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.
Addressing Spider Line Weld Quality
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.
Design Consequences
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.
Technical Points:
Application: Single Wall PVC Pipe (Common)
Mandrel Fixing: Spider Legs
Melt Feed: Central, Symmetrical
Advantages: Short Residence Time, Low Back Pressure/Shear for PO
Weld Quality Factor: Melt Deformation
Modern Design: Short Land Length
FAQ Section
Why are spider dies preferred for PVC pipe extrusion compared to other die types?
Spider dies are commonly used for single wall PVC pipe because they provide a central, symmetrical melt feed and offer a short residence time. Short residence time is beneficial for PVC due to its quick degradation at high temperatures.
What is the main factor determining the quality of the spider line weld in a spider die?
The quality of the spider line weld is primarily determined by the deformation the melt undergoes as it passes through the die after being split by the spider legs. Greater deformation energizes molecules, helping them recombine effectively.
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Saurabh Pawar
