Abstract
The mechanical properties of polylactic acid (PLA), polyethylene terephthalate glycol (PETG), and PLA/PETG structures manufactured using the multi-material additive manufacturing (MMAM) method were studied in this work. Material extrusion additive manufacturing was used to print PLA/PETG samples with various PLA and PETG layer numbers. By varying the top and bottom layer numbers of two thermoplastics, the effect of layer number on the mechanical properties of 3D-printed structures was investigated. The chemical and thermal characteristics of PLA and PETG were investigated using Fourier transform infrared spectroscopy and differential scanning calorimetry. Tensile and flexural strength of 3D-printed PLA, PETG, and PLA/PETG samples were determined using tensile and three-point bending tests. The fracture surfaces of the samples were evaluated using optical microscopy. The results indicated that multi-material part containing 13 layers of PLA and 3 layers of PETG exhibited the highest ultimate tensile strength (65.4 MPa) and a good flexural strength (91.4 MPa). MMAM was discovered to be a viable way for producing PLA/PETG materials with great mechanical performance.
Keywords: 3D printing, material extrusion, tensile properties, flexural properties, PLA, PETG
Introduction
Material extrusion (MEX) is one of the most important 3D printing methods for the fast and high-quality fabrication of polymeric models and engineering components.1,2 Prototypes and final products can be easily fabricated using MEX for aerospace, automobile, maritime, and biomedical applications.2–4 MEX is widely used in additive manufacturing of thermoplastic polymers with low melting point, such as polylactic acid (PLA), polyethylene terephthalate (PET), acrylonitrile butadiene styrene (ABS), polycarbonate, and polyamide.1 It was also shown that reinforced polymers with conductive additives and waste thermoplastics could also be used in MEX for different application areas.5
In MEX process, polymer filament is fed into the heated nozzle for melting and the molten polymer is added over a preheated bed by the movement of the nozzle.1,6 After layer-by-layer printing, cooling down, and solidification, the final product is obtained.2,7 As a facile and fast technique, multi-material additive manufacturing (MMAM) allows the fabrication of high-performance materials by combining different materials together in a single part.1–4 In this technique, it is an important factor to obtain a good interlayer bonding between printed materials, which possess different physical and chemical characteristics.8
PLA, a biodegradable and a recyclable thermoplastic polymer and frequently used in MEX, is produced from corn starch, wheat, and sugarcane and consists of l- and D-lactic acid units.9,10 Polycondensation and ring opening polymerization processes are widely used to fabricate PLA from lactic acid.10,11 PLA is especially used in medical applications as a nontoxic, eco-friendly, and biocompatible polymer.9–12 However, PLA is a relatively brittle material with a low toughness and exhibits a lower elongation at break than 10%.11,12 Polyethylene terephthalate glycol (PETG), as an amorphous thermoplastic polyester with good chemical resistance, durability, and easy processability,13 is used generally to produce packaging foils, thermoformed parts, and injection molded parts.14 PETG possesses better chemical properties, flexibility, and shock resistance compared with PET.13–16
It was seen that some studies on the multi-material additive production of PLA with other thermoplastics have been reported in the literature. Singh et al.17 used the MEX method to fabricate multi-material components consisting of ABS, PLA, and high impact polystyrene (HIPS) and investigated the mechanical properties. They printed 4 layers for each polymer to fabricate multi-material with 12 layers in total. They found that multi-material specimens show higher tensile strength than HIPS, but lower than ABS and PLA.
However, higher peak elongation was reported for multi-material specimens compared with single printed specimens. In another work regarding MMAM of PLA, Lopez and Ahmad18 printed single and multi-material components from PLA, ABS, and HIPS by MEX to investigate mechanical properties. They reported that the sandwich structure with PLA outer skins and ABS core showed the highest tensile strength and Young's modulus. They also claimed that elongation at break increased in this combination compared with the single PLA specimen.
