Many polymer-forming processes are largely 2 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 1st stage can involve the mixing with the polymer of appropriate additions and other polymers so that the completed material should have the required properties. Second-stage processes for thermoplastics forming usually involve heating the powder, granule or sheet material until it softens, shaping the slipped 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 in order that they undergo a chemical change to cross-link polymer chains into a very linked polymer. The key second-stage processes utilised for forming polymers are :
A very wide variety of plastic products are made of extruded sections, e.g. Curtain rails, household guttering, window frames, carrier bags and film. Extrusion involves the causing of the molten thermoplastic polymer through a die. The polymer is fed into a screw mechanism which takes the polymer through the heated area 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 necessary, a die might 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 growth of the material is accompanied by a reduction in thickness. Such film can immediately be modified into bags.
Polyethylene is readily processed to give tubular sleeves by this technique but polypropylene presents a problem in that the rate of cooling is insufficient to stop 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 utilisation of rollers, can be made quick enough to stop crystallisation happening with polypropylene. The extrusion process can be used with most thermoplastics and yields constant lengths of product. Complicated 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 like 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 mold. Pressure is applied to the interior of the tube to inflate it in order that it fills the mould. Blow moulding may also be used with injection moulding.
Plastic vacuum forming
Vacuum forming is a typical strategy of thermoforming. It utilises a vacuum on one side of a sheet of heat-softened thermoplastic to force it against a cooled mold and thus produce the required shape. Sheets, for example six mm thick acrylic, are likely to be heated in ovens then was clamped, but thinner sheets are probably going to be heated by radiant heaters positioned over the mold. Vacuum Forming Companies 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.
welcome to Insurances.net (https://www.insurances.net)
