Comparisons of microcellular polylactic acid parts injection molded with supercritical nitrogen and expandable thermoplastic microspheres: Surface roughness, tensile properties, and morphology

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Abstract
Microcellular injection molding is capable of fabricating light weight, dimensionally stable plastic parts while using less material and energy. Two kinds of blowing agents, namely, supercritical nitrogen and expandable thermoplastic microspheres, were employed to produce foamed polylactic acid parts. The surface characteristics were evaluated with a two-dimensional surface roughness analyzer and a three-dimensional white-light interferometer surface profiler. Through surface roughness comparisons, injection molded American Society for Testing Material tensile test bars with expandable thermoplastic microspheres exhibited better surface quality than their supercritical nitrogen counterparts. The tensile properties of injection molded polylactic acid tensile bars with nitrogen and expandable thermoplastic microspheres at various weight concentrations were investigated. The results showed that the polylactic acid/nitrogen parts possessed a better Young’s modulus and tensile strength. The microstructure on the fractured cross-sectional surfaces was characterized using a scanning electron microscope. As reflected by the testing results, the cell microstructure – such as cell size and cell density, and multi-layered structure with a foamed core sandwiched by skin layers – played an important role in the surface quality and mechanical properties. In addition, while an appropriate expandable thermoplastic microspheres content had a positive effect on the cell microstructure and weight reduction, too high of a concentration of expandable thermoplastic microspheres adversely affected the tensile properties and surface roughness of the microcellular polylactic acid tensile test bars.

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