1. INTRODUCTION
⌅Three-dimensional printer (3DP) technology is one of the important products that technology creates and its area of usage is increasing day by day. Because the first examples of products and prototypes can be manufactured by 3DP technology at low cost and it is advantage in production. Today, production with 3DP technology is called additive manufacturing (AM). AM is developing very fast with Fused Deposition Modeling (FDM) within the additive manufacturing technologies. Although AM technologies are expected to replace traditional manufacturing methods or have a supportive technology, this technology is open to development and continues to mature.
In AM technology, mainly plastics and metals are used. Polylactic acid (PLA) polymer is preferred as a filament in 3D printers due to its organic and easy process. In our study, one of the thermoplastic materials (PLA) was also used.
There are several studies in the literature about 3DP. Fernandez-Vicente et al. (2016)Fernandez-Vicente, M., Calle, W., Ferrandiz, S., Conejero, A. (2016). Effect of Infill Parameters on Tensile Mechanical Behavior in Desktop 3D Printing. 3D Printing and Additive Manufacturing, 3 (3), 183-192. https://doi.org/10.1089/3dp.2015.0036. investigated the effects of 3 infill geometries/density of internal infill and the tensile stress values were compared. It was reported that the change in the density of the internal infill caused a change in the tensile strength between 20% and 50%. The effect of printing position, printing speed and layer thickness on mechanical properties was examined in the study of Chacón et al. (2017)Chacón, J.M., Caminero, M.A., García-Plaza, E., Núñez, P.J. (2017). Additive manufacturing of PLA structures using fused deposition modelling: Effect of process parameters on mechanical properties and their optimal selection. Mater. Design 124, 143-157. https://doi.org/10.1016/j.matdes.2017.03.065.. As the layer thickness decreases, higher tensile stress and lower bending stress were obtained. It was also concluded that increasing the printing speed caused a decrease in tensile and bending stresses. Rajpurohit and Dave (2016)Rajpurohit, S.R., Dave, H.K. (2016). Parametric studies on quality of PLA parts produced using fused deposition modeling. 6th International & 27th All India Manufacturing Technology, Design and Research Conference (AIMTDR-2016), pp. 59-62. worked on the tensile strength values of different layer thicknesses and infill geometry angles using PLA material. It was concluded that the highest tensile stress was achieved by changing the angle of the infill geometry parallel to the tensile force, but in this case the discontinuity in the fibers makes the 3d printed part more brittle. Durgun and Ertan (2014)Durgun, I., Ertan, R. (2014). Experimental investigation of FDM process for improvement of mechanical properties and production cost. Rapid Prototyp. J. 20 (3), 228-235. https://doi.org/10.1108/RPJ-10-2012-0091. investigated the different positions of the samples in the printing area and the tensile/ 3-point bending tests and measurement of surface roughness were performed. It was found that the horizontally produced sample exhibited optimum mechanical properties with optimum production time and cost and bonding between the layers in the vertically printed samples is weaker. The layer thickness, printing speed and the print position of the sample were studied by Jaya Christiyan et al. (2018)Jaya Christiyan, K.G., Chandrasekhar, U., Venkateswarlu, K. (2018). Flexural Properties of PLA Components Under Various Test Condition Manufactured by 3D Printer. J. Inst. Eng. India Ser. C 99, 363-367. https://doi.org/10.1007/s40032-016-0344-8.. The 3-point bending tests were carried out and the highest bending strength was obtained in the printing process with a layer thickness of 0.2 mm.
