Thickness control system for an extrusion die

In an extrusion die for continuously producing a sheet of material that has a substantial width in comparison to its thickness, the combination which comprises:

• means forming a slit which extends along the width of said die, said slit having an opening of variable thickness, means continuously forcing said material through said slit to form said sheet, slit opening control means including a plurality of spaced-apart elongated members each having capacity to expand and contract thermally, and each operatively arranged to adjust the slit opening at an individual location to control the sheet thickness at said location, a plurality of individual cooling means each connected and structurally located in heat exchange relationship with one of said elongated members upstream of said slit opening for positively cooling said elongated member, and a plurality of individual heating means also connected and structurally located in heat exchange relationship with each said elongated member to heat each said elongated member, means for individually alternately actuating and deactivating said heating means, said heating means having capacity to overcome the cooling of said cooling means on the elongated member with which they are associated, insulating means interposed between said die and both said heating and cooling means, such that the functions of said elongated members and the remainder of the die are substantially thermally independent of each other.