Continuous method for forming thermoplastic slab stock

A continuous method for forming void free thermoplastic polymer slabs which comprises:

• a. at a first station placing molten polymer composition, sufficient to provide a formed article of predetermined dimensions, into an open mold cavity defined by a portion of a smooth, continuous belt capable of transmitting heat and by a deformable heat-insulating body which rests on said belt and which defines the perimeter of said article, is essentially rigid when not subjected to force, is capable of flow under molding pressure and is provided with means for venting trapped air;
• b. advancing said belt in stepwise fashion to successively move empty molds first to said first station for filling with polymer and thereafter to a multiplicity of heat exchange and compression stations;
• c. in the first of said heat exchange and compression stations supporting said mold on a lower rigid planar surface at a temperature within 50.degree. C. above the melting point of said polymer, and gradually forcing into contact with said polymer and said deformable body an upper rigid planar surface, heated to a temperature in said range and substantially parallel to said lower surface, while permitting trapped air to escape from the mold;
• d. maintaining sufficient force on said rigid planar surfaces to maintain unsolidified polymer in said mold cavity under compression and thereafter lifting said upper surface;
• e. in subsequent stations, supporting said belt on a lower rigid planar surface maintained at a temperature substantially below the freezing point of said polymer and maintained under compression by an upper rigid planar surface which is also maintained at temperature substantially below the freezing point of said polymer;
• f. cooling said molten polymer through at least one of said rigid planar surfaces while under molding pressure for a time sufficient to result in complete solidification of the polymer, a portion adjacent said heat-insulating body remaining relatively soft and deformable so long as any substantial portion of the total polymer mass is still molten;
• and
• g. removing the cooled molded article, sufficiently solidified to maintain its shape, from the mold after it leaves the last of said cooling stations.