Materials for engineering, 3rd Edition - (Malestrom)

162
Materials for engineering
screw, which compacts and mixes them and transports them to the heated
region where melting occurs, Fig. 5.2.
When the temperature is in the region of 1.3 to 1.6 T g , the melt is forced
through a die and then cooled to give tubes, sheet, ribbon or rod. Post-die
devices, which draw down or increase the dimensions of emerging simple
shapes, are widely used. For example, air can be blown into a formed tube
to expand it to several times its diameter.
The deformation and flow behaviour of a given polymer (known as its
rheological properties) are thus of paramount importance and considerable
emphasis is given by engineers to the measurement, presentation and use of
rheological data for polymer melts.
5.2.2 Injection moulding
In injection moulding the emerging melt from a screw extrusion device is
injected under pressure into a cold split mould. Pressures up to 120 MPa are
required and mouldings of high dimensional precision may be produced,
although the process has a slow cycle time, since the product has to cool
before its removal from the mould in order that its shape is maintained.
5.2.3 Blow moulding
In the blow moulding of plastic bottles, melt is extruded out of an annular die
to form a parison. Air is then injected into the centre of the annulus and a
Hopper
Gear reducer
Screw
Barrel heaters
Head clamp
Barrel
5.2 Schematic view of a single screw extruder.