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:
This includes injection moulding, reaction compression moulding and transfer moulding.
This includes such processes as extrusion, plastic vacuum forming, plastic blow moulding and calendering.
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 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 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.