Extrusion - Pipe and Tube

Extrusion is one of the most common plastic processing method that is used to create rod, sheet, film, profile or pipes & tubes.

It is a continuous, high temperature and high pressure manufacturing method that results in lengths or rolls/coils of finished or semi-finished product.

A pipe or tube die requires a pin and mandrel which provides the core of the pipe. The molten plastic flows around the mandrel support legs and rewelds as it flows through the front section of the die.

To prevent the molten pipe from collapsing on exit from the die, positive air pressure is applied to the inside of the tube via the mandrel.

Pipe diameter and wall thickness can be adjusted by controlling the plastic feed rate into the hopper, screw speed or the haul-off speed.

The extruder

Extruders range in size and capability to suit the material or end product being manufactured. Typical hourly outputs range from a few kgs per hour to up to 20 tonnes per hour.

The key components in an extruder are a barrel and an Archimedian screw. The screw is coupled to an electric motor that drives it within the barrel.
The plastic, usually in the form of powder or granules, is fed from a hopper onto the screw. It is then conveyed along the barrel where it is heated by conduction from the barrel heaters and shear due to its movement across the screw flights. The depth of the screw flights channel is reduced along the length of the screw so as to compact the material.

A die is fitted to the end of the barrel and the molten plastic is formed into the desired shape. Before the plastic is formed by the die it passes through a breaker and/or screen which serves to create back pressure that ensures a consistent linear flow of plastic to the die. The screen and breaker plate assembly also removes impurities from the plastic melt.

Extruders can be single or twin screw. The twin screw and barrel design can be parallel or conical and the screws can co-rotate or counter-rotate.

Screw design

As extruder screw has three different zones along its length. Namely feed, compression and metering.
The feed zone pre-heats and conveys the plastic to the compression zone. In the compression zone the flight channel depth decreases to compact the plastic and squeeze out any trapped air pockets. In the metering zone the flight channel depth is constant but more shallow than the feed zone which ensures a constant homogenous supply of molten plastic to the die.
Different types of plastic require different screw designs and for certain materials additional zones to decompress and re-meter the plastic are present to allow for a vent port in the extruder barrel. Venting is necessary to remove volatiles such as moisture that has turned into steam from the melt. If not removed, these gases can accumulate in the finished product and cause a defect such as a surface blemish or a weaknesss.

Haul-off unit

The haul-off serves to pull the plastic through the water bath at a constant speed. A common haul-off design comprises 2 sets of electric motor driven caterpillar style tracks that are set one above the other with an adjustable gap between them through which the extrusion passes. Lighter duty applications use rubber belts instead of the caterpillar tracks. On exist from the haul-off, the extrusion is cut into lengths or wound onto a drum or into a coil.