HDPE pipe for the duration of extrusion. The HDPE product is coming from the heater, in to the die, then in to the cooling tank. This Acu-Electrical power conduit pipe is co-extruded - black within with a skinny orange jacket, to designate electricity cables. Also often called "gap flanging", hollow https://aluminiumfabrication67889.myparisblog.com/29882715/the-best-side-of-aluminium-extrusion