Foam Injection molding with physical blowing agents

Abstract

Foam injection molding uses environmental friendly blowing agents under high pressure and temperature to produce parts having a cellular core and a compact solid skin (the so-called “structural foam”). The addition of a supercritical gas reduces the part weight and at the same time improves some physical properties of the material through the promotion of a faster crystallization; it also leads to the reduction of both the viscosity and the glass transition temperature of the polymer melt, which therefore can be injection molded adopting lower temperatures and pressures. In this work, the effect of the addition of a blowing agent within a polymeric matrix and the influence of the process conditions on the rheology of the melt, on the physical and mechanical properties and on the morphology of the final product was analyzed. Several polymeric materials were adopted in this work: two thermoplastic polymers commonly used for conventional injection molding and previously well characterized, namely a semi-crystalline polypropylene and an amorphous polystyrene, and two grades of a biodegradable polymer, polylactic acid (PLA). With particular reference to biodegradable polymers, the utilization of the foam injection molding process with physical blowing agents seems the ideal solution to problems of moldability caused by the high viscosity and operative condition very close to those of degradation for this class of materials. Before the foam injection molding, the PLA was foamed by means of a batch foaming system. In particular, the effect of foaming temperature, solubilization time and cooling rate on the morphology of the samples and on their density was analyzed. Several foam injection molding experiments were carried out by using cavities with two different thicknesses and under different experimental conditions. Rheological measurements of the polymer/gas solutions were also obtained by means of a modified nozzle with a slit rheometer with pressure transducers which allow to obtain on-line viscosity measurements. Rheological measurements conducted on the polymer-gas mixtures, showed a significant reduction in viscosity. Furthermore, reduction in density of the foamed samples compared to the unfoamed ones varies with increasing amount of gas injected and increases with increasing injection flow rate, reaching values higher than 40% for polystyrene and of almost 50% in the case of PLA. The analysis of the mechanical properties for both materials showed that the values of Young's modulus were lower than that of the molded part without gas. However, the reduction in Young’s modulus of the foamed parts compared to the Young’s modulus of the unfoamed ones is almost entirely compensated by the reduction in density. On increasing the amount of gas, the morphology of the samples becomes more homogeneous, with an increasing void percentage and smaller bubbles radius. However, there seems to be an optimal physical blowing agent content that leads to the best microcellular structure and the maximum density reduction and mechanical properties. Finally, a study of the effect of gas on the crystallinity of the PLA was carried out by Differential Scanning Calorimetry (DSC) and Wide Angle X-ray Scattering. Results shown a higher cristallinity of the foamed core with respect to the compact skin and the unfoamed part. This is an aspect of considerable importance for biodegradable polymers, for which the crystallinity has a marked effect on properties. [edited by Author]