Composites Communications 24 (2021) 100605

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Composites Communications
journal homepage: www.elsevier.com/locate/coco

An investigation to study the combined effect of different infill pattern and

infill density on the impact strength of 3D printed polylactic acid parts
Pradeep Kumar Mishra, P. Senthil *, S. Adarsh, M.S. Anoop
Department of Production Engineering, National Institute of Technology, Tiruchirappalli, 620015, India

A R T I C L E I N F O A B S T R A C T

Keywords: The emanation of 3D printing technology has enabled researchers to manufacture complex geometrical struc­
Additive manufacturing tures to design high impact energy-absorbing structure for different laboratory and industrial applications. In the
Fused deposition modeling present investigation, an Impact test had been performed to measure the absorbed energy during the plastic
Impact strength
deformation of the PLA (Polylactic acid) 3D printed components with the combination of different infill patterns
Izod impact testing
and infill densities. Experimental investigation on specimens through the Izod impact test concluded that at 85%
infill density for each infill pattern, the energy absorption is maximum. Results showed that the fracture resis­
tance of specimens was a mutual function of induced stress and crack propagation at the notch area during the
impact test and the dynamics of impact failure outlined by mechanics of laminate composite.

1. Introduction O. S. Es-Said et al. [9] found that compared with other raster ori­
entations, parts in which the raster orientations are parallel to the
The market for the design of durable polymeric structures has grown loading axis showed higher tensile stiffness and bending stiffness. A.
significantly in many fields, such as sports, automobile, biomedical, Tsouknidas et al. [10] studied the impact of layer height on the dissi­
packaging and civil, etc. [1–3] today. Thus, several experiments have pating energy characteristics of the printed PLA structure and found that
been carried out on those sections to build such a structure to make them an increase in layer height increased the risk of premature failures. As
light with enhanced strength and resistance to impact. Such kinds of the layer height increases, the increased distance between the neigh­
resilient structures often provide the engineer or researcher with the boring beads raises the level of stress concentration during impact
challenge of designing and manufacturing the component to meet design loading. Compared to the thicker layer, the thinner layer takes more
requirements. Due to the flexibility of the FDM (fused deposition time to create the component, and the thinner layer holds a temperature
modeling) process in making the mesostructure, this problem can be above the melting point for extended period of time [11]. The shorter
easily solved by printing different densities of infill (packing density) layer height, therefore, increased the impact strength by generating less
and patterns to establish the optimum structure. The FDM process was stress concentration and building excellent diffusion bonds among the
primarily used for prototyping by manufacturers, but due to the advent beads which ultimately improved impact resistance. Fillip Gorski et al.
of manufacturing techniques and the use of fiber-reinforced plastics as [12] investigated that compared to the same monolithic part generated
filaments, the process can print load-bearing components with the by the injection moulding process, the impact strength of FDM-based
resistance to higher strain rate [4]. The arrangement of rasters and samples decreased drastically. Tanveer et al. [13] measured the PLA
layers in the FDM process forms a structural framework otherwise called specimen’s impact strength at 50%, 70%, and 100% infill density, and
mesostructure, and the G. Alaimo et al. reported that it had a significant considered the impact strength to be directly proportionate to infill
impact on the mechanical properties of the components [5]. Many density. The study predicted that due to the increase in material packing
process parameters control the functional properties of the component density in the specimen, the rise in impact resistance was observed and
manufactured by the FDM process and among them air gap, layer thus the stress intensity factor decreased. It was found from the above
thickness, raster angle, raster width, infill density and build orientations literature data that the impact resistance of the 3D printed structure is
play a crucial role in influencing those properties [6,7,8]. (see Tables 1 highly dependent on the mesostructure geometry. The mesostructure is
and 2) a function of the density of infills and the pattern of infills in each

* Corresponding author.
E-mail address: senthil@nitt.edu (P. Senthil).

https://doi.org/10.1016/j.coco.2020.100605
Received 23 April 2020; Received in revised form 12 December 2020; Accepted 22 December 2020
Available online 29 December 2020
2452-2139/© 2020 Elsevier Ltd. All rights reserved.
P.K. Mishra et al. Composites Communications 24 (2021) 100605

