Spiral fed multi-layer tubular die

In an extrusion die apparatus for extruding a multiple layer tubular product including an elongated cylindrical mandrel having a forward and a rearward end;

• a plurality of annular die elements mounted co-axially one behind the other on said mandrel and forming contiguous annular thickness control passages between said die elements and said mandrel, said die elements including inner and outer conical surfaces inclined rearwardly along the longitudinal axis of said mandrel at acute angles thereto, said die elements being nested together with said outer conical surfaces facing said inner conical surfaces and forming conical passages therebetween communicating with said annular thickness control passages, and means for delivering a molten thermoplastic material to said conical passages, the improvement wherein each of said die elements has its inner conical surface inclined at substantially the same acute angle with respect to said longitudinal axis of said mandrel and its outer conical surface inclined at an acute angle with respect to said longitudinal axis which is less than that of said inner conical surface;
• said angle of inclination of said inner conical surface being between about 20 and 80 degrees with respect to said longitudinal axis;
• and wherein each of said die elements has a helical groove within its outer conical surface, the depth of said helical groove decreasing as said groove approaches said annular thickness control passages.

An extrusion die apparatus for extruding a multiple layer tubular product comprising, in combination:

• an elongated cylindrical mandrel having a forward and a rearward end;
• a plurality of annular die elements of unitary structure mounted co-axially one behind the other on said mandrel and forming contiguous annular thickness control passages between said die elements and said mandrel, said die elements including inner and outer conical surfaces inclined rearwardly along the longitudinal axis of said mandrel and forming acute angles therewith, said die elements being nested together with said outer conical surfaces facing said inner conical surfaces and forming conical passages therebetween communicating with said annular thickness control passages;
• said annular die elements each including an outer cylindrical portion located behind said outer conical surface, said outer cylindrical portion having a larger diameter than said outer conical surface forming substantially flat annular forward and rearward surfaces extending radially outward from said outer conical surface, said die elements being stacked one behind the other in said nested assembly of said die elements with said substantially flat annular forward surface of one die element mating with said substantially flat annular rearward surface of an adjacent die element,
• said die elements each having the inner conical surface thereof inclined at substantially the same acute angle with respect to said longitudinal axis of said mandrel and being straight and smooth, said angle of inclination of said inner conical surface being between about 20 and 80 degrees with respect to said longitudinal axis;
• said die elements each having the outer conical surface thereof inclined at an acute angle with respect to said longitudinal axis which is less than that of said inner conical surface;
• said die elements each having a helical groove within the outer conical surface thereof, the depth of said helical groove decreasing as said groove approaches said annular thickness control passages,
• a base having a forward end and an outer conical surface which is inclined rearwardly from said forward end of said base at a predetermined angle with respect to said longitudinal axis of said base, said base supporting said mandrel at said forward end thereof with its outer conical surface facing said inner conical surface of an adjacent die element forming a conical passage therebetween which communicates with said annular thickness control passages,
• said base having a helical groove within said outer conical surface of said base;
• a circular inner lip attached to said forward end of said mandrel;
• an annular outer lip supported by said mandrel within said inner lip forming therebetween an annular extrusion orifice, said extrusion orifice communicating with said annular thickness control passages;
• an annular spacer member co-axially mounted around said mandrel between said annular outer lip and an adjacent annular die element, said spacer member having an inner conical surface which is inclined rearwardly at a predetermined angle with respect to said longitudinal axis of said mandrel, said inner conical surface of said spacer member being straight and smooth and facing said outer conical surface of an adjacent die element forming a conical passage therebetween which communicates with said thickness control passages:
• and
• means for delivering a molten thermoplastic material to said conical passages including an inlet port located within said cylindrical portion of each of said die elements and at least one channel communicating between said inlet port and said helical groove on said outer conical surface of said die elements wherein said material is substantially uniformly distributed over said conical surfaces of said die elements, entering said annular thickness control passages to form overlapping tubular layers, said overlapping tubular layers being extruded from the die in the form of a multiple layer tube.