Processing methods for thermoplastics

11.2.2 Processing methods for thermoplastics

Processing technology has a special place in the remarkable history of the polymer industry: polymer

Figure 11.5 Deformation map for PMMA showing deformation regions as a function of normalized stress versus normalized temperature (from Ashby and Jones, 1986; permission of Elsevier Science Ltd, UK) .

chemistry decides the character of individual molecules but it is the processing stage which enables them to be arranged to maximum advantage. Despite the variety of methods available for converting feedstock pow- ders and granules of thermoplastics into useful shapes, these methods usually share up to four common stages of production; that is, (1) mixing, melting and homog- enization, (2) transport and shaping of a melt, (3) drawing or blowing, and (4) finishing.

Processing brings about physical, and often chemi- cal, changes. In comparison with energy requirements for processing other materials, those for polymers are relatively low. Temperature control is vital because it decides melt fluidity. There is also a risk of ther- mal degradation because, in addition to having limited thermal stability, polymers have a low thermal con- ductivity and readily overheat. Processing is usually rapid, involving high rates of shear. The main methods that will be used to illustrate technological aspects of processing thermoplastics are depicted in Figure 11.6.

Injection-moulding of thermoplastics, such as PE and PS, is broadly similar in principle to the pressure die-casting of light metals, being capable of produc- ing mouldings of engineering components rapidly with repeatable precision (Figure 11.6a). In each cycle, a metered amount (shot) of polymer melt is forcibly injected by the reciprocating screw into a ‘cold’ cavity (cooled by oil or water channels). When solidifica- tion is complete, the two-part mould opens and the moulded shape is ejected. Cooling rates are faster than with parison moulds in blow-moulding because heat is removed from two surfaces. The capital out- lay for injection-moulding tends to be high because of the high pressures involved and machining costs for multi-impression moulding dies. In die design, special attention is given to the location of weld lines, where different flows coalesce, and of feeding gates. Com- puter modelling can be used to simulate the melt flow and distributions of temperature and pressure within the mould cavity. This prior simulation helps to lessen dependence upon traditional moulding trials, which are costly. Microprocessors are used to monitor and con- trol pressure and feed rates continuously during the moulding process; for example, the flow rates into a complex cavity can be rapid initially and then reduced to ensure that flow-dividing obstructions do not pro- duce weakening weld lines.

Extrusion is widely used to shape thermoplastics into continuous lengths of sheet, tube, bar, filament, etc. with a constant and exact cross-sectional profile (Figure 11.6b). A long Archimedean screw (auger) rotates and conveys feedstock through carefully pro- portioned feed, compression and metering sections. The polymer is electrically heated in each of the three barrel sections and frictionally heated as it is ‘shear- thinned’ by the screw. Finally, it is forced through a die orifice. Microprocessor control systems are avail- able to measure pressure at the die inlet and to keep it constant by ‘trimming’ the rotational speed of the screw. Dimensional control of the product benefits

Plastics and composites 357 from an annular die is drawn upwards and inflated

with air to form thin film: stretching and thinning cease when crystallization is complete at about 120 ° C. Similarly, in the blow-moulding of bottles and air- ducting, etc., tubular extrudate (parison) moves ver- tically downwards into an open split-mould. As the mould closes, the parison is inflated with air at a pres- sure of about 5 atmospheres and assumes the shape of the cooled mould surfaces. Relatively inexpensive alu- minium moulds can be used because stresses are low.

Thermoforming (Figure 11.6c) is another secondary method for processing extruded thermoplastic sheet, being particularly suitable for large thin-walled hol- low shapes such as baths, boat hulls and car bodies (e.g. ABS, PS, PVC, PMMA). In the basic version of the thermoforming, a frame-held sheet is located above the mould, heated by infrared radiation until rubbery and then drawn by vacuum into close contact with the mould surface. The hot sheet is deformed and thinned by biaxial stresses. In a high-pressure version of ther- moforming, air at a pressure of several atmospheres acts on the opposite side of the sheet to the vacuum and improves the ability of the sheet to register fine mould detail. The draw ratio, which is the ratio of mould depth to mould width, is a useful control param- eter. For a given polymer, it is possible to construct

a plot of draw ratio versus temperature which can be used as a ‘map’ to show various regions where there are risks of incomplete corner filling, bursts and pin- holes. Unfortunately, thinning is most pronounced at vulnerable corners. Thermoforming offers an econom- ical alternative to moulding but cycle times are rather long and the final shape needs trimming.