The invention relates to a process for the preparation of thermoplastic molding materials comprising A) from 39 to 99% by weight of at least one thermoplastic polymer, B) from 1 to 60% by weight of a flameproofing component comprising B1) an expandable graphite, and C) from 0 to 60% by weight of further additives, the percentages by weight being based in each case on the total weight of the components A) to C) and together giving 100% by weight, by mixing melts of the components A), B) and, if present, C) in a screw extruder, the screw extruder comprising at least one metering zone, one plasticating zone, one homogenization zone and one discharge zone in this sequence along the conveying direction, wherein the metering of the expandable graphite B1) into the screw extruder is effected at a point behind which—viewed in the conveying direction—the extruder screw comprises substantially no shearing elements, and thermoplastic molding materials which can be prepared by these processes, the use of said thermoplastic molding materials for the production of fibers, films, moldings and foams, and these fibers, films, moldings and foams themselves.