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Comparative Performance Analysis of Polylactic Acid

Parts Fabricated by 3D Printing and Injection Molding
Ujendra Kumar Komal, Bal Kishan Kasaudhan, and Inderdeep Singh

Submitted: 17 December 2020 / Revised: 17 April 2021 / Accepted: 7 May 2021

In the present experimental investigation, the specimens were fabricated using 3D printing (fused depo-
sition modeling) and injection molding techniques. The process parameters were optimized to fabricate the
good quality polylactic acid (PLA) specimens as per ASTM standards. The effect of variation (80, 90, and
100%) of the infill density on the mechanical performance of developed specimens was analyzed. The
mechanical behavior of the fabricated specimens was compared in the context of tensile, flexural, and
impact properties. The thermal stability and crystallinity of the PLA specimens have been investigated
using thermogravimetric and XRD analysis, respectively. After tensile testing, the surface of the fractured
specimens was observed using a scanning electron microscope. The tensile and flexural strength of the 3D-
printed specimens was superior to the injection-molded specimens. An improvement in stiffness of the 3D-
printed specimens has been observed. Moreover, the printed specimens showed better thermal stability than
the molded specimens. There was no significant variation in the crystallinity of the printed and molded
specimens. It can be concluded that the tensile, flexural, and thermal responses of the 3D-printed specimens
are better than injection-molded specimens at the optimal combination of process parameters.

(mold) in injection molding can only be justified for a

Keywords 3D printing, FDM, injection molding, mechanical
properties, PLA, thermal properties significantly large production volume. With the current focus
of the industry toward customization and offer variety to the
customers, there is a huge potential for employing additive
manufacturing for the fabrication of functional products with a
high degree of design flexibility. FDM is generally considered
the most commonly used additive manufacturing technique for
1. Introduction thermoplastics due to its simple working principles, ease of
handling, rapid operation, and lower power consumption (Ref
Additive manufacturing has revolutionized the new product 1-3). However, there is a general belief that the parts/compo-
development cycle by significantly reducing the time frame nents made of the FDM process lack properties due to the
between the conceptualization of ideas and the launch of formation of voids and air gaps during fabrication. A significant
tangible products. The application spectrum of various additive number of investigations focussing on optimizing the operating
manufacturing technologies has slowly shifted from creating parameters like raster width and angle, infill orientation, layer
prototypes to the fabrication of fully functional products. height and thickness, and bed temperature have already been
However, the mechanical performance of the parts/components reported to enhance the quality of the plastic components (Ref
produced by various layered-manufacturing techniques has 4, 5). Tymrak et al. (Ref 6) proposed that the raster angle of 0°/
been an area of concern and research focus. Also, the time 90° could be preferred to improve the tensile behavior of the
required to create 3D parts using additive manufacturing PLA and acrylonitrile butadiene styrene (ABS). Aw et al.
methods, such as stereolithography (SLA), fused deposition investigated the effect of 100% infill density and line pattern on
modeling (FDM), is high. In order to compete with the the mechanical behavior of the products. The authors con-
commercial polymer processing methods, such as injection cluded that 100% infill density and line pattern significantly
molding, the comparative/higher mechanical performance at a remove the air voids between the layers and improve the
lower cost is expected to tilt the decision in favor of additive adhesion between the layers, leading to improved mechanical
manufacturing technologies. The higher cost of the tooling properties (Ref 7). The various types of thermoplastics such as
PLA, ABS, polycarbonate (PC), and polypropylene (PP) have
been used in the FDM process, and their effect on the behavior
This invited article is part of a special topical focus in the Journal of of the parts/components has been investigated (Ref 8-10).
Materials Engineering and Performance on Additive Manufacturing. Material selection is at the core of any design innovation. The
The issue was organized by Dr. William Frazier, Pilgrim Consulting, researchers have widely explored the biopolymers like PLA
LLC; Mr. Rick Russell, NASA; Dr. Yan Lu, NIST; Dr. Brandon D.
Ribic, America Makes; and Caroline Vail, NSWC Carderock. due to their biodegradability, renewability, and high strength
and suggested that it has tremendous potential as a substitute
Ujendra Kumar Komal and Inderdeep Singh, Department of for petroleum-derived polymers for various applications (Ref
Mechanical and Industrial Engineering, Indian Institute of 11, 12). Form the last few years, PLA has been successfully
Technology Roorkee, Roorkee, Uttarakhand 247667, India; and employed in packaging, industrial, and biomedical applications
Bal Kishan Kasaudhan, Department of Chemical Engineering, due to its biodegradability, compostability, and biocompatibility
Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh
208016, India. Contact e-mail: inderdeep.singh@me.iitr.ac.in.