Lin et al.8 investigated the mechanical properties of polycaprolactone (PCL)/PLA structures fabricated by MEX. They stated that tensile strength of PCL/PLA increases 28% with the multi-material fabrication method. They also found that bond strength between PCL and PLA increased with the increasing bonding layer temperature. Baca and Ahmad19 also studied the relation between mechanical properties and interlayer interactions of ABS/PLA, PLA/HIPS, and ABS/HIPS combinations. They determined that the highest tensile strength, elongation at break, and Young's modulus were observed in ABS/PLA specimen.
There are several studies related to the fabrication of PLA or PETG by means of MEX technique.20–26 Hsueh et al.27 investigated separately the mechanical properties of 3D-printed PLA and PETG in terms of printing speed and temperature. They found that PLA showed higher tensile, compression, and bending performance than PETG. While the combination of PLA with other thermoplastics by the MEX method was investigated in terms of mechanical properties and interlayer interactions, there is no study presenting detailed investigation of multi-material PLA/PETG components and their mechanical behavior. It was seen that PLA and PETG could be fabricated as multi-material parts, and higher mechanical properties could be obtained than neat PLA and PETG if a good adhesion between depositing layers is obtained.
Therefore, in this study, the effect of PETG on the mechanical properties of PLA-based multi-material parts was shown. In addition, there is a lack of information in the literature about the effect of PLA and PETG layer number change on the mechanical properties of 3D-printed multi-material parts. With these aspects, this study could be an important contribution for the literature regarding 3D-printing of high-performance multi-material parts. MEX is a cost-effective technique and paves the way for the fabrication of structural prototypes and products for end use. PLA and PETG as recyclable polymers offer a possibility for the production of high strength materials and their different structural designs.
Combining the high tensile properties of PLA and the good flexibility of PETG materials in a single composite by MEX could be an efficient way to obtain high stiffness and advanced mechanical properties, which may be necessary for tissue engineering scaffolds8 or other engineering components. Thus, it is important to elaborate tensile and flexural properties of PLA/PETG multi-material components by revealing the relation between layer numbers of each filament and mechanical behavior.
As a result, the mechanical properties of single material PLA, PETG, and multi-material PLA/PETG specimens fabricated by MEX were investigated in terms of tensile strength and flexural strength in this study. By altering the number of layers of filaments in the upper and bottom regions of the samples, the effect of different layouts on mechanical properties was studied. As a result, the mechanical properties of two distinct polymers combined as a multi-material component using the MEX process were compared with single material specimens.
Materials and Methods
Materials
In this work, commercial PLA and PETG filaments were purchased from Porima company and were used to obtain tensile and three-point bending test specimens by MEX technique. The PLA and PETG filaments are 1.75 mm in diameter. The melting temperature of PLA and PETG is ∼180°C and ∼260°C, respectively. The main characteristics of PLA and PETG are given in Table 1.
Table 1.
| PLA | PETG | |
|---|---|---|
| Density (g/cm3) | 1.25 | 1.27 |
| Tensile strength (MPa) | 60 | 55 |
| Elongation at break (%) | 0.5–430 | 120 |
| Young's modulus (MPa) | 3600 | 2200 |
| Melting temperature (°C) | 180 | 260 |
| Glass Tg (°C) | 60 | 80 |
PETG, polyethylene terephthalate glycol; PLA, polylactic acid.
3D printing process
Within the scope of this study, MEX class Bluer 3D Printer (Two Trees) was used for PLA and PETG sample printing in additive manufacturing production. The nozzle diameter of the 3D printer was 0.2 mm. The process parameters for PLA and PETG printing are given in Table 2. When PETG layers were first printed, adhesion problems in interlayer section of multi-material parts occurred because of the shrinkage of PETG layer after printing. This shrinkage problem resulted in an insufficient interlayer bonding between PETG and PLA layers and low mechanical stability in multi-material structure.
Table 2.
