Extrusion is the process of pushing molten plastic under pressure through a shaped die to produce a product with a specific cross-section. Extrusion is used for the production of a wide range of plastic products and can be applied to shorter products cut to length, such as piping, or continuous length products, such as plastic sheeting. Plastic extrusion can be highly automated, resulting in nearly continuous and consistent production.
Standard Plastic Extrusion
At its simplest, plastic extrusion begins with a granulated plastic and ends with a part that has a consistent cross-section. Plastic resin is gravity-fed into a hopper, which is connected to a screw drive. The screw drive has two purposes: to mix the plastic, particularly if multiple plastics are used, and to melt the plastic before it reaches the die. After the plastic moves through the screw drive, it passes through a filter that screens out any solids that remain in the liquid. The screened plastic is forced through the die, then passes into a cooling bath to remove the heat from the extruded plastic. The extrusion can then be cut to length or wound onto a mandrel, depending on the product.
Tubing is extruded in the same manner as solid extrusions, but the die incorporates a pin in the center to create a hollow cross-section. The pin can be connected to a positive pressure source to keep the tubing from collapsing when it enters the cooling bath.
To create thin sheets of material, the plastic is put through one or more sets of rollers after exiting the die to gradually thin the material to the desired size. In some cases, the rollers can be used to mold two or more sheets of different material together to take advantage of the properties of the two materials.
Blown Film Extrusion
In blown film extrusion, the die creates a cylinder of thin film that is inflated via compressed air. Blown film extrusion methods are used to create plastic shopping bags and result in more consistent wall thickness than sheet extrusion, but the total width of the material is limited.
Jacketing extrusion is the process of extruding a plastic as a coating over another product, particularly wire. Plastic insulation can be extruded over a single conductor or a bundle of wires.
Read more: Kinds of Plastic Extrusion | eHow.com