Cuspated sheet forming

A continuous process of deforming a sheet of thermoplastic material by sequentially pressing against one face of a molten sheet of the material at a temperature below its decomposition point the cold tips of projections set in and extending normally from a first series of separately moveable combs and sequentially pressing against the second face of the material the cold tips of projections set in and extending normally from a second series of separately moveable combs so that the projections on the first series of combs interpenetrate with the projections on the second series of combs in such a manner that the projections of the first series are spaced from the projections of the second series by a distance greater than the thickness of the sheet;

• the interpenetrated projections are substantially parallel during the act of interpenetration;
• and the combs of any one series form a block pressing against the sheet of thermoplastic material, wherein the block so formed by the combs of any one series has two straight parallel side, parallel with the sides of the thermoplastic sheet and has non-linear projections running across the block having a median normal to the parallel sides of the block said projections characterized in that an envelope around any one projection overlaps with the envelope around an adjacent projection;
• the combs each contain one or more complete projections;
• and wherein the interpenetrated combs are moved along at the speed of the sheet until the sheet is set and the combs are then removed and recycled and wherein the temperature of the cold tips of the projection is at all times during the interdigitation process less than the softening point of the thermoplastic material.