Coextrusion is the process of forming extrudate composed of more than one thermoplastic polymer. The process came about to address the needs of packaging applications where a single thermoplastic polymer could not satisfy the application requirements. Coextruded packaging may be very complex structures comprised of many different thermoplastic layers, often including tie layers whose purpose is to bond the adjacent layers that have limited compatibility. Each thermoplastic polymer is melted in a separate extruder and combined in such a way to that the individual resin layers retain their identity and properties.