In this study, coreback foam injection molding experiments were thoroughly conducted to seek appropriate conditions to expand polylactide (PLA) at the maximum level and form a higher open cell structure. The experiments demonstrated that two ways of cell opening existed in the foam injection process. One way occurs at high temperatures, at which the number of nucleated cells is low, but the cell growth rate is high due to the lower melt viscosity of PLA. Lower melt viscosity accelerates the cell growth rate and makes bubbles coalesce with each other more easily. The foams tend to have either large bubbles or a hollow structure. Coalescence also occurs when the number of cells is increased, the cell size is decreased, and the cell wall becomes thinner. As the number of nucleated cells increases, the cell wall becomes so thin that the cells can easily coalesce or form a fibrillated structure. By manipulating the foaming temperature and the expansion ratio in coreback operation, threefold expansion foams with 67% porosity and 60% open cell content were produced without adding any nucleating agents or rheological reinforcement additives.

In this study, to reduce the slippage of Pulp-Injection-Molding (PIM) materials during flow, we analyzed flow behaviors by the dynamic visualization of changes in the surface roughness of molds and materials inserted for in-mold-decoration, e.g., paper, cotton cloth, and leather inserts. In the dynamic observation, we confirmed that in the case of materials with high water content ratio, the flow-front shape during the flow process becomes stable due to constraint by large flow resistance from the embossed surface. We studied several emboss shapes and all degrees of embossed surface roughness were found to be very effective for stabilizing the flow-front behavior. Especially in the case of non-embossed mold surface, the changes in the flow-front shape became unstable near the gate. We also clarified that for embossed mold surfaces, the material tends to be constrained, which often results in rotating flow of the PIM material from the non-embossed side towards the embossed side surfaces. The PIM material was also seen to slip on the non-embossed surface. When the in-mold-decoration molding of paper insert was carried out on PIM materials, surface roughness was found to improve dramatically. Cross-sectional observation of the molded samples confirmed that because water evaporation did not occur on the coated side of the paper insert during the drying process, there were no vent holes on molded surfaces. Dynamic observation also confirmed the constraint of 7 material flow on the inserted paper surface, resulting in the generation of continuous rotating flow towards the paper insert side, which contributed to the stabilization of the flow-front material behaviors.