Thermosetting Plastics


  • Bonding by Covalent bonds.
  • Primary bonds held together by strong cross links.

Process: Condensation Polymerisation

  • Forms strong primary bonds between chains.
  • Two monomers react to form a new molecule with water or alcohol emitted as a bi-product.
  • The polymer cannot be resoftened.
  • E.g. Phenol Formaldehyde


  • High melting points.
  • High tensile strength.
  • Can withstand high temperatures without losing their rigidity.
  • Primary bonds between molecule chains.

Compression Moulding

  • Split formed mould.
  • Polymer can be in powder or slug form.
  • Combination of heat and pressure (coalescence) allows piece to be formed.
  • Triggers chemical reaction cross linking and the object is removed.
  • High quality finish
  • E.g. Electrical fittings, Bottle tops.

Transfer Moulding

  • The moulding powder is placed in a compartment above the mould where it is heated.
  • The plunger forces the molten polymer into a cooled cavity.
  • The polymer solidifies in the mould which is then opened and the product is removed.
  • Used to make complex products.
  • E.g. socket covers.
Compression moulding (above) & Transfer moulding (below)



  • Bonded by covalent bonds.
  • Secondary bonds with weak Van Der Waals forces which can be broken down by heat.

Process: Addition Polymerisation

  • Monomers join up to form long chain like molecules called polymers.
  • These are arranged like spaghetti and when each polymer overlaps, weak temporary bonds called Van Der Waals forces are formed.
  • E.g. Polyethylene


  • Low melting points.
  • Easy to mould
  • Can be remoulded and are subject to disruption by heat.
  • Can be recycled .
  • Low tensile strength.
  • Secondary bonds between molecule chains.


  • The thermoplastic moulding powder is fed from a hopper into a heated chamber.
  • A large archimedian screw moves the softening plastic through the chamber.
  • This plastic is forced through a die at the end of the machine. The die gives the desired extruded shape which is then cooled by air or water and cut into lengths.
  • It may also be cooled in a vacuum chamber.
  • E.g. Piping.

Injection Moulding

  • The thermoplastic in granule form is fed into a heated compartment by a hopper.
  • A plunger forces the plastic along the machine barrel where they are melted by heaters.
  • A torpedo compacts the materials.
  • The softened materials are then forced into the mould by the torpedo where it cools and solidifies.
  • The mould is opened and the plastic product is ejected.
  • E.g. Lego


  • Continuous lengths of sheets are produced by calendaring.
  • The material passes through a series of heated rollers to produce the desired thickness of the material.
  • It is the cut to size or collected on a roll.
  • E.g. Cling film.