Thermoforming* is a polymer processing technique in which the combination of pressure and heat is used to stretch plastic into or onto a mould to take on a desired shape. The process is considered a secondary process as sheet polymer must be produced before thermoforming can occur. Products made by thermoforming range from low value packaging for the food industry to bespoke or more expensive heavy duty parts such as car dashboards or fridge liners.
* Animation from British Plastics Federation www.bpf.co.uk
A thermoforming procedure usually consists of the following steps.
The first stage of thermoforming is producing sheet to the correct thickness from which to make the final part. This is typically created using extrusion and calendaring techniques so as to achieve a constant gauge. It is at this stage that additives and colour are added to determine the desired properties of the final product.
Material can be prepared for thermoforming in two ways, dependant of the thickness of the part to be manufactured. Thin gauge parts using lighter material can be fed from a roll or formed inline directly from the extruder and indexed through the forming station. Sheet for thicker gauge parts will be fed into the thermoformer in separate sections pre-cut to size.
Production volumes are usually much higher for thin gauge parts as heating and forming time is lower, and roll fed thermoforming is typically highly automated and continuous.
Before the forming process takes place the sheet of plastic is heated to a predetermined temperature to achieve pliability. This temperature is critical as cool sheet will not form into integrate detail in the mould and too much heat will result in the material sticking to the mould. The sheet can be heated unevenly across the sheet as required to suit the complexity of the mould.
There are four main methods of creating the force required to stretch the polymer over the mould.
- Mechanical - an object applies a force to the sheet to create the desired shape
- Atmospheric Pressure – a vacuum creates a pressure difference above and below the sheet and atmospheric pressure forces the plastic into the mould
- Pressure forming – the sheet is subjected to a compressed air supply that forces it onto the mould. This can provide much greater force that atmospheric pressure.
- A combination of the above – when one of the above methods does not generate enough force to create a desired shape, they can be combined to
Cooling of the plastic occurs by the use of a cooling system in the mould or by waiting on the polymer to cool toward ambient temperature depending on production requirements. It is vital the material cools enough to maintain its shape, yet remain pliable enough to be removed from the mould.
The part is ejected from the mould either by physically pushing the part with an ejector or blowing it out with a jet of compressed air. This ejection can be before or after the final shape is cut from the surrounding material. This trimming process is the final stage of thermoforming, and is simply cutting the desired shape away from the surrounding parent sheet.
The sheet that is left by the thermoforming process is usually granulated and returned to the extruder to start the process again.