Process Parameters for 3D Printing of Polylactic Acid and Polyethylene Terephthalate Glycol
| PLA | PETG | |
|---|---|---|
| Nozzle diameter (mm) | 0.2 | 0.2 |
| Filament diameter (mm) | 1.75 | 1.75 |
| Printing rate (mm/s) | 40 | 30 |
| Nozzle temperature (°C) | 220 | 260 |
| Bed temperature (°C) | 60 | 70 |
| Raster angle (°) | (−45/+45) | −45/+45) |
Therefore, to obtain tensile and three-point bending test samples, PLA layers were first printed as a bottom section of printed parts and PETG was printed over PLA layers to develop multi-material parts. A good interlayer bonding and structural stability were obtained by this arrangement. For comparison, 16 layers (layer height of 0.2 mm for each printed layer and total specimen thickness of 3.2 mm) were determined as the total layer number of printed samples. The layer number of each printed filament was shown next to its name. The multi-material parts with different layer numbers were named as 13PLA/3PETG, 10PLA/6PETG, 8PLA/8PETG, 6PLA/10PETG, and 3PLA/13PETG. The cross-sectional images of the 3D-printed samples are given in Figure 1.
FIG. 1.
The optical microscopy images of printed sample cross sections.
The tensile and three-point bending test specimens were prepared according to ASTM D638-4 and ASTM D790, respectively. The dimensions of tensile and three-point bending test specimens are given in Figure 2a and b, respectively.
FIG. 2.
Tensile test specimen dimensions (mm) (a) and three-point bending test specimen dimensions (mm) (b).
Fourier transform infrared spectroscopy
Thermo Scientific Nicolet I550 Fourier transform infrared spectroscopy (FTIR) spectrometer equipped with the Smart Orbit-Diamond model ATR was used to characterize chemical structure of PLA and PETG filaments before printing. The FTIR characterizations were conducted in the range of 4000–400 cm−1 with a spectral resolution of 4 cm−1.
Differential scanning calorimetry
Differential scanning calorimetry (DSC) was carried out to determine thermal properties of PLA and PETG filaments before printing using TA Instrument/DSC 25 under 10 mm min−1 N2 atmosphere. For each specimen, ∼7 mg material was weighed and aluminum pan was used. DSC tests for PLA and PETG specimens were performed under 3°C min−1 heating rate from 20°C to 250°C.
Mechanical tests
All the mechanical tests were performed with the universal tensile test machine with capacity of 10 kN (SHIMADZU—AGS-X). Tensile tests were carried out according to the ASTM D638-4 procedure with 5 kN load cell in 5 mm min−1 tensile rate. For each multi-material part, five specimens were manufactured and tested. Statistical errors were also given in the results of the tests.
Three-point bending tests were conducted by installing three-point bending test apparatus. The tests were carried out following the standard ASTM D790 procedure at a speed of 10 mm min−1 with a 5 kN load cell. The tests were carried out three times for each multi-material part and average values were calculated.
Optical microscopy analysis
Optical microscope was used to analyze qualitatively the fracture surfaces of the tensile specimens. The images of interlayer section were taken, to determine the filament adhesion and fracture type of the filament layers after tensile test.
Results and Discussion
FTIR analysis
The FTIR spectra for PLA and PETG filaments are given in Figure 3. The characteristic stretching peaks of PLA are the C-O stretching at 1080 cm−1, the C-O-C stretching at 1182 cm−1, the C-H bending at 1361 cm−1 and 1455 cm−1, the C-O-O stretching at 1119 cm−1, and the C = O stretching at 1745 cm−1. The CH3 asymmetric and CH3 symmetric bending frequencies were observed at 2993 and 2947 cm−1, respectively.28 In case of PETG, the characteristic bands of -CH were observed at 725 cm−1. The peaks at 1018 and 1100 cm−1 are related with C-H bond vibrations. The peaks at 1409 and 1245 cm−1 are associated with the -CH2 deformation band and C-O-O stretching of ester groups, respectively. The peak at 1717 cm−1 frequency shows C = O ester groups. Asymmetric and symmetrical aliphatic C-H stretching are identified at frequencies of 2850 and 2956 cm−1, respectively.15,29
FIG. 3.
FTIR spectra of PLA and PETG filaments. FTIR, Fourier transform infrared spectroscopy; PETG, polyethylene terephthalate glycol; PLA, polylactic acid.
