Effect of Printing Parameters on the Tensile Mechanical Properties of 3D-Printed Thermoplastic Polyurethane ^†

Abstract

Thermoplastic polyurethane (TPU) filament was used to fabricate specimens through material extrusion (MEX)-based 3D printing technique with varying printing parameters. Nozzle diameters of 0.4 mm and 0.8 mm were used, while the printing infill orientation (also denoted as raster angle) was either parallel (0°) to the length of the specimens, perpendicular to it (90°), or at a 45° angle with alternating direction in each layer (±45°). Tensile tests were conducted to determine tensile strength, Young’s modulus, and elongation at break of the samples. The highest tensile strength was achieved using a 0.8 mm nozzle diameter and 0° raster angle, reaching 32.5 MPa, with a corresponding Young’s modulus of 145.8 MPa. Meanwhile, the sample with the lowest modulus (100.4 MPa) and tensile strength (17.8 MPa) was the one 3D-printed with a 0.4 mm nozzle and 90° raster angle.

1. Introduction

Currently, there is a steadily increasing demand for faster production and more reliable products. Various techniques are being utilized to deliver products to customers within shorter timeframes. Lean management helps improving manufacturing, CAD software facilitates design, and rapid prototyping allows tangible evaluation of the product. Among rapid prototyping methods, additive manufacturing has evolved beyond merely a design-testing tool and nowadays plays a role in actual production. Its market value was estimated to be 13.89 billion USD in 2021 and is forecasted to grow to 44.03 billion USD by 2027 [1]. Additive manufacturing is widely used in aerospace [2], biomedicine [3], and even in construction [4]. There is also deep interest in using this technology for military purposes [5]. There are many types of additive manufacturing technologies, varying based on the methods and materials used. These include stereolithography (the first commercial AM process, patented in 1986), material extrusion (MEX)—commonly known as fused deposition modeling (FDM) or fused filament fabrication (FFF)—selective laser sintering, and binder jetting [6].

MEX is the most widespread technology due to its low cost and minimal tooling requirements [7]. The material for MEX comes in the form of thermoplastic filaments, which are strings with circular cross-sections and a constant diameter. The filament is fed into a printer head, which consists of two main parts: gears and a nozzle. The gears feed the filament into the nozzle, which then melts it and deposits the molten polymer onto the build platform in the form of small strands called beads. These beads are laid side by side to form a layer, with subsequent layers deposited in a similar fashion. Both the printer head and the gears are typically controlled by servomotors, which are governed by the internal code of the 3D printer’s mainframe. The printer head moves in the horizontal plane (X-Y), while either the printer head or the build platform moves along the Z axis, allowing the fabrication of three-dimensional objects. The foundation for 3D printing is a Computer-Aided Design (CAD) model [8], which is converted to a standard tessellation language (.STL) format containing triangular facet data. With the help of a slicer software, configured with the 3D printer’s periphery data, a G-code can be generated, which specifies the movement of the printer head and the build platform along with other printing parameters such as movement speed, feed rate, and temperature.

While the MEX 3D printing technology appears highly versatile, it is not without limitations [9]. For example, overhangs below the highest built layer cannot be printed without support. Furthermore, the accuracy of the printing process is heavily dependent on the nozzle diameter and the precision of the servomotors controlling the printer head and the build platform. Additionally, interlayer and inter-bead adhesion considerably influence the mechanical properties of 3D-printed objects. An inherent phenomenon in MEX 3D printing is the formation of voids between beads. Voids may also occur within the beads if the filament quality is poor, which further reduces mechanical properties.

Regarding the applied filament grade, incompatible fillers [10], moisture content [11], and inconsistent diameter [12] greatly influence the quality of the product. The most common filament materials are poly(lactic acid) (PLA) [11], acrylonitrile butadiene styrene (ABS) [13], and glycol-modified poly(ethylene terephthalate) (PET-G) [14]. Other filaments include polystyrene (PS) [15], polyethylene (PE) [16], and thermoplastic polyurethane (TPU) [17].

TPU has a wide range of applications due to its high flexibility and chemical resistance [18]. It is generally produced from petrochemical sources, comprising a mixture of diisocyanate, polyol, and chain extenders. Fortunately, biologically produced counterparts are already available, made from plant-based polyols. Depending on whether the polyol is polyester-, polyether-, or polycarbonate-based, the resulting TPU can exhibit different properties. Polyether-based TPU has good flexibility at room temperature but is prone to oxidative degradation. Its processing temperature is relatively high (210–240 °C) [19]. For proper MEX 3D printing, the temperature of the build plate should be set to 60 °C, and the printing speed must be slow (10–20 mm/s) to ensure proper spreading of the molten filament [20].

