In this paper, the effect of saturation pressure on the time-temperature equivalent law of the decompression rate (decompression time) and foaming temperature of the cell density, the number of cells per unit volume remaining in foamed plastic was discussed. The foaming was carried out in the method described be by using batch foaming process. The blowing agent was soaked into the resin as a solid state at various high saturation pressures under temperatures higher than the glass transition temperature of the resin. After foaming agent reached its saturation state, cell nucleation and cell growth were accelerated by decompression. Finally, cell growth was halted by cooling. The polystyrene (PS) specimens were foamed under the various saturation pressures, foaming temperatures and decompression rates. The following results were obtained. (1) Cell density of foamed PS shows time and temperature dependence as follows. The cell density increases when the decompression rate is quick, i.e. the decompression time is shortened at the condition of low foaming temperature, and cell density decreases when the decompression rate is slow, i.e. decompression time is lengthened at the condition of high foaming temperature under various saturation pressures. (2) The time-temperature equivalent law is maintained between the time dependence and temperature dependence of the cell density of foamed PS, and it can expressed with the same time-temperature shift factor if the decompression rate is the same even if saturation pressure changes.
