A Complete Guide to Vacuum Forming

Vacuum forming takes its name from the fact that a vacuum is used to form a sheet of plastic into a desired shape. It is popular in industry because it can produce detailed shapes quickly and affordably.

This process is mostly suitable for low to medium-volume batch production, or very large-format assemblies. Plastic vacuum forming can also offer a cost-effective alternative to injection moulding, which often involves a significant investment in tooling. Discover more about vacuum forming in our ultimate guide, where we go through common practices and methods of vacuum forming.

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The basics of vacuum forming

Plastics can be shaped using a process called thermoforming. This is simply the technique of applying heat to plastic so it becomes soft and malleable. Currently, the three main approaches to thermoforming are vacuum forming, pressure forming and injection moulding.

Vacuum forming versus pressure forming

Vacuum forming and pressure forming take the opposite approach to getting the plastic into the mould. With vacuum forming, a vacuum pump sucks the plastic into the mould. With pressure forming, compressed air pushes the plastic down. Vacuum forming is quicker and more affordable, but pressure forming can produce a higher level of detail.

Vacuum forming versus injection moulding

Injection moulding works on essentially the same basis as pressure forming. It is just much more targeted. This means that it can achieve the highest level of detail. It is, however, both slower and more expensive than either pressure forming or vacuum forming.

Vacuum forming versus 3D printing

Vacuum forming and 3D printing are completely different technologies. Where vacuum forming uses moulds, 3D printing uses “cookie cutters”. It essentially cuts out slice after slice of the object and stacks each slice on top of the next until the desired result is achieved.

3D printing is, currently, best suited to low-volume applications. It’s particularly useful for jobs which need to be completed quickly. For example, you could use a 3D printer to create a plastic mould for vacuum forming a batch of prototypes. Once you had the design fixed, however, you’d probably spend the time and money needed to create a more robust mould.

The process of vacuum forming

The process of vacuum forming consists of five main stages. These are:

  • Making the mould
  • Setting up the thermoforming machine
  • Heating the plastic
  • Applying the vacuum
  • Cooling, trimming and finishing the plastic

Here is a more detailed guide to each stage.

Making the mould

Thermoforming moulds can be made from a wide variety of materials of various levels of durability. These include:

  • Modelling clay
  • Structural foam
  • MDF/Wood
  • Plaster of Paris
  • Fibreglass
  • Resin
  • Plastic
  • Aluminium
  • Cast iron

There are four main criteria that will determine the most appropriate material for any given job. These are:

  • The type of plastic being used for the job
  • Whether the part is 3D-modelled or just drawn
  • The level of accuracy needed
  • The level of robustness needed

In some cases, speed may also be a factor at least in the short term. For example, a production run may start with plastic moulds as these can be produced quickly. It may then move on to using aluminium moulds for their robustness.

Setting up the thermoforming machine

Once the mould is created, it needs to be put into the thermoforming machine. The plastic sheet is placed above the mould but not on it. There needs to be a small gap between the base of the plastic sheet and the top of the mould. The plastic is then clamped securely into place. Finally, the heater is positioned above, but not on, the plastic sheet.

Heating the plastic

In thermoforming, it is vital that the correct temperature is maintained across the entire plastic sheet. Even minor variations in temperature could ruin the outcome of the process.

For this reason, modern vacuum forming machines use pyrometers to monitor the operating temperature. The pyrometers interact with the process controls to ensure that the heating is promptly adjusted if the temperature varies.

Some vacuum forming machines can also support various other strategies for ensuring a consistent temperature. The two main ones are sheet-level monitoring and pre-stretching.

Sheet level monitoring

A photo-electronic beam is projected into the gap between the heater and the plastic. If this beam is interrupted, it means that the sheet of plastic has started to sag downwards. The machine will therefore counteract this by blowing air upwards to lift the plastic back into position.

Pre-stretching

After the initial heating stage, the plastic sheet is stretched to ensure that its thickness is exactly consistent. This means that if the temperature is applied consistently, the results should be the same across the whole sheet.

Applying the vacuum

Vacuum Forming VisualOnce the plastic reaches the correct temperature, the mould will be moved upwards towards the plastic. The vacuum former will then be activated. This will suck out all the air from the machine. As it does so, the plastic will be drawn to the mould. This process has to be done quickly so the plastic stays warm and therefore soft.

Cooling, trimming and finishing the plastic

The plastic has to harden before it can be released. High-speed fans are used to shorten the time this takes. Some machines also spray chilled water onto the plastic. This reduces the cooling time even further.

Once the sheet has cooled, it usually needs to be split out into individual components. This generally leaves some excess plastic that needs to be trimmed away. The components may then need some finishing touches before they are considered ready to be used. For example, packaging may need to be printed and/or decorated in some way.

Types of vacuum forming machines

Fundamentally, all vacuum forming machines operate on the same basis. In practical terms, however, there are wide variations between the capabilities of different vacuum forming machines. In broad terms, current vacuum forming machines can be divided into four main types. These are:

  • DIY machines
  • Tabletop machines
  • Mid-size machines
  • Industrial machines

The capabilities of the machines go up with size and price. For example, DIY machines might use ceramic heaters. These have a relatively slow response time. Industrial machines, by contrast, are much more likely to use quartz heaters.

