POLYMER PROCESSES

Question 1

Explain vacuum forming?

Answer 1

• A mould is placed on the bed of a machine, called a platen, which is then lowered to the bottom of the machine.
  -A polymer sheet is clamped over the mould and a heater is pulled over the polymer sheet.
  -When the sheet is softened the platen is raised into the polymer and the heat is removed.
  -The vacuum is switched on, which sucks the polymer over the mould.
  -Once it has cooled and returned to a solid state the vacuum is switched off and the platen is lowered.
  -The polymer is removed from the mould and any excess trimmed off.

-The mould must have a draft angle so that the polymer can be removed.
-It is a slow process, so it is limited to small scale batch production.

Question 1

Explain injection moulding?

Answer 1

-Thermoplastic granules are poured into the hopper.
-The screw thread rotates, this pulls the granules passed the heaters, which melts the polymer.
-When a sufficient charge of polymer has formed at the end of the screw, a hydraulic ram forces the screw thread forward, which injects the polymer into the mould.
-The mould is water cooled, which causes the polymer to harden.
-The mould opens and ejector pins push the moulding out.
-Any excess polymer is trimmed off the moulding.

-Used for large scale mass production due to the high set up costs.
-Used to produce casings on electrical products like computers and vacuum cleaners.

Question 2

Explain extrusion?

Answer 2

-Polymer granules are loaded into the hopper.
-The screw moves the polymer past heaters, which soften the polymer.
-The hydraulic ram pushes the screw, forcing the polymer through a steel die.
-The extrusion may be supported by rollers as it leaves the die and is cooled by water or air.
-The extrusion is then cut to the desired length.

-It is used to produced either solid rods or hollow tubes or sections such as I and T.
-Similar to injection moulding but it is forced through a die instead of into a mould.