Search

Plastic pellets contain a certain percentage of moisture. During injection molding, the moisture appears as silver streaks on the molded part surface. Therefore, drying operation before molding is an indispensable requirement. This is not only time-consuming but also requires additional energy consumption and cost. This study investigates whether silver streak can be minimized and improved by using gas counter pressure (GCP) to restrict the fountain flow effect in the melt filling process. Effect of GCP values on the silver-streak appearance and the associated surface gross were investigated. The higher the GCP pressure used, the less the silver streaks appeared. When the used GCP pressure reached 50 bar, the silver streaks were no longer seen on the surface, regardless of the moisture content. Moreover, a new monitoring method based on the integral value of the pressure was also built up to assist in plasticizing parameter optimization. Molded parts with a pressure integral value higher than a critical value were no longer troubled by sliver streaks from moisture. This can be a useful way to optimize process parameters when molding moisture contained plastic material.

Microcellular injection molding technology (MuCell) using supercritical fluid (SCF) as a foaming agent is one of the important green molding solutions for reducing the part weight, saving cycle time, and molding energy, and improving dimensional stability. In view of the environmental issues, the successful application of MuCell is becoming increasingly important. However, the molding process encounters difficulties including the sliver flow marks on the surface and unstable mechanical properties that are caused by the uneven foaming cell sizes within the part. In our previous studies, gas counter-pressure combined with dynamic molding temperature control was observed to be an effective and promising way of improving product quality. In this study, we extend this concept by incorporating additional parameters, such as gas pressure holding time and release time, and taking the mold cooling speed into account to form a P(pressure)-T(temperature) path in the SCF PT diagram. This study demonstrates the successful control of foaming cell size and uniformity in size distribution in microcellular injection molding of polystyrene (PS). A preliminary study in the molding of elastomer thermoplastic polyurethanes (TPU) using the P-T path also shows promising results.