Size and Print Path Effects on Mechanical Properties of Materials Extrusion 3D Printed Plastics

Optimization of mechanical properties through print conditions of thermoplastics by Material Extrusion (MatEx) has been commonly utilized, but this optimization tends to neglect dependence of properties on the local process history. Here, we systematically investigate effect of part size with standard tensile test specimens (ASTM D638 Type IV or V), print orientation and printing multiplicity on mechanical properties. Both polycarbonate (PC) and polypropylene (PP) were printed to understand crystalline vs. glassy plastics. For both printed PP and PC, elastic modulus was independent of specimen type as well as multiplicity, and weakly dependent on the print orientations. However, failure of PC was strongly dependent on the size of specimen with the smaller Type V exhibiting ductile failure while brittle failure was only observed with the Type IV specimens. Through examination of single orientation prints, the ductility of the Type V specimens is associated with the intralayer cooling time between adjacent extruded roads. In contrast, all of PP specimens exhibited ductile failure. These differences between PP and PC can be attributed to the mechanism of solidification, which changes the requirements for a strong interface as well as the temperature dependence of the viscosity.