Production of expanded thermoplastic product

Claim 1

A PROCESS FOR PRODUCING EXPANDED POLYSTYRENE, COMPRISING (1) INTRODUCING POLYSTYRENE AND FOAM NUCLEATING AGENT INTO THE FIRST ZONE OF A SCREW EXTRUDER WHICH HAS A LENGTH-TO-DIAMETER RATIO IN THERANGE FROM 20 TO 36 AND CONTAINS A SINGLE SCREW IN A SINGLE CYLINDER, AT LEAST THE LAST FOUR DIAMETER LENGTHS OF SAID FIRST ZONE BEING THE SOLE METERING SECTION OF SAID EXTRUDER: (2) CONVEYING SAID POLYSTRENE FORWARD IN SAID FIRST ZONE TOWARD A SECOND ZONE OF SAID EXTRUDER BY ROTATION OF SAID SCREW, WHILE HEATING AND COMPRESSING THE POLYSTRENE TO A TEMPERATURE IN THE RANGE FROM 150 TO 300*C. AND A PRESSURE OF AT LEAST ABOUT 1500 P.S.I.; (3) PASSING THE RESULTING PLASTIC MASS INTO SAID SECOND ZONE OF SAID EXTRUDER, SAID SECOND ZONE COMPRISING IN SEQUENCE A MIXING XONE AND A COOLING ZNE AND TERMINATING IN AN EXTRUSION DIE; (4) INJECTING NEAR THE INLET OF SAID SECOND ZONE A CONTINOUSLY FLOWING STREAM OF VOLATILE LIQUID FOAMING AGENT INTO SAID FLOWING PLASTIC MASS; (5) THE PRESSURE OF THE PLASTIC MASS THROUGHOUT SAID SECOND ZONE BEING NO HIGHER THAN THE HIGHEST PRESSURE IN SAID EXTRUDER UPSTREAM FROM THE POINT AT WHICH SAID FOAMING AGENT IS INJECTED; (6) INTIMATELY MIXING SAID FOAMING AGENT AND SAID PLASTIC MASS IN SAID SECOND ZONE AT A TEMPERATURE IN THE RANGE FROM 150 TO 300*C., SAID MIXING COMPRISING REPEATING AT LEAST FOUR TIMES THE STEPS OF DIVIDING THE FORWARD-FLOWING PLASTIC MASS AND ADMIXED FOAMING AGENT INTO A MULTIPLICITY OF SEGREGATED FORWARDFLOWING PORTIONS AND RECOMBINING SAID SEGREGATED PORTIONS, WHEREBY A MIXTURE OF SUBSTANTIALLY UNIFORM COMPOSITION IS FORMED; (7) COOLING THE RESULTING MIXTURE IN SAID COOLING ZONE TO A TEMPERATURE IN THE RANGE FROM 100 TO 160*C., AND (8) EXTRUDING THE COOLED MIXTURE THROUGH SAID DIE INTO A SPACE AT SUBSTANTIALLY LOWER PRESSURE, WHEREBY THE EXTRUDED MASS IS EXPANDED BY VIRTUE OF THE EXPANSION OF SAID FOAMING AGENT.