A very wide variety of plastic products are made from extruded sections, e.g. curtain rails, household guttering, window frames, polythene bags and film n&k . 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 n&k 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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