A die assembly for the pipe extrusion line of thermoplastic pipe from a plastic exhibiting an appreciable amount of memory and thus having an elongated heated portion defined by an elongated outer bushing ring with an inside diameter substantially equal to the nominal outside diameter of the pipe to be produced and an inner mandrel ring having an outside diameter smaller than the inside diameter of the bushing ring and disposed within the bushing ring to form an annular extrusion channel, the essentially straight portion of the extrusion channel defining the land length of the die and the width of the extrusion channel defining the die opening of the die, the improvement comprising a mandrel formed in two sections, including a longer upstream section, fixedly mounted in the die and having a fixed mandrel outside diameter and a downstream mandrel section removably mounted in the die as an extension of the upstream section and replaceable by downstream mandrel sections of equal length but different mandrel outside diameters. The improved die assembly is capable of producing a given diameter pipe having a plurality of different SDR Ratings which closely approximates a predetermined land length to die opening ratio for each of the different SDR ratings, thereby eliminating the necessity of providing a complete new die assembly for each different SDR rating desired.
1. In a die assembly for the pipe extrusion line of thermoplastic pipe from a plastic exhibiting an appreciable amount of memory and thus having an elongated, heated portion defined by an elongated outer bushing ring having an inside diameter approximately equal to the nominal outside diameter of the pipe to be produced and an inner mandrel ring having a length substantially equal to the length of said bushing ring and an outside diameter smaller than said inside diameter of said bushing ring to form an annular extrusion channel therebetween; the improvement, comprising:
(a) said bushing ring being a single bushing ring having an essentially constant inside diameter throughout substantially its entire length constituting the downstream portion thereof; and
(b) said mandrel ring being formed of two sections comprising (1) a longer upstream section fixedly mounted in said die and having an essentially constant outside diameter throughout substantially its entire length constituting the downstream portion thereof, which is opposite the constant inside diameter portion of said bushing ring and (2) at least one replacable, shorter, downstream section removably mounted in said die as an extension of said upstream mandrel section having an essentially constant outside diameter throughout substantially its entire length constituting the downstream portion thereof and differing from said outside diameter of said upstream mandrel section;
(c) the length of the constant inside diameter portion of said bushing ring and thus the constant outside diameter portion of said upstream section of said mandrel ring plus the entire length of said downstream section of said mandrel ring defining the land length of said die; and
(d) the difference between said inside diameter of said bushing ring and said outside diameter of said downstream portion of said mandrel ring defining the die opening of the die.
2. In a die assembly for the pipe extrusion line of thermoplastic pipe from a plastic exhibiting an appreciable amount of memory and thus having an elongated, heated portion defined by an elongated outer bushing ring having an inside diameter approximately equal to the nominal outside diameter of the pipe to be produced and an inner mandrel ring having a length substantially equal to the length of said bushing ring and an outside diameter smaller than said inside diameter of said bushing ring to form an annular extrusion channel therebetween; the improvement, comprising:
(a) said bushing ring being a single bushing ring having an essentially constant inside diameter throughout substantially its entire length constituting the downstream portion thereof; and
(b) said mandrel ring being formed of two sections comprising (1) a longer, upstream section fixedly mounted in said die and having an essentially constant outside diameter throughout substantially its entire length constituting the downstream portion thereof which is opposite the constant inside diameter portion of said bushing ring and (2) at least three alternately replacable, shorter downstream sections removably mounted in said die as extensions of said upstream mandrel section, of equal length and having essentially constant outside diameters throughout substantially their entire length constituting the downstream portions thereof and including (A) a single first section having an outside diameter substantially equal to said outside diameter of said upstream section of said mandrel ring, (B) at least one second section having an outside diameter greater than said outside diameter of said upstream section of said mandrel ring and (C) a least one third section having an outside diameter less than said outside diameter of said upstream section;