These are much quicker to respond to instructions. Industrial machines are also more likely to have twin heaters, rather than just one.

Likewise, DIY machines may have a limited number of heating zones compared to industrial machines. This can make a huge difference to the consistency of the temperature, especially when working with large batches. Of course, DIY machines are not really intended for making large batches.

Similarly, DIY machines are unlikely to be able to use complex tools such as plugs. Plug tools essentially add “push” to the “pull” of the vacuum. The aim is to ensure that the mould is filled with enough plastic for the job.

Firstly this ensures that the plastic goes where it is supposed to go, for example into all corners.

Secondly, it ensures that the plastic has a consistent thickness.

Plug tools are most useful when moulds are particularly deep and/or particularly complex. Again, DIY machines are not really intended for these kinds of jobs.

Vacuum forming tool advice

Thermoforming moulds come in two main forms. Technically, these are known as negative and positive. They are, however, often known as male and female.

Positive (male) moulds are convex. This means that the plastic is formed over them. As a result, positive moulds prioritize the interior dimensions of the parts. Negative (female) moulds are concave. This means that the plastic is formed inside them. As a result, negative moulds prioritize the exterior dimensions of the parts.

Regardless of which type of mould is used, it needs to be designed in a way that makes it possible to release the plastic without damaging it. Here are some important points to consider.

Draft angles are a must

It is extremely difficult to release plastic from moulds which only use perfectly straight lines. This means that draft angles (taper) should be added to all sides of the mould. With male mounds, there should be a minimum of 3° of taper. With female moulds, there should be at least 5° of taper. As moulds get taller/deeper, the degree of taper should be increased.

Radii corners are helpful

On a similar note, it is much easier to extract a part from a mould that uses rounded corners than from a mould that uses straight corners.

The draw ratio is important

In simple terms, you should generally aim for balance. For example, if you’re designing a mould with tall/deep features, try to spread them apart from each other. Also, keep in mind that the taller/deeper a mould is, the more plastic it will need. In other words, the more it will cost to produce.

Textured moulds are often more expensive

Creating a textured mould is often more complex (and hence expensive) than creating a plain one. This may be justified over a large production run as it could allow for the use of smooth plastic. Over shorter production runs, however, it may be more economical (and quicker) to keep the mould smooth and use textured plastic.

Ribs and bosses can be added later

Similar comments apply to the use of ribs and bosses. They can be included in the design of the mould. This will, however, increase its complexity and thus the cost and time needed to produce it. This may be justified over large production runs. For short production runs, it may be easier, more economical and quicker to add them afterwards with adhesive.

Vents help with definition

Vents help with the process of air removal. It is therefore recommended to place them in strategic positions such as at edges, in corners and in cavities.

Undercuts are best avoided

Including undercuts almost inevitably raises production costs. Firstly, they may require the creation of a more complex tool than would otherwise be required. Secondly, they make it more difficult to extract the plastic from the mould.

If you absolutely must use undercuts, they should be kept to a maximum of 15mm. If at all possible, they should be placed at one end of the design. The opposite end of the design should have an angle at least equal to the undercut. If possible it should be bigger.

Materials used in vacuum forming

Here is a quick guide to the plastics most commonly used in vacuum forming.

Plastic Key properties Key characteristics Common applications
Acrylic – Perspex (PMMA)
Hygroscopic

Medium/strong

Temperature-sensitive, can become brittle

0.3 – 0.8% shrinkage rate

Prone to shattering but good to work manually and takes cellulose and enamel sprays

Available in multiple colours, can be transparent or opaque

Very suited to applications where clarity is important. Lights and light diffusers, roof domes, sanitary ware (including baths) and signs
Acrylonitrile Butadiene Styrene (ABS)
Hygroscopic

Strong

Forms easily to a high definition

0.3 – 0.8% shrinkage rate

Easy to saw and cut and takes all sprays

Mostly black, white and grey, limited colour range

Hard and rigid, resists both weather and impact. Electrical enclosures, luggage, sanitary parts and vehicle parts
Polycarbonate (PC / LEXAN / MAKROLON)
Hygroscopic

Very strong

Forms well to a high definition

0.6 – 0.8% shrinkage rate

Can be machined, ultrasonically welded, taped and drilled and takes spray

Clear, translucent and solid colours, embossed textures, opal and diffuser patterns

Great resistance against both fire and impact. Aircraft trim, guards/visors/shields, light diffusers, signage and skylights
Polyethylene (HDPE)
Not Hygroscopic

Very strong

PE itself is challenging. PE FOAM is easier to manage but needs to be formed at low temperatures

2.0 – 3.5% shrinkage rate

Cannot be sprayed but can be printed with certain inks

Black, white and colours

Very similar to PP. Has high shrinking rates but good chemical resistance Caravan parts, enclosures and housings, vehicle parts
Polyethylene Terephthalate Glycol/Co-Polyester (PETG)
Can generally be used without pre-drying