Table 1 are very few investigations available to explain the function of the
Representation of different design mix for impact specimens. mesostructure in the propagation of cracks and the effect of stress in­
Serial Number Combination of different Infill pattern & Number of Samples tensity on the impact strength of the printed parts. For this purpose,
Infill density specimens were printed with different combinations of infill densities
1 Line 7 and infill patterns (line, zigzag, concentric, cross, concentric 3d, cross
{50, 75, 80, 85, 90, 95, 100} 3d, cubic subdivision, circle, octet, quarter cubic, triangular, and tri-
2 Zig-zag 7 hexagon pattern) to research the effect of mesostructure. From the
{50, 75, 80, 85, 90, 95, 100} initial investigation it was found that line, zigzag and concentric
3 Concentric 7
{50, 75, 80, 85, 90, 95, 100}
exhibited excellent energy-absorbing capability impact over other pat­
4 Cross 2 terns of infill. Therefore, the printing of infill patterns with multiple
{50,75} infill densities was confined to the line, zigzag and concentric patterns.
5 Cross 3D 2 Regarding the material selection for fabricating the specimen, the semi-
{50,75}
crystalline polymer ‘PLA’ was preferred for investigation because of its
6 Concentric 3D 2
{50,75} remarkable biocompatibility and biodegradability characteristics [14].
7 Cubic subdivision 2 The major objectives of the present investigation are:
{50,75}
8 Grid 2 • Representation of a significant impact energy dataset for multiple
{50,75}
9 Octet 2
combinations of infill density and infill geometry pattern.
{50,75} • Comparison of different infill density and geometry under the same
10 Quarter Cubic 2 experimental constraints.
{50,75} • Discussion of the inferences on impact strength obtained from the
11 Triangular 2
experiment in the context of stress intensity and crack propagation.
{50,75}
12 Tri-hexagon pattern 2
{50,75} 2. Materials and methods

2.1. Design and manufacturing of samples

Table 2
Fixed process parameters for printing. The impact specimen’s 3D model (Fig. 1a) was developed in accor­
Serial Number Printing parameters Value
dance with the ASTM D256 specification (Fig. 1b) in the CREO para­
metric solid modeling program, and a total of 117 specimens were
1 Layer height 0.2 mm
selected from the combinations of infill density and infill pattern (Table-
2 Initial layer height 0.2 mm
3 Line width 0.35 mm 1). The specimens were manufactured using the FDM technique using
4 Wall line width 0.35 mm PLA feedstock material in the ‘Ultimaker 3 Extended’ 3D printer ac­
5 Outer wall line width 0.35 mm cording to the appropriate process parameters (Table − 2).
6 Inner wall line width 0.3 mm
7 Wall thickness 1 mm
8 Wall line count 3
2.2. Experimental set-up
9 Printing temperature 215 ◦ C
10 Build plate temperature 60 ◦ C
11 Nozzle Flow 100% For a given group of process parameters, each sample set consisted of
12 Build plate adhesion type None three specimens. The results were taken as the mean impact strength
values of the mechanical test. Since the physical properties of many
materials (especially thermoplastics) can vary depending on ambient
component. Hence, its geometry can influence the propagation of cracks
temperature, room temperature tests were performed according to the
and the factor of stress intensity regarding impact loading and influence
standards. Izod impact tests were performed to study the energy ab­
the impact strength of the component. The dynamics of failure due to
sorption and characterize the type of damage of the different configu­
impact load on specimens were described by the mechanics of laminated
rations. The Izod impact strength test is a typical ASTM method of
composite structure [20].
assessing material impact resistance. A pivoting arm (constant potential
The reported resistance of the Izod impact is defined as this energy
energy) (Fig. 2a) is raised to a specific height and then released. The arm
divided by the region of the mid-section. While there are inconsistencies
swings down and reaches a notched sample and breaks the sample. The
in the calculations of energy dissipation due to the kinetic energy of
energy which the sample absorbs is determined from the height at which
broken specimens and multiple fractures and delamination, impact en­
the arm swings after reaching the sample. The total energy of fracture is
ergy offers a useful calculation for comparative purposes at least. There
determined by

Fig. 1. (a) 3D model of Impact Test Sample and (b) Dimensions of Impact Test Sample according to ASTM D 256.

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P.K. Mishra et al. Composites Communications 24 (2021) 100605

Fig. 2. (a) Schematic of impact testing equipment and (b) Geometrical arrangement of different infill patterns.