DSC analysis
Thermal properties of the filaments were analyzed by means of DSC. To determine the thermal properties of PLA and PETG, heating was carried out from 20°C to 250°C at a heating rate of 3°C/min. DSC results of PLA and PETG are presented in Figure 4. Glass transition temperature (Tg) value of PLA was determined at 55.2°C, and cold crystallization temperature (Tc) was observed at 96.9°C. It was seen that PLA showed two different melting temperatures at 144.3°C and 152.1°C. This may be caused by pigments or other additives, as well as the presence of multiple crystalline states in PLA structure.30,31 The melting enthalpy value of the PLA sample was determined as 35.17 J/g. Unlike PLA, PETG did not show crystallization and melting peaks. The Tg value for PETG was determined as 69.3°C.
FIG. 4.
DSC curves of PLA and PETG filaments. DSC, differential scanning calorimetry.
Tensile test results
The tensile properties of PLA, PETG, and PLA/PETG layered structures were determined and compared with each other. The stress–strain curves and tensile strength of each specimen are given in Figure 5. The average values and statistical errors of tensile properties are given in Table 3. It can be clearly seen that neat PLA shows higher tensile strength (59.9 MPa) compared with the neat PETG (54.9 MPa), as can be expected.5 However, multi-material structures of PLA and PETG exhibit higher tensile strength and strain values than those of neat PLA and PETG. According to Figure 5a, as the PLA layer number increases in the structure, tensile strength of the samples also increases.
FIG. 5.
Stress–strain curves (a), tensile strength (b), tensile modulus (c), and elongation at break (%) (d) of specimens.
Table 3.
Tensile Properties of Polylactic Acid (PLA), Polyethylene Terephthalate Glycol (PETG), and PLA/PETG Multi-Material Samples
| Sample | Ultimate tensile strength (MPa) | Young's modulus (MPa) | Elongation at break (%) |
|---|---|---|---|
| PETG | 54.9 ± 2.6 | 1008.3 ± 47.6 | 7.4 ± 0.3 |
| PLA | 59.9 ± 2.9 | 1102.3 ± 17.8 | 8.1 ± 0.6 |
| 13PLA/3PETG | 65.4 ± 1.9 | 1223.8 ± 32.8 | 7.6 ± 0.2 |
| 10PLA/6PETG | 62.2 ± 3.3 | 1077.8 ± 31.5 | 8.6 ± 0.2 |
| 8PLA/8PETG | 60.3 ± 1.5 | 1031.1 ± 74.7 | 8.9 ± 0.2 |
| 6PLA/10PETG | 58.8 ± 0.7 | 986.5 ± 29.5 | 8.6 ± 0.2 |
| 3PLA/13PETG | 58.4 ± 1.5 | 1020.5 ± 52.7 | 8.5 ± 0.9 |
The average ultimate tensile strength of 13PLA/3PETG is 65.4 MPa, and this is the highest value among the multi-material samples. The average tensile modulus values are also in agreement with the tensile strength results of the samples. The highest tensile modulus was attained in 13PLA/3PETG sample (1223.8 MPa). In addition, tensile modulus decreases notably with increasing PETG layer number likewise tensile strength values. However, increasing PETG layer number results in relatively higher elongation at break values, as can be seen in Figure 5d. The highest elongation at break (8.9%) was obtained in 8PETG/8PLA specimen. The previous studies show that the layered 3D-printed structures can be an efficient way to increase tensile properties.
Li et al.32 investigated the mechanical properties of five layer-PLA and five layer-PLA/carbon fiber (CF) 3D-printed multi-material samples. They observed that the tensile strength and Young's modulus of PLA/PLA-CF sample are 55.7 and 1980 MPa, respectively. In another work, Baca and Ahmad19 investigated the mechanical properties of 3D-printed ABS/PLA and PLA/HIPS two-layered multi-material structures. They found that the average tensile strength of ABS/PLA and PLA/HIPS samples is 40.4 and 29.6 MPa, respectively. Furthermore, Young's modulus of ABS/PLA and PLA/HIPS is 1256.4 and 1027.0 MPa, respectively. It can be concluded that the PLA/PETG structures in this study exhibit relatively higher tensile strength and moderate tensile modulus values compared with the mentioned previous studies.