In the study of the 3D printing of glass, kevlar and carbon fibers (Caminero et al., 2018Caminero, M.A., Chacón, J.M., García-Moreno, I., Rodríguez, G.P. (2018). Impact damage resistance of 3D printed continuous fibre reinforced thermoplastic composites using fused deposition modeling. Compos. Part B-Eng. 148, 93-103. https://doi.org/10.1016/j.compositesb.2018.04.054.), the printing area layouts and impact strengths of the composite structures were investigated. It was concluded that glass fiber composites showed the best impact strength and the carbon fiber reinforced composites are more brittle. Sezer et al. (2019)Sezer, H.K., Eren, O., Börklü, H.R., Özdemir, V. (2019). Additive manufacturing of carbon fiber reinforced plastic composites by fused deposition modelling: Effect of fiber content and process parameters on mechanical properties. Journal of the Faculty of Engineering and Architecture of Gazi University 34 (2), 663-674. https://doi.org/10.17341/gazimmfd.416523. produced composite materials using carbon fiber and ABS material and studied the effects of print head temperature on the mechanical properties of the sample during the 3D printing process. In the study by Rajpurohit and Dave (2018)Rajpurohit, S.R., Dave, H.K. (2018). Flexural strength of fused filament fabricated (FFF) PLA parts on an open-source 3D printer. Adv. Manuf. 6, 430-441. https://doi.org/10.1007/s40436-018-0237-6., the bending strength for different layer thickness and infill geometry angles were examined. They explained that the bending stress was inversely proportional to the layer thickness.
When these studies are evaluated, it is thought that the effect of additive manufacturing technology will increase progress further.
The concept of cross-section is an important subject to consider in the design of structural parts. Because the different section geometries could have different mechanical properties. On the other hand, the structural elements used in automobiles and machines have different strength values depending on their location and the load acting on them. The aim of this study is to examine the effect of section geometry, especially when plastic structural parts used in automobiles are subjected to different loads. In the current study, the 3D printer using FDM technology, with print dimensions of 200x200x210 mm, was designed and manufactured. Tensile and 3-point bending samples with using 5 different infill geometries were 3D printed and then tested by using PLA polymer filament. The most important difference is the new information about the comparison of tensile and bending strengths of hourglass, gyroid, octahedral, triangle filling geometries, which are rarely studied.
2. EQUIPMENT AND MATERIALS
⌅The designed and manufactured plastic parts of the 3D printer were produced using ‘Tronxy P802E 3D printer’ and PLA (polylactic acid) material was used. The print dimensions of the designed printer are 200x200x210 mm. The printing head moves along the X and Z axes, and the print area moves along the Y axis. One and two NEMA 17 stepper motors are used for X-Y and Z axis movements, respectively. All stepper motors are controlled by A4988 stepper motor driver and Ardunio mega 2560 and Ramps 1.4 combination are used as control card. A 12 volt 360 Watt power supply is used in the printer. The print head reaches a maximum temperature of 300 °C and the print area to a maximum of 150 °C. The manufactured 3D printer is shown in Fig. 1.
In this study, 3D printed samples were produced using five different infill geometries and all the infill geometries are shown in Fig. 2. The printing parameters of the samples are given in Table 1. All samples were produced one by one, using the same parameters, in the same position and printing area. The thickness of all samples is 4 mm and the material used in the production of all samples is PLA. Polylactic Acid (PLA) polymer filament of 1.75 mm in diameter.
| Internal infill rate | 20% |
| Layer height | 0.2 mm |
| Wall thickness | 1.2 mm |
| Lower and upper surface thickness | 0.6 mm |
| Printing speed | 50 mm·s-1 |
| First layer printing temperature | 200 °C |
| Printing area temperature | 40 °C |
3. EXPERIMENTAL RESULTS AND DISCUSSION
⌅3.1. Tensile properties
⌅The tensile test samples were prepared according to ISO 527-2 type 1B. Sample sizes are shown in Fig. 3. The tensile tests were carried out on the Shimadzu AG-X plus tester at cross head speed of 5 mm/min. The strength values obtained from the tensile test are given in Fig. 4. According to the results, no excessive difference was observed between the maximum and minimum tensile and yield strength values. There is a difference of approximately 17% between the maximum and minimum values in the tensile stress, while the increment in yield stress is determined as approximately 14%. It was also determined that the yield and tensile strengths were close to each other (Rodríguez-Panes et al., 2018Rodríguez-Panes, A., Claver, J., Camacho, A.M. (2018). The Influence of Manufacturing Parameters on the Mechanical Behaviour of PLA and ABS Pieces Manufactured by FDM: A Comparative Analysis. Materials 11 (8), 1333. https://doi.org/10.3390/ma11081333.). The maximum tensile strength was obtained with triangular infill geometry. This result is in accordance with the study by Chadha et al. (2019)Chadha, A., Ul Haq, M.I., Raina, A., Singh, R.R., Penumarti, N.B., Bishnoi, M.S. (2019). Effect of fused deposition modelling process parameters on mechanical properties of 3D printed part. World J. Eng. 16 (4), 550-559. https://doi.org/10.1108/WJE-09-2018-0329..