Journal of Materials Engineering and Performance

(Ref 13, 14). The mechanical behavior of PLA parts fabricated mens fabricated using FDM where D1 = Infill density, 80%,
using the FDM process has been widely explored (Ref 15-19). D2 = Infill density, 90%, and D3 = Infill density, 100%.
The comparative investigation of the parts fabricated by
FDM and injection molding has also been reported in some 2.3 Mechanical Performance
articles (Ref 3, 20). However, the majority of the literature is
The tensile, flexural, and impact test specimens were
focused on the tensile behavior of the polymers. A robust
fabricated as per ASTM D3039, D7264, and D256, respec-
understanding of the comparative performance analysis of the
tively. The tensile and flexural behavior of the PLA specimens
parts manufactured using FDM and injection molding is rarely
was investigated in the context of ultimate strength and
available.
modulus using a Universal Testing Machine (UTM) (Instron:
The judicious selection of materials is essential for ensuring
5982, USA, Load Cell = 10 kN equipped with Blue Hill
the concept of ’Design for Environment’. Therefore, the
Software). Both the tensile and flexural tests were conducted at
sustainable material option, PLA, was selected for the current
a loading rate of 2 mm/min. The impact test was conducted
research endeavor. The current investigation highlights the
using a low energy impact tester.
impact of infill density on the thermal, mechanical, and
crystalline behavior of the PLA specimens fabricated using
2.4 Thermal Analysis
the FDM technique. The comparative performance analysis (in
terms of thermal, mechanical, and crystalline) of the parts The thermal behavior of the specimens was investigated
manufactured using FDM and injection molding has also been using a thermogravimetric analyzer. Both the thermogravimet-
conducted. ric analysis (TGA) and differential thermal analysis (DTA)
were executed using the same equipment. A metered sample

2. Materials and Methods

2.1 Fused Deposition Modeling (FDM)

The blue-colored PLA filament having a diameter of
1.75mm was purchased from Novabeans, Gurugram, Haryana,
India. The PLA specimens were printed according to ASTM
D3039, ASTM D7264, and ASTM D256 standards for tensile,
flexural, and impact tests, respectively. The specimens with
varying infill densities (80, 90, and 100%) were printed using
FDM.

2.2 Injection Molding (IM)

The PLA filament was pelletized into an average length of
4-5 mm. These pellets were used to fabricate test specimens
using a commercial scale injection molding machine. The
details of the process parameters used during both the processes
are given in Table 1. Figure 1 depicts the test specimens
fabricated using both techniques.
The abbreviations used in Table 1 are; IM = Specimen
fabricated using injection molding. D1, D2, and D3 = Speci-
Fig. 1. (a) injection-molded and (b) 3D-printed specimens

Table 1 Details of process parameters

3D Printing Injection molding

Parameters D1 D2 D3 Parameters IM

Layer height, mm 0.1 0.1 0.1 Temperature profile 170-165-160-155 °C

Shell thickness, mm 1.2 1.2 1.2
Bottom/top thickness, mm 1 1 1 Injection pressure, MPa 50
Fill density, % 80 90 100
Print speed, mm/s 60 60 60 Holding pressure, MPa 40
Printing temperature, °C 210 210 210
Bed temperature, °C 50 50 50 Holding time, s 15
Platform adhesion type Raft Raft Raft
Diameter, mm 1.8 1.8 1.8 Cooling time, s 30
Flow, % 107 107 107
Nozzle, mm 0.4 0.4 0.4

Journal of Materials Engineering and Performance

was placed in a pan and heated from room temperature (25-
30 °C) to 600 °C with a heating rate of 5 °C/min. All the tests
were conducted in a nitrogen atmosphere, and the loss of mass
was recorded with respect to temperature. The data obtained
from the test were used to plot the curves and compare the glass
transition, crystallization, and melting temperature of the
specimens.

