Effect of infill density on mechanical and fire properties of polylactic acid composites produced by FDM 3D-printing technology

Abstract: 3D-printing is a new and upcoming manufacturing technique that can significantly reduce time and material losses in production. Fused deposition modeling (FDM) is one of the most commonly used 3D-printing methods for processing conventional thermoplastic polymers. To reduce the printing time and usage of material via FDM technology, a user typically specifies infill density. Therefore, it is important to understand how this printing parameter affects the fire and mechanical properties of the 3D-printed object.  This study aims to investigate the effect of various infill densities on mechanical and fire properties of polylactic acid (PLA) composites produced by FDM 3D-printing technology. PLA composites of five different infill densities were 3D-printed: 20%, 40%, 60%, 80% and 100%. The samples for all tests were designed in AutoCAD and then imported into the slicing software, Ultimaker Cura. The 3D-printer used for printing was the Ultimaker S3 which uses FDM technology. To test the fire and mechanical behavior of 3D-printed PLA composites three tests were conducted: cone calorimeter test, tensile test and UL-94 flammability test. The cone calorimeter testing was done using the incident radiation of 35 kW/m2. The results showed that the trend of HHR curves of all infill densities are akin to each other, though the peak heat release rate and total heat released increases with higher infill density. Time to ignition was also longer for samples with higher infill density. Tensile testing was conducted according to the ASTM D638 standard. The results showed that with increasing infill density mechanical properties improved, with 100% infill density having the highest tensile strength (58.15 MPa) and elastic modulus (1472.1 MPa). From the UL-94 test results no difference in flammability could be observed. Every sample had no rating, which implies that PLA specimens of all infill densities are very flammable, with long afterflame and heavy flammable dripping. The study concludes that among the examined infill densities, no ideal percentage of infill density could be found. Requirements based on application will determine what infill density is most appropriate. Nevertheless, the data collected can hopefully provide a useful reference in designing and manufacturing 3D-printed PLA composites.