate tensile strength than that of the 45º-direction in the first experiment. Figure 8 shows the optical surface image of specimen extruded as different orientation angles. In addition, a roughness of each surface is measured to obtain the difference. As shown in Figure 8, horizontal and vertical direction roughness were measured. Ra is the numerical value of the measured roughness. The horizontal and vertical Ra of the specimen with respect to the x-direction were 1.938 ȝm and 2.052 ȝm. The horizontal and vertical Ra of the specimen with respect to the 45º -direction were 0.147 ȝm and 3.014 ȝm. In addition, the gap between extruded lines could be obtained since the roughness graph had distinct periodic signal.

Fig. 8. Roughness measurement

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Fig. 9. Cross-sectional schematic drawing of melted thread

Vn

P cos T A'

P cos T A cos T

P cos 2 T A

Fig. 10. Statics schematic for tensile stress theory

V x cos 2 T

(1)

It is possible to observe the direction of extruded lines about x-direction is diagonal since the nozzle of printer moves diagonal on the bed. On the other hand, the orientation as 45º-direction has horizontal extruded lines. As shown in Figure 9, FDM printing method is stacking melted thread extruded filaments by applying a little pressure as nozzle between layer-by-layers [10]. Due to the fact that the contact area between each layer is the smallest in A-section, the relative adhesive force is likely to be weaker than the other sections. When a tensile force is applied to the specimen, the adhesive force is a crucial factor that determines the ability to withstand [11]. The stress diagram within a specimen when a uniaxial tensile stress is applied is shown in Figure 10. For a given inclination of ș, the stress on the inclined plane V n can be calculated as Equation (1). It can be noticed that there is a cosine term on the right side of the equation, meaning that the value of V n will always be relatively smaller than or equal to V x . Based on such findings, it can be said that the specimen with respect to the x-direction receives less tensile strength on A-section. The fact that the specimen with respect to the x-direction receives less tensile strength provides an explanation for the reason why the specimen with x-direction has a stronger ultimate tensile strength than that of the 45º-direction in the first experiment. Infill Rate Figure 11 shows the internal cross section of the extruded specimen depending on the infill rate. The extruded filament lines exhibit the difference in density for the two specimens. The 50%-infill rate printed-specimen has a sparsely extruded filament, while the 100%-infill has no gap between the lines. Moreover, while the melted thread of the 50%-infill rate printed-specimen remained as a form of a cylinder, the internal section of the 100%-infill rate printed-specimen shows that the layers are stacked well. This can sufficiently explain why the specimens extruded with a higher infill rate have the better mechanical properties.

Fig. 11. Optical images of internal section of printed specimen (a) infill rate 50%; (b) infill rate 100%

Heechang Kim et al. / Procedia Manufacturing 10 (2017) 887 – 897

Material Optical images

PLA

ABS

Fig. 12. Optical images of printed specimen with orientation

To see why the PLA specimen exhibits a stronger tensile strength than ABS, magnified images of the failure surface for each specimen after the tensile test are shown in Figure 12. While stretched extruded lines are found on the failure surface of the PLA specimen, the ABS specimen has a relatively neat failure surface. This implies that the melted PLA thread has a stronger bonding than the melted ABS thread. The optical images also provide additional evidence to the fact that PLA usually has a better ultimate tensile strength than ABS. 4. 3D Printing with Dual Materials 4.1. Dual materials The multiple-material printed specimen was printed as one body part extruded partially with ABS and PLA to verify the effect of a percentage of each material by using FDM 3D printer which can extrude dual filaments. Other parameters except for proportion of materials were fixed (orientation angle: x-direction, infill rate: 100%). All of specimens in multiple-material printed experiment were designed as symmetric structure which has 3 lines as shown in Figure 13. The middle line of the specimen was extruded different material with 2 edge lines. Percentages of specimen were shaped as 20%:80%, 40%:60% and 50%:50%. Table 5 lists the number of specimen designed by proportion of ABS and PLA.

