Each method is intended to create a different kind of plastic shape. Molding, for example, is the process best suited for the creation of complex, discrete products.
Examples of discrete products include fasteners, pallets and cases for small electronics. Extrusion, on the other hand, allows for the creation of complex, continuous products. Examples of continuous products are trim, hoses and tubes.
Industry and commerce require broad access to discrete plastic shapes as well as plastic shapes produced by continuous manufacturing processes like extrusion. Extruded plastic products like PVC pipes, plastic hoses, plastic tubes, trim, channels and coextrusions are used extensively in all kinds of applications.
Plumbing, gardening, chemical processing, building construction and furnishing depend heavily on extruded plastics. A wide variety of plastic materials can be extruded including high and low density polyethylene, butyrate, vinyl, polypropylene, polystyrene and acrylic materials. The extrusion process involves heating and pressurizing these raw materials and then shaping them with a die into usable products.
The plastic extrusion process takes place in a few steps. First, the raw thermoplastic material, in flake or pellet form, is poured into a hopper. From there it is fed down into a conveyance channel where it is heated by the turning of a large shearing screw.
The friction caused by the turning of the screw combined with heat from electric heating elements causes the plastic to become molten by the time it reaches the end of the channel.
At the end of the channel is a die, which is a tool used to shape raw materials into useful products. In the case of plastic extrusion, the die is a specially-cut shape in a metal plate through which the molten plastic passes.
Once the plastic has become molten, it is forced through the die and emerges on the other side as a newly extruded plastic shape. Once extruded, the shape is allowed to cool and harden. It can then be cut to the appropriate length and either sent for shipment or prepared for additional processing like painting or labeling.