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Vol. 8. Issue 5.
Pages 3741-3751 (September - October 2019)
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Vol. 8. Issue 5.
Pages 3741-3751 (September - October 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.06.034
Open Access
Mechanical characteristics of wood, ceramic, metal and carbon fiber-based PLA composites fabricated by FDM
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Zhaobing Liua,b,c, Qian Leid,
Corresponding author
leiqian_9036@163.com

Corresponding author.
, Shuaiqi Xinga
a School of Mechanical and Electronic Engineering, Wuhan University of Technology, Wuhan 430070, China
b Hubei Digital Manufacturing Key Laboratory, Wuhan University of Technology, Wuhan 430070, China
c Institute of Advanced Materials and Manufacturing Technology, Wuhan University of Technology, Wuhan 430070, China
d R&D Department, Shandong Kinshi Bitumen Co., Ltd, Rizhao 276806, China
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Tables (4)
Table 1. Key printing parameters used in this work.
Table 2. Formability of all materials under different printing conditions.
Table 3. Mechanical properties of tensile tests for different PLA composites.
Table 4. Mechanical properties of flexural tests for different PLA composites.
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Abstract

Fused deposition modeling (FDM) has gained much attention in recent years, as it revolutionizes the rapid manufacturing of customized polymer-based composite components. To facilitate the engineering applications of these FDM-printed components, understanding their basic mechanical behaviors is necessary. In this paper, the mechanical characteristics, including tensile and flexural properties of samples fabricated by FDM with different additives, i.e. wood, ceramic, copper, aluminum and carbon fiber, based polylactic acid (PLA) composites are comprehensively investigated. The effects of different PLA composites, build orientations and raster angles on mechanical responses are compared and analyzed in detail. It is found that ceramic, copper and aluminum-based PLA composite parts have similar or even increased mechanical properties compared with virgin PLA made parts. In most cases, PLA composite samples that are FDM-printed in on-edge orientation with +45°/−45° raster angles have the highest mechanical strength and modulus. It is worth noting that the results in this research provide a useful guideline for fabricating complex functional PLA composite components with optimized mechanical properties.

Keywords:
3D printing
Composites
FDM
Mechanical properties
PLA
Full Text
1Introduction

Three-dimensional (3D) printing, also known as additive manufacturing (AM) has been developed to enable the manufacturing of complex 3D components without expensive molds based on layer by layer fabrication using computer-aided design (CAD) models. Over the past decades, this technology has flourished in various industrial sectors such as aerospace, civil, biomedical, food and others. A series of critical reviews relating to the above fields can be found in Refs. [1–4].

In 3D printing, a technique commonly adopted for components to obtain some mechanical response is the so-called fused deposition modeling (FDM) [5]. In FDM, a filament of thermoplastic printing material is melted through an extrusion nozzle of printer and deposited line by line and further layer by layer to form a 3D geometry of the designed component. During the printing process, the mechanical characteristics of 3D printed materials by FDM are greatly affected by several variables, like nozzle and platform temperature, printing speed and orientation, layer deposition height, raster angle, infill pattern and so on [6,7].

To achieve desired performance of 3D printed components, different thermoplastic polymers have been employed in FDM, including PLA (polylactic acid), ABS (acrylonitrile butadiene styrene), nylon (a kind of polyamide), PETG (polyethylene terephthalate glycol-modified), PEI (polyethylenimine) and PEEK (polyether ether ketone) [8]. Among these materials, PLA, as a biodegradable green material has received considerable attentions in biopolymer research owing to its excellent biocompatibility and sustainability [9]. Since the beginning of its commercialization in 2002, researchers have been trying to explore its potential applications, such as tissue engineering scaffolds in medical field [10,11]. However, compared with traditional engineering materials, like metals and fiber reinforced composites, the mechanical properties of PLA are intrinsically lower [12–14]. In order to modify the capabilities of virgin PLA, researchers have endeavored to investigate PLA-polymer matrix reinforced composites by adding various additive materials, like natural or synthetic fibers, metals, ceramics into PLA [11,15–21]. However, the emerging of these new PLA composites brings new challenges to the FDM-printing of functional components with satisfactory mechanical properties. To the best of authors’ knowledge, research on the comparing and understanding of basic mechanical responses of various PLA composites is still limited.

