In order to create pressure in the extruder the pitch height of the screw is reduced along the screw from the feeding zone to the melt zone and to the metering zone. For an extruder, the cylinder barrel ends in a filter package and a die to shape the extrudate. This creates a back pressure that helps to melt and homogenize the plastic material. Injection moulding is also based on the Archimedes screw with the added feature to stop the screw and move it axially forward to push molten plastic into a mould in order for it to cool down and take on the shape dictated by the mould cavity.

The shape of the extruder die determines the shape of the extrudate that may be further manipulated by, for instance, an inner air pressure of a tubular extrudate to make plastic film or by pulling of the multifilament that comes out during melt spinning in order to orient the polymers for improved strength and stiffness. Blow moulding is an intermittent technique used to make hollow objects such as bottles that share some elements of the extrusion process. During the intermittent processes where the screw is moved axially in order to push molten plastic out, there are possibilities and limitations to the process cycle. In particular the injection moulding cycle is rather special. Once the mould is filled, the material shrinks during cooling. In order to handle that shrinkage and make sure the mould is properly filled every time, additional material is squeezed in by overfilling it by adding an after pressure that compresses the molten plastic. A molten plastic is, unlike most low molecular fluids, actually compressible.