2.5 Crystallinity
The XRD analysis was performed using a Bruker diffrac-
tometer (D8-Advance) for the angle range (2h) of 5°-60° with a
scanning speed of 1°/min. Copper was used as a target material.
The data have been analyzed using X’Pert Highscore software,
and the spectra were plotted from the analyzed data using
Origin software (Ver. 8.5). The total area under the curve and
crystalline area were measured using Origin software. Finally, Fig. 3. Variation in modulus
the crystallinity of the specimens was quantified using Eq 1
suggested by Vonk (Ref 21).
strength (tensile, flexural, and impact) with higher infill density
Crystallinearea is attributed to the strong bond created between the rasters,
Crystallinity ð%Þ ¼  100 which required a comparative higher force to break. Similar
Total area under the curve
ðEq 1Þ types of observation have also been reported previously (Ref
22, 23). The increment in tensile and flexural modulus with
infill density is correlated with the improvement in stiffness of
2.6 Microstructural Analysis the PLA. The improvement in modulus is due to the
comparative lower voids and lower porosity present in the
The surfaces of fractured specimens were observed using an
specimens. Moreover, the clustered strands in a single direction
optical camera and scanning electron microscope (SEM). The
arrest the plastic deformation leading to an increase in the
specimens’ failure characteristics under tensile loading and
stiffness of PLA specimens. A similar trend of variation in
variation in properties were understood using these images.
modulus with infill density was also reported by Carneiro et al.
(Ref 22).
The tensile and flexural responses (strength and modulus) of
3. Results and Discussion D3 were recorded higher than the IM specimens. The
improvement in properties is due to the strong bonding
3.1 Mechanical Performance between the rasters and the presence of clusters of strands.
Moreover, the unique direction of the raster throughout the
Mechanical behavior of the injection-molded (IM) and 3D- printing improves the polymer molecules’ orientation, which
printed (D1, D2, and D3) specimens was evaluated in the also contributes to improving these properties. The lower
context of tensile, flexural, and impact responses. The data tensile and flexural properties of IM can also be due to the
obtained from the tests were plotted, as shown in Figs. 2 and 3. heterogeneous orientation of polymeric molecules during
All the specimens demonstrated a brittle failure during tensile injection molding. Near the mold surfaces, the polymer
testing. The mechanical properties of 3D-printed specimens’ molecules align in flow direction and randomly at the core
mechanical properties were found to increase with infill density, regions, leading to a decrease in the specimens’ load-bearing
and maximum properties were recorded for D3. The increase in capabilities. The variation in the orientation of polymeric
molecules near the surface and core regions is due to the
variation in friction force (higher near the mold surfaces and
lower at the core regions) during injection molding (Ref 24).
Moreover, the degradation of the PLA due to high shearing
action during pelletizing and injection molding could be the
reason for the reduction in tensile and flexural properties.
Contrary to tensile and flexural strength, the impact strength
of the 3D-printed specimen was found to be lower than the IM.
This reduction in the impact strength of 3D-printed specimens
could be due to the poor fusion between the layers due to lower
bed temperature. When the bed temperature decreases, the
interlayer temperature at the fusion sites also decreases. At
lower temperature, the mobility of the polymeric chain
decreases. Therefore, the inter-diffusion between the layers
becomes difficult (Ref 25). Moreover, when the temperature at
which the diffusion starts is lower, there is less time available
for inter-diffusion between the layers before solidification
(reaching glass transition temperature). This improper inter-
Fig. 2. Variation in mechanical strength diffusion between the layers resulted in the smaller number of

