Spider die heads are commonly used for single wall PVC pipe extrusion, fixing the mandrel using spider legs. These legs cut the polymer melt (or rubber-like solid in the case of PVC) into sectors, which must then heal, or weld, together downstream in the die. The quality of this weld, known as the spider line weld quality, is crucial for the final pipe integrity.
Factors Influencing Spider Line Weld Quality
Melt Deformation and Relaxation
Historically, it was thought that high die pressure and long residence time improved welding. However, observation showed that small, thin pipes had good spider lines despite short residence times, while big pipes suffered. The key difference was the amount of deformation the melt underwent from the spider to the die exit. Deformation energizes molecules to cross the split created by the spider legs and reduces melt roughness. For a good weld, the melt needs to remain in the die for a certain residence time expressed in the relative relaxation time. PO resins have a longer relaxation time than PVC.
Die Design and Temperature
A bigger and deeper spider can improve weld quality by creating more melt deformation. However, this increases die inventory and melt velocity near the spider legs, requiring more heat stabilizer and increasing the risk of burning, especially for PVC. Higher melt temperatures generally facilitate better welding by speeding up molecular diffusion and relaxation.
Double Compression
A better solution than larger spiders is using double compression tooling in the die. This applies compression twice, allowing the melt to relax between squeezes. This design increases melt deformation without excessively increasing die inventory or requiring a larger spider, reducing the risk of burning and improving spider line welding. It results in improved spider line welding and a wider processing window.
FAQ Section
Why are spider line welds a concern in pipe extrusion?
Spider legs in the die cut the polymer melt, creating weld lines that need to fully heal to ensure the pipe's structural integrity. If the weld quality is poor, these lines can become weak points, potentially affecting the pipe's performance, such as its resistance to bursting or impact [Source implies this through discussion of optimizing weld quality].
How does melt deformation improve spider line quality?
Melt deformation from the spider legs to the die exit energizes the polymer molecules, encouraging them to bridge the gaps created by the spider legs and effectively weld together. Sufficient deformation, rather than just pressure or time, is crucial for achieving a good spider line weld quality.
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Saurabh Pawar
