Tensile Testing of Polymer Material Specimens Obtained by Fused Deposition Modeling ^†

Abstract

In this work, a comparative analysis of polymer test specimens from different types of filaments, manufactured using FDM technology, was performed. A tensile strength test was executed on test specimens after 3D additive printing, made from different groups of materials—PLA, PLA Wood, PETG, PC, PA6, ASA, CPE HG100 and FilaFlex SEBS. Test specimens from the same materials were subjected to accelerated aging, after which they were tested again for tensile strength. The results of all tests were analyzed and compared.

1. Introduction

Three-dimensional printing, also known as additive manufacturing, is an increasingly used technology for rapid prototyping, the production of parts with complex geometries, and mass production of objects, applicable in all areas of industry, including medicine and the food and beverage industry [1,2,3]. It is based on three-dimensional printing of a digital model, which, by adding layer by layer of material, produces a three-dimensional object [4]. The first 3D printing technology used a photo-curing polymer, with control of the area exposed to ultraviolet rays [5,6]. A few years later, fused deposition modeling (FDM), which uses a thermoplastic continuous filament, became the most widespread technology. It became the most widely used technology due to the affordable prices of the devices and the wide variety of consumables, as well as the easier principle of operation [7,8].

Like any type of material, products produced through FDM printing age, especially under the influence of external atmospheric conditions such as exposure to the sun and elevated temperatures [9,10]. Polymeric materials are not resistant to temperature fluctuations due to their low melting point. One of the most important indicators for any material is its mechanical characteristics, which are known during its production. The data is established by applying a tensile strength test on test specimens, which is a destructive method [11,12].

Most thermoplastics exhibit non-Newtonian and viscoelastic behavior, meaning they exhibit elastic and plastic properties after the application of an external force. However, depending on the 3D printing method, extrusion temperature, orientation and percentage of filling of the object, these characteristics change. Accelerated aging processes are performed to verify the changes in the mechanical and physical characteristics of products exposed to sunlight and elevated temperatures [13,14]. This way, it is possible to know in advance how long their life would be and for what purposes a product made of each type of material can be used.

For tensile testing, standard testing machines are used to test the specimen [15,16,17]. An extensometer is mounted on the specimen to measure longitudinal deformation [18,19,20]. The results obtained are used to construct the deformation curve of the material and determine mechanical characteristics such as yield strength (σy), tensile strength (σm) and modulus of elasticity (E). Figure 1 shows typical deformation stress–strain curves of plastics [21]. Curve (1) is typical for brittle plastics, (2) for tough and strong plastics with strain hardening, (3) for hard and tough plastics with yield strain and (4) for elastomers.

The aim of this work is to perform tensile strength tests on test specimens manufactured from different materials using FDM technology. Some of the manufactured test specimens will be subjected to accelerated aging, after which tensile strength tests will be performed again and the results obtained will be compared.

2. Materials and Methods

2.1. Used Materials

For the needs of tensile strength testing under normal conditions, standard test specimens were made by FDM printing. The test specimen was designed using a CAD software program 2020, and the file was saved with the .stl extension for subsequent 3D printing. The dimensions of the test specimen are shown in Figure 2, according to the standard BDS EN ISO 527-2:2012 [22].

The Tevo Tornado (produced by company TEVO 3D Electronic Technology Co., Ltd., in Zhanjiang, China) and Ultimaker S5 (manufactured by Ultimaker, Zaltbommel, Netherlands) printers were used, depending on the 3D printing filament. The printing settings were set with the Cura program. Six test specimens were produced from each of the materials, namely PLA, PLA Wood, PETG, PC, PA6, ASA, CPE HG100 and FilaFlex SEBS, (manufactured by 3D Jake, Paldau, Austria) according to the settings described in [23]. For two of the materials—PC and PA6—heating was performed at a temperature of 90 °C for 2 h, recommended by the filament manufacturer. This was done in order to achieve the best mechanical performance of the printed object. One more test body was made for these two materials, which was not heated, in order to test and determine how heating affects the mechanical characteristics. The test bodies were marked as “unannealed”. All test bodies were produced with 30% filling with a Geroid grid and 0.1 mm layer height.

2.2. Methods and Equipment Used

The tensile tests were conducted with a modernized Zwick 1474 testing machine, manufactured by Zwick Roell, Germany and revised by the Department of Mechanics of Technical University, Sofia. There is with a servo-controlled loading mechanism, which ensures precise loading through digital control. The experimental setup for testing the tensile strength of the specimens is presented in Figure 3. A self-clamping extensometer for measuring longitudinal deformation with a measuring length of 25 mm is mounted on the test specimens.

Three of the test bodies were tested for tensile strength after printing (marked only with the name of the material), and the remaining three were subjected to accelerated aging (marked as “after aging”), according to the standard ASTM D3045 [24], which simulates temperature conditions under prolonged exposure to hot air alone, which are intended to be used to determine the durability and expected life of materials to prevent losses. An electric resistance furnace with a contact mercury thermometer was used for the applied accelerated aging. The heating period and temperatures were calculated based on the duration and temperature of sunshine in the city of Sofia for one calendar year, which covers the four seasons. According to an established methodology for accelerated aging in [25], the test bodies were heated at a temperature of 50 degrees Celsius for a period of 11 days.

3. Results

Tensile tests were conducted according to the standard BDS EN ISO 527-1 [7]. During the tests, the values of force, extensometer elongation and displacement of the movable gripper of the machine were continuously recorded. The obtained results were used to construct the deformation curves of the studied polymer materials. From the constructed deformation curves, the tensile strength σm and the modulus of elasticity E were determined (

Table 1. Tensile test, PC.

