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In its advanced form, the vacuum forming process utilizes sophisticated pneumatic, hydraulic and heating controls thus enabling higher production speeds and more detailed vacuum formed applications.
Commonly used plastic materials are:
- ABS - Acrylonitrile Butadiene Styrene
- PETG - Polyester Copolymer
- PS - Polystyrene
- PC - Polycarbonate
- PP - Polypropylene
- PE - Polyethylene (sheet and foamed sheet)
- PVC - Polyvinyl Chloride
- PMMA - Acrylic
Common products made from thermoformed plastics are:
- Bath Tubs Shower Trays
- Ski-Boxes Small Boat Hulls
- Machinery Guards Hygenic Door Liners
- Refrigerator Liners Surf Boards
- Parts of vehicles Exterior Shop Signs
Materials - The process uses a group of plastics known as 'thermoplastics', such as: acrylic, polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP) and crystalline polyester (CPET). Fibre reinforcing within the material will give added strength and rigidity to the finished product.
Applications - The most familiar products made from vacuum forming include bath tubs, sink units, shower trays, open plastic containers and panels, food packaging and containers, automotive parts and electrical cabinets and enclosures.
The Process - The typical process steps can be identified as sheet heating, sheet transfer, clamping, pre-stretch, forming, cooling, release and trimming. They are examined more closely under the sub headings listed below.
Process Variations - The heating method used can vary from small infrared heaters to large forced air convection ovens with gas or electric heating. The sheet should be heated from both the top and bottom of the sheet. The use of thick sheets requires a top enclosure and a clamping process. Compressed air forms part of the process, as additional force may be required to force the heated plastic sheet down over the cold mould, which is porous and is subject to a vacuum from beneath.
Quality Issues - The process requires materials, which exhibit high uniform elongation otherwise tearing can occur at critical points and there is a danger of excessive thinning at sharp corners.
Strict control of sheet temperature is absolutely vital and this is why the choice of Sheet Heating Oven is of paramount importance. Clamping force and vacuum pressure are both important to minimise variability in the final component.
As sheet material has plastic memory at high temperatures it is vital to get an optimum operating and uniform temperature of the sheet of plastic. Surface finish on the underside may or may not be critical along with tolerances these factors are largely dependent on the mould design and surface finish.
Economic Factors - Equipment and tooling costs for vacuum forming are moderate to high but depend largely on the complexity of the mould. There is some skilled labour required for the process but this can be reduced with process automation.
Production rates are 20-300 components per hour dependant on product size. The process lends itself to excellent material utilisation with less than 3% of material wasted however the waste material is not directly recyclable. Material wastage can be minimised by the use of a high quality Sheet Heating Oven, which ensures even and repeatable heating of the sheet.
Implications for Product Design - Relatively complex geometries and even hollow shapes are possible with vacuum forming. Holes, inserts, threads, ribs, lettering, cores, moulding around inserts are possible but come at an extra cost.
Sheet Heating - The single most important factor in developing a successful and productive vacuum forming process is the even and complete through heating of the sheet.
Radiant heaters provide a rapid warm up but can only heat the sheet from one side. Multiple radiant units are required for large sheets and issues associated with heat intensity and lamp overlap can cause localised hot spots or cold spots, which result in thinning or stress cracking.
The use of a forced air Sheet Heating Oven, designed by an experienced sheet heating oven manufacturer can return dividends in terms of product quality in a very short period. The incorporation of high mass, low adhesion product platens ensure that both sides of the sheet are evenly heated over the entire sheet area.
The ambient sheets are loaded into the oven via manual or pneumatically operated doors. The Sheet Heating Oven incorporates a PLC process controller to indicate when the sheet is ready for forming. In high volume environments the ovens incorporate a vertically powered indexing sheet lift, which presents a correctly heated sheet to a single automatic door at a pre-defined time interval.
The fact that the air temperature is rapidly recirculating at a precisely controlled temperature ensures that no part of the sheet can ever fail to evenly heat or locally overheat. Airflow Products Ltd in Sheffield, UK have been manufacturing Sheet Heating Ovens for many years and are acknowledged as world leaders in this field.
Clamping - 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
Forming - Once the material is suitably clamped a vacuum can be applied to assist in forming the sheet. In some cases a pre-stretch is performed by applying compressed air to the underside of the sheet, which blows a bubble inside the clamp. The pressure is then exchanged for vacuum and the forming begins. 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.
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 a reverse pressure activated through the vacuum system can release the sheet. 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.
Please browse our website for more information about our rapid specification, quotation, manufacturing, installation & after sales Sheet Heating Oven service then contact us on 01142 327788 to speak to one of our team or e-mail us on sales@airflow-group.com.
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