Monday, 30 September 2013

Plastic Moulding

Injection Moulding-



Extrusion Moulding-
This process is a continuous process used to produce solid and hollow products that  have a constant cross-section.
  • Thermoplastic granules are fed from a hopper through a heated cycinder by a rotational screw
  • the tapered shape of the screw compacts the plastic as it travels through the the heated cylinder towards the die
  • the die determineds the shape of the cross section of the plastic when it is extruded through the die opening
  • Extrusion rates vary from 9 to 60 metres p/minute. The thinker the cross section the longer it will take to extrude
  • as the extrusion is complete the product passes through a cooling trough which contains cold water
  • after cooling the product is cut to specification/ requirements automatically

Vacuum Moulding-
This process is used to manufacture a wide range of thermoplastic products. these products vary in size from food trays to interior vehicle panels

  • A desired mould is placed upon a platen (support plate)
  • the platen and mould are lowered, Rigid thermoplastic sheet material is then clamped onto an air tight gasket and heated from above
  • After the sheet has softened, air is blown uder the sheet to raise it into a bubble shape before the platen is raised bringing the mould into contact with the plastic
  • Any trapped air is then removed using a vaccum pump which ensures that the plastic has taken complete shape of the mould
  • Once the plastic sheet has cooled to below its freezing point, the air flow is reversed to remove the plastic from the mould
  • Once completed the waste material is then cut from the plastic sheets and recycled

Rotational Moulding-
This process is mainly used to manufacture hollow objects such as footballs, road cones and storage tanks

  • Moulds are mounted and rotated contnuously by three rotational arms
  • Firstly, The moulds are filled with thermoplastic powder, which is measured only to coat the outer edges before closing the mould for the next stage
  • whilst being rotated (both horizontal and vertically) the mould is subjected to 230°C - 400°C
  • as tyhe mould rotates it's inner surfaces are coated with the powder which turns to plastic with the extreme heat applied. Producing an even thickness of plastic inside the mould
  • Whilst still rotating the mould is then moved into a cooling area where it is cooled by air or water jets
  • Once cooler enough to retain it's own shape, the products are removed from the moulds and in some cases to prevent distrosion to the shape during final cooling. A jig can be applied, this is often with flat surfaces (eg storage containers)

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