A study of strain-induced nucleation in thermoplastic foam extrusion

The conditions that induce the phase separation and the bubble nucleation for the thermoplastic foam extrusion process in which physical foaming agents (PFA) are involved are obviously linked to the solubility parameters: temperature, PFA content, and pressure. However, it has been reported that flow or shear can significantly modify these degassing conditions. An in-line detection method based on ultrasonic sensors was used to investigate the influence of the flow on the foaming conditions of polystyrene/HFC134a mixtures, for PS resins of various melt flow rates. An increase of the apparent degassing pressure at low melt temperature was observed for high viscosity resins. Deviations from solubility data have been attributed to the combined effects of elongational and shear stresses.