The extrusion proces is used to push molten viscous material through a mould or die. This proces is used to make profiles from plastics or metals (aluminum) in one continues go or until the material runs out. This limitation is overcome in the plastics industry by continues supply of raw material. For this it is possible the create very long products like hoses and tubes that are only limited in length by the possibility of shipping it. In case of plastics production the raw material is in the form of pellets or granulate which is transported and pressurized by a screw (see image) in side a heated barrel. The molten viscous material is pushed out trough the die and rapidly cooled after that.
The proces gives relatively much form freedom. Profiles with several enclosed compartments or chambers can be processed. Because the technique is relatively cheap, it is commonly used for the production of high numbers of products or partial products.
The accuracy of the proces depends on several factors. When the material exits the die it starts to swell a little bit because of it's elastic behavior. This means that the original form will not hold perfectly. This phenomena is called ""die swell"". Another thing that induces deformation after exiting the tool or die is difference in wall thickness. Non-uniform geometry is for almost all techniques (eg. injection moulding) that use heat to make material moldable a big problem. The material does not cool down evenly so differences in shrinkage will be seen. Quickly cooling the material down will freeze it in it's form but will leave residual stress in the material that could come out in a later stage.