Extrusion dies for forming dual wall corrugated plastic pipe and dual wall
plastic pipe having a foam annular core that provides homogenous melt
flow to each axially displaced orifice by means of an individual
concentric spiral mandrel are disclosed. Spiral mandrels transport the
melt from the extruders initially in spiral paths and then to spiral
paths having diminishing depths so that the melt from adjacent spiral
paths mix axially resulting in an annular path of homogenous melt.
Extrusion dies for real time, continuous and remote control of the
openings of the axially displaced orifices by moving the die lip closest
and furthest from the extruder in a precise manner are also disclosed. A
means for controlling separation of the die lips for both the inner and
outer wall walls of the pipe gives the pipe manufacturer the benefit of
better controlling quality in real time and changing grades on the fly by
controlling wall thickness.