Three-point bending test results
The flexural properties of PLA, PETG, and PLA/PETG layered structures were determined by means of three-point bending test. The flexural stress–strain curves and flexural strength of each specimen are given in Figure 6. The average values and statistical errors of flexural properties are presented in Table 4. As could be observed in tensile behavior of the samples, three-point bending test results show a similar trend in terms of flexural properties. Flexural stress–strain curves indicate that PLA displays the highest average flexural strength (96.9 MPa) among the other samples. However, PETG sample has the lowest average flexural strength (69.8 MPa).
FIG. 6.
Flexural stress–strain curves (a), flexural strength (b), flexural modulus (c), and displacement (d) of 3-point bending specimens.
Table 4.
Flexural Properties of Polylactic Acid (PLA), Polyethylene Terephthalate Glycol (PETG), and PLA/PETG Multi-Material Samples
| Sample | Flexural strength (MPa) | Flexural modulus (MPa) | Displacement (mm) |
|---|---|---|---|
| PETG | 69.8 ± 2.1 | 1760.1 ± 53.1 | 7.4 ± 0.3 |
| PLA | 96.9 ± 0.5 | 3033.9 ± 21.5 | 6.2 ± 0.6 |
| 13PLA/3PETG | 91.4 ± 0.4 | 2393.2 ± 281.3 | 6.4 ± 0.2 |
| 10PLA/6PETG | 86.1 ± 1.4 | 1693.9 ± 133.1 | 7.6 ± 0.2 |
| 8PLA/8PETG | 82.1 ± 0.4 | 1575.6 ± 87.7 | 7.7 ± 0.2 |
| 6PLA/10PETG | 84.0 ± 3.5 | 1767.0 ± 151.2 | 7.6 ± 0.2 |
| 3PLA/13PETG | 79.0 ± 3.2 | 1554.6 ± 105.3 | 7.8 ± 0.9 |
Hsueh et al.27 have found that PLA exhibits higher bending strength and Young's modulus than PETG. The effect of increasing PLA layer number can also be seen in higher flexural strength (91.4 MPa) of 13PLA/3PETG compared with the other multi-material samples. This result also supports the strong interlayer bonding between PETG and PLA filaments.8,17 In other words, in this case, PETG layers make an important contribution to the flexural strength of multi-material structures. However, flexural characteristics of multi-materials samples also lower significantly with the increasing PETG layer number.
It can be noticed that 10PLA/6PETG, 8PLA/8PETG, 6PLA/10PETG, and 13PLA/3PETG samples show a similar tendency in terms of flexural strength values. However, flexural modulus of neat PLA sample was obtained as 3033.9 MPa as an average value. In the case of multi-material samples, 13PLA/3PETG exhibits the highest flexural modulus (2393.2 MPa).
During three-point bending test, it was observed that PETG sample does not break under applied bending force. Furthermore, neat PETG sample has a high flexural strain, as can be seen in Figure 6a. It is also noteworthy that increasing PETG layer number contributes to higher strain values in PLA/PETG multi-material structures. The highest strain value (7.8%) was observed in 3PLA/13PETG. On the contrary, PLA and 13PLA/3PETG samples exhibit 6.2% and 6.4% strain, respectively. It was assessed that this resulted from high flexibility of PETG polymer compared with PLA.33 These results indicate that PLA/PETG multi-material structures are remarkably affected by the layer numbers of both materials. Besides, both materials obviously contribute to a different characteristic of multi-material structure when a strong bond can be obtained in interlayer section.
Optical microscopy analysis
The filament adhesion and fracture surface investigation were carried out by optical microscopy analysis. Fracture surface images of the neat PETG and PLA samples after tensile test are given in Figure 7. According to Figure 7a, PETG fracture surface shows relatively sufficient interfilament interaction. In addition, there is not an obvious crack formation in PETG fracture surface because of tensile load. This situation resulted from the high flexibility of PETG.13–16 Conversely, PLA fracture surface exhibits micro-crack formations, which can be seen among rasters, as can be seen in Figure 7b. It can be concluded that fracture propagates through the weakest bonds of PLA rasters until the failure, as expected.34
FIG. 7.