Fig. 13. Specimen design used for multiple materials

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Heechang Kim et al. / Procedia Manufacturing 10 (2017) 887 – 897 Table 5. Experiment design for multiple-material printing experiment Percentage (%) No.

ABS

PLA

1

100

0

2

80

20

3

60

40

4

50

50

5

40

60

6

20

80

7

0

100

Fig. 14. The result of strain-stress experiment for multiple materials

Figure 14 shows the average ultimate tensile stresses of each extruded specimen with respect to different proportion of ABS and PLA. A total of seven specimens were used for the tensile test. The test results of the average stresses per strain were expressed as a strain versus stress graph. Table 6. ANOVA results Control factors

Sum of square

Degree of freedom

F

Prob > F

Significance

Ratio Error

209.736

6

32.2

2.04707e-07

***

15.196

14

Total

224.932

20

Fig. 15. Change in tensile strength according to the material proportion

To test the effect of material ratios on the tensile strength, the same method used in the single material test was conducted. Table 6 shows the results of ANOVA for the tensile strength. It can be inferred that the ratio has a statistical significant effect F(6, 32.2), p = 0.00. The test results of the tensile test for the multiple-material printing experiment are demonstrated in Figure 15. According to the first experiment (single material experiment), PLA has a larger ultimate tensile strength than ABS. Higher proportions of PLA lead to stronger tensile strengths of the specimen. Such trends can be confirmed from the bar graph in Figure 15. However, considering the

Heechang Kim et al. / Procedia Manufacturing 10 (2017) 887 – 897

variance of each component, the difference in tensile strength does not seem to have much of significance. Unlike the noticeable difference between the tensile strength values of pure ABS and PLA, the multiple-material printed specimen did not show much difference in consideration of the variance. We observed the fracture section of the multiple-material printed specimen through the optical microscope to analyze the results.

Fig. 16. Optical images of printed specimen with multiple materials

Fig. 17. Schematic drawing of printing process with multiple materials

Many cases of fracture are found especially at the interface of the two materials for multiple-material printed specimens. The voids (or porosity) and overlaps found between the two materials can be seen in Figure 16. Such factors may affect the adhesion of the specimen. These defects are either caused by the error of the printer or operator which results in the weakening of the tensile strength. Another reason why such void and overlapping phenomena are present may be due to the characteristics of the FDM printing method. A general FDM type printer initially prints the borders of the target as shown in Figure 17, followed by the filling up of the inner sides. Although this may not be much of an issue when printing single-material products, some problems may be expected from the FDM printing method when multiple-materials are involved in the printing process. If one material is printed between two printed parts of another material, the adjacent edges tend to show some structural errors such as void or overlap. Therefore, we conducted the next experiment to verify if the problem of the adhesion can be solved by re-designing the structural arrangement of multiple materials. 4.2. Structure effect of dual materials The specimens for the third experiment were prepared to verify that the change of tensile strength depends on the structural arrangement design. The material proportion between ABS and PLA was fixed at 50%. Likewise, other parameters were fixed except for the structural arrangement. There are 4 types of specimens with different structures, and those are shown in Figure 18. The 3 horizontal lines type specimen has the same design with the second experiment (dual material experiment), while the 4 horizontal lines type specimen has an additional line. In addition, two more specimens with two vertical layers are prepared, where each specimen has 6 lines and 8 lines.

