Clamping frame for plastic vacuum forming machine

Claim 1

In a vacuum forming machine for molding a thermoplastic sheet about a suitable die and including a vacuum table adapted to support a die thereon and heating means positioned above said table and defining an oven into which the plastic sheet is received for heating prior to forming, the improvement comprising clamping frame means for securely supporting a sheet of plastic during heating and forming comprising a lower rectangular clamping member having a substantially continuous and planar upper clamping face, an upper rectangular clamping member having a substantially continuous lower clamping face and being mounted for movement relative to the lower clamping member between an open position to admit a plastic sheet therebetween and a closed position to grip and retain the peripheral edge of the sheet between said clamping faces, a plurality of parallel, elongate ridges carried by one of said clamping faces and extending longitudinally of and substantially continuously throughout the longitudinal extent of said one of the clamping faces to substantially continuously grip the plastic sheet around its peripheral edge, a coating of heat-absorbent material covering a substantial portion of the exposed surface of said upper clamping member to increase the heat transfer to the plastic sheet through the clamping frame member, means for lifting said clamping frame into close proximity to said heating apparatus such that a supported plastic sheet may be heated, and for lowering said frame into close proximity to said vacuum table such that a supported sheet will contact a die positioned on said vacuum table, and vacuum means operatively associated with said vacuum table for drawing a supported plastic sheet onto a die positioned on said vacuum table.