Forming objects through the use of a vacuum is also called thermo-forming, because it’s a process that uses heat. It’s the way that many objects that we use every day are developed and made. At its simplest, a sheet of plastic is heated and then draped over a mould. Next, a vacuum is used to suck the plastic sheet into the mould. After this, the sheet is ejected, now in the shape of the mould.
Obviously, there are variations to this basic process that are more sophisticated. Special pneumatic and heating controls can deliver high production speeds and produce very complex and detailed moulds in a range of finishes.
Clamping and Heating
From Polyester Copolymer to Polystyrene, Polypropylene, PVC, Polyethylene and Acrylic, the process can use a diverse range of thermoplastic materials. Two of the main processes these are subjected to are clamping and heating. See the pictures in this document to get an idea of the components used in the forming set-up: https://docs.google.com/document/d/1XeWDEWVADTHM1fkZCcQGk45XMeJ5cwC1MF4FkgCEgFw/edit#.
The plastic sheet has to be held firmly in place during the process. A sheet of up to 6mm thickness can be vacuum-formed using one heater, and twin heaters can form a sheet of up to 10mm wide.
The heaters themselves have an infrared element situated within a reflector plate, usually made of aluminium. The sheet has to be evenly heated, with the entirety of its surface heated, and heat transported through the whole thickness of the sheet. To achieve this, the heater has a number of zones controlled by regulators. Quartz heaters are generally faster to heat up than ceramic heaters and are therefore preferred in high-temperature applications.
How the Melting Is Controlled
The accurate control of the heating temperature depends on a sensor called a pyrometer. This picks up the melting temperature of the plastic sheet and passes this information to a process controller. According to https://www.bridgewooduk.com/production-assembly/vacuum-forming, when the vacuum forming needs to be carried out at a very precise temperature, twin quartz heaters are used because they can accurately form material at high temperatures. The heaters have to compensate for loss of heat from the plastic sheet caused by air currents, for example.
This is a great example of a basic process that has been adapted and refined to enable the high volume, low-cost production of sophisticated moulded products.