Extruders typically consist of one or two motor-driven screws rotating inside a heated barrel. One screw is the standard design for most applications, while two screws are used for powder raw materials or compounding processes. The design of the screws follows the key requirements of the process, such as output rate, melt quality, and the raw materials used.
The molten strand as produced by the extruder normally has a circular shape. The extrusion die element is used to re-shape this simple flow into a more complex geometry corresponding with the design of the final product.
The molten profile leaving the die orifice already has a shape similar to that of the final product. However, in some instances of higher wall thicknesses and difficult to maintain shapes, such as cables, pipes, and profiles, the molten profile must be fed into a pre-cooling chamber.  
Direct contact with water-cooled metal walls or disks enables the creation of a solid skin layer strong enough to introduce the necessary haul-off forces and fix the final shape