Novel injection molding foaming approaches using gas-laden pellets

A novel method of producing injection molded parts with a foamed micro structure has been developed. It has been named supercritical fluid-laden pellet injection molding foaming technology (SIFT). Compared with conventional microcellular injection molding (MIM) tech-nologies, SIFT lowers the equipment costs significantly without sacrificing the production rate, making it a very promising candidate for mass producing foamed injection molded parts. In this study, both nitrogen (N2) and carbon dioxide (CO2) were verified by experiments to be suitable for this process as physical blowing agents. A novel foam injection molding approach using CO2+N2 co-blowing agents was proposed in this study. It has been discovered that when CO2 and N2 were introduced together as co-blowing agents at appropriate ratios, foamed parts with much finer morphology can be produced using this approach compared with using either blowing agent alone. It was first realized by combining the SIFT gas-laden pellets with the microcellular injection molding (MIM), such that N2 and CO2 can be introduced to the foaming process independently in two steps with precise dosage control. This approach has been implemented on several types of commodity polymers with equal success including low density polyethylene (LDPE), poly¬propylene (PP), high impact polystyrene (HIPS), and thermoplastic polyurethane (TPU) with two distinct mold geometries, and all lead to extraordinarily fine cell structures and/or high foam expansion ratio. To enhance the performance of the SIFT technology for future stringent industrial applications, several improvements were made. Methods to further enhance the cell nucleation rate were investigated. For the purpose of prolonging the gas-laden pellets shelf life, low molecular weight (LMW) additives were compounded with the polymer matrix. The experimental results indicate that LMW additives can effectively reduce the gas desorption rate from the pellets and lead to a longer shelf. It was found that at certain blend compositions, the ductility of the molded samples can be dramatically improved while achieving significant weight reduction. This novel ductility enhancing mechanism was investigated in this study. The analysis indicates that to achieve improved ductility, it is important to create a microcellular structure with a sub-micron scale immiscible secondary phase.