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Fig. 18. Specimen design for various structures

Fig. 19. Change in tensile strength according to the structural arrangement

Table 7. ANOVA results Control factors

Sum of square

Degree of freedom

F

Prob > F

Significance

Structural arrangement Error

205.852

3

116.42

6.13277e-07

***

4.715

8

Total

210.568

11

Fig. 20. Optical images of various boundaries for multiple materials

Table 7 shows the result of ANOVA for tensile strength with respect to the structural arrangement. A statistical significant effect F(3, 116.42), p = 0.00 is displayed for the structural arrangement. The difference in tensile strength depending on the structural arrangement design of the specimen is shown in Figure 19. Interestingly, even when the material proportion of each specimen was kept to 50%-50%, the tensile strength exhibits a comparatively large difference depending on the design of the structural arrangement. Considering the fact that the 3 lines type and 4 lines type specimens tested for the experiment do not show much difference in ultimate tensile strength, we may infer that one additional line does not have much influence on the overall strength of the specimen. However, the specimen shows increased tensile strengths as the layer in the 3 lines type specimen is split into two vertical layers, creating a 6 lines type. The same holds for the 4 lines type specimen being split into an 8 lines type specimen. To analyze the reasons for the increase in tensile strength despite the same horizontal structural arrangement, we observed the structure of the specimen with an optical microscope. Figure 20 shows the optical images of the specimens printed with multiple materials. Figure 20 (a) and (b) show the adjacent edges between one layer and another, while Figure 20 (c) shows the vertical interface of each extruded part. Although all three images show the boundaries between two material edges, it is important to notice the fact that Figure 20 (a) and (b) show the edges between the horizontal layers, while Figure 20 (c) shows the edges between vertically extruded parts. The planes between the two materials in Figure 20 (c) do not seem to be in perfect contact with each other as expected. On the other hand, the planes between the two materials in Figure 20 (a) and (b) show better contact compared to those of Figure 20 (c). Regarding the previously mentioned characteristics of the FDM printing method, we can conclude that the adhesion between the horizontal layers are stronger than the vertical boundaries since slight pressure is applied from the nozzle to the melted filament while the filament is being extruded. Therefore, even if the same proportion is applied for a multiple-material FDM 3D printing, the tensile strength shows a great variation depending on the structural arrangement. This result provides us with design insights for multiple-material 3D printing process.

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5. Conclusion Acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA) specimens were created according to the ASTM standards D-638 to determine which factors affect the tensile strength in the first experiment (single material experiment). The orientation angle, infill rate and type of material were selected as the parameters, and through ANOVA, we could observe which parameters had effects on the mechanical properties of the specimens. The optical microscope was used to verify each of the effects on the extruded filament. Several results were drawn from the analysis. Materials in the x-direction with fill rates of 100% using PLA exhibited the best mechanical properties, and it was also possible to use these factors to print products with improved mechanical properties. The 3D printer used in this research was capable of printing products using multiple materials. Considering the fact that ABS and PLA are the mostly used filaments, we conducted experiments to find the difference in tensile strength regarding the proportion of two the materials. However, the results from the ANOVA showed the unstable extruding as FDM 3D printing method. We discovered the problem that voids and overlaps may occur in the boundary between two materials. This problem should be considered when printing a product using multiple materials since it can lead to waste materials or inefficient costs in the manufacturing industry. In order to solve the problems mentioned above, we considered the structural arrangement as a factor to change between the two materials. We modified the structural design by adding vertical lines and horizontal layers. Simply adding additional vertical lines to the product can still be ineffective since voids and overlapping may exist between the materials. Nevertheless, by adding an additional horizontal layer, improved results in terms of the mechanical properties were obtained. Due to the characteristics of the FDM method, instead of printing sections with different materials in a consecutive manner, it is easier for the nozzle to print the sections on an additional layer. For an additional layer with multiple material printing, the horizontal interfaces were expected to have better adhesive forces than the vertical interfaces. The results showed that the structural arrangement design for multiple materials in FDM can affect the mechanical properties. This means that the operator should consider the structural design when printing, for a fixed material ratio. Based on such considerations, the efficiency can be enhanced in terms of mechanical properties even with the same ratio of materials. Acknowledgements This work was partially supported by the SW fusion technology upgrading (R&D) for new industry creation (S0177-16-1002) and the regional industry manpower fostering project (2016H1D5A1910285) of Ministry of Science, ICT and Future Planning of Korea, and by technology innovation project (S2439592) of Small and Medium Business Administration

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