Method of extruding a foamed thermoplastic product

Claim 1

METHOD OF PRODUCING A FOAMED THERMOPLASTIC PRODUCT FROM A POLYMER OF A COMPOUND FROM THE CLASS CONSISTING OF MONOVINYL AROMATIC COMPOUNDS AND THEIR HALOGENATED DERIVATIVES, COMPRISING, HEAT PLASTICIZING AND MIXING SAID POLYMER AND A CELL-SIZE CONTROL ADDITIVE UNDR PRESSURE IN THE ABSENCE OF BLOWING AGENTS TO FORM A HEAT PLASTICIZED MIXTURE THEREOF; FEEDING SAID MIXTURE UNDER PRESSURE TO AN INJECTION ZONE; DECOMPRESSING SAID MIXTURE AND INJECTING A LOW BOILING NORMALLY LIQUID SUBSTANCE, BOILING BETWEEN 80*F. AND 200*F. INTO SAID MIXTURE; THEREAFTERR INCREASING THE PRESSURE AND HEATING SAID MIXTURE WHILE SUBJECTING IT TO A FORWARDING MIXING ACTION; THEREAFTER SUBJECTING THE MIXTURE TO A FORWARDING MIXING ACTION WHILE COOLING THE MIXTURE TO A TEMPERATURE OF WHICH IT IS FLOWABLE BUT BELOW THE BOILING POINT OF THE LOW BOILING NORMALLY LIQUID SUBSTANCE AT THE PRESSURE OF SAID MIXTURE; THEREAFTER INCREASING THE PRESSURE OF SAID MIXTURE WHILE CONTINUING TO COOL; AND THEREAFTER CONTINUOUSLY EXTRUDING THE MIXTURE INTO A ZONE OF CONSIDERABLY LOWER PRESSURE TO CAUSE THE LOW BOILING NORMALLY LIQUID SUBSTANCE TO VOLATILIZE AND THUS EXPAND SAID MIXTURE.