Determining the mechanical characteristics of some tensile specimens, depending on the material and the printing position

3D printing is finding more and more applications in the industrial field and represents a modern additive manufacturing process based on a digital model. There are a number of advantages of additive manufacturing through 3D printing compared to classic manufacturing processes. Additive manufacturing through 3D printing allows material savings compared to classic manufacturing processes. Material consumption is punctual according to the project, without unnecessary losses and excess material. Through additive manufacturing, the design can be optimized in the sense that rapid changes can be made to the prototype in the CAD file. Also, through additive manufacturing through 3D printing, the principle of sustainability is promoted. We can say that additive manufacturing through 3D printing ensures rapid prototyping, which leads to the development of new products, shortens the design and manufacturing cycle, improves the quality and precision of models, eliminates costly mistakes, and optimizes the way of collaboration between engineers, marketing departments and sales and management team. In addition to these advantages, it must be seen if the 3D printed elements can replace the elements manufactured by classical methods in terms of mechanical resistance. In this sense, an investigation of some mechanical properties of some samples manufactured by 3D printing from various types of materials is required. The samples will be subjected to tension and bending. In this paper, only the tensile tests of some specimens, obtained by 3D printing in three positions: horizontal, vertical and in height, using three categories of printing filaments of the type: ABS+, PETG, PLA+, are presented. They have been made 5 samples for each material and printing direction, resulting in a total of 45 samples. The samples made from the three categories of material and in the three printing positions were subjected to tension until breaking. The formulas that were the basis of the tests are presented and graphs are drawn that represent the average characteristic curves of the samples for each category of material and printing guidelines.


1.
Introduction The 3D printing is finding more and more applications in the industrial field and represents a modern additive manufacturing process based on a digital model [1].There are a number of advantages of additive manufacturing through 3D printing compared to classic manufacturing processes.Cost reduction is a first advantage [2].Costs of thousands of euros can be reduced by eliminating some operations during the classic production process such as casting, processing by plastic deformation, processing by cutting and welding, allowing at the same time to make the required changes much faster and cheaper.
Another important advantage of additive manufacturing through 3D printing is related to design optimization in the sense that rapid changes can be made to the prototype in the CAD file [3].
Rendering 3D printed parts to a high degree of complexity is another important advantage [4].Each layer of material that the printer deposits on the printing surface is made sequentially, which allows the creation of complex internal structures.In the additive manufacturing process, the 3D printer can create partial interior voids, filled with a honeycomb structure, resulting in particularly light and at the same time rigid parts.If the parts to be manufactured exceed in size the surfaces intended for printing, they can be printed separately in pieces, later being glued to form a whole.
Reducing manufacturing time is another very important advantage of additive manufacturing through 3D printing [5].Depending on the complexity of the model and the printing time of the printer, the parts can be manufactured in a few hours or days.
Additive manufacturing through 3D printing allows material savings compared to classic manufacturing processes.Material consumption is punctual according to the project, without unnecessary losses and excess material.
Through additive manufacturing through 3D printing, the principle of sustainability is promoted.There is no need for a planned production.The products can be made to order according to needs and easy to recycle thanks to the versatile materials.In addition, some of the materials are ecological, compatible with the environment and resources are not consumed in excess.
In this work, the breaking tests of some specimens subjected to tension, obtained by 3D printing in three positions: horizontal, vertical and in height, using three categories of printing filaments of the type: ABS+, PETG, PLA+ [6], are presented.[7].They have been made 5 samples for each material and printing direction, resulting in a total of 45 samples.The samples made from the three categories of material and in the three printing positions were subjected to tension until breaking.The formulas that were the basis of the tests are presented and graphs are drawn that represent the average characteristic curves of the samples for each category of material and printing direction.

