Multi-staged foaming of thermoplastic resin

Claim 1

In a method of preparing a thermoplastic foam article wherein a heat foamable thermoplastic preform is heated to a temperature at or above the decomposition temperature of a blowing agent compounded in the preform and foamed, the improvement wherein the foamable preform is heated and foamed in a series of foaming stages, said series comprising at least one stress-relieving foaming stage in which the preform is partially foamed at a temperature equal to or above the decomposition temperature of the blowing agent (T d ) but below the temperature at which distortion and tearing occurs, and at least one additional foaming stage which is conducted subsequent to said stress-relieving foaming stage and at a temperature higher than that of said stress-relieving foaming stage, and in which foaming is substantially completed at a foaming rate faster than the rate of foaming during said prior stress-relieving foaming stage.