( )
Et = mg ho − hf ± 1.5 J section region and the stress-induced significantly. Furthermore, the
higher porosity in the mesostructure of printed specimen limited the
where Et is the total energy, m is the mass, g is gravitational acceleration, interfacial bonding strength among the beads and resulted in enhancing
h₀ is the original height and hf is the final height. The absorbed energy the fracture energy of specimen [21,22]. Therefore, this coupled factor
per unit cross-sectional area (kJ/m^2) or impact strength Ec is defined as drastically reduced the impact strength of specimens with 50% infill
density. In the case of infill density of 85%, the number of beads is 23
Et
Ec = closes to the specimen’s notch as shown in Fig. 3c. The resisting region is
wt
more in this case, and the propagation of cracks is not constant through
Where the width and thickness of the specimen are w and t, respectively. the geometry of the infill. This form of mesostructure thus absorbed a
Owing to their limited contribution to the energy balance, energy losses greater amount of energy from impacts, as shown in Fig. 4. The spec­
due to bearing friction and air resistance were overlooked. imen printed with an infill density of 100% has 28 number of beads as
shown in Fig. 3c. These 28 beads resist the impact effect, and each bead
3. Results and discussion is in contact with the neighboring beads. The crack will easily propagate
through the specimen once nucleated, as the resistant region is contin­
The details of the impact resistance of all infill patterns are shown in uous (Fig. 5a & b). Thus, while the number of beads are more, the crack
Fig. 3a & b. As per the results the line, zigzag and concentric patterns are propagation rate moves rapidly and therefore the impact intensity has
showing a better impact-resisting performance (Fig. 3a & b) as begun to decrease with each pattern of infilling.
compared to others. Therefore, the focus of the investigation was The impact resistance result of each combination of infill density and
restricted to test the combination of line, zigzag and concentric patterns pattern was carefully analyzed to prove the requirements for the crack
(Fig. 2b) with infill densities (50%, 75%, 80%, 85%, 90%, 95%, and propagation in the concern for the specimen’s impact-absorbing ability.
100%). Information on energy absorption due to impact testing on It is observed that zig-zag and concentric patterns from 50% to 85% infill
different infill density combinations and patterns of 3D-printed PLA density (Fig. 4) more follow the impact resistance of the infill line
specimens are recorded in Fig. 4. The findings initially showed that there pattern. After that, the zig-zag pattern displayed greater impact-resistant
is a growing trend in the energy absorption of impacts as the percentage capability than the other two patterns. At 100%,95%, and 90% infill
of infill density increases, but the trend decreases after 85% of infill density, the line infill pattern specimens showed continuous crack
volume, as shown in Fig. 4. As the mesostructure of the printed specimen propagation (Fig. 5a to c) at the surface region for which the fracture
plays a vital role in determining the factor of stress intensity and crack rate was high and eventually resulted in less impact power. In the case of
propagation and indirectly determining the force of impact. So, this the zig-zag pattern, the length of the crack is very long (Fig. 5b to d)
study considered the mechanism of the element of stress intensity and compared to the pattern of line infill, and this delayed the rate of
the principle of crack propagation in the impact specimens for fracture propagation of the crack. Hence this phenomenon leads to an
analysis. enhancement of the zig-zag pattern effect resistance after infill density of
The mesostructure of the specimens was taken into consideration to 85% over the other two patterns. Compared to the line and the zig-zag
explore the impact of stress intensity factor and crack propagation in the infill pattern, the impact energy of the concentric pattern is lower for
specimens. The line infill pattern was taken into the investigation for its all infill density (Fig. 4). This is because sharp bends are present in a
basic geometrical topology and studied the impact of variance in the concentric pattern closer to the structure of the notch (Fig. 3d), which
percentage of infill on stress intensity and propagation of cracks. Three serves as a concentrator stress region. The specimen was exposed to
percentages of infills (50%, 85%, and 100%) had been considered, and more stress due to the presence of stress concentration zones and ulti­
the mesostructure specifics are shown in Fig. 3c. For the case of infill mately ended with less impact intensity relative to other infill patterns.
density of 50%, the number of printing beads closer to the notch region The impact resistance for all infill patterns showed the highest
is 14 numbers (other than the perimeter) as shown in Fig. 3c. Since the impact resistance value compared to other densities of infill at 85% infill
number of beads are smaller and there are large gaps in between printed density. At an infill density of 85%, some layer has undergone delami­
beads, the higher stress induced by the impact load in the total cross- nation (Fig. 5d) and this process reduced the fracture energy carried out

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P.K. Mishra et al. Composites Communications 24 (2021) 100605

Fig. 3. (a) Energy absorption in specimen with various patterns at 50% infill density (b) Energy absorption in specimen with various patterns at 75% infill density (c)
Mesostructure at different infill density of line pattern and (d) Mesostructure of concentric pattern showing sharp bends at notch zone.

by the crack tip and allowed a greater amount of energy to be absorbed the delamination at the fracture zone disappeared in each infill pattern.
by the specimens [15–19]. The difference between the maximum and Consequently, the impact resistance of specimens decreased sharply
minimum value of impact resistance at the infill density of 85% was after the infill density of 85% and even dipped below impact resistance
about 55.07% and the difference value decreases as the value of infill at 50%.
density rises. The lowest difference value at 100% infill density was
recorded to be 26.02% (Fig. 4). The result showed that the maximum 4. Conclusions
impact resistance (13.91 kJ/m^2) was recorded in the combination of
85% infill density and line infill pattern (Fig. 4) and this value was 3.49 • It is observed that impact strength depends strictly on the nature of
times the impact resistance provided by neat PLA [17]. At 100% infill the mesostructure which plays the balance between the factor of
density, the crack length has almost the same magnitude for each infill stress intensity and the phenomenon of crack propagation. Thus,
pattern, and its impact resistance values have confirmed it (Fig. 4). This 85% demonstrated the highest energy absorbing potential across the
phenomenon showed that the specimens appear to be more homoge­ infill density range (from 50 to 100%), with a combination of each
neous as infill density increases and crack is easily propagated through infill template.
the notch field. In addition, at 100% and 95% infill density (Fig. 5a & b),

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P.K. Mishra et al. Composites Communications 24 (2021) 100605

Fig. 4. Impact energy absorption at combination of different infill density and patterns.

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Fig. 5. Fracture behaviour of specimens after impact test at different infill density (a) 100% (b) 95% (c) 90% and (d) 85%.