Page 281 - Plastics Engineering

264 Processing of Plastics

Fig. 4.16 Material flow path with co-rotating scres
The following table compares the single screw extruder with the main types
of twin screw extruders.

4.2.7 Processing Methods Based on the Extruder
Extrusion is an extremely versatile process in that it can be adapted, by the use
of appropriate dies, to produce a wide range of products. Some of the more
common of these production techniques will now be described.

(a) Granule ProductiodCompounding
In the simplest case an extruder may be used to convert polymer formulations
and additives into a form (usually granules) which is more convenient for use
in other processing methods, such as injection moulding. In the extruder the
feedstock is melted, homogenised and forced through a capillary shaped die.
It emerges as a continuous lace which is cooled in a long water bath so that
it may be chopped into short granules and packed into sacks. The haul-off
apparatus shown in Fig. 4.17 is used to draw down the extrudate to the required
dimensions. The granules are typically 3 mm diameter and about 4 mm long.
In most cases a multi-hole die is used to increase the production rate.

(b) Profile Production
Extrusion, by its nature, is ideally suited to the production of continuous
lengths of plastic mouldings with a uniform cross-section. Therefore as well as
producing the laces as described in the previous section, the simple operation
of a die change can provide a wide range of profiled shapes such as pipes,
sheets, rods, curtain track, edging stsips, window frames, etc (see Fig. 4.18).
The successful manufacture of profiled sections depends to a very large extent
on good die design. Generally this is not straightforward, even for a simple
cross-section such as a square, due to the interacting effects of post-extrusion
swelling and the flow characteristics of complex viscoelastic fluids. Most dies
are designed from experience to give approximately the correct shape and then
sizing units are used to control precisely the desired shape. The extrudate is then
cooled as quickly as possible. This is usually done in a water bath the length
of which depends on the section and the material being cooled. For example,