Process for producing pre-foamed particles of polypropylene resin

Claim 1

A process for producing pre-foamed particles of a polypropylene resin which comprises expanding original pre-foamed particles of a polypropylene resin, said original pre-foamed particles having the following relation 2<E.sup.1/3 ×n.sup.1/2 <45 wherein E is the expansion ratio of the original pre-foamed particles, and n is the number of cells per mm 2 of the cross section of the original pre-foamed particles, by (1) including an inorganic gas, a volatile blowing agent, or a mixture of these into the original pre-foamed particles so that the internal pressure reaches 1.5 to 10 kg/cm 2 (abs), and expanding ability is imparted to the original pre-foamed particles and then heating them with a heated gas to a temperature of from the heat distortion temperature of the base resin of the pre-foamed particles, which is from 80° to 130° C., to a temperature 15° C. higher than the melting point of the base resin of the pre-foamed particles of (2) heating the original pre-foamed particles with steam to a temperature of from the heat distortion temperature of the base resin of the particles which is from 80° to 130° C. to a temperature 15° C. higher than the melting point of the base resin of the pre-foamed particles with or without imparting expanding ability thereto by including an inorganic gas, a volatile blowing agent, or a mixture of these into the original pre-foamed particles so that the internal pressure reaches 1.5 to 10 kg/cm 2 (abs), whereby pre-foamed particles having an expansion ratio higher than the original expansion ratio E are obtained.