Cuspated sheet forming

DETAILED DESCRIPTION What is claimed is: 1.
A forming process of manufacturing an unperforated cuspated sheet comprising deforming a sheet of melt spinnable thermoplastic material by pressing against one face of a hot sheet of the material the cold outer tips of a first array of pointed pins and simultaneously pressing against the second face of the material the cold outer tips of a second array of projections so that the arrays interpenetrate in such a manner that the projections are spaced from the pins by a distance greater than the thickness of the sheet, thereby deforming the sheet so as to exhibit a plurality of hollow cusps having tips and side walls, the initial temperature of the hot sheet being in the range between the melting point of the thermoplastic material and the temperature at which the material becomes unworkable, the initial temperature of the tips of the arrays being less than the melting point of the thermoplastic material and the speed of interpenetration of the arrays being in the range above the speed at which the tips of the cusps of the product are punctured and below the speed at which the side walls of the cusps of the product are perforated, and subsequently withdrawing the pins and projections from the sheet.
2.
A process according to claim 1 wherein the tips of the projections on each array interpenetrate by a distance twice the nearest distance between adjacent pins on the first array.
3.
A process according to claim 1 for producing a singly cuspated sheet wherein the projections of the second array are in the form of an apertured grid through the centres of which apertures the pins of the first array penetrate.
4.
A process according to claim 3 wherein the apertured grid is a hexagonal honeycomb and the array of pointed pins is arranged in a triangular lattice.
5.
A process according to claim 1 for producing a doubly cuspated sheet wherein the projections of the second array are pointed pins.
6.
A process according to claim 5 wherein both arrays of pointed pins are arranged in a square lattice and the pointed pins of one array pass through the centres of the unit squares of the second array