Rapid rotational foam molding of integral skin polypropylene cellular composites

Abstract

Rapid Rotational Foam Molding (RRFM) is a novel patent-pending process that was designed and developed to maximize the synergistic effects resulting from the deliberate combination of extrusion and rotational foam molding and thereby serve as a time-andenergy efficient technology for the manufacture of integral-skin rotationally molded foams of high quality. This thesis presents a thorough study of the scientific and engineering aspects related to the evolution of the RRFM process and its feasibility. This innovative processing technology was assessed and verified through a battery of planned experimental trials conducted utilizing an in-house custom-built industrial-grade lab-scale experimental setup. The experimental trials involved a variety of polypropylene (PP)- based foamable formulations with a chemical blowing agent (CBA) that were compounded and processed by utilizing an extruder and then foamed and injected as a foamed core, instantly, into the cavity of a suitable non-chilled rotationally molded hollow shell made of non-foamed pulverized PP grades. The investigated mold shapes included a cylindrical shaped mold and a rectangular flat shaped mold. The obtained moldings were examined for the quality of the skin surface, the skin-foam interface, and the achieved foam morphologies that were characterized in terms of foam density, average cell size, and average cell density. Optimal processing parameters were successfully determined for three different PP skin-foam formulation combinations. The accomplished reduction in processing time and energy consumption by implementing RRFM were substantial. A variety of processing impediments that hindered the efficiency of the single-charge conventional rotational foam molding practice were resolved by implementing RRFM; these include: the foam/skin invasion into the skin/foam layer of the manufactured article and the premature decomposition of CBA during compounding or subsequent rotational foam molding processing steps.