An Experimental Study of Water-Assisted Injection Molding of Plastic Tubes with Dimensional Transitions

Abstract

This report was made to experimentally study water assisted injection molding of plastic tubes with dimensional transitions. Experiments were carried out on an 80-ton injection-molding machine equipped with a lab scale water injection system, which included a water pump, a pressure accumulator, a water injection pin, a water tank equipped with a temperature regulator, and a control circuit. The materials used for the experiments included an amorphous polystyrene and a semi-crystalline polypropylene. Two types of water injection pins, the orifice and the porous type, combined with two ``float-shape'' axisymmetric cavities were used to mold the plastic parts. After molding, the hollowed core ratios of molded parts were measured. Various processing variables were studied in terms of their influence on the residual wall thickness and hollowed core ratio of water assisted injection molded parts: melt temperature, mold temperature, water temperature, water pressure, water injection delay time and water hold time, and melt short shot size. The core-out geometries in water-assisted injection molded parts were also compared to those in gas-assisted injection molded products. It was found that the porous type water pin molded plastic tubes had a more uniform thickness distribution than the orifice type water pin and the ring type gas pin. Additionally, the experimental result suggests that the water injection delay time and short shot size were the principal parameters affecting the core out behavior of molded d-2d-d tubes, while the core-out geometries of the d-4d-d tubes were mainly influenced by the melt and mold temperatures, water pressure and water injection delay time, and melt short shot size.