The aim of this paper is to understand the mechanical behaviors of FDM-printed wood, ceramic, copper, aluminum and carbon fiber-based PLA composite parts. In addition to the type of PLA composites, the effects of two important printing variables in FDM, i.e. raster angles and building orientations on mechanical properties are comprehensively investigated and analyzed. Finally, the above analysis is further verified by the observations of optical images from fracture surfaces of tensile and flexural tests.

2Materials and methods2.1Materials

The filaments of virgin PLA (Ingeo 4043D, NatureWorks LLC) and PLA with different additive powders (wood, ceramic, copper, aluminum and carbon fiber) with the diameter of 1.75 mm were used as printing materials, which were purchased from Dongguan Zhehan Plastic & Metal Manufacture Co., Ltd. The blend ratio of PLA and each additive was chosen as approximately 3:2 in this research, which is believed to have the effectively modified mechanical properties compared to the virgin PLA.

2.2Equipment

The FDM printer (Model: Creator Pro, Flashforge Co., Ltd., China) was used in this research. The control accuracy of the printer is about ±0.1–0.2 mm. Mechanical properties were tested in a universal testing machine (Model: DNS-100, manufactured by Sinotest Equipment Co., Ltd., China) with a load of 100 kN. The samples were loaded up to material failure at a displacement rate of 50 mm/min and 10 mm/min for tensile tests and flexural tests, respectively. In flexural tests, the support separation was set to 80 mm. The data acquisition rate for both kinds of tests was set to 100 Hz for loads and displacements. After mechanical testing, the fracture morphology of all the samples were evaluated by an optical microscopy (Model: ICX41M, Sunny Optical Technology Co., Ltd., China).

2.3Test sample preparation

To evaluate the mechanical properties, dog-bone and rectangular shaped parts were fabricated by the FDM printer according to ASTM D638 and ASTM D790 standards. For each test, five same samples were prepared to make sure the obtained values of mechanical properties reliable. The dimensions of tested samples are presented in Fig. 1. In this study, three printing orientations (Namely, Flat, On-edge, and Upright) were considered to build the samples as illustrated in Fig. 1. For each printing orientation, two kinds of printing paths were designed with raster angles of 0°/90° and +45°/−45°, as depicted in Fig. 2, where, for instance, 0° means the axial direction along the sample length in the corresponding printing orientation, and 90° is the transverse direction. The printing path with +45°/−45° raster angles follows the similar routine as that of 0°/90° does. Key printing parameters adopted in this work are provided in Table 1.

Fig. 1.

Dimensions of tested samples and printing orientations (unit: mm).

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Fig. 2.

Layer deposition with different raster angles.

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Table 1.

Key printing parameters used in this work.

Materials  Virgin PLA, wood, ceramic, copper, aluminum and carbon fiber-based PLA 
Platform temperature  65° 
Nozzle temperature  200° 
Printing speed  80 mm/min 
Nozzle diameter  0.4 mm 
Infill pattern  Linear 
Object infill density  100% 
Layer height  0.3 mm 
Raster angle  0°/90°, +45°/−45° 
3Results and discussion3.1Formability

In this section, printing formability of all the PLA composite parts is evaluated in terms of printing raster angles as well as orientations. For example, successful FDM-printed samples of wood, ceramic, aluminum and copper-based PLA in flat orientation are presented in Fig. 3. In particular, Fig. 4 shows that FDM-printed samples of carbon fiber-based PLA in flat, on-edge and upright orientations. It is worth noting that printing in upright orientation was performed without any printing of support structure, as shown in Fig. 4(c). Moreover, in FDM-printing of these PLA composite parts, wood-based PLA sample is the most difficult one to be fabricated. Delamination defect, as illustrated in Fig. 5 usually occurs if the printing variables cannot be controlled properly. A summary of printing formability of virgin PLA and its composites under different printing conditions is given in Table 2. It is observed that ceramic, copper and aluminum-based PLA samples could not be completely printed in upright orientation.

