Effect of variable infill density on mechanical behaviour of 3-D printed PLA specimen: an experimental investigation
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Fused deposition modelling (FDM) is one of the additive manufacturing technique, widely accepted due to its ‘easy to use’ technology. The set process parameters during FDM printing, affect the mechanical behaviour of the built part. The objective of this study is to investigate the effect of variable infill density on the tensile and impact strength of polylactic acid (PLA) specimens printed by an open-source 3D printer. The tensile and impact test specimens are printed according to the DIN EN ISO 527-2, ASTM D256 (Izod) and ASTM D6110 (Charpy) respectively. A computerised tensometer and impactometer used to conduct the tensile and the impact strength of the PLA specimens. The tensile strength with the highest value of 46.3 N/mm2 and the lowest value of 29.9 N/mm2 are found. The results demonstrate that the specimens with varying infill density exhibit better tensile strength than the specimens with single infill density. The varying infill density specimens weigh lighter as compared to single infill density, which saves the raw material. The highest Charpy and lowest Izod strength found to be 4.72 kJ/m2 and 1.7 kJ/m2. The impact test experimental data establishes that impact strength is directly proportional to the infill density. Thus, by varying the infill density, there is a reduction in the impact strength of the printed specimens.
KeywordsFDM PLA Infill density Tensile properties Impact strength
Fused deposition modeling (FDM), is an additive manufacturing (AM) technology used for printing three-dimensional part. It is based on the extrusion process where heated thermoplastic filament is “selectively dispensed through a nozzle or orifice” . FDM is less expensive and easier to maintain as compared to other AM printing technologies and has a lower cost of hardware maintenance . FDM seems to be a useful process for domestic as well as small industrial learning. It cannot be denied that FDM performance such as system cost, build time, raw material used, and waste are better, when compared with other AM techniques [3, 4].
Product developed through FDM process is mainly used to understand the three-dimensional complex and undertake simple prototypes . Along with visual aid, it is also essential to evaluate the mechanical behaviour of the three dimensional (3D) printed part when subjected to various mechanical loads. Hence, the investigation of the mechanical behaviour of 3D printed parts is a subject of importance and research .
Various studies have been done on the process parameters to enhance mechanical properties. The primary process parameters which influence FDM are raster angle, build orientation, layer thickness, infill density and raster gap. Considering these factors, Sood et al. , studied their influence on impact, flexural and tensile strengths of FDM specimen. Empirical models developed, relating response and process parameters and the developed model were tested using the ANOVA technique. Masood et al. studied the process parameters such as air gap, raster angle and raster width on the FDM part and compared the tensile properties with moulded specimens. The results show that the tensile strength of the FDM part is 70–75% of the moulded part . Li et al.  used open-source FDM printer to study the effect bonding degree on the tensile strength of the PLA specimen. The result showed that bonding strength effects significantly by layer thickness, followed by deposition velocity.
Tymrak et al. compared the tensile properties of polylactic acid (PLA) and acrylonitrile butadiene styrene (ABS) on an open-source 3D printer. The parameters considered for the study were raster angle and layer height . Cantrell et al. compared the tensile properties as well as shear properties of PLA and ABS on two different commercially available printers. The result indicated that build orientation and raster had a negligible effect on tensile specimens. ABS specimen shows shear yield strength varied by up to 33% . Ziemian et al. studied five different mechanical properties of ABS specimen: compression, tension, 3-point bending, impact and fatigue, considering build orientation and raster angle. The results show that specimens build with raster angle direction parallel to applied force gives better mechanical properties . Few authors studied the effect of Temperature, print speed, infill direction, relative density, and layer thickness on the PLA specimen and developed an empirical formula to optimise the process parameter [13, 14, 15, 16]. Authors studied the impact of print orientation [17, 18], filament recycled PLA  on the mechanical properties of FDM developed parts. Lanzotti et al. used an open-source 3D printer to study the effect of layer thickness, the number of shell perimeters and infill orientation of PLA specimen. The results were prepositioned with an empirical model to understand the impact of each process parameter . However, by scrutinising literature, it is found that the display of mechanical properties of specimen produced with PLA exhibits better results as compared to ABS specimen . During the review, it is found that there are very few papers on the mechanical behaviour of PLA parts processed by low-cost 3D printers. Mainly, the parameter used during slicing aspect calls for further research. This study will focus on the effect of variable infill density, and their combination on the tensile strength and impact strength of 3D printed PLA specimen. The experimental data of the variable infill density also compared with single infill density specimen. The purpose of this study is to improve our understanding of optimal infill density and assist users in the selection of correct combination. It is crucial to get an optimum strenght to weight ratio. The understanding build during work done by various researchers is that the mechanical strength is directly proportional to the infill density. However, if the structure is large, 100% fill density will increase the raw material and built time increasing printing cost. In industry-standard practice, to save the build time and the running cost, higher shell perimeter and reduced in infill density is opted, which subsequently reduces the mechanical properties of the FDM parts.
