The Single-screw Extruder

A hollow cylindrical enclosure, called the barrel. The barrel can be smooth or grooved. Conical barrels have been described (Meuser and Wiedmann, 1989) but they are not common in food extruders

A sturdy Archimedean screw or worm, with a thick root and shallow flights, turning inside the barrel. The flights of the rotating screw propel the material along a helicoidal channel (flow channel) formed between the screw root and the barrel. The width of the flow channel, resulting from the screw pitch, is considerably larger than its thickness. The gap between the screw tip and the barrel surface is made as narrow as possible

A restricted passage element, known as the die at the exit end of the extruder. The functions of the die are to serve as a pressure release valve and to impart to the extrudate the desired shape, determined by the cross-section of the aperture(s). The die is sometimes preceded by a perforated plate (breaker plate) which helps distribute the compressed material evenly across the die

A device for cutting the extrudate emerging from the die. In its simplest form this consists of a rotating knife

Different kinds of devices for heating or cooling the barrel (steam or water jackets, electrical resistance heaters, induction heaters etc.). These elements, external to the barrel, are usually divided into individual segments, in order to impose different temperature conditions at different sections of the extruder

A hopper for gravity feeding or an auger for positive feeding

Ports for the injection of steam, water and other fluids as needed. Ports for pressure release

Measurement instruments (feed rate, temperature, pressure) and controls

A drive, usually with speed variation capability and torque control.