Fig. 3.

3D printed samples of wood, ceramic, aluminum and copper-based PLA in flat orientation: (a) tensile tests, (b) flexural tests.

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Fig. 4.

3D printed samples of carbon fiber-based PLA in flat, on-edge and upright orientations: (a) tensile tests, (b) flexural tests, (c) an example of successfully printed sample in upright orientation.

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Fig. 5.

Delamination defect in the printing of wood-based PLA.

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Table 2.

Formability of all materials under different printing conditions.

Samples for mechanical testingPrinting orientationsLayer depositionFormability
PLA  PLA+wood  PLA+ceramic  PLA+copper  PLA+aluminum  PLA+carbon fiber 
Tensile testsFlat0°/90°  √  √  √  √  √  √ 
+45°/−45°  √  √  √  √  √  √ 
On-edge0°/90°  √  √  √  √  √  √ 
+45°/−45°  √  √  √  √  √  √ 
Upright0°/90°  √  √  ×  ×  ×  √ 
+45°/−45°  √  √  ×  ×  ×  √ 
Flexural testsFlat0°/90°  √  √  √  √  √  √ 
+45°/−45°  √  √  √  √  √  √ 
On-edge0°/90°  √  √  √  √  √  √ 
+45°/−45°  √  √  √  √  √  √ 
Upright0°/90°  √  √  ×  ×  ×  √ 
+45°/−45°  √  √  ×  ×  ×  √ 

Note: √ denotes successful printing, × means failure printing.

3.2Mechanical properties

Most previous works reported that mechanical properties of FDM-printed polymer components rely on the printing variables, in which printing raster angle and orientation play an important role on mechanical performance of these components. In order to assess the mechanical characteristics of PLA composites, the effects of printing raster angle and build orientation on mechanical properties were analyzed in this study through tensile and flexural tests. The stress–strain curves for tensile and flexural tests are presented in Figs. 6 and 7, respectively. As shown, for PLA and its composites, the stress–strain curves, in the initial stage, follow the Hooke’s law (strain proportional to the stress) until ultimate yield stress. After the ultimate yield point, negligible necking continued until the brittle fracture occurred without visible strain hardening. Moreover, from the results, it can be concluded that tensile and flexural properties strongly depend on the variation of raster angle as well as build orientation. For PLA and its composites, the best tensile and flexural properties could be obtained when external loading direction is parallel to the build orientations and raster angles of printed filaments are oriented longitudinally (e.g. the cases in flat and on-edge orientations). On the contrary, the worse tensile and flexural properties could be obtained when the tested specimen are loaded along the build orientation (e.g. the cases in upright orientations) due to weak interlayer bonding.

Fig. 6.

Tensile stress–strain curves of different PLA composites with two raster angle combinations under different printing orientations: (a) flat, (b) on-edge, (c) upright.

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Fig. 7.

Flexural stress–strain curves of different PLA composites with two raster angle combinations under different printing orientations: (a) flat, (b) on-edge, (c) upright.

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As for tensile and flexural properties between PLA and its composites with different filler materials, an obvious difference of mechanical behaviors would be witnessed. In order to quantify and facilitate the analysis of tensile and flexural properties, the tensile and flexural modulus, ultimate strength, and strain at break were calculated and summarized in Tables 3 and 4. Results have demonstrated that tensile and flexural properties are very sensitive to the filler materials. Overall, by adding ceramic, copper and aluminum powders into virgin PLA, the tensile and flexural modulus, strength as well as strain at break are still close or even increased compared to that of virgin PLA under different printing orientations and raster angles. The maximum tensile modulus of 1056.31 MPa and flexural modulus of 4621.37 MPa for ceramic-based PLA samples were obtained. While, the second largest tensile strength of 58.28 MPa and the maximum flexural strength of 118.67 MPa were obtained for copper-based PLA samples. As for wood and chopped carbon fiber-based PLA samples, the tensile and flexural modulus and strength are both decreased to some extent. It is evident that there is no obvious improvement on elongation or deflection at break for all PLA composites compared to that of virgin PLA.