2.
3D printed samples for tensile tests Figure 1 shows the 3D modeling in the Autodesk Fusion program of the specimen for the tensile tests, which results in the file in STL format for slicing before its 3D printing.The samples were manufactured by 3D printing from three types of filaments: ABS+, PETG and PLA+.The physical and mechanical characteristics of the filaments used to print the samples are presented in table 1.The following notations were used in the table: TF-Type of fillament; TDCP-Print head nozzle temperature; TPI-Printer bed temperature; TS-Tensile Strentgh; EB-Elongation at break; FS-Flexural Strentgh; FM-Flexural Modulus; IS-Impact Strentgh.
The tensile samples were made from three categories of material (ABS+, PETG and PLA+) and in three printing directions (A-horizontal direction, B-vertical direction and C-lateral direction) resulting in 45 pieces, according to the experimental plan presented in table 2. The realization of the tensile specimens taking into account the three printing directions A, B and C is presented in figure 2.

Tensile tests
The tests were carried out with an Instron 8875 electrohydraulic testing machine.The force was read using the machine's force cell.
The deformation was recorded using the digital correlation method, on a surface in the calibrated area, painted with a white background and black spots.
The tension in the specimen was calculated using the relationship below where F is the force measured with the force cell and A is the cross-sectional area of the specimen, which was considered to be 40 mm 2 . (1) The yield stress is the apparent yield stress, calculated as the stress value at a permissible plastic deformation of 0.2%.
The breaking limit is considered as the highest value of the stress recorded.The results are not presented for some samples, for which the digital image correlation equipment did not record, as a result of a software error.

Experimental results
The results of tensile tests for the samples made of ABS+ material; PETG and PLA+, for the three printing directions are presented in the tables below.
The results of the ABS material tensile tests, printing position A, are presented in table 3.   The following notations were made in the tables: E-modulus of elasticity; Ɣ-Poisson's ratio; σ Cflow stress; σ R -breaking stress.
The results of the tensile specimens made of PETG material position A are presented in table 6.The results of the tensile specimens made of PETG material position C are presented in table 8.The results of the tensile specimens made of PLA+ material position B are presented in Table 10.The results of the tensile specimens made of PLA+ material position C are presented in table 11.

Interpretation of the results
The centralization of data from tables 3-11 is done graphically by drawing the average characteristic curves for the tensile samples, depending on the material and the three printing positions.
For the tensile specimens made of ABS+ material, the average curves in figure 3 result.

Conclusions
From the average characteristic curves in figures 3; 4 and 5, it can be seen that regardless of the type of material used to manufacture the tensile test specimens, the specimens printed in the lateral position C have the highest breaking resistance, the specimens printed in the horizontal position A have an intermediate breaking resistance and the minimum resistance is the samples printed in the vertical B position.
For the samples printed in the horizontal position A, it is found that the highest resistance was obtained by the samples from PLA+ materials, i.e. over 40 MPa, an intermediate resistance by the samples from PETG material, i.e. around 40 MPa, and the lowest resistance around 30 MPa the samples from the ABS+ material obtained it.
For the samples printed in position B, the highest resistances are obtained by the samples from the PETG and PLA materials, i.e. a little under 20 MPa, and the lowest resistance is obtained by the ABS+ material, i.e. a little over 10 MPa.
For the samples printed in the lateral C position, the highest but close resistances were obtained by the samples printed from the PETG and PLA materials, i.e. over 40 MPa, and the lowest resistance was obtained by the ABS material, i.e. a little over 30 MPa.
We can conclude that the samples printed in lateral C position have the highest tensile strength and the most resistant material is PLA+, followed by PETG and ABS+.

Figure 1 .
Figure 1.Modeling the tensile test specimen in the Autodesk Fusion program

Figure 2 .
Figure 2. Printing the tensile samples in the three printing directions

Figure 3 .
Figure 3. Average curves for tensile specimens printed from ABS+

Figure 4 .
Figure 4. Average curves for the tensile specimens printed from PETG

Table 1
The mechanical and physical characteristics of the filaments

Table 2
Experimental plan for tensile tests

Table 3
The results of the ABS material tensile tests, printing position B, are presented in table 4.

Table 4
ABS+ test results, position BThe results of samples from ABS material, printing position C, are presented in table 5.

Table 5
ABS+ test results, position C

Table 6
The results of the tensile specimens made of PETG material position B are presented in table 7.

Table 8
The results of the tensile specimens made of PLA+ material position A are presented in table 9.