Researchers Investigate Tensile Properties of 3D Printed PLA Specimens: Is 80% Infill Best?

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Comparison between 3D printed products from both recycled and virgin PLA.

Plastic is still one of the most popular, least expensive 3D printing materials, and polylactic acid, better known as PLA, is at the top of the bunch, due to its high strength, biocompatibility, and biodegradable nature. A lot of studies have been completed regarding PLA 3D printing filament, and a team of researchers from the Universiti Malaysia Pahang (UMP) recently published a paper, titled “Preliminary investigations of polylactic acid (PLA) properties,” that details their investigation of the tensile property of a PLA specimen 3D printed using FDM technology, along with figuring out the optimum combination of 3D printing parameters to achieve maximum mechanical properties.

The abstract reads, “This research work aims to investigate the tensile property of Polylactic Acid (PLA) and to determine optimum printing parameter combinations with the aim of acquiring maximum response using low-cost fused filament printer. Two parameters chosen to be varied in this research are raster angle and infill density, with the value of 20°, 40°, 60° and 20%, 50%, 80% respectively. Tensile specimens with a combination of these two parameters were printed according to ASTM D638 type 1 standard. Three mechanical properties were analysed, namely ultimate tensile strength, elastic modulus and yield strength. It was found that the tensile property increases with the infill density. Meanwhile, both high and low raster angle have shown the considerably high mechanical properties. The optimum parameters combination is 40° raster angle, and 80% infill density. Its optimum mechanical property is 32.938 MPa for ultimate tensile strength, 807.489 MPa for elastic modulus and 26.082 MPa for yield strength.”

Applications for FDM 3D printing include more load-bearing parts for specific requirements, and many of these demand a “certain level of mechanical property information” to be set as a benchmark in order to assess a 3D printed PLA part’s strength.

This strength typically relies on some specific 3D printing parameters, including layer height, infill density, and raster angle, which refers to the direction of the beads of material in regards to loading of a part or component. In the experiment, several parameters were kept constant in order to “avoid mislead of result obtained.”

“The design of the experiment includes the parameter and its value selected to be investigated,” the researchers wrote. “Two chosen parameters are raster angle and infill density, with a value of 40°, 60°, 80° and 20%, 50%, 80% respectively. Layer height is kept constant at 0.1 mm throughout this whole experiment. The total number of parameter combinations are 9. Since the sample size is 5, the total number of specimens printed are 45 specimens.”

While the layer height remained 0.1 mm for each of the nine parameter combinations, the raster angle was split up into three groups: three at 40°, three at 60°, and three at 80°. The three infill densities tested were 20%, 50%, and 80%. The team used a Rainstorm Desktop 2D Multicolor Printing Printer Reprap Prusa i3, with 0.4 mm nozzle, to manufacture the tensile test specimen out of 1.75 mm PLA, which was designed using SOLIDWORKS and sliced with Repetier Host software.

Average stress strain curve for all 9 parameter combinations.

The team completed tensile testing, with a maximum load of 50 kN, on all nine of the 3D printed, 3.2 mm thick PLA specimens at a speed of 5 mm/min, according to ASTM D638 standard. The ultimate tensile strength, yield strength, and elastic modulus were extracted from multiple regions and points: proportional limit, elastic limit, yield point, ultimate stress point, and fracture point.

“Ultimate tensile strength (UTS) is the maximum stress that a material can withstand without undergoing plastic deformation while being stretched or pulled,” the researchers explained. “Elastic modulus is the ratio of the force exerted upon a substance to the resultant deformation it experiences, or also known as stiffness. Meanwhile, yield strength is the stress required to produce a small specified amount of plastic deformation.”

The team discovered that when the infill increased, so too did all the property values, which means it’s possible to choose the right infill percentage in order to achieve economic material use.

“Upon analysis of all obtained data, the best-suited parameter combination that results in optimum mechanical property have been identified, which is 3rd parameter combination of 40° raster angle and 80% infill density,” the researchers noted. “It has resulted in the ultimate tensile strength of 32.938 MPa, elastic modulus of 807.489 MPa and yield strength of 26.082 MPa. The 9th combination was not chosen as the optimum parameter condition due to its lower yield strength comparatively. Selection of optimum parameter combination was made based on the criteria of possible maximum mechanical property.”

(a) Fabricated PLA tensile specimen, (b) working mechanism of FDM

The researchers did not investigate the effect of infill density or raster angle on the specimen’s UTS, yield strength, or elastic modulus for this paper.

“The tensile properties of PLA 3D printed specimens were successfully extracted from the plotted stress-strain graphs upon tensile tests. From the obtained experimental data, the optimum parameter combination with the maximum mechanical property was determined. However, these research results are not adequate for 3D printer user to explore the options they have based on their specific need,” the researchers concluded. “Therefore, this scope of research must be extended by including more ranges of parameter values to be investigated.”

Co-authors of the paper are S. R. SubramaniamM. SamykanoS. K. SelvamaniW. K. NguiK. KadirgamaK. Sudhakar, and M. S. Idris.

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