Table 3.

Mechanical properties of tensile tests for different PLA composites.

Mechanical properties of tensile testsPrinting orientationsLayer depositionMaterials
PLA  PLA+wood  PLA+ceramic  PLA+copper  PLA+aluminum  PLA+carbon fiber 
Tensile modulus, MPaFlat0°/90°  707.1  656.8  1042.5  1002.6  838.4  596.4 
+45°/−45°  712.2  696.7  1056.3  1016.9  779.5  670.0 
On-edge0°/90°  901.0  808.1  962.4  883.2  649.7  745.7 
+45°/−45°  800.5  784.1  907.2  836.2  760.9  717.4 
Upright0°/90°  696.2  645.0  –  –  –  427.1 
+45°/−45°  738.7  673.0  –  –  –  584.3 
Tensile strength, MPaFlat0°/90°  42.4  29.5  43.2  40.3  40.2  32.8 
+45°/−45°  47.0  33.4  46.3  49.3  42.8  31.6 
On-edge0°/90°  63.4  35.9  46.5  53.7  46.8  38.2 
+45°/−45°  67.6  38.7  45.0  58.3  51.1  41.3 
Upright0°/90°  30.5  19.1  –  –  –  16.8 
+45°/−45°  27.8  19.1  –  –  –  20.0 
Elongation at break, %Flat0°/90°  6%  5%  5%  8%  5%  8% 
+45°/−45°  5%  5%  5%  5%  7%  6% 
On-edge0°/90°  7%  5%  6%  6%  7%  6% 
+45°/−45°  8%  6%  7%  7%  7%  6% 
Upright0°/90°  3%  4%  –  –  –  5% 
+45°/−45°  4%  3%  –  –  –  4% 
Table 4.

Mechanical properties of flexural tests for different PLA composites.

Mechanical properties of flexural testsPrinting orientationsLayer depositionMaterials
PLA  PLA+wood  PLA+ceramic  PLA+copper  PLA+aluminum  PLA+carbon fiber 
Flexural modulus, MPaFlat0°/90°  2075.2  1696.8  3022.3  2174.0  2492.1  1738.4 
+45°/−45°  2446.4  1749.8  3128.3  2227.0  2470.9  1729.7 
On-edge0°/90°  2286.8  2704.3  4503.0  3118.9  3207.5  2844.3 
+45°/−45°  2538.4  2651.3  4621.4  3845.1  3275.8  2939.2 
Upright0°/90°  2605.9  1992.8  –  –  –  1325.7 
+45°/−45°  2478.3  1653.0  –  –  –  1272.6 
Flexural strength, MPaFlat0°/90°  69.7  40.4  57.8  55.5  62.8  50.3 
+45°/−45°  79.5  52.1  70.1  66.3  64.7  47.6 
On-edge0°/90°  104.5  70.4  97.2  107.6  91.5  74.2 
+45°/−45°  109.5  71.0  100.1  118.7  97.8  75.6 
Upright0°/90°  51.9  37.9  –  –  –  30.9 
+45°/−45°  42.4  27.1  –  –  –  25.9 
Deflection at break, %Flat0°/90°  3%  2%  2%  3%  4%  4% 
+45°/−45°  4%  4%  3%  3%  5%  5% 
On-edge0°/90°  4%  3%  3%  3%  6%  5% 
+45°/−45°  3%  4%  3%  3%  7%  5% 
Upright0°/90°  2%  3%  –  –  –  3% 
+45°/−45°  2%  2%  –  –  –  3% 

In addition to the effects of filler materials, mechanical properties are also greatly affected by printing orientations and raster angles. For example, in flat orientation, tensile modulus and tensile strength in layer deposition (+45°/−45°) are higher than those in layer deposition (0°/90°). It is noted that, in most cases, layer deposition (+45°/−45°) can be adopted in flat and on-edge orientations to increase the mechanical properties, expect the case in upright orientation.

