15 - Polymer processes

Question 1

Vaccum forming process (6)

Answer 1

• Mould is placed on the bed of the platen which is then lowered to the bottom
• 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 over the sheet
• Vacuum pump is switched on which sucks the polymer onto the mould
• Once polymer has cooled and returned to a solidified state, the platen is lowered and vacuum is switched off
• Mould is removed from the moulding and excess polymer is trimmed off

Question 2

Vacuum forming:

• Polymer stock form
• Products

Question 3

Key features of vacuum forming (3)

Answer 3

• Moulds are relatively easy to make but need a draft angle, air holes and rounded corners
• Not cost effective for one-off manufacture
• Small-scale batch and mass production

Question 4

Thermoforming process (2)

Answer 4

• Similar to vacuum forming but there is an additional mould that is pressed onto the surface of the polymer sheet at the same time the vacuum is applied
• The two moulds trap the softened polymer in between them giving extra detail to the moulding

Question 5

Thermoforming:

• polymer stock form
• products

Answer 5

• Thermoplastic sheet approx 6mm
• Bath, food packaging (fruit punnets, cake boxes, sandwich boxes)

Question 6

Key features of thermoforming

Answer 6

• Good for fine detail such as moulding in logos
• Slow process used in industrial production

Question 7

Calendering process (4)

Answer 7

• Pellets of thermoplastic is heated so that it melts into a dough-like consistency
• It is then extruded between a series of heated rollers to make it thinner
• Its then finished on cooling rollers
• It is then chopped into standard stock sheet sizes, or in the case of polymer film it is rolled up for storage and distribution

Question 8

Calendering:

• Polymer stock form
• products

Answer 8

• Thermoplastic pellets
• Thin polymer sheet and film

Question 9

Key features of calendering

Answer 9

• Is an industrial production method, carried out by specialist manufacturers
• Used for continuous production

Question 10

Line bending process

Answer 10

Uses an electrically heated element that provides heat along a line to bend sheet of termoplasric shuck as acrylic

Question 11

Line bending:

• Polymer stock form
• Products

Answer 11

• Thermoplastic sheet
• Acrylic boxes, sheet or point-of-sale displays

Question 12

Key features of line bending (2)

Answer 12

• Quite slow and labour intensive - commonly used in school or college workshops
• Suitable for one-off or limited batch production

Question 13

Lamination (lay-up) process (6)

Answer 13

• a mould or former in the shape of the product or component is prepared - made from timber, manufactured boards or high density foam
• Mould or former coated in related agent e.g wax or PVA or parcel tape
• A top layer of gel (gel is polyester resin mixed with a pigment) is applied to give colour and addititives such as prevent UV degradation and hydrolysis
• Fibreglass matting is cut to size and laid over the former then begins the stippling process 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
• 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
• GRP is allowed to set

Question 14

What is the stippling process that takes place during lamination?

Answer 14

polyester resin in brushed onto matting and a small roller is used to push out any air bubbles that may form and to smooth out the matting

Question 15

Lamination:

• polymer stock form
• products

Answer 15

• Fibre-based composite sheets (GRP, CFRP)
• Boats hulls, kayaks, train carriages, theatre and film props, theme park rides

Question 16

Key features of lamination (lay-up) (4)

Answer 16

• Requires a former to mould around
• Polymer resin used as the polymer
• A top gelcoat is applied which may contain pigment
• CFRP requires an autoclave for the product to ‘cure’

Question 18

Injection moulding process (5)

Answer 18

• Thermoplastic granules are poured into the hopper
• A screw thread is rotated by a motor thus pulling the granules through the chamber and past electric heaters
• The heaters melt the polymer
• When a sufficient charge of polymer has been melted and formed at the end of the screw, a hydraulic ram forces the screw thread forward injecting the polymer into the mould
• The mould is water cooled to allow it to harden quicker
• Any excess polymer is trimmed of the moulding - formers or jigs May be used the dimensional accuracy of the moulding while it cools and gardens completely

Question 19

Injection moulding:

• polymer stock form
• products

Answer 19

• Thermoplastic granules
• Electrical product casings (computers, TVs, vacuum cleaners, calculators

Question 20

Key features of injection moulding (4)

Answer 20

• Complex 3D shapes can be easily made
• Fast industrial production method
• Used for large-scale mass or production
• High initial tooling cost

Question 21

Blow moulding process (5)

Answer 21

• The polymer is fed into the hopper
• An Archimedean screw pulls the polymer through a heated section melting the polymer
• The melted polymer is extruded as a tube called a parison
• The mould slides close around the parison and air is injected into the mould, forcing the polymer to the sides
• The polymer is allowed to cool for a few seconds the mould opens and the finished bottled is ejected

Question 22

Blow moulding:

• polymer stock for
• products

Answer 22

• Thermoplastic tube ‘parison’
• Drink bottles, shampoo bottles, detergent bottles etc

Question 23

Key features of blow moulding (3)

Answer 23

• Produces hollow thin-walled components
• High initial tooling cost
• Used for continuous production

Question 24

Rotational moulding process (4)

Answer 24

• Polymer powder or granules are loaded into a mould, which is clamped and sealed
• Mould is transferred to an oven where it is heated to 260-370°c, depending upon the polymer used then the mould is slowly rotated around two axes and as it’s heated it costs the inside of the mould
• Once the polymer has achieved the correct thickness the mould is cooled - usually a fan and/or water is used to cool the polymer
• When polymer has solidified, the part will shrink slightly, allowing it to be removed

Question 25

Rotational moulding: - polymer stock form - products

Answer 25

* Thermoplastic powder or granules * Traffic cones, kayaks, water tanks, kerosene tanks, children's play equipment

Question 26

Key features of rotational moulding (3)

Answer 26

* Seamless hollow components with a large wall thickness * Mould continuously rotated through heating and cooling * Large-scale batch of mass production

Question 27

Extrusion process (6)

Answer 27

* Polymer granules are loaded into the hopper * The Archimedean screw moves the polymer granules past heaters * The heaters soften the polymer * When sufficient polymer has melted, the hydraulic ram pushes the Archimedean screw, forcing the polymer through a steel die the shape of the die determines the shape of the extrusion * 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

Question 28

Extrusion: - Polymer stock form - Products

Answer 28

* Thermoplastic granules * Solid rods, tubing, angle sections, curtain track

Question 29

Key features of extrusion

Answer 29

* Industrial production method * Continuous production

Question 30

Compression moulding process (5)

Answer 30

* A 'slug' of ore-weighed thermoset polymer is inserted into pre-heated moulds * The moulds are closed and hydraulic pressure is applied this pressure ensures that the polymer takes the shape of the mould * The moulds remain closed while cross-linking takes place and the thermoset 'cures' * When the moulding has cured, the machine opens and the product is removed * Excess polymer known as 'flash' is removed

Question 31

Compression moulding: - polymer stock form - Products

Answer 31

* Thermoset polymer charge 'slug' * Electrical light fittings, switches, plugs, picnic plates and cups

Question 32

Key features of compression moulding (3)

Answer 32

* Uses a pre-weighed 'slug' of polymer * Polymer removed from moulds * Large-cale batch production