Polymer processes Flashcards
What are all the polymer processes?
Vacuum forming, thermoforming, line bending, laminating, injection moulding, blow moulding, rotational moulding, extrusion, compression moulding
Describe the vacuum forming process
- The mould is placed on the bed of the machine, which is called the ‘platen’. The platen is lowered to the bottom of the machine.
- Polymer sheet is clamped over the mould and a heater is pulled over the polymer sheet.
- When the polymer sheet has softened, the platen is raised into the polymer and the heat is removed.
- The vacuum pump is switched on, which sucks the polymer onto the mould.
- Once the polymer has cooled and returned to a solidified state, the platen is lowered and vacuum swicthed off.
- The mould is removed from the moulding. Excess polymer is trimmed off.
Thermoforming examples
It is used in food packaging such as polymer fruit punnets, cake boxes and sandwitch boxes can be thermoformed, especially where logos or product names might need to be moulded into polymer.
Thermoforming process
It is similar to vacuum forming, except there is an additional mould that is pressed onto the surface of the polymer sheet at the same time as the vacuum is applied, sucking the polymer down on to the mould below. The two moulds trap the softened polymer in between them, giving extra detail to the moulding.
When is calendering used?
Used towards the end of manufacturing paper. It is also used in making thin polymer sheet and film as stock material for further processing into products later in the production cycle.
Process of calendering
Calendering works by heating pellets of thermoplastic so that it melts into a dough-like consistency. It is then extruded between a series of heated rollers so that it becomes squashed and stretched to make it thinner. It is then finished on cooling rollers, before being chopped into standard stock sheet sizes, or in case of polymer film, it is rolled up for storage and distribution.
What is line bending supposed to make?
Typically, line bending might be used to make products such as acrylic boxes, shelves or point of sale displays.
What is the process of line bending?
Typically, line bending might be used to make products such as acrylic boxes, shelves or point of sale displays.
Used this in light project
Lamination (lay-up) process
1 A mould or former in the shape of the product or component is prepared. This might be made from timber, manufactured boards such as plywood or high density foam.
2 The mould or former is coated with a release agent such as wax or PVA, or is covered with parcel tape.
3 A top layer of gel coat is applied. The gel coat is a coating of polyester resin often mixed with a pigment to give a colour to the moulding. The resin might also have additives included to prevent degradation from UV and hydrolysis. It using CFRP then curing takes place in a specialist oven; know as an autoclave.
4 Fibreglass matting is cut to size and laid over the former. Polyester resin is brushed onto the matting and a small roller is used to push out any air bubbles that may form and to smooth out the matting (this process is called stippling). The matting is available in a range of stock forms including chopped strand, woven and ‘tissue’, which is a fine grade matting for covering the surface.
5 Step 3 is repeated until the desired thickness is achieved, and fine tissue matting is used on the top layers. A vacuum bag may be used to compress the layers of GRP before it sets.
6 The GRP is allowed to set.
Products from lamination
boats and yachts, kayaks, trains, scenery or props for theatre and films, as well as cars for theme park rides.
Process of injection moulding
1 Thermoplastic granules are poured into the hopper.
2 A screw thread is rotated by a motor. This pulls the granules through the chamber and past electric heaters.
3 The heaters melt the polymer.
4 When a sufficient charge of polymer has melted and formed 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 The mould opens and ejector pins push the moulding out.
7 Any excess polymer is trimmed off the moulding. Formers or jigs may be used to maintain the dimensional accuracy of the moulding while it cools and hardens completely.
Products of injection moulding
For example, the casing on electrical products such as computers, TVs and vacuum cleaners would be injection moulded. Such products have very complex parts that may include integral clip fastenings, screw posts, battery housing and circuit board holders.
Blow moulding process
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 the polymer to the sides.
5 The polymer is allowed to cool for a few seconds, the mould opens and the finished bottle is ejected.
Products of blow moulding
Blow moulding is the common process used to produce bottles and a wide range of other hollow products. The most common polymers are PET, LPDE, HDPE and PP.
Rotational moulding process
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 upon the polymer used. The mould is rotated slowly (fewer than 20 rotations per minute) 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.
