Method for producing sheet material

A method of making a bilaminar sheet material having uniform thickness which comprises:

• (a) extruding a molten thermoplastic material through an annular slotted extrusion means,
• (b) extruding the thermoplastic material through the annular slotted extrusion means such that said molten material forms a generally tubular hollow element having a plurality of linear side wall portions, each of said side wall portions having a constant thickness along its entire length, a number of said linear side wall portions having unequal thicknesses relative to other side wall portions,
• (c) discharging a gas at a slight positive pressure into said element as it is formed whereby said gas fills said element,
• (d) passing said tubular hollow element containing said gas between two juxtaposed parallel cylindrical rollers,
• (e) rotating said rollers in opposite directions so as to draw said tubular hollow element into the nip between the rollers, at least one of said rollers having a plurality of spaced apart recesses on its outer surface, whereby said tubular hollow element is collapsed and flattened into a continuous bilaminar planar sheet element with raised closed pockets containing said gas, the opposed flattened surfaces of said element being autogenously welded to each other in the areas of contact,
• (f) maintaining the temperature of the thermoplastic material around the annular slotted extrusion means by means of a plurality of heater elements positioned around and in close proximity to the annular slot, the plurality of heater elements comprising four heater elements, said four heater elements comprising a first pair of oppositely positioned heaters and a second pair of oppositely positioned heaters around said annular slot, the first pair of heater elements being substantially parallel to the horizontal axis of said juxtaposed parallel rollers and said second pair of heater elements being substantially perpendicular to said horizontal axis of said juxtaposed roller.