Polymer	Test Piece №	E, GPa	σm, MPa
PC unannealed	1	1.75	44
PC	1	1.83	46
	2	1.91	51
	3	1.87	48
	Average	1.87	48
PC after aging	1	1.97	45
	2	1.84	48
	3	1.95	48
	Average	1.92	47

,

Table 2. Tensile test, PA6.

Polymer	Test Piece №	E, GPa	σm, MPa
PA6 unannealed	1	1.71	34
PA6	1	1.93	37
	2	2.04	38
	3	1.80	36
	Average	1.92	37
PA6 after aging	1	1.87	35
	2	1.81	36
	3	1.89	36
	Average	1.86	36

,

Table 3. Tensile test, PETG.

Polymer	Test Piece №	E, GPa	σm, MPa
PETG	1	1.60	35
	2	1.64	34
	3	1.59	40
	Average	1.61	36
PETG after aging	1	1.62	35
	2	1.66	36
	3	1.61	34
	Average	1.63	35

,

Table 4. Tensile test, PLA Wood.

Polymer	Test Piece №	E, GPa	σm, MPa
PLA Wood	1	2.73	33
	2	2.67	33
	3	2.52	33
	Average	2.64	33
PLA Wood after aging	1	2.52	35
	2	2.48	33
	3	2.58	34
	Average	2.53	34

,

Table 5. Tensile test, ASA.

Polymer	Test Piece №	E, GPa	σm, MPa
ASA	1	1.14	22
	2	1.16	21
	3	1.12	23
	Average	1.14	22
ASA after aging	1	1.17	23
	2	1.16	23
	3	1.16	23
	Average	1.16	23

,

Table 6. Tensile test, FilaFlex SEBS.

Polymer	Test Piece №	E, GPa	σm, MPa
FilaFlex SEBS	1	0.11	3.73
	2	0.13	3.51
	3	0.15	4.02
	Average	0.13	3.75
FilaFlex SEBS after aging	1	0.12	3.65
	2	0.14	3.78
	3	0.16	3.81
	Average	0.14	3.75

,

Table 7. Tensile test, PLA.

Polymer	Test Piece №	E, GPa	σm, MPa
PLA	1	2.33	40
	2	2.27	36
	3	2.31	37
	Average	2.30	38
PLA after aging	1	2.22	38
	2	2.26	38
	3	2.25	39
	Average	2.24	38

and

Table 8. Tensile test, CPE HG100.

Polymer	Test Piece №	E, GPa	σm, MPa
CPE HG 100	1	1.19	33
	2	1.20	27
	3	1.15	28
	Average	1.18	29
CPE HG 100 after aging	1	1.17	33
	2	1.18	32
	3	1.17	26
	Average	1.17	30

). When testing FilaFlex SEBS specimens, the extensometer was removed when its dissolution limit was reached, due to its high elasticity.

After conducting tensile strength tests on PC and PA6 test specimens, it was found that the manufacturer’s recommended annealing resulted in an increase in mechanical properties. The tensile strength increased by 4 MPa for PC and by 3 MPa for PA6. Increases of 9.1% and 8.8% were recorded for PC and PA6, respectively.

The results of the tensile strength tests conducted on the test specimens after applied aging, equivalent to 1 year of exposure to elevated temperatures, show no deterioration in the mechanical characteristics—tensile strength and modulus of elasticity—compared to those to which no aging was applied.

4. Discussion

After processing all the results, diagrams were generated with the results of the tensile strength test of the tested materials, presented in Figure 4, and the results for the modulus of elasticity, presented in Figure 5, in ascending order.

The results show that the lowest values of mechanical characteristics are obtained with the FilaFlex SEBS material. This material is not suitable for use in products that require greater strength. With a relatively low value of 22 MPA, the strength of the ASA material is also high. In the range from 30 MPa to 40 MPa are the majority of the studied materials in the following sequence: CPE HG 100, PLA Wood, PETG, PA6 and PLA. The material with the highest tensile strength values compared to the studied ones is PC with σ_m = 48 Mpa.

Stress (MPa)–percentage extension (%) diagrams with one deformation curve each were constructed for materials PC after heating, PA6 after heating, PTEG, PLA Wood, ASA, PLA and CPE HG 100, presented in Figure 6.

The materials FilaFlex SEBS, ASA and CPE HG100 exhibit low elasticity and yield strength. FilaFlex SEBS has great plastic properties. PLA Wood, PETG and CPE HG100 have short extensions and the moment of failure occurs after reaching the maximum stress. The materials PLA and PLA Wood have a clearly pronounced yield strength and exhibit the qualities of a tough material. PA6 after heating and PC after heating are characterized as brittle materials, but PA6 has greater plastic properties. PLA and ASA are characterized by a steady flow regime with softening before breakage.

5. Conclusions

The tensile strength and the modulus of elasticity of test specimens made from various types of polymer materials obtained by fused deposition modeling were determined. The results obtained can be used in the strength–strain calculation of structural elements made from the studied materials.

All materials tested showed no deterioration in mechanical properties within the study period of 1 calendar year. They are suitable for exposure to UV radiation and elevated temperatures up to 50 degrees Celsius, which makes them thermally resistant and suitable for the manufacture of 3D parts for outdoor use, with the exception of PLA, PLA Wood and FilaFlex SEBS materials, which shrink and distort after exposure to elevated temperatures.

The results obtained will be used to establish a relationship between the shear strength and tensile strength of the polymer materials studied in this article.