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 process is shown in the following diagram form.
In its advanced stage, pneumatic and hydraulic systems, complimented with sophisticated heat and process controllers, allow high speed and accurate vacuum forming for those heavy duty and high end volume applications.
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. Throughout this manual you will ﬁnd useful hints and techniques to assist in maximising the potential from this process. 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. These are examined more closely under the following sub headings.
CLAMPING The clamp frame needs to be sufﬁciently powerful to handle the thickest material likely to be formed on the machine — up to 8 mm with our single heater models and up to 12 mm 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. 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.
HEATING 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 disadvantages: 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. A cooling jacket is required for the pyrometer. Precise temperature readout is also available with a computer controlled system working in unison with the pyrometer(s). The mega point system was devised by Formech for the accurate heating of large areas, using a standard PC and a minimum of exterior electronics. The heating control system is an extension of the process controller, allowing rapid visual interpretation of the heater zoning. Temperatures are controlled precisely using thyristor modules. Full feedback is available to allow zones to be banked up or down by percentage amounts. Twin heaters are also recommended when forming thicker materials as they assist in providing more uniform heat penetration and faster cycle times.
SHEET LEVEL 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 prestretched 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.
VACUUM 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).
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. 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. See the relevant section for further details on trimming and ﬁnishing.
TWIN SHEET THERMOFORMING Two sheets are heated and thermoformed. One sheet is formed in the top mould and the other in the bottom mould. After forming the two sheets are brought together. A game of temperature and pressure causes a micro-fusion of material that welds the two sheets together. This technology gives the possibility to produce hollow products. A good design of points where the two sheets weld together can give a very rigid but light product.
MAIN ADVANTAGES OF TWIN-SHEET:
Increased Structural integrity and Rigidity.
Enclosed Cross-Section Capability.
low Tooling Cost.
lnternal Reinforcement Options: Structural Member, Rigid Foam, Etc.
The process has some distinct advantages over blow - moulding and rotomoulding.
Most thermoplastics can be used.
This technology found lot of applications in the packaging industry because the hollow section gives a very good impact strength in comparison with a solid plastic pallet. Minimum 50% weight reduction makes the packaging much more ergonomic and the overall loading capacity is more than sufﬁcient for at least 60% of the industrial applications. Many other products ﬁnd their way to Twin-Sheet thermoforming. Those grey tiles are used to protect the stadium floor while building up the stages for a concert at Welmbley Stadium (and many other places). They are made out of HDPE, 2m by 2m twin-sheeted and support a 8 tonnes load. Those performances are created by a well deﬁned cone pattern that gives an unbelievable compression-strength.