Shaping and Finishing

The techniques used for shaping and finishing plas­tics depend on three factors: time, temperature, and flow (also known as deformation). Many of the processes are cyclic in nature, although some fall into the categories of continuous or semicontinuous operation.

One of the most widely used operations is that of ex­trusion. An extruder is a device that pumps a plastic through a desired die or shape. Extrusion products, such as pipes, have a regularly shaped cross section. The ex­truder itself also serves as the means to carry out other operations, such as blow moulding and injection mould-

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ing. In extrusion blow moulding, the extruder fills the mould with a tube, which is then cut off and clamped to form a hollow shape called a parison. The hot, molten parison is then blown like a balloon and forced against the walls of the mould to form the desired shape. In injection moulding, one or more extruders are used with recipro­cating screws that move forwards to inject the melt and then retract to take on new molten material to continue the process. In injection blow moulding, which is used in making bottles for carbonated drinks, the parison is first injection moulded and then reheated and blown.

In compression moulding, pressure forces the plastic into a given shape. Another process, transfer moulding, is a hybrid of injection and compression moulding: the molten plastic is forced by a ram into a mould. Other fin­ishing processes include calendering, in which plastic sheets are formed, and sheet forming, in which the plas­tic sheets are formed into a desired shape. Some plastics, particularly those with very high temperature resist­ance, require special fabrication procedures. For exam­ple, polytetrafluoroethene (Teflon) has such a high melt viscosity that it is first pressed into shape and then sintered —exposed to extremely high temperatures that bond it into a cohesive mass without melting it. Some polyamides are produced by a similar process.

Uses

Plastics have an ever-widening range of uses in both the industrial and consumer sectors.

Packaging

The packaging industry is a leading user of plastics. Much LDPE (low-density polyethene) is marketed in rolls of clear-plastic wrap. High-density polyethene

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(HPDE) is used for some thicker plastic films, such as those used for plastic waste bags and containers. Other packaging plastics include polypropene, polystyrene, polyvinyl chloride (PVC), and polyvinylidene chloride. Polyvinylidene chloride is used primarily for its barrier properties, which can keep gases such as oxygen from passing into or out of a package. Similarly, polypropene is an effective barrier against water vapour. Polypropene is also often used in housewares and as a fibre for carpet­ing and rope.