For single wall PVC pipe, a spider die head is commonly employed. This design features spider legs that secure the mandrel to the outer cone, enabling a central, symmetrical feed of the melt at the die's center, which is advantageous for round pipe production. Spider dies are favored for PVC pipes due to their short residence time compared to spiral mandrel or coat hanger dies. This short residence time is increasingly important for high-grade Polyolefin (PO) materials like HDPE100 and low MFI PP grades, where spider dies are gaining popularity because they result in low back pressure, shear, and beard grow.
A historical challenge with spider dies is the quality of the weld where the melt sectors, separated by the spider legs, recombine. Early thinking suggested high pressure and long residence time improved welding. However, observation showed small pipes with thin walls had good spiderlines despite short residence times, while large pipes with thick walls suffered. The key difference was the greater deformation the melt underwent in the die for small pipes. This deformation energizes molecules to cross the split and also reduces melt roughness.
These insights lead to significant design consequences: a long land length to create pressure is superfluous, resulting in lower die pressure, easier extrusion, and simpler tool exchange for Quick Tool Change (QTC). While dual spider legs are a possible solution, going halfway through the cross-section, they have drawbacks like potential tears in the socket outside and higher formulation costs due to increased die inventory needing more heat stabilizer.