Abstract
In microcellular injection molds molded using cavities with an obstacle pin, a non-foamed area is formed from the back of the obstacle pin to the weld-line area, and on both sides of this non-foamed area, a complicated structure consisting of minute bubbles layers and large bubbles layers is formed. In this study, attempts were made to clarify the development process of foamed structures by the cross-sectional observation of high impact polystyrene samples, and by the visualization of the cavity filling process of general purpose polystyrene using a glass-inserted mold. Based on the above experimental analyses, we proposed the following model on the development process of the foamed structure around the obstacle pin. First, a triangular non-foamed area was formed behind the pin. Next, the area where bubbles swarm continued flowing downstream while the resident non-foamed melt flowed out was gradually cooled and gained high viscosity. This caused the flow line of large bubbles from the pin circumference to the triangular non-foamed melt residence area to shift toward to the outer side of the non-foamed area. Large bubbles moving from the upstream cut across the swarm area of minute bubbles. As a result, a specific complicated sandwich structure consisting of the minute and large bubbles layers was formed on both sides of the non-foamed weld-line area.
