Casting and moulding

Casting and moulding

1. these processes can be used to make 3D parts in a single operation

   • all involve some form of mould in the shape of the desired product
     • they also all involve a change in state of the material used
       • e.g allowing a liquid to set or cool, forming a solid

2. metal casting

   • melted metal is poured into moulds
     • and allowing it to cool
       • can be used to make complicated 3D shapes
   • sand casting
     • mould is made form sand, with an added bonding agent
       • the shape needed is pressed into the sand and then removed, leaving a shape for the metal to fill
         • each mould is only used once
           • the sand is shaken off the cast product after it has been soilidified
           • over 70% of castings are made from sand casting
             • iron is often cast using sand eg for car engine parts
   • die casting
     • where lots of products are to be made
       • car door handles, camera bodies
     • moulds typically made from steel
       • can bes used again and again but are very expensive to make
     • metals cast by this process must have a low melting point eg aluminium and zinc, pewter

3. if a large number of plastic products are to be made

   • injection moulding
     • used to make many different plastic parts
     • injection moulding uses an expensive metal die
       • the die might contain space for many small parts, or one large part
     • plastic is injected into the mould under pressure
     • only used with thermoplastics!
     • process
       • 1) Raw material: granules of thermoplastic
         • these are fed into a hopper
           • a rotating feed screw
             • takes the granules through a heater where they are melted
               • when enough melted plastic has reached the shot chamber, the feed screw is rammed forward
                 • this injects the plastic into the die under pressure
                   • after the plastic has cooled the mould is opened to release the finished mouldings
       • may be repeated many time a minute

4. extrusion process

   • used to make continuous lengths of thermoplastic product
     • eg pipes/ guttering
       • alos used to provide parison used in blow moulding
   • lie injection, uses a hopper, heating chamber and rotating feed screw
   • Melted thermoplastic is forced through a die
     • it is not rammed to increase the pressure
       • the extruded shape cools after leaving the die
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