The typical process steps can be identified as clamping, heating with sheet level activated , pre-stretch, forming with plug assist, cooling with air and spray mist, release and trimming They are examined more closely under the following sub headings;
The clamp frame needs to be sufficiently powerful enough to ensure the plastic sheet is firmly held during the forming process. It should be able to handle the thickest material likely to be formed on the machine – up to 6mm with a single heater model and up to 1Omm with the twin heater machines. If an automated process is used the operation of the moving parts must be guarded and interlocked to avoid accidental damage.
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.
More sophisticated quartz heaters are available which have less thermal mass enabling more rapid response time. Pyrometers enable accurate heat temperature control by sensing the melting temperature of the sheet and interacting with the operating process control. Precise temperature readout is also available with a computer controlled system working in unison with the pyrometer(s). Twin heaters are also recommended when forming thicker materials as they assist in providing more uniform heat penetration and faster cycle times.
Twin quartz heaters are advisable when forming high temperature materials with critical forming temperatures. By close control of areas of heat intensity, heat losses around the edges caused by convection air currents and absorption from clamp areas can be fully compensated for and consistent results achieved on a continuous basis. Cost savings can also be considerable if Quartz heaters are specified as there is an adjustable percentage power drop when the heaters are in the rear position during the forming process.
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.
Pre-stretch ( Bubble)
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.
Once the material is suitably pre-stretched a vacuum can be applied to assist in forming the sheet. 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.
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
Once cooled sufficiently the sheet can be released by a reverse pressure activated through the vacuum system. The part is then stripped from the mould and transferred to the trimming station.
Trimming and Finishing
Once the formed part has cooled and been removed from the machine the excess material is removed. Holes, slots and cut-outs are then drilled into the part. Other post-forming processes include decoration, printing, strengthening, reinforcing and assembly.
A variety of different trimming methods are used to trim the product from the sheet. The type of equipment best suited depends largely on the type of cut, size of the part, draw ratio, thickness of material and the production quantity required. They are also factors to consider when determining the investment cost of such equipment. Thin gauge parts are normally trimmed on a mechanical trim press – otherwise known as a Rollerpress. Heavy gauge parts can be removed, placed into trim “jigs” or fixtures and trimmed with any of the following; Horizontal / Vertical Bandsaw, Hand Held Air Powered Router, CNC Routers - 3, 4 and 5 axis.