Many polymer-forming processes are essentially two stage; the first stage with polymers being the production of the polymer in a powder, granule or sheet form and the second stage being the shaping of this material into the required shape. The first stage can involve the mixing with the polymer of suitable additives and other polymers in order that the finished material should have the required properties. Second-stage processes for thermoplastics forming generally involve heating the powder, granule or sheet material until it softens, shaping the softened material to the required shape and then cooling it. For thermosets the second-stage processes involve forming the thermosetting materials to the required shape and then heating them so that they undergo a chemical change to cross-link polymer chains into a highly linked polymer. The main second-stage processes used for forming polymers are:
Plastics Moulding
This includes injection moulding, reaction compression moulding and transfer moulding.
Plastics Forming
This includes such processes as extrusion, plastic vacuum forming, plastic blow moulding and calendering.
Plastics Cutting
Injection moulding,
In addition, products may be formed by polymer joining. The processes are:
Adhesives, Plastic Welding,Fastening systems such as riveting, press and snap fits and screws.
The choice of process will depend on a number of factors, such as:
The quantity of items required The size of the items
The rate at which the items are to be produced, i.e. cycle time The requirements for holes, inserts, enclosed volumes, threads Whether the material is thermoplastic or thermoset
Plastic Injection moulding
Moulding uses a hollow mould to form the product. The main processes are injection moulding, reaction injection moulding, compression moulding and transfer moulding.
A widely used process for thermoplastics, though it can also used for rubbers, thermosets and composites, is injection moulding. With this process, the polymer raw material is pushed into a cylinder by a screw or plunger, heated and then pushed, i.e. injected, into the cold metal mould. The pressure on the material in the mould is maintained while it cools and sets. The mould is then opened and the component extracted, and then the entire process repeats itself. High production rates can be achieved and complex shapes with inserts, threads, holes, etc. produced; sizes range from about 10 g to 25 kg in weight. Typical products are beer or milk bottle crates, toys,
control knobs for electronic equipment, tool handles, pipe fittings.
Reaction injection moulding
Reaction injection moulding involves the reactants being combined in the mould to react and produce the polymer. The choice of materials that are processed in this way is determined by the reaction time, this must be short, e.g. 30 seconds, so that cycle times are short. It is mainly used with polyurethanes, polyamides and polypropylene oxide and composites incorporating glass fibres. The preheated reactants are injected at high speed into a closed mould where they fill the mould and
combine to produce the finished product. This method is used for large automotive parts such as spoilers, bumpers and front and rear fascia.
Compression moulding
Compression moulding is widely used for thermosets. The powdered polymer is compressed between the two parts of the mould and heated under pressure to initiate the polymerisation reaction. The process is limited to relatively simple shapes from a 2-3 g to 15 kg in weight. Typical products are dishes, handles and electrical fittings.
Transfer moulding
Transfer moulding differs from compression moulding in that the powdered polymer is heated in a chamber before being transferred by a plunger into the heated mould.
Plastic Forming processes
Forming processes involve the flow of a polymer through a die to form the required shape.
Plastic Extrusion forming
A very wide variety of plastic products are made from extruded sections, e.g. curtain rails, household guttering, window frames, polythene bags and film. Extrusion involves the forcing of the molten thermoplastic polymer through a die. The polymer is fed into a screw mechanism which takes the polymer through the heated zone and forces it out through the die. In the case of an extruded product such as curtain rail, the extruded material is just cooled.
If thin film or sheet is required, a die may be used which gives an extruded cylinder of material. This cylindrical extruded material is inflated by compressed air while still hot to give a tubular sleeve of thin film . The expansion of the material is accompanied by a reduction in thickness. Such film can readily be converted into bags.
Polyethylene is readily processed to give tubular sleeves by this method but polypropylene presents a problem in that the rate of cooling is inadequate to prevent crystallisation and so the film is opaque and rather brittle. Flat film extrusion can be produced using a slit-die. The rate of cooling, by the use of rollers, can be made fast enough to prevent crystallisation occurring with polypropylene. The extrusion process can be used with most thermoplastics and yields continuous lengths of product. Intricate shapes can be produced and a high output rate is possible.
Plastic Blow moulding
Blow moulding is a process used widely for the production of hollow articles such as plastic bottles from thermoplastics. Containers of a wide range of sizes can be produced. With extrusions blow moulding the process involves the extrusion of a hollow thick-walled tube which is then clamped in a mould. Pressure is applied to the inside of the tube to inflate it so that it fills the mould. Blow moulding can also be used with injection moulding.
Plastic vacuum forming
Vacuum forming is a common method of thermoforming. It uses a vacuum on one side of a sheet of heat-softened thermoplastic to force it against a cooled mould and hence produce the required shape. Sheets, such as 6 mm thick acrylic, are likely to be preheated in ovens before being clamped, but thinner sheets are likely to be heated by radiant heaters positioned over the mould. Vacuum forming can have a high output rate, but dimensional accuracy is not too good and such items as holes, threads and enclosed shapes cannot be produced.
Calendering
Calendering is a process used to form thermoplastic films, sheets and coated fabrics. The most common use has been for plasticised PVC. Calendering consists of feeding a heated paste-like mass of the plastic into the gap between two rolls, termed nip rolls. It is squeezed into a film which then passes over cooling rolls before being wound round a wind-up roll. This process can also be used to coat a fabric with a polymer.
Plastic machining
The processes used to shape a polymer generally produce the finished article with no further, or little, need for machining or any other process. With injection moulding, compression moulding and blow moulding there is a need to cut off gates and flashing; with extrusion, lengths have to be cut off. As with metals, single-point and multi-point cutting tools can be used with polymers. Where discontinuous, rather than continuous, chips are produced and the machined surface becomes excessively rough as a result of chips being sheared off. It is thus desirable to select cutting conditions which result in the formation of continuous chips.
Polymers tend to have low melting points and thus machining conditions which do not result in high temperatures being produced are vital if material is not to soften and deform.
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