3.3Fracture morphology

To further understand the mechanical behaviors of FDM-printed PLA composite parts, an analysis of fracture morphology for all samples was performed. Figs. 8 and 9 show the optical images of fracture morphology of samples after tensile and flexural tests, respectively. It is evident that more dense interlayer structures that are very close to the virgin PLA were observed from images in the cases of ceramic, copper and aluminum-based PLA samples than those of wood and carbon fiber-based PLA ones. In other words, the low mechanical strength of wood and carbon fiber-based PLA samples could be attributed to defects such as high porosity, poor compaction and adhesion between filament layers. Take FDM-printed wood-based PLA samples for instance, the above-mentioned defects are witnessed more obviously from images B1–B6 in Fig. 8 for tensile tests and b1–b6 in Fig. 9 for flexural tests. This would explain the reasons why the mechanical strength of ceramic, copper and aluminum-based PLA composites, no matter in tensile or flexural tests, is higher than that of wood and carbon fiber-based PLA composites.

Fig. 8.

Optical images of fracture morphology for all printed samples after tensile tests.

(1.47MB).
Fig. 9.

Optical images of fracture morphology for all printed samples after flexural tests.

(1.37MB).

As discussed previously, raster angle and build orientation also lead to the variation of mechanical behaviors for PLA parts with the same shape. Optical images have demonstrated that in flat and on-edge orientations, low porosity could be noticed for the samples with +45°/−45° raster angles compared to the ones with 0°/90° raster angles. This phenomenon could further confirm the high mechanical strength achieved by samples with +45°/−45° raster angles in most cases of PLA composite parts in Tables 3 and 4. Furthermore, the interlayer structure of samples printed in on-edge orientation usually has better compaction and adhesion than the counterparts in flat orientation with the same raster angle, as more layers need to be printed in this scheme but the layer bonding direction is perpendicular to the loading direction, which would carry more loading force. For the samples in upright case, there is no obvious difference in optical images observed between 0°/90° raster angles and +45°/−45° raster angles. However, due to the weak interlayer bonding, the upright samples cannot carry more load, thereby leading to the weakest mechanical strength among the samples in all build orientations.

4Conclusions

In this paper, the mechanical properties of PLA and its composites have been evaluated taking the effects of important printing variables, i.e. build orientation and raster angle into consideration. The key research findings are summarized as follows:

  • i)

    Virgin PLA, wood and carbon fiber-based PLA have better printing formability than the ceramic, copper and aluminum-based PLA in upright orientation.

  • ii)

    The similar or even enhanced mechanical properties of FDM-printed ceramic, copper and aluminum-based PLA samples have been obtained compared with virgin PLA. Adding wood and chopped carbon fiber into virgin PLA significantly lower its mechanical properties. Moreover, PLA composite samples that are FDM-printed in on-edge orientation with +45°/−45° raster angles have the highest mechanical strength in most cases. While, all the samples printed along upright orientation have the weakest mechanical strength and modulus due to weak interlayer bonding.

  • iii)

    The fracture surfaces for the samples after tensile and flexural tests have been analyzed in detail. The low mechanical properties of wood and carbon fiber-based PLA composite parts would be attributed to defects such as high porosity, poor compaction and adhesion between filament layers, compared to virgin PLA, as well as ceramic, copper and aluminum-based PLA composite samples.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgment

The authors would like to thank the financial support from the Fundamental Research Funds for the Central Universities (WUT:2017IVA017 and 2017III047) for undertaking this work.

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Copyright © 2019. The Authors
Journal of Materials Research and Technology

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