1. Introduction
Currently, additive manufacturing technologies are identified as promising areas in the production of machine elements, parts, and components due to the reduction in production costs, waste, or availability constraints [
1]. The most common 3D printing technology is Fused Deposition Modeling (FDM). In this technology, thermoplastic materials are used in the manufacturing process. The thermoplastic materials utilized in 3D printing include ABS (acrylonitrile–butadiene–styrene), which is deemed a universal material and is employed in various industries such as toy manufacturing; ASA (acrylonitrile–styrene–acrylate), due to its UV resistance, is used in the automotive industry and the production of bumper covers or side mirror housings; HIPS (high-impact polystyrene), due to its high impact strength, is used in the production of helmets and protective elements; PLA (poly(lactic acid)), as a biodegradable material, is suitable for the production of disposable vessels; PETG (polyethylene terephthalate glycol) is a material from which plastic bottles are made; PC (polycarbonate), due to its high mechanical resistance, is frequently encountered in the automotive industry; TPU (thermoplastic polyurethane), as a flexible material, can be a material for the production of toys and elements that cannot be brittle. Many materials are not suitable for 3D printing, including liquid materials, pastes, silicones, and photopolymer resins; usually, a different technology is used for these materials [
2,
3,
4,
5]. Knowledge of the environmental and site-dependent impacts of 3D-printed products contributes to an increase in the possible service life. New materials and their modifications are being developed that increasingly meet user expectations in terms of specified properties [
6,
7,
8,
9]. As a result, the range of applications of plastics is also expanding, and they are increasingly used, for example, for parts such as gears or the bodies and covers of mechanisms. As a result, plastic parts operating in this capacity are exposed to environmental factors such as engine oil, mineral oil, and lubricating agents [
10]. There are also other applications, primarily in the automotive industry, where 3D printing is used to produce lightweight spare parts and prototypes [
11]; in the electronics industry, where cases and packaging are designed as people’s interest turns toward wearable electronics [
12]; in the furniture industry, where spare parts and various accessories are produced; as well as in the food industry, where packaging and storage containers are produced [
13]. Engine oil is one of the most common substances found in the operating environment of machinery and equipment. There are three types of oil available on the market: synthetic oil, semi-synthetic oil, and mineral oil. The difference between these types of oil is related to the level of viscosity [
14]. Oil is used for lubrication, that is, to reduce the frictional resistance of machine components moving against each other, and to cool them, which increases their lifespan. Depending on the needs, it can contain various additives, which can have a negative impact on the durability of the components through the possible migration, swelling, or change in their desired characteristics, such as mechanical and thermal properties [
15].
This paper describes the effect of mineral oil on the durability of the following four commonly used plastics for 3D printing: ABS, ASA, HIPS, which belongs to the group of styrene materials, and PLA, which belongs to the group of biopolymers and is biodegradable, which is an important feature nowadays [
16,
17]. These materials were also selected because they are among the most frequently used in FMA technology production [
18]. They are widely available and often used by small- and medium-sized enterprises.
Most typical structural components are usually subject to time-varying loads during operation. The phenomenon of material deterioration due to such loads is called fatigue. These loads are generally lower than the static strength of the material. A large number of cycles of such loading causes non-reversible damage before reaching its maximum strength [
19].
The most commonly described studies of the mechanical properties for 3D-printed parts focus on evaluating the relationship of the printing parameters to the mechanical properties of the printed samples [
13]. The influence of the nozzle diameter, sample placement on the table, layer thickness, fill type, or raster width is evaluated [
20]. There are also articles in which cyclic tests were used to evaluate the strength. Articles [
21,
22] focused on PLA material and its modified counterpart, where it was shown that, under the same loading conditions, the fatigue life of the 3D-printed samples was similar. Additionally, the presence of porosity in the material was demonstrated by conducting SEM (scanning electron microscope) analyses [
23,
24]. Another paper performed a low-cycle fatigue strength analysis for TPU (thermoplastic polyurethane). The material, in the form of pellets, along with paraffin, was extruded into filament form to be able to print test shapes using a 3D printer. The results showed a reduction in the fatigue life [
25]. The applications of the described materials varied from use in sports equipment, shoe insoles for runners, or designing scaffolds that can replace human bone. Among the studies mentioned, most were concerned with evaluating the low-cycle fatigue strength of specimens that were tested without the influence of environmental factors on the structure. So far, there is little or no information reported in the literature presenting the effect of environmental factors on the mechanical properties determined in low-cycle tests. This raises the question of how the number of cycles can be affected by factors in the industrial environment, e.g., by exposing the material to mineral/engine oil [
26]. The results of the research may result in a better representation of the various real mechanical behaviors of the materials during cyclic loading [
27]. Such studies can also give information about the behavior of the invariability of the performance properties of the parts made by 3D printing from polymers in specific operating environments [
28].
In the present study, 3D-fabricated samples of commonly used 3D printing materials such as ABS, ASA, HIPS, and PLA were placed in machine oil for up to 60 days at 23 °C and 70 °C. Mineral oil was selected because of its widespread use as a lubricant and cooling agent in many industrial applications. The purpose of this study is to determine how environmental conditions in the form of the exposure of fittings to mineral oil affect the mechanical properties in low-cycle fatigue cyclic tests of polymeric materials for 3D printing. Such an evaluation will allow for a more precise selection of materials for specific applications. This, in turn, could lead to more efficient, durable, and safety-free products and structures incorporating 3D-printed parts.
