polymer processes

Question 1

extrusion process

Answer 1

1) granular plastic is held in a hopper
2) it is moved by an archimedean screw along a heated tube, called the heating chamber
3) once the polymer has been melted, it is pushed through a die mould, which will form the shape of the extrusion
4) because there is no hydraulic ram in this process, molten plastic can be fed through the die continuously

Question 2

injecting moulding process

Answer 2

1) thermoplastic granules are poured into the hopper

2) a screw thread is rotated by the motor. this pulls the granules through the chamber and past the electric heaters

3) the heater melts the polymer

4) when a sufficient charge of polymer has melted forward at the end of the screw, a hydraulic ram forces the screw thread forward. this injects the polymer into the mould

5) the mould is water cooled, which enables the molten polymer to harden quickly

6) any excess polymer is trimmed off. formers or jigs may be used to maintain the dimensional accuracy of the moulding while it cools and hardens

Question 3

blow moulding process

Answer 3

1) the polymer is fed into the hopper

2) an archimedean screw pulls the polymer through a heated section, melting the polymer

3) the melted polymer is extruded as a tube, which is called a ‘parison’

4) the mould sides close around the parison and air is injected into the mould, forcing polymer to the sides

5) the polymer is allowed to cool for a few seconds, the mould opens and the finished product is ejected

Question 4

vacuum forming processes

Answer 4

1) the mould is placed on the bed of the machine, which is called a ‘platen’. the platen is lowered to the bottom of the machine

2) polymer sheet is clamped over the mould and a heater is pulled over the polymer sheet

3) when the polymer sheet has softened, the platen is raised into the polymer and then is removed

4) the vacuum pump is switched on, which sucks the polymer onto the mould

5) once the polymer has cooled and returned to a solidified state, the platen is lowered and vacuum is switched off

6) the mould is removed from the moulding. excess polymer is then trimmed off

Question 5

compression moulding process

Answer 5

1) a ‘slug’ of pre weighed thermoset polymer is inserted into pre-heated moulds

2) the moulds are closed and hydraulic pressure is applied. the pressure ensures that the polymer takes the shape of the mould

3) the moulds remain closed while cross linking takes place and the thermoset ‘cures’

4) when the moulding has cured, the machine opens and the product is removed

5) excess polymer known as ‘flash’ is removed

Question 6

rotational moulding process

Answer 6

1) polymer powder or granules are loaded into a mould, which is clamped and sealed

2) the mould is transferred to an oven where it is heated to 260-370 C , depending on the polymer used. the mould is rotated slowly around two axes, and as the polymer is heated, it coats the inside of the mould

3) once the polymer has achieved the correct thickness, the mould is cooled. usually a fan and/or water is used to cool the polymer

4) when the polymer has solidified, the part will shrink slightly, allowing it to be removed