Thermosetting Plastics
Bonding
- 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
Properties
- 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.
Thermoplastics
Bonding
- 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
Properties
- 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.
Extrusion
- 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
Calendering
- 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.