2 Materials and methods
In the present study, a total of thirty-six number of specimens for tensile test, twenty-seven number of specimens for Izod and Charpy test printed by FDM technique, respectively. A set of three infill density percentage considered for the experimental work. A set of four specimens of tensile test and a set of three for Izod and Charpy test printed by fixing one infill density respectively. The sample preparation and testing of the specimen discussed in subsequent section as follows.
2.1 Sample preparation
Infill Density Arrangement
Infill density percentage
Parameters used for a 3D printer for PLA
50%, 75%, 100% and combinations
This study demonstrates the mechanical behaviour of the 3D printed part under two different test conditions, mainly tensile test and impact test. The criterion for the selection of test is to understand the response of PLA fibres in two diverse test conditions. Firstly, the direction of the applied force is parallel of 3D printed fibres in the tensile test and perpendicular in case of impact test. Next, the parallel forces try to elongate the fibre, whereas perpendicular forces will try to bend the fibres before failure. And lastly, the performance of fibres with the change in the duration of applied forces, i.e. gradual loading and impact loading.
The weight of the specimens is also considered to find the optimum weight to strength ratio. The experimental data shows that a better tensile strength achieved with the use of lesser raw material. The specimen ABA displays tensile strength of 48.2 N/mm2 with 3.42 g of weight which comparatively better than specimen A (46.6 N/mm2) with a weight of 3.60 g. This difference seems less as the specimens are small but will drastically if the size of the 3D printed part increases. The results also show that the tensile strength of specimens ACA (44.7 N/mm2) and BAB (46.75 N/mm2) is approximately equivalent to specimen A (46.6 N/mm2) with lesser weight which is 3.36 g and 3.35 g compared to 3.60 g respectively. The two major factors considered for lesser material weight during the printing of the 3D part are the selection of appropriate combinations of infill density and their position.
The results show that higher the infill density, better the impact strength. There is significant deprivation of impact strength by lowering the infill density or by combining the variable infill density. Hence it can be stated that weight is directly proportional to the impact strength—the optimum weight of the raw material not found as in case of tensile test.
Relative percentage change of weight and tensile strength of specimen
Relative strength percentage change
Relative weight percentage change
Figure 10 also represents the strength to weight ratio (SWR) of the specimens. The curve shows that the maximum value of SWR value of specimen ABA (14.07) followed by specimen BAB (13.91) and specimen ACA (13.27). The results depict that the tensile strength of the 3D printed part mainly depends on the two factors. Firstly, the denser infill density at the outer periphery and secondly on a lesser dense infill density at the inner region. The dense outer shell provides resistance from crack propagation, whereas, sparse-dense infill provides better flexibility, which eventually enhances the tensile strength of the specimen. The sequence of the specimen, according to SWR, is ABA, BAB and ACA.
Relative percentage change of weight and impact strength of specimen
Izod strength (kJ/m2)
Relative strength percentage change
Relative weight percentage change
Charpy strength (kJ/m2)
Relative strength percentage change
Relative weight percentage change
The experimental data indicates that the higher tensile strength achieved by stacking layers of different infill density eventually reduces the raw material to an extent.
The tensile strength of the specimens with variable infill density is increased by keeping dense infill at the outer periphery, and less dense infill at the inner area and this arrangement provides initial resistance to crack propagation on boundary and flexibility from the inner structure.
The experimental data shows an increase of 10% Charpy impact strength as compared to Izod impact strength with a 50% reduction in the weight of the specimens.
In the case of the impact test, the infill density is directly proportional to the impact strength, which results in the weight of raw material. The strength depletes by combining the varying infill density.
Contrary to tensile strength, the impact strength increases if the inner layer is of higher infill density and outer shell are of lesser density.
Fused deposition modelling (FDM) is one of the important and commonly used technologies of 3D printing. The need was to identify process parameters that affect the mechanical behaviour of the built part. This study has the objective of investigating the effect of variable infill density on the tensile and impact strength of Polylactic Acid (PLA) specimens printed by an open-source 3D printer. The tensile and impact test specimen are made according to the DIN EN ISO 527-2, ASTM D256 (Izod) and ASTM D6110 (Charpy) respectively. In general, single infill density directly influences the tensile and impact strength of the 3D printed specimen. However, here in above mentioned situations, the tensile and the impact strength show a mixed response with combined infill density. The results obtained are useful for the selection of the appropriate weight of the 3D printed part, which will be undergoing a calculated tensile as well as impact loading. The present study helps to find the optimum strength to weight ratio for the PLA specimens printed by an open-source 3D printer.
The present work gives a good platform for future work as the influence of other factors along with variable infill density on the tensile and impact can also be considered. One can also study the orientation of the combinations. Other additional parameters like build orientation, the part layer thickness can also be combined for future applications and software development. The results can be more elaborative and conducive for further research with the use of an empirical relationship.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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