Production of foam webs from high-temperature-resistant polysulfones or polyether sulfones

A process for producing foam webs by mixing thermoplastic polymers and from 1 to 10% by weight, based on the thermoplastic polymers, of a volatile blowing agent at above 300° C. under pressure and extruding the mixture into the open atmosphere, which comprises using as a blowing agent, water or a mixture of water with up to 200 parts by weight, based on 100 parts by weight of water, of an inert gas or organic liquid as ancillary blowing agent, wherein the thermoplastic polymers are a mixture of a high-molecular weight and a low-molecular weight polysulfone or polyether sulfone and wherein the flowability MVR 360° C./10 kg to ISO 1133 of the high-molecular weight polysulfone or polyether sulfone is below 75 cm3/10 min and the flowability of the low-molecular weight polysulfone or polyether sulfone is above 80 cm3/10 min.