During 3D printing, the printing parameters such as nozzle diameter, temperature, and speed should be properly selected to ensure the highest quality of the fabricated product. Gonzalez et al. [21] conducted a study investigating the effects of infill percentage, layer height, and shell number. The authors used PLA and applied a grid-like infill pattern. The research concluded that infill percentage has the most significant effect on tensile and flexural strength. The minimum tensile strength (~9 MPa) was achieved with a 0.15 mm layer height, two shells, and 30% infill, while the maximum tensile strength (~34 MPa) was obtained with a 0.2 mm layer height, four shells, and 90% infill. Ameen et al. [22] tested TPU by 3D printing rectangular scaffold specimens to determine their compressive strength, varying three parameters: nozzle temperature, layer thickness, and printing speed. The study concluded that the maximum collapse stress (0.61 MPa) can be achieved with a 0.1 mm layer height, 230 °C nozzle temperature, and 20 mm/s printing speed.

Based on the literature, there is a growing interest in comprehensive investigations of the effect of 3D printing parameters on products created through the MEX technology. Manufacturers and members of the scientific community are deeply interested in the possibilities of well-optimized 3D printing settings. The current study aims to further explore the tensile mechanical properties of thermoplastic polyurethane by 3D printing specimens with three different infill orientations and two different nozzle diameters. The produced samples were characterized based on their tensile strength, Young’s modulus, and elongation at break. The results can be used to expand the knowledge on MEX 3D printing as well as to provide guidance for optimal printing.

2. Materials and Methods

2.1. Material

The filament used in this study was 3D Jake’s (Niceshops GmbH, Paldau, Austria) translucent A95-type thermoplastic polyurethane (TPU) filament with a diameter of 1.75 mm. The A95 designation refers to the Shore A hardness of the material. According to the supplier, it has a high degree of flexibility and good layer adhesion, which makes it ideal for applications where impact resistance is required. It is also described as easy to print. The recommended processing temperature is between 200 and 230 °C.

2.2. Processing

Immediately after unpacking, the filament was placed in a Sunlu FilaDryer S1 type filament dryer (Zhuhai, China), set to 50 °C in order to reduce moisture absorption, which, according to the supplier, would negatively affect the print quality. A Craftbot Plus MEX-based 3D printer (Budapest, Hungary) was used for specimen production. The parameters of the 3D printing process are collected in Table 1.

Table 1. Three-dimensional printing parameters.

Description	Unit	Value
Nozzle temperature	°C	230
Bed temperature	°C	60
Average speed	mm/s	50
Nozzle diameter	mm	0.4/0.8
Layer height	mm	0.2
Infill density	%	100
Shell layer number	-	1
Shell layer thickness	mm	0.8/1.6

In addition, the test pieces were 3D-printed using three different raster angles, as shown in Figure 1: 0° (infill parallel to the length of the specimen), ±45°, and 90° (infill perpendicular to the length of the specimen). The printed parts owned a geometry corresponding to the 1BA specimen according to ISO 527-2 standard [23]. The SuperSlicer software was applied to generate the toolpath.

Figure 1. The geometry and the applied infill raster angles of the specimens.

2.3. Characterization

For the tensile tests, an Instron 5582 universal testing machine (Norwood, MA, USA) was used. The initial tensile speed was set to 1 mm/min (to determine Young’s modulus), which was maintained until 0.3% elongation. After that point, it was increased to 50 mm/min. Four specimens were tested for each nozzle diameter/raster angle combination, and the mean and standard deviation were calculated from the results. To compare the samples, a one-factor analysis of variance (ANOVA) followed by a Tukey test was used to determine whether significant differences existed between the samples prepared with different nozzle diameters and infill raster angles.

3. Results

The results of the tensile tests are shown in Figure 2, while Figure 3 depicts the tensile diagrams recorded during the measurements. Figure 2a presents the Young’s modulus values, indicating that as the raster angle increases, the modulus gradually decreases. The sample 3D-printed with a 0.8 mm nozzle and a 0° raster angle yielded a modulus of 145.8 MPa, while those fabricated with ±45° and 90° raster angles exhibited a lower value of ~120 MPa. When using a 0.4 mm nozzle, the samples 3D-printed at 0° and ±45° orientations had a modulus of ~120 MPa, which dropped to 100.7 MPa at 90° raster angle. Accordingly, a nozzle diameter of 0.8 mm consistently outperformed the 0.4 mm. A similar trend, namely an increase in Young’s modulus with a larger nozzle diameter, was previously observed by Czyżewski et al. [24], who fabricated PLA-based samples using a raster angle of ±45°. The authors found that a nozzle diameter of 0.2 mm resulted in a modulus of 2.20 GPa, while increasing the diameter to 0.4 mm improved the modulus to 3.15 GPa. The observed reduction in Young’s modulus with increasing raster angle is also in good agreement with the existing literature. For instance, Khosravani et al. [25] reported a gradual decrease in the modulus of PLA samples 3D-printed with raster angles of 0°, 30°, 45°, 60°, and 90°, with values dropping from ~3.25 GPa to ~1.8 GPa. Sugavaneswaran et al. [26] concluded that the decrease in elastic modulus with increasing raster angle is due to the influence of printing orientation, which allows greater elongation of the material.

