Cooling extrudate distortions

Pipe haul-off units form a key part of the extrusion process since they have the function of exerting a steady, uniform pull back through the cooling and calibration zones to supplement the extrusion pressure at the die head. The haul-off unit effectively creates a tensile stress in the solid material that balances the hydrostatic and viscous forces in the liquid state of the extrudate and overcomes the resistance of external friction. Haul-off units may be belt driven or multi-pad caterpillar systems. Large diameter pipes require multiple caterpillar units and thin wall pipe also require more units to avoid distortion. 

When not being used in conjunction with another process, like thermoforming or blow molding, profile extrusions are the variety most often used by product designers. They can be either solid or hollow and, for the most part, are made of thermoplastic materials (although thermoset materials can be extruded with special equipment). Therefore, the effect of nonuniform cooling is precisely the same for extrusion as it is for the molding processes sink and distortion. 

Extrusion. Extrusion is a major technique which is favored for the production of parts having a constant cross-section, such as electrodes. The mix is cooled to just above the softening point (approximately 125°C), then extruded through steel dies, cut to length and rapidly cooled to solidify the pitch before distortion occurs. The resulting shape is known in the industry as a green shape . 

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