Effects of Mechanical Vibration on Cell Density and Cell Morphology in the Dynamic Microcellular Foaming Process

Abstract

During the microcellular foaming extrusion process, the cell density in the foam could be increased by shear effects due to improved shear energy, which contributes to the free energy for bubble nucleation. However, the cells might also be deformed when the shear stress is high. In order to increase the cell density without destroying the cell structure, mechanical vibration induced by electromagnetic field is introduced into the foaming process. A novel electromagnetic dynamic foaming simulator has been designed to investigate the effects of vibration amplitude and frequency on cell density and cell morphology of foam samples during the dynamic foaming process for polystyrene (PS) microcellular plastics when using supercritical carbon dioxide (ScCO2)as a blowing agent. Optimal foaming temperature, gas mixing time, and appropriate shear rate are also investigated when using the dynamic foaming simulator. It was found that excessive shear effects in the stable foaming process will stretch the cells in the direction of shear stress and decrease the cell density. However, when the vibration is introduced into the foaming process at optimal foaming temperature and appropriate gas mixing time, the cell diameter can be decreased and cell density can be increased at a lower shear rate, while the cells can maintain the spherical shape, since the total shear energy is increased, but the shear stress in the direction of the melt flow is not sharply increased.