Extrusion is an important and manufacturing process which is widely used to create a number of everyday items. Manufacturing companies employ extrusion to make products with a consistent cross-section. Typical products are pipes, profiles for window frames or door seals, sheets boards for construction and decking, films and wraps from which bags can be made.
The process of extrusion requires materials to be extruded through a die to create the final product. During the plastics extrusion process, raw thermoplastic material – also known as resin- is gravity fed from a top-mounted hopper into the barrel of an extruder. The resin is melted and continually extruded through a die by a screw in order to form shaped product. Additives, such as colourants or UV inhibitors, can be introduced into the material.
The full extrusion process:
Resin is gravity fed from a top-mounted hopper into the barrel of an extruder. The process is similar to plastics injection moulding but fundamentally different as extrusion is a continual process. As the material enters the feed it comes in contact with the screw. The rotating screw forces the plastic resin to move forward into the heated barrel. In most processes, a heating profile is set for the barrel utilizing three or more independent PID (proportional-integral-derivative controller) controlled heat zones. These heat zones gradually increase the temperature of the barrel from the rear where the resin has entered to the front. This allows the plastic resin to melt gradually as it is pushed through the barrel and lowers the risk of overheating which may cause degradation in the polymer.
At the front of the barrel, the resin leaves the screw and travels through a reinforced screen to remove contaminants. This screen is generally reinforced by a breaker plate due to the high levels of pressure experienced.
After passing through the breaker plate resin enters the die. The die is what gives the final product its profile or shape and is designed in such a way that the molten plastic flows evenly from a cylindrical profile to the product’s profile shape. An uneven flow would result in a product with unwanted stresses at certain points in the profile. These stresses can cause warping upon cooling.
Thereafter the product is cooled, often by pulling the extruded product through a water bath.
The process of extrusion moulding can be used to manufacture products such as sheets, films, pipes, wire covering and rods. The extrusion process can create almost any shape imaginable, as long as it is a continuous profile.
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