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.

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PET materials contain small concentrations of monomers and low molecular-weight oligomers formed during the condensation polymerization of ethylene glycol and terephthalic acid as byproducts. All those low molecular weight oligomers can migrate to the surface if the film is treated at high temperatures for tens of minutes. The presence of these oligomers on the surface can interfere with the manufacturing process, so it is important to reduce the concentration of oligomers for improving properties of PET films. In this study, we first confirmed that the principal component of the crystalline deposits on the film surface after the thermal treatment was cyclic trimer. To remove this component from PET resins, extraction experiments of PET resins with supercritical carbon dioxide (sc-CO₂) were carried out using a semi-batch apparatus under various temperatures and extraction times. As a result, it was found that the cyclic trimer in the PET residue was partially removed by this technique for long processing times (2-8h). The decrease in cyclic trimer content in the PET resins was approximately 10% of that in untreated PET resins even if the crystallinity of PET resins was changed. This finding indicates that the production of low-oligomer-content PET resins was realized regardless of the crystallinity of starting materials. It was also found that this technique may be applicable to the polyester film industry.

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