Concentric co-extrusion die for extruding a multilayer thermoplastic film

A concentric co-extrusion die system comprising of:

• a co-extrusion die, the die having
• an annular extrusion outlet;
• and
• a plurality of annular or conical extrusion layers, each layer defined by a pair of adjacent annular or conical die mandrels defining between them a flow path for molten thermoplastics material, from a layer inlet to an annular extrusion layer outlet, through which a thermoplastics tubular extrusion is formed in use, extrusion through each of the plurality of annular layer outlets feeding the annular extrusion outlet, thereby forming a multi-layered product, wherein:
• at least one of said extrusion layers has a plurality of molten material inlets arranged spaced apart around the external circumference of the co-extrusion die,
• each molten material inlet being connected to a feed channel which has a plurality of bifurcations providing 2n subsidiary outlet feed channels, with n being the number of bifurcations,
• each subsidiary outlet feed channel is connected to a corresponding helical outlet channel so as to provide a continuous flow path along the subsidiary outlet feed channel and the corresponding helical outlet channel,
• each of the plurality of molten material inlets is connected to a respective group of the helical outlet channels, and
• the molten material inlets arranged around the external circumference of the co-extrusion die are connected to the corresponding feed channels via respective inlet paths passing through a central die block separate from the mandrels and from a main body of the die, and
• a plurality of thermosplastic material extruders for supplying the at least one of said layers with molten material.