According to Fig. 5, there is an increase of approximately 30% between the minimum and maximum elastic modulus. This shows that the infill geometry has a significant effect on the elastic modulus. While the maximum value is obtained as 1852 MPa with the triangle infill geometry, the minimum value is obtained with gyroid infill geometry as 1420 MPa. The highest value followed by octahedral (OC), hourglass (HG) and grid (GR) infill geometries. It shows that bonding between the PLA polymer materials and the different infill patterns are very important to the Elastic Modulus in the 3D printing process (Fernandez-Vicente et al., 2016Fernandez-Vicente, M., Calle, W., Ferrandiz, S., Conejero, A. (2016). Effect of Infill Parameters on Tensile Mechanical Behavior in Desktop 3D Printing. 3D Printing and Additive Manufacturing, 3 (3), 183-192. https://doi.org/10.1089/3dp.2015.0036.; Khan et al., 2018Khan, S.F., Zakaria, H., Chong, Y.L., Saad, M.A.M., Basaruddin, K. (2018). Effect of infill on tensile and flexural strength of 3D printed PLA parts. IOP Conf. Ser.: Mater. Sci. Eng. 429, 012101. https://doi.org/10.1088/1757-899X/429/1/012101.).
In tensile test, the fracture region of the samples gives information about strength of the specimens. According to the literature, there are two types of failure, tensile and delamination. Since the tensile failure creates higher strength and therefore it is a desired type of failure but the delamination failure causes low strength (Schmitt et al., 2020Schmitt, M., Mehta, R.M., Kim, I.Y. (2020). Additive manufacturing infill optimization for automotive 3D-printed ABS components. Rapid Prototyp. J. 26 (1), 89-99. https://doi.org/10.1108/RPJ-01-2019-0007.). Figure 6 shows the photographs of the fracture regions of the infill geometries with the highest and lowest strength. Accordingly, the tensile and delamination fractures were observed in triangular and gyroid infill geometries, respectively. In the tensile failure type, the fracture is along infill rasters while the delamination failure is between rasters (Schmitt et al., 2020Schmitt, M., Mehta, R.M., Kim, I.Y. (2020). Additive manufacturing infill optimization for automotive 3D-printed ABS components. Rapid Prototyp. J. 26 (1), 89-99. https://doi.org/10.1108/RPJ-01-2019-0007.).
As it is known from the literature, while displacement is a measure of ductility, toughness is a measure of the energy absorbed by the material (Galeja et al., 2020Galeja, M., Hejna, A., Kosmela, P., Kulawik, A. (2020). Static and Dynamic Mechanical Properties of 3D Printed ABS as a Function of Raster Angle. Materials 13 (297). https://doi.org/10.3390/ma13020297.). For this reason, the variation of toughness and maximum displacement depending on the geometry was investigated in this study. The modulus of toughness was calculated from the area under stress-strain curve (Wan Muhamad et al., 2020Wan Muhamad, W.M., Reshid, M.N., Abd Wahid, K.A., Saniman, M.N.F. (2020). Mass Reduction of a Jet Engine Bracket using Topology Optimisation for Additive Manufacturing Application. Int. J. Adv. Sci. Technol. 29 (8s), 4438-4444. http://sersc.org/journals/index.php/IJAST/article/view/25492/13653.). Toughness vs. maximum displacement values are seen in Fig. 7. Accordingly, the ductility increased in infill geometries where the toughness also increased. The ductility of the samples decreased as the displacement was also decreased.
