Foamed plastic sheet material and system for its manufacture

Claim 1

A FOAMED PLASTIC SHEET MATERIAL OF IMPROVED FLEXIBILITY AND REDUCED BRITTLENESS SUITED FOR USE IN PLACE OF FIBER BOARED MATERIAL FOR BOOK COVER BACKINGS AND THE LIKE CONSISTING ESSENTIALLY OF COMPRESSED POLYVINYL AROMATIC PLASTIC PARTICLES OF CLOSED-CELL STRUCTURE HAVING A DENSITY OF 5-15 POUNDS PER CUBIC FOOT, SAID SHEETING HAVING PLANAR EXPOSED SURFACES, A THICKNESS OF ABOUT 0.50-0.25 INCH AND A CROSS-SECTION OF UNIFORM THICKNESS CONTAINING A LATTICEWORK COMPRISED OF A MULTIPLICITY OF CONTINUOUS, DIAGONALLY EXTENDING AND INTERMESHED AREAS OF CELL-SEPARATING FUSED PARTICLE INTERFACE WINDING THROUGH THE INTERIOR OF THE THIN SHEET WHEREIN THE AREAS OF FUSED PARTICLE INTERFACE EXTEND BETWEEN THE EXPOSED SURFACE OF THE SHEET AT SHARP ACUTE ANGLES THERETO AND INTERSECT TO PROVIDE A HINGE EFFECT AT THEIR POINTS OF INTERSECTION, THE CLOSE AND SEPARATED CELLS BEING ELONGATED ALONG THE PLANE OF THE SHEET AND OF A GENERALLY ELLIPTICAL CONFIGURATION, EACH EXPOSED SURFACE EXHIBITING A SMOOTH, CONTINUOUS UNINTERRUPED SURFACE SKIN OF SAID THERMOPLASTIC MATERIAL, SAID SHEET EXHIBITING A DENSITY GRADIANT ACROSS ITS THICKNESS WITH THE HIGHER DENSITY BEING AT BOTH SAID PLANAR SURFACES.