During the process of extrusion of plastics, raw thermoplastic material in the form of small beads (often called resin in the industry) are gravity fed from a top mounted hopper into a barrel of an extruder. Additives such as colorants and UV inhibitors (in either liquid or pellet form) are often used and can be mixed into the resin prior to arriving at the hopper or added through a coextruder.
The material enters through the feed throat (an opening near the rear of the barrel) and comes into contact with the screw. The rotating screw (normally turning at up to 120 rpm) forces the plastic beads forward into the barrel which is heated to the desired melt temperature of the molten plastic (which can range from 200°C to 325°C depending on the polymer).
In most processes, a heating profile is set for the barrel in which three or more independent PID controlled heater zones gradually increase the temperature of the barrel from the rear (where the plastic enters) to the front. This allows the plastic beads to melt gradually as they are pushed through the barrel and lowers the risk of overheating which may cause degradation in the polymer.
Coextrusion and multi-layer sheet
Coextrusion refers to the simultaneously extrusion of multiple layers of material. This type of extrusion utilizes two or more extruders to melt and deliver a steady volumetric throughput of different molten plastics to a feedblock (blackbox), which combines the materials and transport the material to the diehead, which molds the material into the desired shape. The different layers thickness are controlled by the relative speeds and sizes of the individual extruders delivering the materials.