Production of thermoplastic sheet

Claim 1

A PROCESS FOR PRODUCING A THERMOPLASTIC SHEET COMPRISING: A. EXTRUDING A THEMOPLASTIC RESIN IN THE FORM OF A MOLTEN THERMOPLASTIC SHEET; B. PASSING SAID SHEET THROUGH AN AIR GAP SIZED TO PROVIDE MINIMUM COOLING BY AIR, AND THEREAFTER C. PASSING SAID SHEET DOWNWARDLY THROUGH A PRIMARY COOLING ZONE, ENTERING SAID ZONE THROUGH AN EXPOSED UPPER SURFACE AND EXITING SAID ZONE THROUGH A SUBMERGED BOTTOM EXIT, SAID ZONE BEING CHARACTERIZED BY CONTINOUS STREAMS OF COOLING LIQUID ENTERING SAID PRIMARY ZONE ON BOTH SIDES OF SAID SHEET AND DIRECTED TO FLOW PARALLEL TO SAID EXPOSED UPPER SURFACE AND TOWARD SAID ENTERING SHEET, WHEREBY SAID EXPOSED UPPER SURFACE IS MOVING IN LAMINAR FLOW, SAID COOLING STREAM PASSING COCURRENTLY DOWNWARD IN LAMINAR FLOW ADJACENT SAID SHEET ON BOTH SIDES THEREOF AND EXITING SAID PRIMARY ZONE ALONG WITH SAID SHEET; AND THEREAFTER D. PASSING SAID SHEET DOWNWARDLY THROUGH A RELIEF ZONE BELOW SAID PRIMARY ZONE AND CONTIGUOUS THEREWITH AND COOLING SAID SHEET THEREIN; AND THEREAFTER E. PASSING SAID SHEET DOWNWARDLY INTO A SECONDARY COOLING ZONE BELOW SAID RELIEF ZONE OF (D) AND CONTIGUOUS THEREWITH AND FURTHER COOLING SAID SHEET BY CONTERCURRENTLY FLOWING SECONDARY COOLING LIQUID STREAMS FLOWING UPWARDLY ADJACENT BOTH SIDES OF SAID SHEET,SAID SECONDARY COOLING STREAMS ENTERING NEAR THE OUTLET OF SAID SHEET FROM SAID ZONE AND EXITING INTO SAID RELIEF ZONE OF (D) AND THEREAFTER F. WITHDRAWING SAID PRIMARY AND SECONDARY LIQUIDS FROM SAID RELIEF ZONE AT A RATE SUFFICIENT TO MAINTAIN A DESIRED FLUID DEPTH IN SAID PRIMARY ZONE AND TO ESTABLISH THE MINIMUM AIR GAP OF (B); G. WITHDRAWING SAID COOLED SHEET FROM SAID SECONDARY ZONE.