Polymer extrusion continuously pushes molten polymer through a die to create products with approximately constant cross-section, such as pipe, sheet, film or profile.

A rotating screw performs several functions at once: it conveys solid feed, compresses and melts it, mixes the melt and generates pressure that drives flow through the die.

The die shape is not always identical to the desired final cross-section. Polymer melts store elastic deformation while flowing through the die. After leaving confinement, some of that deformation recovers and the extrudate can expand. This is die swell.

A simple swell ratio is

$$B=\frac{D_e}{D_d},$$

where $D_e$ is extrudate diameter after leaving the die and $D_d$ is die diameter. If a 10 mm die produces an 11.5 mm strand,

$$B=\frac{11.5}{10}=1.15.$$

Cooling and downstream pulling also affect final dimensions. Pulling faster than the natural extrusion velocity can reduce cross-section, while uneven cooling can distort the profile.

Stable extrusion therefore requires control of melt temperature, screw speed, pressure, die geometry, cooling and take-up speed.

Unlike injection molding, extrusion is a continuous process. Its economics and control problems are therefore tied to steady throughput and downstream handling rather than repeated mould filling and ejection.