According to Plastrading.com:
The plastic extrusion process involves melting and forming raw plastic into a continuous profile. It is most commonly used to make items such as pipe/tubing, weatherstripping, fencing, deck railings, window frames, plastic films and sheeting, thermoplastic coatings, and wire insulation.
Although all of the things mention above are created using the extrusion processes most of them are used one of the many types of extrusion processes available. In this two-part post we’ll look at six types of extrusion process and what they entail.
When it comes to extruding tubes for straws or medical tubing the process works the same as normal extrusion up until a certain point. Hollow sections are usually extruded by placing a pin or mandrel inside of the die, and in most cases positive pressure is applied to the internal cavities through the pin.
In the co-extrusion process two or more materials are pressed through the same die into a single piece. When multiple plastics are combined, the result can yield properties distinct from those of a single material which is why manufacturers would use this technique. One of the ways co-extrusion is used is in the vinyl fencing industry, where co-extrusion is used to tailor the layers based on whether they are exposed to the weather or not.
In the extrusion coating process, the blown or cast film method is used to coat an additional layer onto an existing roll stock of paper, foil or film. One use of this process is to improve the characteristics of paper by coating it with polyethylene to make it more resistant to water. The extruded layer can also be used as an adhesive to bring two other materials together. A famous product that uses this technology is Tetrapak.