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Vacuum Forming
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Virtually all thermoplastics can be supplied as sheet and vacuum formed. The more commonly used materials are listed below.
Typical Materials UsedAcrylonitrile Butadiene Styrene ABSPolyester Copolymer PETG Polystyrene PS Polycarbonate PC Polypropylene PP Polyethylene (sheet and foamed sheet) PE Polyvinyl Chloride PVC Acrylic PMMA Typical Products Produced
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Overview
Thermoforming is one of the oldest and most common methods of processing plastic materials. Vacuum formed products are all around us and play a major part in our daily lives.The process involves heating a plastic sheet until soft and then draping it over a mould. A vacuum is applied sucking the sheet into the mould. The sheet is then ejected from the mould. In its advanced form, the vacuum forming process utilizes sophisticated pneumatic, hydraulic and heat controls thus enabling higher production speeds and more detailed vacuum formed applications.
Nature of Use and LimitationsVacuum forming offers several processing advantages over other forming processes. Low forming pressures are used thus enabling comparatively low cost tooling.Since the process uses low pressures, the moulds can be made of inexpensive materials and mould fabrication time can be reasonably short. Prototype and low quantity requirements of large parts, as well as medium size runs therefore become economical. More sophisticated machines and moulds are used for continuous automated production of high volume items like yoghurt pots, disposable cups and sandwich packs. Unlike other thermoplastic forming processes, where powder or resin are the starting point, vacuum forming uses extruded plastic sheet. With vacuum forming a secondary process may be required to trim the formed sheet to arrive at the finished part. The trimmed waste can then be re-ground and recycled. |
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The Process
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;
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 – 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.
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 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.
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 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
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
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.
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.
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 – 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.
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 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.
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 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
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
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.
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.
Contact a Moulder
If you are looking for a product or a moulder, please complete your details below and the BPF will send the detail, on your behalf, to the most relevant companies within the plastics industry who will contact you directly with more details.Links
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This process page is kindly sponsored by Formech International Ltd. For more information please e-mail ... sales@formech.com or visit www.formech.com |
About Formech International Ltd
Formech recognise the importance that plastics play in our society today and have been manufacturing and selling thermoforming equipment worldwide for over twenty years. The Company has provided industrial and educational users with a wide range of high precision engineered, easily maintained and cost effective vacuum forming machines to turn plastic sheet into innumerable shapes, objects and commercial products.
Formech's commitment is to support their customers at the forefront of technological change in implementing their product ideas. Through prudent monitoring of the industry and a constant awareness of customer requirements Formech has provided the vacuum forming solution to over 6000 customers worldwide.
Formech strive to ensure our machines and support services continue to meet the ever growing and rigorous demands of the international market. That combined with an innovative approach ensures Formech is now firmly established as the most popular producer of manual, semi and fully automatic thermoforming machines on the market.
Formech's commitment is to support their customers at the forefront of technological change in implementing their product ideas. Through prudent monitoring of the industry and a constant awareness of customer requirements Formech has provided the vacuum forming solution to over 6000 customers worldwide.
Formech strive to ensure our machines and support services continue to meet the ever growing and rigorous demands of the international market. That combined with an innovative approach ensures Formech is now firmly established as the most popular producer of manual, semi and fully automatic thermoforming machines on the market.
