Effect of expandable thermoplastic microspheres on microcellular injection molded polylactic acid (PLA): Microstructure, surface roughness, and tensile properties

Microcellular injection molding is capable of producing light weight, dimensionally stable plastic parts while using less material and energy. Expandable thermoplastic microspheres (ETM) were employed as chemical blowing agents to produce biodegradable polylactic acid (PLA) parts. The surface characteristics of the samples were evaluated with a 2D surface roughness analyzer and a white-light 3D surface profiler (weight-light interferometer). It was found that microcellular injection molded parts with ETM exhibit good surface quality, similar to conventional solid injection molded parts. The tensile properties of injection molded PLA samples with variable ETM weight ratios have been investigated. The microstructure and cell density of the fractured cross-sectional surfaces were characterized by using a scanning electron microscope (SEM) and an image analysis tool. As shown by the testing results, the cell microstructures - such as cell size, cell density, and a layered structure with a foamed core sandwiched by solid skin layers - play an important role in the surface quality and mechanical properties. A smaller cell diameter and a thicker skin layer help to improve the surface smoothness and tensile properties of the microcellular injection molded parts with ETM. In addition, an appropriate microsphere content has a positive effect on cell microstructure and weight reduction, whereas too high a concentration of microspheres adversely affects the tensile properties and surface roughness.