Fracture surfaces of PETG (a) and PLA (b) after tensile test.
The fracture surface images of PLA/PETG structures with different layer numbers are given in Figure 8. The PETG part of multi-material samples reveals that rasters cannot be distinguished from each other in the fracture surface. This proves that an adequate interaction takes place among PETG rasters during 3D printing. However, PLA fractured surfaces show the distinguishable PLA rasters, as can be seen in Figure 8c–e. These weak bonds are prone to the formation of micro-cracks as a result of applied tensile load. It is also evident that micro-crack formations support the fracture propagation.35 However, it can be clearly seen that a good adhesion between PLA and PETG filaments were obtained in 13PLA/3PETG, 10PLA/6PETG, 8PLA/8PETG, and 6PLA/10PETG samples.
FIG. 8.
Fracture surfaces of 13PLA/3PETG (a), 10PLA/6PETG (b), 8PLA/8PETG (c), 6PLA/10PETG (d), and 3PLA/13PETG (e) after tensile test.
The effect of filament adhesion on the mechanical properties of the samples is also obvious from the fracture surface images. The fracture surfaces of the multi-material samples are in good agreement with the tensile test results. 13PLA/3PETG, which has a good adhesion in interlayer section, exhibits a higher tensile strength and Young's modulus compared with 3PLA/13PETG, which has a relatively weaker bond between PLA and PETG. Although increasing PETG layer number causes a decrease in tensile properties, its positive effect can be seen in strain values and elongation at break. It can be stated that PETG layers result in higher elongation values in comparison with the samples containing less PETG layers. This situation is evident particularly in 8PLA/8PETG, 6PLA/10PETG, and 3PLA/13PETG.
Conclusions
The effect of the layer numbers of PLA and PETG in multi-material samples on the mechanical properties was investigated by tensile test, three-point bending test, and optical microscopy analysis. According to the results of this study, it was seen that MEX is an appropriate method to obtain PLA/PETG multi-materials, which exhibit a convincing structural stability. Furthermore, it was also seen that increasing PLA layer numbers improve the tensile and bending properties of multi-material samples. It was observed that PLA/PETG multi-material parts have favorable mechanical behavior under tensile and bending forces. However, further work is necessary to assess the effect of raster angle of printed filaments on the mechanical properties. The results of this study can be summarized as follows:
FTIR analysis of the filaments reveals that both materials show the characteristic peaks of PLA and PETG thermoplastics.
According to DSC analysis, the Tg and Tc values of PLA were determined at 55.2°C and 96.9°C, respectively. It was also observed that PLA showed two different melting temperatures at 144.3°C and 152.1°C, possibly because of the multiple crystalline states in the structure. In addition, PETG did not show crystallization and melting peaks. The Tg value for PETG was observed at 69.3°C.
3D-printed multi-material samples show relatively higher mechanical properties compared with 3D-printed single material PLA and PETG. Higher PLA layer numbers improve the tensile properties of multi-material samples. The highest ultimate tensile strength (65.4 MPa) and Young's modulus (1223.8 MPa) were obtained in 13PLA/3PETG sample. Moreover, increasing PETG layer numbers up to eight layers cause a higher strain, as can be seen in 8PLA/8PETG (8.9%).
3D-printed PLA exhibits the highest average flexural strength (96.9 MPa) and flexural modulus (3033.9 MPa). Increasing PLA layer number causes an increase in flexural strength and flexural modulus, while increasing PETG layer number gives rise to an increase in flexural strain, as can be seen in 3PLA/13PETG (7.8%).
The optical microscopy images of the sample fracture surfaces reveal that PETG has a better interfilament bonding during 3D-printing process compared with PLA. It is also evident that a good adhesion in the interlayer section of PLA and PETG is highly determinative for the mechanical properties of PLA/PETG multi-material samples.
Author Disclosure Statement
No competing financial interests exist.
Funding Information
No funding was received for this article.
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