What Is Extrusion? The Film, Pipe and Profile Production Process

Extrusion is a production method in which pelletized plastic is continuously melted in an extruder and pushed through a die to form a continuous profile. Unlike injection molding, the process is continuous rather than intermittent — the material leaving the die is cooled along a long line before being cut or wound onto a reel.

Single-screw extruders are the basic equipment most commonly used in film, pipe and profile production, combining the melting and pushing function with a single rotating screw.

Twin-screw extruders use two intermeshing screws and generally offer greater mixing/blending power — for this reason they are preferred for processes requiring homogeneity, such as masterbatch production and additive compounding.

In the blown film method, the melt leaving the extruder is inflated with air through a circular die into a 'bubble,' drawn upward, cooled, and then flattened and wound onto a reel. Products such as shopping bags, shrink film and agricultural mulch film are produced this way; AYPE and LLDPE are commonly preferred materials for this application (see the 'What Is PE' article).

In the cast film method, the melt exits through a flat slot and is rapidly cooled on chill rolls — this method generally provides higher transparency and dimensional stability.

In pipe production, the melt exits through a circular die and is typically passed through a vacuum- or pressure-assisted calibration/cooling tank to gain dimensional accuracy. YYPE is typically preferred for drinking-water and natural-gas pipes — rigidity and pressure resistance are decisive in this application.

In profile extrusion, changing the die geometry allows production of window frames, gaskets or specially shaped parts; material selection here is also made according to the stiffness/flexibility balance the application requires.