What is single screw extruder? How thermoplastic material is processed through the extrusion?

SINGLE SCREW EXTRUSION
Extrusion may be defined as a process for making a product (an extrudate) by forcing a material through an orifice or die to form a shape, or alternatively, the production of finished, or semi-finished products, using an extruder.

Extrusion and Thermoplastics

In extrusion of a thermoplastic, heating first softens the material so that it can be shaped. The extrusion machine, or extruder does this process. This heat softening is called by various names, such as ‘plastication’, ‘plasticization’ or ‘thermal softening’. Most extruders are single screw machines. It is the screw which forces the material towards, and then through, the die. Shape is imparted by the die, and/or by post-extrusion forming, and then the product is set to shape by cooling it while maintaining its shape. The equipment that does this is called the post extrusion equipment, while the whole system is called an extrusion line.

Extruder Classification

Extruders may be classified by three figures, for example, 1-60-24. The first number states how many screws the machine has, the second number specifies the screw diameter in millimeters (mm) and, the third number specifies the effective screw length as a multiple of the screw diameter. In the example given therefore, a single screw machine is being described that has a screw of diameter 60 mm and a length of 24 screw diameters (that is, a L/D ratio of 24/1).

Machine Construction

A cut-away drawing of a simple single screw machine,. This shows the arrangement of the different parts of the machine. The screw and barrel are the two units that interact to convey the plastic material, melt the material and then force it through the die. The electric motor drive unit and gearbox rotate the screw at a predetermined speed. Temperature controllers

Profile Extrusion.

Profiles of open or hollow cross section can be extruded from specially designed dies. Each shape presents its own problems of die design and post- extrusion handling. Profile dies are frequently intricate and thus difficult and expensive to make. Unless CAE is used, a lengthy process of trial and error is required to establish both die design and extrusion conditions (A case in point, is to prevent preferential flow in the thicker sections).

Such dies can be used for small sections, but a simpler technique is desirable for larger profiles (for example, for diffusers for fluorescent light- ing fittings). One technique involves post-forming sections extruded from tube dies. Compared with profile dies, these large tube dies can be made at low cost and their symmetry facilitates uniform flow.

The inner lip of the die can have a reeded surface that imparts a pattern to the extrudate. A knife mounted in the die face, slits the emerging reeded tube and the tube passes to a shaping and cooling jig. Alternate inner and outer formers do the shaping with polished forming surfaces. Cooling is accomplished by internal and external perforated copper rings through which is directed a gentle flow of air. The haul-off may be the rubber caterpillar-band type, and an on-demand knife cutter can cut off sections. Trial and error will again be necessary to establish conditions and former design, but adjustments to formers are made more quickly and cheaply than changes to a pro- file die.