Materials used in vacuum forming are usually thermoplastics. Almost all thermoplastics can be supplied in sheets and so can be vacuum formed. It’s also suitable for transparent materials such as acrylic, making it ideal in the aeronautical industry creating cabin windows and cockpit canopies.
HIPS – High Impact Polystyrene Sheeting
ABS – Acrylonitrile butadiene Styrene
PMMA – Acrylic
PE – Polyethylene
PETG – Polyester Copolymer
PS – Polystyrene
PC – Polycarbonate
PVC – Polyvinyl Choride
PP – Polypropylene
If you aren’t sure which plastic is the best type to use for your application then Bray Plastics can help. Our expertise means that we can advise you of the best material for the end product and for the mould to ensure a cost effective, quality production.
Recycled plastic can be used which is an environmentally friendly option. It’s usually quite adequate for most applications and cheaper than polycarbonate or carbon fibre.
Some plastics don’t form very well and these should be avoided. Bray Plastics can advise you which these are.
There are also a type of plastic called thermo-setting plastics. These once heated and solidified again can’t then be remelted. Bakelite is an example of this – it’s a liquid resin, can be heated, but then can’t be remelted and reformed once set.
Vacuum forming is a type of thermoforming, where a plastic sheet is heated until it becomes mouldable, when it is then stretched over a mould and the force of the vacuum pulls the plastic against the mould.
Vacuum forming can be used for a number of purposes. Bray Plastics often use vacuum forming to create very large scale components. The largest to date was the size of a bath tub. It is also possible to lay a number of moulds on the same bed and create many smaller components at the same time. Depending on size up to 80 components could be created from one plastic sheet.
Before any vacuum forming can take place a mould is required. This can be made from wood, aluminium, foam, resin. When creating the mould the plastic engineer must consider the shrinkage of the plastic when it is heated and cooled. Most plastics shrink by different amounts so the mould must be made larger by the same percentage to allow for the shrinkage. ABS shrinks by 0.8% so the internal dimensions of the mould must be 0.8% larger to allow for the shrinkage whilst cooling.
The mould is placed on the machine bed and held in place. The plastic sheet is clamped firmly in place. The clamps need to handle the thickest material (between 6-10mm). Some plastic sheets have undergone screen printing prior to being formed. In these cases the screen print file is created from the end product and worked backwards to form a flat print.
The plastic sheet is heated so the sheet reaches a uniform temperature throughout its thickness. Different plastics have different temperatures they are required to reach before they become malleable.
The heaters are usually infra-red elements set in an aluminium reflector plate.
Once the plastic has reached it’s ‘plastic’ state or forming temperature the mould is pushed up into the sheet. The vacuum is applied to draw the air out between the sheet and the mould.
Only once the plastic has been formed around the mould is it allowed to cool. Once cooled the plastic returns to its solid state. If the mould is released before the plastic is fully cooled then the plastic may become deformed and the component will be spoiled.
Once the formed part has been cooled and removed from the machine it needs to be cut from the excess material and then have a finishing process if necessary. This can include cutting holes or slots, printing, coating, strengthening, assembly or reinforcing.
Vacuum forming originated in the 1930s and is still a hugely relevant engineering process due to its low cost, efficiency and speed of replication, and the cost of creating the tooling.
New thermo-formable plastics and composites are being developed to be more environmentally friendly and more efficient to process.
Vacuum forming can be used in conjunction with a number of other processes including injection moulding, 3D printing and casting to form a number of components that will all be assembled together to form a final product.
The terms vacuum forming and thermoforming are usually interchangeable, however thermoforming is a more generic term and refers to a range of fabrication processing where a plastic sheet has been heated to its ‘plastic’ malleable state and then shaped using a mould. The forming (pushing the plastic on to the mould) can be via vacuum or air pressure.
A male or female mould can be used. A male mould is the same shape as the object being formed, and the plastic sheet would be placed over it. A female mould forms the exterior of a shape and is the reverse of the object being made.
Once the plastic has cooled it will be permanently in the shape of the mould unless it is heated to a high temperature again.
Thermoforming can include pressure forming, vacuum forming and twin sheet forming.
Vacuum forming is a type of thermoforming. The plastic is heated to it’s plastic, soft, malleable shape and placed over a mould. Then the air is vacuumed out, sucking the plastic onto the mould. Then the plastic is cooled back to a solid state, forming the new shape. The air can then be blown back to free the plastic from its mould.
Here are some examples of vacuum forming.
Thermoforming is a term used in plastic engineering to cover a variety of processes. All thermoforming involves heating a plastic sheet to its pliable plastic state. Different plastics require a different temperature and length of heating to reach their malleable state.
Once heated the plastic is placed over an existing mould. Moulds can be made of a variety of different materials including MDF, aluminium, epoxy and foam. Then the plastic sheet is pushed or sucked around the mould to form the new shape. This can be done via air pressure (pressure forming) or vacuum (vacuum forming).
Once the plastic cools it becomes solid again and will hold its new shape. It can then be removed from the mould and a variety of finishes can be applied if necessary. These can include different coatings, screen printing, lamination etc.
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