Просмотр исходного текста страницы Plastics and Plastic Manufacturing3320366

Many polymer-forming processes are essentially two stage; the 1st stage with polymers being the production of the polymer in a very powder, granule or sheet form and the second stage being the shaping of this material in the required shape. The very first stage can involve the blending together with the polymer of suitable additives along with polymers to ensure the finished material should have the desired properties. Second-stage processes for thermoplastics forming generally involve heating the powder, granule or sheet material until it softens, shaping the softened material on the required shape then cooling it. For thermosets the second-stage processes involve forming the thermosetting materials to the required shape and after that heating them so that they undergo a compound switch the signal from cross-link polymer chains in a highly linked polymer. The leading second-stage processes employed for forming polymers are:

Plastics Moulding

This includes injection moulding, reaction compression moulding and transfer moulding.

Plastics Forming

For example such processes as extrusion, plastic vacuum forming, plastic blow moulding and calendering.

Plastics Cutting

Injection moulding,

Moreover, products can be formed by polymer joining. The processes are:

Adhesives, Plastic Welding,Fastening systems such as riveting, press and snap fits and screws.

Selecting process will depend on quite a few factors, like:

The number of items required How big those items

The pace of which those items can be produced, i.e. cycle time The prerequisites for holes, inserts, enclosed volumes, threads Perhaps the materials are thermoplastic or thermoset

Plastic Injection moulding

Moulding works on the hollow mould in order to create the item. The key processes are injection moulding, reaction injection moulding, compression moulding and transfer moulding.

A widespread process for thermoplastics, eventhough it could also used for rubbers, thermosets and composites, is injection moulding. Using this process, the polymer raw materials are pushed right into a cylinder using a screw or plunger, heated then pushed, i.e. injected, into the cold metal mould. For most within the material inside the mould is maintained while it cools and sets. The mould will be opened as well as the component extracted, and then the entire process repeats itself. High production rates can be performed and complicated shapes with inserts, threads, holes, etc. produced; sizes range between about 10 g to 25 kg in weight. Typical items are beer or milk bottle crates, toys,

control knobs for electronics, tool handles, pipe fittings.

Reaction injection moulding

Reaction injection moulding involves the reactants being combined inside mould to react and create the polymer. The choice of materials which might be processed like this is determined by the response time, this must be short, e.g. Half a minute, in order that cycle times are short. It is mainly in combination with polyurethanes, polyamides and polypropylene oxide and composites incorporating glass fibres. The preheated reactants are injected at top speed right into a closed mould where they fill the mould and

combine to make the end product. This method can be used for big automotive parts like spoilers, bumpers and front and back fascia.

Compression moulding

Compression moulding is traditionally used for thermosets. The powdered polymer is compressed backward and forward parts of the mould and heated pressurized to initiate the polymerisation reaction. The procedure is limited to not too difficult shapes at a 2-3 g to fifteen kg in weight. Typical products are dishes, handles and electrical fittings.

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