Improving Spiderline Weld Quality with Double Compression

Overcoming Spiderline Weakness

The quality of the weld line formed by the spider legs in a die is crucial for pipe integrity. While melt deformation improves welding, achieving sufficient deformation evenly across the melt stream can be challenging, particularly for larger pipes. Merely increasing pressure or residence time, or using dual spider legs, presents limitations and drawbacks.

The Double Compression Approach

A more effective solution to improve spiderline weld quality is implementing double compression in the die design. Instead of compressing the melt only once, this approach involves squeezing the melt in two distinct steps, allowing the melt to relax between compressions. This method significantly enhances weld quality compared to single compression or dual spider leg designs.

Benefits of Double Compression

Double compression tooling provides a wide processing window and ensures melt stability across various formulations. It leads to improved spider line welding and a smoother inner bore, especially at higher output levels. This design can roughly double the potential output and diameter range achievable with a single die. Furthermore, it allows for a short land length at the die exit, which improves wall distribution and facilitates Quick Tool Change (QTC). The reduced reliance on high single-stage compression also lowers the risk of burning, even when using formulations with higher melting point external lubricants. The first compression gap ensures the minimum required back pressure for the extruder, particularly for larger pipe sizes. This technology is also beneficial for PO resins, which have a longer relaxation time than PVC, by ensuring sufficient compression for good weld quality.

Technical Points:

• Method: Two-step melt compression with intermediate relaxation
• Advantage: Improved spiderline weld quality over single compression/dual legs
• Processing: Wide window, melt stability
• Output/Range: Roughly doubles potential
• Die Exit: Short land length improves wall distribution, eases QTC
• Reduced Risks: Burning
• Applicability: PVC and PO

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