Working With Polymers Flashcards
(40 cards)
Vacuum forming process
- The mould is placed on the bed of the machine. The mould must have a draft angle so it be easily removed.
- Polymer sheet is clamped over the mould and a heater is pulled over the polymer sheet.
- When the polymer sheet has softened, the mould is raised into the polymer and the heat is removed.
- The vacuum is switched on, which forces the polymer over the mould.
- Once the polymer has cooled, the mould is lowered and vacuum is switched off.
- The mould is removed from the moulding. Excess polymer is then trimmed off.
Thermoforming
Similar to vacuum forming but used to mould additional detail into the polymer or to mould thicker sheets.
Thermoforming process
- The polymer is clamped between a negative mould and a positive mould.
- The polymer is heated and the positive mould is pressed into the polymer. At the same time, a vacuum is applied, sucking the polymer down on the mould below.
- The two moulds trap the softened polymer between them, giving extra detail to the mould.
Calendering
Is a smoothing and rolling process used at the end of the paper manufacturing process and to make think polymer sheets and film.
Calendaring process
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 the finished on a cooling roller before being chopped into standard stock sizes.
Type of production for calendaring
Is a purely industrial production method, carried out by specialist manufacturers. Usually used for continuous production.
Line bending
Uses an electrically heated element to heat the plastic along a line. This allows the thermoplastic to be bent at that line.
Line bending scale of production
Is suitable for one off production or limited batch production as it is quite slow and labour intensive.
Lamination (lay-up)
Used to make 3D products from fibre based composites, such as GRP and CFRP. Examples include boat hulls, scenery, props for theatres.
Lamination (lay up) process
- A mould in the shape of the prodduct is prepared. This may be made from timber, manufactured boards and high density foam.
- The mould is covered in a releasing agent, such as wax or PVA, or covered in parcel tape.
- A top layer of gel coat is applied. The gel coat is a coating of polyester resin often mixed with a pigment to give colour to the moulding. The resin may also have additives to prevent degradation from UV and hydrolysis.
- Fibreglass matting is cut to size and laid over the former. Polyester resin is brushed into the matting and a small roller is used to push out any air bubbles that may form and to smooth out the matting.
- The previous steps are repeated until the desired thickness is achieved, and fine tissue matting is used on top layers. A vacuum bag may be used to compress the layers ofGRP before it sets.
- The GRP is then allowed to set.
Injection moulding scale of production
Large scale mass production or continuous production, due to the high cost of the equipment and the moulds.
Injection moulding process
- Thermoplastic granules are poured into the hopper.
- A screw thread is rotated by a motor. This pulls the granules through the chamber and past electric heaters.
- The heaters melt the polymer.
- 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.
- The mould is water cooled, which enables the molten polymer to harden quickly.
- The mould opens ad ejector pins puch the moulding out.
- Any excess polymer is trimmed off the moulding. Formers and jigs may be used to maintain dimensional accuracy of the moulding while it cools and hardens completely.
Blow moulding
A process used to form hollow forms out of thermoplastics.
Blow moulding processes
- The polymer is fed into the hopper.
- An Archimedean screw pulls the polymer through a heated section, melting the polymer.
- The molten polymer is extruded as a tube called a parison. The mould sides close around the parison, and air is injected into the mould, forcing the polymer on the sides.
- The polymer is allowed to cool for a few seconds, the mould opens and the finished bottle is ejected.
Pro of blow moulding
It is very accurate, so is used with products with a high level of tolerance (threads for example).
Rotational moulding
Used to produce heavy duty, seamless hollow objects that need a large wall thickness. Blow moulding is not suitable as it would produce a product that have a wall that is too thin.
Rotational moulding pros and cons
The moulds can be changed relatively quickly, but the high set up costs mean it can only be used for large scale batch or mass production.
Rotational moulding process
- Polymer powder or granules are loaded into the 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 (fewer than 20 rotations per minute) around the two axes, and as the polymer is heated, it coats 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 the polymer has solidified, the part will shrink slightly, allowing it to be removed.
Extrusion moulding process
- Polymer granules are loaded into a hopper.
- The Archimedean screw moves the polymer granules past the heater,
- The heaters soften the polymer.
- When sufficient polymer has melted, the hydraulic ram pushes the Archimedean screw, forcing a 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.
Compression moulding
Is a common process to mould thermoset polymers like UF and MF. Used for electrical plugs and light fittings.
Compression moulding process
- A slug of pre-weighed thermoset polymers like UF is inserted to a set of preheated moulds.
- The moulds are closed and the hydraulic pressure is applied. The pressure ensures that the polymer takes the shape of the mould.
- The mould remains closed while the 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.
Why are acrylic spray paints are used on polymers?
- provides protection against the effects of UV light
- improves aesthetics
Acrylic spray paints
- fast drying
- water soluble
- resistant to water when dry
Over moulding
Is moulding a second polymer over specific parts of a product. TPE is often used, and over mouldings are primarily used to provide areas of grip and texture.