This study determined the differences in cyclic strength by measuring the number of load cycles performed to specimen failure and evaluated the changes in the specimen porosity to determine the effects of mineral oil and temperature on the mechanical properties of 3D-printed parts operating in contact with oil.
4. Discussion
For the ABS specimens, the position of the fatigue curves (
Figure 2) also indicates an increase in the fatigue life from the range of the lower
σm values at both tested temperatures (
Table 3). For the higher stress values, above 13–14 MPa, different results were obtained for both temperatures. At 23 °C, the fatigue life decreased in comparison with that of the reference samples, while, at 70 °C degrees, it was close to it. Below this value, the fatigue life increased for all groups stored in oil for both temperatures. For this material, the highest increase in durability was obtained for the samples stored in oil for 60 days.
The results of the cyclic tensile fatigue tests showed, for the ASA material, an increase in the fatigue strength as a result of the mineral oil (
Table 4). At room temperature, the number of cycles the material withstood after a storage period in oil of 15 days was the highest. At elevated temperatures, the best results were achieved for the samples that were exposed to mineral oil for 60 days. At this temperature, there was an apparent tendency for the fatigue life to increase as the
σm stress decreased. The position of the fatigue curves (
Figure 3) indicates that the increase in the durability is more significant than the lower value of
σm than the value close to the strength
σm of the reference samples. This suggests that increased temperature may increase the penetration of the oil into the structure of the material, and thus the plasticity. This, in turn, increases its cyclic stress strength. With elevated temperature, the effect of the increased strength was even more pronounced. For the third cycle of the test at 50% and 60% of the
σm, the values reached the limit of the test, which was one million cycles, and the samples did not fail.
For the PLA (
Figure 3) specimens at 23 degrees Celsius, no effect of the oil bath and its duration on the change in the fatigue strength was detected, except for the G15 group (
Table 5). This suggests that the effect of the oil initially increases, while, in the long term, it decreases the fatigue life. In addition, at 70 °C, changes were observed in the geometry of the specimens, and there was a slight deformation of the shape. This was probably due to the fact that 70 °C is a temperature close to the softening temperature of the material. This suggests that PLA should not be used in working environments where oil is present, especially at elevated temperatures.
For HIPS, the realization of the test at the 80% and 90%
σm value levels of the reference group was not possible (
Figure 5). The samples deteriorated already in the first loading cycle. The probable reason for such a phenomenon was a significant reduction in the
σm value of the samples stored in oil compared to the
σm value of the samples in the reference group (
Table 6). In all probability, the
σm value of 90 and 80% of the reference group’s
σm exceeded that of the samples stored in oil. Tests were only possible at lower load values, suggesting that the HIPS material may not be suitable for applications that require high load values.
The porosity was presented in
Figure 6, and
Table 8 and
Table 9. The temperature of the oil had a generally negative impact on the porosity. At the temperature of 70 °C, the porosity in almost all groups of the materials decreased or was unchanged in comparison to the oil bath at a temperature 23 °C. In the case of ABS G15 and G30, and HIPS G60, the porosity increased with the oil temperature. The oil bath caused the decreased porosity for all materials, except the ASA, where the porosity at G0 was lower than for the other groups. During the investigations, it was not possible to determine the specific impact of the porosity on the fatigue strength. The samples require further chemical analysis to determine the degradation changes. This will be the focus of future research.
5. Conclusions
Mineral oil can act as a kind of “lubrication”, reducing the friction between material particles and filament stitches, and increasing the plasticity of the sample and its cyclic stress toughness. This increase is particularly noticeable and stable for ABS throughout the σm stress range at which the tests were conducted. For ASA, it is also visible, but only in the range of Sa stress values below 12–14 MPa, which stands at about 75% of the σm value measured for the reference samples made of this material.
The results obtained for PLA and HIPS indicate that there is no clear relationship between the changes in the cyclic strength and the length of the oil exposure time and temperature. For this reason, based on the results obtained, it is not possible to assess their suitability for such conditions.
Elevated temperature caused a decrease in the porosity of the samples in most cases. Thus, the homogeneity of the material increased, and the number of geometric notches inside the samples decreased, which also had a beneficial effect on the increase in their cyclic strength.
Based on the results obtained, it can be assumed that the material most suitable for the manufacture of parts operating in the presence of oil and elevated temperatures is ABS. ASA is also suitable, but only in the load range below 75% of the initial σm value, as there is a tendency that, the lower the stress, the greater the increase in the cyclic life for ASA. It should be assumed that such high results for ABS were influenced by the reduction in the porosity (much higher than in the case of ASA) and the simultaneous lack of the chemical degradation of the material. The influence of chemical degradation was indicated by the decrease in the fatigue strength for the materials from the G30 and G60 groups compared to the results obtained for the G15 group. However, taking into account the presented research as a whole, it is difficult to determine the cause of this condition. For this purpose, it is necessary to examine and analyze the microstructure of the individual samples. The presented results show that porosity analysis alone is not sufficient in this case. Such research (microstructure analysis) is planned by the authors of this paper for the future.
The above results may have significant implications for industry, particularly in the context of material selection and operation in environments with cyclic stresses and elevated operating temperatures. It should be noted here that other material properties that are relevant to the characteristics of manufactured products, such as hardness, were not studied. In the opinion of the authors, the results obtained can be a starting point for further studies of other mechanical characteristics that are important from the point of view of the requirements for structural elements and machine parts.