Journal of Materials Engineering and Performance

entangled polymeric molecules between the layers, which 3.3 Crystallinity
resulted in poor adhesion between the layers, and ultimately
The crystallinity of the specimens plays an essential role in
leading to poor impact resistance (Ref 25).
understanding the mechanical behavior of polymers and
The finding is also in line with the findings reported by
polymer-based composites. The XRD analysis was conducted
Aliheidari et al. (Ref 25). The authors investigated and
to observe the variation in crystallinity of the specimens. The
concluded that the increase in nozzle and bed temperature
degree of crystallinity (%) was found to be 4.93, 4.68, 5.41, and
increases the fracture resistance of the 3D-printed parts.
4.10 for IM, D1, D2, and D3, respectively. The lower
Benwood et al. (Ref 19) recently suggested that the optimum
crystallinity for all the specimens confirms the highly amor-
temperature of bed while printing the PLA should be 105 °C.
phous nature of PLA. No significant variation in the crys-
Moreover, the loading direction (transverse with respect to
tallinity was observed for all the specimens, which indicates
raster) during the impact test can be another cause for the
that the fabrication techniques used in the current investigation
reduction in impact strength.
have no significant influence on the crystallinity. The speci-
mens’ lower crystallinity is evident by the absence of any sharp
3.2 Thermal Properties
peak in the XRD spectra (Fig. 5). The broader peak at
The thermal behavior of the PLA was investigated by 2h = 16.5 °C confirms the amorphous region, which is in
calculating the loss of mass with respect to temperature. The agreement with the previously reported results (Ref 3, 24).
data obtained from the test have been plotted, as shown in
Fig. 4(a) and (b). The TGA curves for all the specimens 3.4 Microstructural Analysis
showed a single-stage degradation, which confirms the absence
The images taken of the tensile fractured surfaces using an
of fillers or compound in the filament. All the specimens
optical camera and SEM are shown in Fig. 6 and 7, respec-
degraded completely around 450 °C. The shifting of the curves
tively. All the specimens show a brittle failure during tensile
on the right-hand side indicates the increase in thermal stability
loading. Figure 6 and 7 also confirm that there is no extensive
of the specimens with infill density. It can also be confirmed
yielding before failure.
from the data (T5%) presented in Table 2. However, this shifting
The SEM images (Fig. 7) also revealed good printing with
of curves can be seen only after 250 °C.
good interlayer and intralayer bonding, which can be the reason
The improvement in thermal stability of the specimens with
for the good mechanical properties of the 3D-printed speci-
infill density may be attributed to the reduction in air gaps and
mens. The formation of air gaps can be seen between the beads
moisture absorption within the specimens. It is in agreement
in the 3D-printed specimens which is due to printing orientation
with the optical images (Fig. 6) and SEM images (Fig. 7). The
and lower infill density. However, a reduction in size and
investigation revealed a reduction in the thermal stability of IM
than D1, D2, and D3. It could be attributed to PLA’s
degradation during pelletizing and injection molding due to Table 2 Key observations from TGA and DTA data
high shearing action.
The DTA traces specifically emphasizing the glass transition Specimen Tg, °C Tc, °C Tm, °C T5%, °C
(Tg), crystallization (Tc), and melting (Tm) temperature are
shown in Fig. 4(b). The DTA curves show almost similar traces IM 51.34 91.07 169.15 288.39
for all the specimens. The essential data were extracted and are D1 51.77 90.69 168.88 308.59
presented in Table 2. It depicts an insignificant change in Tg, Tc, D2 52.63 92.53 171.67 327.57
and Tm of all the specimens. The results shown in Table 2 are in D3 53.28 93.58 171.41 340.67
line with the previously reported results (Ref 18, 26).

Fig. 4. (a) TGA and (b) DTA curves of the specimens

Journal of Materials Engineering and Performance

intensity of air gaps or voids with the increase in infill density looks almost similar to the fractured surface of IM specimens
was observed. This also confirms that the mechanical properties (Fig. 7a) due to 100% infill density. The illustrative information
of the 3D-printed specimens increase with the decrease in air generated from the SEM images substantiates the variation in
gaps or voids. mechanical properties of the specimens.
The injection-molded specimens show (Fig. 7a) a smooth
surface indicating no air gaps. However, the reasons for the
lower tensile and flexural properties of IM specimens as
compared to D3 specimens have already been explained in
4. Conclusions
sect. 3.1. The fractured surface of the D3 specimens (Fig. 7d)
The conventional injection molding process and advanced
additive manufacturing technique, i.e., FDM, were used to
fabricate good quality PLA specimens as per ASTM standards.
To further strengthen the research and industrial fraternity’s
motivation to adopt additive manufacturing over the conven-
tional methods of fabricating polymeric parts, a comparative
performance analysis of the PLA specimens fabricated using
injection molding and FDM processes was conducted. Based
on the current investigation, it can be concluded that the tensile,
flexural, and thermal responses of the 3D-printed specimens at
the optimal combination of process parameters are better than
injection-molded specimens. However, the impact strength of
the injection-molded specimen was superior to the 3D-printed
specimens. It can also be suggested that the inorganic and
organic fillers can be incorporated while fabricating the
filament for FDM to improve the impact strength of the final
parts/products. Injection molding and FDM seems to have no
significant influence on the crystallinity of the PLA. The current
investigation also highlights that the 3D-printed products
Fig. 5. XRD spectra of the specimens

Fig. 6. Optical images of the tensile fractured surfaces

Journal of Materials Engineering and Performance

Fig. 7. SEM images of (a) IM (b) D1 (c) D2 and (d) D3 specimens

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