Figure 2. Young’s modulus (a), tensile strength (b), and elongation at break (c) values of the 3D-printed TPU specimens (same letters above the bars mean no significant difference at 5% significance level based on the ANOVA and Tukey comparison test).

Figure 3. Tensile curves registered for samples 3D-printed at 0.4 mm nozzle diameter and 0° raster angle (a), 0.8 mm nozzle diameter and 0° raster angle (b), 0.4 mm nozzle diameter and ±45° raster angle (c), 0.8 mm nozzle diameter and ±45° raster angle (d), 0.4 mm nozzle diameter and 90° raster angle (e), 0.8 mm nozzle diameter and 90° raster angle (f).

In terms of tensile strength, a clear trend was observed, namely that with a growing raster angle, the tensile strength decreased. This trend was also confirmed by Tukey’s test. The reason for this is that at 0° raster angle, the entire tensile load is carried by the continuous beads. By contrast, in samples 3D-printed with infill orientations of ±45° and 90°, a significant portion of the load is transferred across bead interfaces, which are inherently weaker due to imperfect adhesion and potential voids [27,28]. Meanwhile, the variation in nozzle diameter did not have a significant effect on the strength values. A similar observation was made in the study of Sudin et al. [29], where the authors tested PLA-based specimens and found no difference in the tensile strength values of samples prepared with 0.4 and 0.8 mm nozzle diameters, both exhibiting ~32 MPa. They attributed this to the layer height/nozzle diameter ratio, which affects anisotropy and void formation, ultimately resulting in similar mechanical behavior between the two configurations. However, their study also noted an increase in modulus with larger nozzle diameters (from 0.3 mm to 0.5 mm), which they attributed to wider extrusion widths producing more stiff material behavior.

As illustrated in Figure 2c, the infill orientation has an evident influence on elongation at break. When the printed beads’ orientation is parallel to the direction of the tensile load (raster angle = 0°), the load is primarily carried by the continuous, unbroken beads, allowing high elongation values. In contrast, when the beads are oriented perpendicular to the tensile load (raster angle = 90°), the load is transferred across the interfaces between the beads, relying on the interlayer adhesion, which is inherently weaker. Consequently, samples 3D-printed with an infill orientation of 90° exhibit significantly lower elongation at break, typically failing before reaching 150% strain, whereas samples 3D-printed with 0° raster angle demonstrated elongation values typically in the range of 250–300%. Tukey’s test also confirmed that printing orientation had a significant effect on elongation at break, whereas nozzle diameter did not exhibit a major influence. Meanwhile, changes in nozzle diameter had little to no effect on elongation. This is in good agreement with the tensile strength values.

Notably, Figure 3 reveals that, although the 0° (Figure 3a,b) and the ±45° (Figure 3c,d) raster angles yield comparable elongations, their tensile curves behave rather differently. Instead of a sudden failure, the 0° samples exhibited gradual fracture, especially at 0.8 mm nozzle diameter (Figure 3b). This behavior can be attributed to the sequential breakage of the individual beads, with each broken bead causing a drop in the tensile stress. Meanwhile, at a raster angle of ±45°, the load is transferred across bead interfaces that are much weaker and more brittle, and their rapid delamination leads to a sudden drop to failure. The samples 3D-printed with a raster angle of 90° (Figure 3e,f) show similar characteristics to those fabricated with ±45°, however, sustaining less deformation before failing. This behavior can be ascribed to the fact that at 90° infill orientation, the tensile stress is applied directly across the layer bonds, while at ±45° the bead paths are angled relative to the loading direction, creating a shear-dominated loading condition, which allows the structure to stretch and accommodate more strain. The work of Lang et al. [30] reported similar behavior and gave the same reasoning for the difference in failure mechanism.

4. Conclusions

In this study, the effects of different infill orientations (0°, ±45°, 90°) and nozzle diameters (0.4 mm, 0.8 mm) on the tensile mechanical properties of specimens fabricated through material extrusion using thermoplastic polyurethane filament were investigated. The results indicated that using a raster angle parallel to the tensile load (0°) allowed the greatest mechanical resistance, resulting in tensile strength of ~32 MPa. This value decreased at ±45° printing infill orientation to 26–27 MPa and reached its lowest at 90° raster angle (~17–18 MPa). Nozzle diameter appeared to have little to no effect on the tensile strength. However, a larger nozzle diameter (0.8 mm) was associated with a higher Young’s modulus compared to the 0.4 mm nozzle. The highest modulus (145.8 MPa) was observed for the sample 3D-printed with 0° raster angle and 0.8 mm nozzle diameter. Elongation at break, similar to the tensile strength, was not affected by the nozzle diameter but was greatly influenced by printing infill orientation. A raster angle of 0° resulted in an elongation of 250–300%, while samples 3D-printed at 90° exhibited elongation values of ~140%. In the future, it is worthwhile to investigate other parameters, aiming to reduce the number of possible variations by applying design of experiment (DoE) methods, such as Taguchi, genetic algorithms, and neural networks.