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What is Extrusion...[continued]
Other special types of extrusion
CoextrusionA coextrusion process combines two or more molten polymer streams from multiple extruders to create distinct layers in the extruded product. For example, in a foam-core pipe, the foamed inner layer is surrounded by two, unfoamed outer layers. Coextrusion is often used to surround regrind or recycled resin with virgin resin. Packaging film often has many layers (up to 9 or more), including a barrier resin layer, a sealant resin layer, and adhesive layers to tie incompatible layers together. Die design is especially critical in coextrusion.
Direct extrusion usually describes a process in which resin, additives, and other ingredients such as fibers or fillers are fed into the extruder, mixed and melted, and sent directly to the forming die (sheet, profile, pipe, etc.), rather than being made into pellets first and then extruded into the end-product in a separate step. Direct extrusion is used for high-volume products like sheet or decking boards.