Toughness is a function depending on the tensile strength and elongation (ductility) of the material. If a material has a higher toughness value, it means that this material has higher strength and/or higher elongation (Ning et al., 2017Ning, F., Cong, W., Hu, Y., Wang, H. (2017). Additive manufacturing of carbon fiber-reinforced plastic composites using fused deposition modeling: Effects of process parameters on tensile properties. Journal of Composite Materials 51 (4), 451-462. https://doi.org/10.1177/0021998316646169.). The effective parameter in obtaining high toughness value was the ductility of the sample in this study. Because low strength was obtained at high elongation values (ductilities).
3.2. Bending properties
⌅The 3-point testing is important for automotive applications and used before manufacturing of automobile parts (Yadav et al., 2020Yadav, D.K., Srivasta, R., Dev, S. (2020). Design & fabrication of ABS part by FDM for automobile application. Mater. Today Proc. 26 (2), 2089-2093. https://doi.org/10.1016/j.matpr.2020.02.451 ). In this study, the 3-point bending test samples shown in Fig. 8 were prepared according to ASTM D790. The tests were carried out on the Shimadzu AG-X tester at a cross speed of 2 mm/min. and terminated when a break occurred.
Figure 9 (a-b) presents the strength results of the tests. According to this, the maximum flexural modulus and maximum stress values are obtained with the triangular infill geometry, while the lowest values are achieved with the grid infill geometry. The difference between the maximum and minimum stress value is approximately 12%. These results are in accordance with the study Chadha et al. (2019)Chadha, A., Ul Haq, M.I., Raina, A., Singh, R.R., Penumarti, N.B., Bishnoi, M.S. (2019). Effect of fused deposition modelling process parameters on mechanical properties of 3D printed part. World J. Eng. 16 (4), 550-559. https://doi.org/10.1108/WJE-09-2018-0329.. The highest bending strength followed by octahedral (OC), hourglass (HG) and gyrodi (GY) infill geometries.
The maximum flexural modulus is 1922 MPa, and the minimum is 1664 MPa, with a decrease of approximately 16%. On the other hand, the strength values obtained by the bending test are higher than the tensile tests. This increase is particularly prominent in HG, GR and GY infill geometries and the increase values vary between 13% and 24%. The bending test results showed that the infill geometry plays an important role for the bending strength of 3D printed PLA materials. Because of the interaction between layers, different infill geometries effect the bending strength as in the tensile strength (Wicaksono et al., 2018Wicaksono, S.T., Ardhyananta, H., Rasyida, A., Hidayat, M.I.P. (2018). Internal geometry effect of structured PLA materials manufactured by dropplet-based 3D printer on its mechanical properties. AIP Conf. Proc. 1945 (1), 020065. https://doi.org/10.1063/1.5030287.). In order to evaluate the ductility, the elongation values of the bending process obtained until fracture were examined. According to Fig. 10, while the similar displacement values are observed in TR, HG, GY and GR fill geometries, the OC infill geometry has the highest value with a significant increase.
4. CONCLUSIONS
⌅In this study, the effect of cross-sectional change on mechanical properties was investigated. For this purpose, the tensile and bending properties of 3D printed specimens having different infill geometries were investigated experimentally. The selected infill geometries were hourglass, gyroid, octahedral, triangle and grid. The original part of the study is the comparison of the most investigated grid section and the less studied hourglass, gyroid, octahedral and triangle sections in terms of mechanical properties.
The results showed that; the maximum strength in bending and tensile samples was obtained with triangular infill geometry. The triangle infill geometry increased elastic modulus by 30% in the tensile test and 16% in the bending test. It also increased the tensile stress by 17% and the maximum bending stress by 12%. As it is seen in the study, the infill geometry change had a more pronounced effect on tensile tests. In addition, in the tensile test, the lowest strength values were obtained with gyroid infill geometry, whereas in the bending experiment, it was obtained with grid. According to the toughness and ductility values obtained in the tensile test, the ductility of the samples was determined to be high while the toughness value was also high. The maximum ductility value obtained in the bending test was achieved with an approximate 99% increase by selecting octahedral infill geometry.