Dissolution Mechanism of Physical Blowing Agent into the Polymer in Low-pressure Physical Foam Injection Molding Process (Part 1) Effect of Molding Conditions on Physical Blowing Agent Concentration

Abstract

Microcellular foam injection molding processes using environmentally benign physical blowing agents, such as nitrogen (N₂) and carbon dioxide (CO₂), are gaining attention as one technology for reducing plastic wastes in the oceans and our daily lives. In physical foam injection molding, the concentration of the blowing agent is critical to the quality of foamed products, but the dissolution mechanism of the blowing agent into polymer remains unclear in any foam injection molding machine, including the low-pressure foam injection molding machine (SOFIT) that we developed recently. This study investigated the dissolution mechanism of CO₂ in polypropylene (PP) in a low-pressure foam injection molding machine using near-infrared spectroscopy. The results revealed that the CO₂ concentration in PP was affected by the screw rotation speed, cooling time, pellet feed rate, back pressure, and barrel temperature. These variables changed the specific surface area of the polymer in the screw zone where the polymer is partially filled (starved), and the pressure gradient in the screw zone where the polymer is filled. The CO₂ concentration was found to vary in association with the increase or decrease in the specific surface area of the polymer in the polymer-starvation zone and the pressure gradient in the polymer-filling zone. It was also found that shear flow due to screw rotation causes surface renewal of the starved polymer and promotes CO₂ dissolution. These findings can lead to determining appropriate molding conditions, screw design improvement, and, finally, optimization of the process based on a numerical model.