Theoretical and experimental research on the mechanical tests of 3D printed samples from various materials

At this moment, a real industrial, scientific and technological revolution is being discussed worldwide, consisting in the development of Additive Manufacturing AM technologies through the use of 3D printing. The FDM (Fused Deposition Modelling) 3D printing technology consists in the deposition of fusible material layer by layer in the XOY plane, the thickness of the 3D printed layer is g = 0.12 ÷ 0.24 mm, the resolution of 3D printing is r = 0.1mm, while the print head advances vertically on the OZ axis. The purpose of the work is to determine the breaking moment through destructive mechanical bending tests of some samples made by 3D printing from the following materials: ABS+, PETG and PLA. 5 samples were made in 3 printing directions: horizontal, vertical and lateral, resulting in 15 samples for each material. The total number of samples used is 45. Knowing these values, depending on the requirements of the specifications, a certain material and a certain printing position can be used.


1.Introduction
The need to obtain components with the highest possible material utilization efficiency, with the lowest possible material consumption and with the lowest possible number of machines represents the future of technology.
FDM (Fused Deposition Modeling) 3D printing technology is considered the most feasible and with the best price / quality ratio 3D printing technology [1,2].This was first developed for desktop 3D printers by the American company Maker Bot, which use PLA+, ABS+ or PETG filament.These filaments have a maximum yield strength of σC = 60 MPa, which is a big disadvantage when 3D printing high strength parts such as shafts, axles, gears, sprockets, couplings, etc. but which is highly valued for most other types of 3D printed landmarks [3].
The multitude of advantages offered by 3D printing manufacturing technology consists in the way of automation from the level of the CAD design system where there are libraries of designed products to the production system using robotic cells recommends the expansion of use in all industrial fields and non-industrial [4,5].
Originating in the early 1980s, this technology has moved from the family of non-conventional technologies to that of conventional technologies, at the moment these printers can be accessed at reasonable prices.

2.Used materials
The 3D printer Creality Ender 3 V2 was used for this printing.The characteristics of the materials used are presented in table 1.
Table 1 The three print direction of the samples are: horizontal, vertical and lateral.

3.Construction of experimental samples
The first material to be printed using the 3 directions is ABS +.The tensile samples are shown in figure1.The second material printed in the three positions is PETG and is presented in figure 2. The third material used for printing in the three positions is PLA, presented in figure 3.

4.Mechanicalbending tests
The stress was calculated as the maximum value of the bending stress, at the extremity of the section where the force is applied, with the below relation 1.
In the relation 1, F is the force read using the force cell, L is distance between supports (60mm), b is section width measured and h is section height, measured.The specific strain was calculated as the maximum value of the longitudinal specific strain, at the extremity of the section where the force is applied, with the relation 2.
In the relation 2, w is the read arrow, with the help of the position transducer of the machine`s crossbar.For samples that did not break during the test, the test was stopped at 12 mm deflection or approximately 8 % specific strain.

5.Experimental data
The results for the ABS+ material in the horizontal printing Ais presented in table 2. The results for the ABS+ material in the lateral printing C is presented in table 4. The results for material PETG material in the horizontal printing A, is presented in table 5.The results for PLA material in the horizontal printing A, is presented in table 8.The results for PLA material in the vertical printing B, is presented in table 9.The results for PLA material in the lateral printing C, is presented in table 10.

6.Interpretation of results
The tests were performed on an INSTRON 8875 universal electrohydraulic testing machine.After processing the results presented in tables 2, 3 and 4, the following curves were generated.For ABS+ material in the 3 printing positions figure 5.

7.Conclusions
Analyzing the graphs in Figures 4, 5 and 6, it can be seen that the PLA material from which the samples are made has a better bending strength than the PETG material and much better than the ABS+ material.
Average value of bending stress as also shown in table 2 to table 10 is 2323 (MPa) for PLA, 1694 (MPa) for PETG and 1657 (MPa) for ABS+.
From the point of view of the 3D printing direction for all 3 materials, the maximum value in the case of bending tests is obtained when the specimen is positioned in variant C, lateral direction.
From the point of view of the PLA material, having a melting temperature between 125-185 0 C, higher than the other materials used, the bonds formed between the deposition layers have greater strength.
Through the 3D printing mode in the C variant -lateral direction -it forms continuous field lines as in the case of hot lamination of metallic materials because between the deposition layers there is no total cooling of the previously deposited row and thus a more stable connection takes place.This phenomenon could be followed in detail through a study at the microcrystalline level, which will be the subject of future research.

Figure 1 .
Figure 1.Printing the tensile samples from ABS+ in horizontal-a, vertical-b and lateral-c direction

Figure 2 .
Figure 2. Printing the tensile samples from PETG in horizontal-a, vertical-b and lateral-c direction

Figure 3 .
Figure 3. Printing the tensile samples from PLA in horizontal-a, vertical-b and lateral-c direction

Figure 4 .
Figure 4. Samples subjected to bending on the universal machine INSTRON 8875

Figure 7 .
Figure 7. Average characteristic curves for bending tests, material PLA . The Characteristics of the materials

Table 2 .
Values for ABS+, horizontal printing AThe results for the ABS+ material in the vertical printing B, is presented in table 3. 4

Table 3 .
Values for ABS+, vertical printing B

Table 4 .
Values for ABS+, lateral printing C

Table 5 .
Values for PETG, horizontal printing A

Table 6 .
Values for PETG, vertical printing BThe results for material PETG in the lateral printing C, is presented in table 7. 5

Table 7 .
Values for PETG, lateral printing C

Table 8 .
Values for PLA, horizontal printing A

Table 9 .
Values for PLA, vertical printing B

Table 10 .
Values for PLA, lateral printing C