Thermoforming is one of the oldest 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.
1- Is Vacuum Forming an out-dated Technology?
Vacuum forming originates from the 1930’s and yet it remains increasingly relevant today.
No alternative process can match the low cost, ease of tooling, efficiency and speed of replication for prototyping and small series of certain shapes. One of the main benefits of vacuum forming is the speed at which tooling can be produced and the cost compared to injection moulding is substantially cheaper.
Combining vacuum forming with new technologies such as 3D printing for tool design, thermo-formable ink technology and In-Mould Decoration bring new possibilities and advantages as employed by the world’s largest brands and institutions. Formech collaborates with Dupont, BASF, Cambridge University and many more international technology centres to conduct research into new advanced composite materials which will bring new possibilities for vacuum forming for high tech industries. The world’s leading material science companies continue to invest in development of thermo-formable plastics which are more efficient to process and kinder to the environment. Engineering grade and composite plastics are typically much more demanding to form successfully or require industrial specification machinery. Formech now offers a range of high performance and compact machines bringing industrial processing capability to lab environments for efficient testing and research with new materials.
2- Vacuum Forming, 3D Printing & Injection Moulding
Every process is there for a good reason and usually each process occupies its own space within the complete manufacturing spectrum. Vacuum forming is a valid alternative to 3D printing and injection moulding when looking at the overall picture, taking into consideration tooling time and cost, complexity of the part required, finish, quantity and delivery time. When you consider all of these important aspects, it will become clear that vacuum forming, 3D printing and injection moulding all have a part to play.
For one-off prototypes of complex shapes 3D printing brings certain advantages over vacuum forming and most other processes by the ability to create a complete, finished component without design constraints and requirement for trimming.
At Formech we embrace 3D printing as a complementary process to vacuum forming for most design challenges. 3D printing brings new possibilities for creativity and versatility in tool design with cost effective and easy replication when used in conjunction with vacuum forming. Most of the world’s leading design labs utilise both techniques back to back.
3- Vacuum Forming Applications
Not every design or shape is suitable for vacuum forming but it remains the most cost effective and efficient method for processing the most diverse range of plastic processing applications. Formech vacuum forming machines are used by high profile customers such as Apple, Google, Disney, Bolshoi Ballet, Ford, Tesla, Ferrero Rocher, Nestle, Makita, Royal Air Force, Philips, Yale University, Dupont and many more across electronics, film & theatre, automotive, aerospace, confectionary, packaging, education and many more applications. The continually growing popularity of vacuum forming amongst these leading companies and institutions demonstrates the process remains the number one choice for a hugely diverse range of applications.
4- Ethical Use of Plastics
The use of plastics in product design, packaging and high volume applications remains a sensitive subject and often attracts debate. Every material choice from paper to plastic to metal, virgin or recycled presents an impact to the environment. For certain applications there are still no suitable alternatives to the use of thermo-plastic materials where cost, product life, and preservation of contents are concerned. Similar to energy sources derived from nuclear, fossil fuels, wind and solar, every method has its advantages and disadvantages when all the facts are considered.
Formech aims to minimise environmental impact of processing thermo-plastic materials by deploying quartz heating with efficient energy management across our range of machines. All Formech machines can be easily fitted with exact reducing windows to minimise plastic use and waste, wherever possible. By working closely with the world’s leading material science companies and institutions we fully support development and use of materials, which reduce environmental impact.
Vacuum Forming and Formech machines feature widely in the higher-education curriculum for the most technically advanced colleges and universities worldwide. Even 80+ years on, vacuum forming is a continually developing technology and we are excited to be at the forefront of new research and capabilities.
5- Nature of use and limitations
Vacuum 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 run therefore become economical. More sophisticated machines and moulds are used for continuous automated production of high volume items (yoghurts pots…).
Vacuum forming uses extruded plastic sheet and a secondary process may be required to trim the formed sheet to arrive at the finished part.
6- The process of Vacuum Forming
The clamp frame needs to be sufficiently powerful enough to ensure the plastic sheet is firmly held during the forming process. It can handle the thickest material likely to be formed on the machine – up to 6mm with a single heater and up to 10mm 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 must be provided to protect the machine operator at all times.
Heaters are generally infra-red elements mounted within an aluminum 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 that 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 pyrometers. 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 (auto-level) – Not available on all the machines.
A photo-electric beam is activated under the sheet of plastic during the heating cycle. If the sheet of plastic sags down and breaks the beam, then a small amount of air is injected into the bottom chamber under the sheet, thus lifting the sheet to stop it from sagging until the sheet clears the beam.
Pre-stretch (bubble) – Not available on all the machines.
Once the plastic has reached its forming temperature or “plastic” state it can be pre-stretched to ensure even wall thickness when the vacuum is applied. The method of controlling the bubble height should be 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.
Plug assist – Only available on the HD and TF Series.
Plug assisted vacuum forming (moulding) is used when straight vacuum forming is unable to distribute the thermoplastic sheet evenly to all areas of the mold. To help spread the sheet out more evenly, a device known as a plug is utilized to push the sheet into the mold before the vacuum is applied. This process enables more of the thermoplastic material to reach the bottom of the mold and thus more material is available to fill the corners of the mould and limit the plastic from thinning out.
Once the material is suitably heated a vacuum can be applied to assist in forming the sheet. A vacuum pump is used to draw the air trapped between the sheet and the mould. The vacuum pumps vary from diaphragm pumps to dry and oil filled rotary vane pumps. With larger machines a vacuum reservoir is used in conjunction with a high volume capacity vacuum pump. This enables a two stage instantaneous vacuum to be applied ensuring rapid moulding of the heated sheet (before the sheet temperature drops below its ideal forming temperature).
Cooling and release – Not available on all the machines
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% and is also beneficial in controlling the shrinkage of the moulded parts.
Mould temperature control units are also available which regulate the temperature within the mould ensuring accurate and consistent cooling times when cooling crystalline and crystallizing polymers such as PP, HDPE and PET.
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. Thin gauge parts are normally trimmed on a mechanical trim press – otherwise known as a rollerpress.