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Vacuum forming, i.e. thermoforming, is one of the oldest and most common methods of processing plastic materials. Thermoformed plastic products are all around us and play a major part in our daily lives. It is a very versatile process used to manufacture a wide range of products from simple packaging trays to high impact aircraft cockpit covers. It is also used extensively to make design prototypes of products to be produced by other processes.
The process, however, is basically the same in each case. In its simplest form thermoforming is the heating of a plastic sheet which is then draped over a mould whilst a vacuum is applied. The moulding is then allowed to cool before it is ejected from the mould using a reverse pressure facility.
The Vacuum Forming Process
In its simplest form the process consists essentially of inserting a thermoplastic sheet in a cold state into the forming clamp area, heating it to the desired temperature either with just a surface heater or with twin heaters and then raising a mould from below. The trapped air is evacuated with the assistance of a vacuum system and once cooled a reverse air supply is activated to release the plastic part from the mould.
The thermoforming industry has developed despite two fundamental shortcomings. Many other thermoforming processes use a resin base in powder or pellet form. Vacuum forming begins further down the line with an extruded plastic sheet which incurs an additional process and therefore an extra cost to reach this stage. In addition, there is generally an area of material which is cut away from the formed part which unless reground and recycled has to be considered as waste and accounted for in any costings made. However these problems have been invariably resolved by strict control of sheet quality and by clever mould design to minimise the amount of waste material. Despite the above disadvantages vacuum forming offers several processing advantages over such others as blow, rotational and injection moulding. Fairly low forming pressures are needed therefore enabling comparatively low cost tooling to be utilised and relatively large size mouldings to be economically fabricated which would be otherwise cost prohibitive with other processes. Since the moulds witness relatively low forces, moulds can be made of relatively inexpensive materials and mould fabrication time reasonably short. This results in comparatively short lead times. It provides the perfect solution for prototype and low quantity requirements of large parts as well as medium size runs utilising multiple moulds.
The typical process steps can be identiﬁed as follows: clamping, heating with sheet level activated , pre-stretch, forming with plug assist, cooling with air and spray mist, release and trimming.
The clamp frame needs to be sufﬁciently powerful enough to handle the thickest material likely to be formed on the machine. If an automated process is used the operation of the moving parts must be guarded and interlocked to avoid accidental damage. In addition a safety guard ( in the form of a fabricated guard or light curtain) must be provided to protect the machine operator at all times.
Heaters are generally infra-red elements mounted within an aluminium reﬂector plate. In order to obtain the best vacuum forming results, using any material, it is essential that the sheet is heated uniformly over its entire surface area and throughout its thickness. In order to achieve this it is necessary to have a series of zones which are controlled by energy regulators. Ceramics do have some disadvantage in that their high thermal mass makes them slow to warm up (approx 15 minutes) and slow in their response time when adjustments are made.
A photo-electric beam is incorporated in the machine to scan between the bottom heater and the sheet of plastic. If the sheet of plastic sags down and breaks the beam then a small amount of air is injected into the bottom chamber, thus lifting the sheet to stop it from sagging.
Once the plastic has reached it's forming temperature or 'plastic' state it can be pre- stretched to ensure even wall thickness when the vacuum is applied. Pre-stretch is an invaluable feature when forming deep draw parts with minimum draft angles and high mould surface detail. The method of controlling the bubble height should be such that consistent results are obtainable. Vacuum, air pressure and optional aids such as a plug assist are then used to assist in moulding the heated, stretched plastic.
Once the material is suitably pre-stretched a vacuum can be applied to assist in forming the sheet. A dry vane vacuum pump is used to draw the air trapped between the sheet and the mould. The vacuum pump should be capable of maintaining a differential pressure of approx 27" mercury. With larger machines a vacuum reservoir is used in conjunction with a high volume capacity vacuum pump. This enables a two stage vacuum to be applied ensuring rapid moulding of the heated sheet ( before the sheet temperature drops below its ideal forming temperature).
Plug-assist forming is the term used to describe the use of a male plug tool, mounted on a pneumatic or hydraulic cylinder situated over the forming area of the machine, to force the material into a female cavity within the moulding area. It enables complicated and deep-draw moulds to be produced without webbing and with even thickness distribution. The idea behind the process is to feed as much material into the cavity prior to the vacuum being applied in order to avoid thinning in that area. Plug moulds are generally made from wood or metal and a smooth surface allows the sheet to slide whilst stretching into the mould. A felt or leather lining ensures that the risk of premature chilling on contact is greatly reduced. Resin plugs provide a good alternative as being good insulators they do not affect the temperature of the sheet.Plug assist is also an essential feature when forming multiple impression male moulds as they can be placed very close together without the fear of the material webbing between the formed parts.
Cooling and Release
Once formed the plastic must be allowed to cool before being released. If released too soon then deformation of the moulding will result in a reject part. To speed up the cooling cycle high speed fans are ﬁtted and activated once the part is formed. A spray mist option is also available whereby nozzles are attached to the fans and a ﬁne mist of chilled water is directed onto the sheet. This, in conjunction with the fans can speed up the cooling cycle by up to 30%. Mould temperature control units are also available which regulate the temperature within the mould ensuring accurate and consistent cooling times when cooling crystalline and crystallising polymers such as PP, HDPE and PET.