Strong

Good formability, capable of high definition

0.3 – 0.7% shrinkage rate

Can be sawed, cut and routered. Can be die-cut and punched to a limited extent. Can be printed using paints and inks intended for polyester

Mostly clear, limited colour range

Sterilizable and resistant to alcohols and acidic oils but not recommended for use with high-alkaline solutions. Attractive and easy to form. Hygienic packaging (e.g. for foods and medicines), plus displays (e.g. point-of-sale displays)
Polypropylene (PP)
Not Hygroscopic

Very strong

Challenging to form. Requires precision control of temperature and sheet level

1.5 – 2.2% shrinkage rate

Cannot be sprayed

Translucent, available in black, white and colours

Challenging to form and prone to sheet sag, but very flexible and non-absorbent. Chemical tanks and enclosures, luggage, packaging for food and medicine, toys
Polystyrene (HIPS)
Not hygroscopic

Medium/strong

Forms well can support high definition

0.3 – 0.5% shrinkage rate

Machines well but needs special primer to be sprayed

Clear and coloured, available in patterned and textured forms.

Has poor UV resistance so best kept for indoor applications. Very easy to form and available in a wide range of colours, patterns and textures Most high-volume/low-value items such as a lot of (non-sterile) packaging

Ethical use of plastics

The use of plastics has become increasingly controversial. They are, however, currently the only practical solution for numerous everyday problems. That being so, the keys to using plastics ethically are to minimize the quantity used and to ensure that the plastic is recycled and/or recyclable.

Using recycled plastics can be challenging if hygiene is paramount. In fact, current regulations may well prohibit it. If so, manufacturers should place even more emphasis on minimizing the quantity of plastic used and ensuring that it can be recycled after use.

Over the long term, industries needs to work together with environmental stakeholders to come up with alternatives. These may include increasing recycling facilities and/or developing new “green” plastics. Both of these processes are already underway.

Industries dependent on plastics can still work towards reducing their overall environmental footprint. In particular, they could look to maximize the efficiency of the heating process. Ideally, this would be powered by renewable energy.

Applications of vacuum forming

It would be literally impossible to list every current application of vacuum forming. Here is just a quick sample of some of the main areas where it is used.

Automotive and transportation

British Airways Lifevest Stowage Top Kydew Material

Vacuum forming is used extensively on the inside and outside of all kinds of vehicles. It can be used to produce parts that are light enough for aeroplanes and parts that are robust enough for agricultural vehicles, buses and HGVs. It can produce water-resistant components for boats.

Vacuum forming plays a huge role in car and van manufacturing. All kinds of interior and exterior parts are made using vacuum forming. This helps to reduce costs for both the manufacturer and the purchaser without compromising on quality.

Industrial

Vacuum forming has all kinds of uses within industrial sectors. At one end of the scale, it can be used to produce strong, weather-resistant parts for heavy-duty machines. For example, it is widely used in agricultural machinery. At the other end of the scale, it can be used to produce small runs of special items such as custom parts or prototypes.

Household items

Ansini Medical Products

The average household probably has vacuum-formed items in every room plus the garden (and garage). Kitchens and bathrooms in particular will be full of them. In fact, most sanitary ware is likely to use vacuum forming to some extent. This includes large items such as baths. In fact, if you have a hot tub in the garden, that was probably vacuum-formed too.

Vacuum forming is increasingly being used as an alternative to glass. So far, it’s only really used in smaller-scale applications such as skylights. This could, potentially, be extended in future.

Protective equipment

Safety guards, safety goggles and visors and even riot shields can all be made using vacuum forming. Making items like these out of a single piece of plastic helps to increase robustness. This is, of course, a huge benefit in these kinds of applications.

Packaging and displays

Vacuum forming has long been used to create packaging for consumer goods, foods and medicine. It’s now increasingly being used to create eye-catching promotional displays.

Pros and cons of vacuum forming

Like all technologies, vacuum forming has its pros and cons. In order to judge their significance, however, you need to look at them in context. With that in mind, here are seven key points to consider before starting a manufacturing process and an explanation of how the use of vacuum forming could influence their outcomes.

Speed

Vacuum forming is one of the fastest production methods used today. If you’re willing to use a fairly lightweight mould, you can get production moving very quickly. This makes vacuum forming a great choice for product development, where you’re probably going to want to make incremental improvements.

Similarly, once you have the mould created, the actual vacuum-forming process itself is very quick. Keep in mind, however, that the vacuum-forming process may not result in a completed item. It is quite common for vacuum-formed products to need further work before they can be used or sold.

Volume

The fact that vacuum forming is so quick means that it’s very scalable. You can use it just as effectively for huge production runs as for agile prototypes. In fact, there’s a strong case for arguing that vacuum forming really comes into its own with smaller jobs.

Mass production, in various forms, has been a reality since the industrial revolution. By contrast, it’s only just becoming feasible to run small production jobs to similar levels of economy.

Even though vacuum forming uses one mould per job, it offers a huge level of versatility in the way it uses moulds. For example, basic templates can be customized into new shapes and sizes. They can also be updated to reflect new developments in their area of use.

Budget

Vacuum forming can be a very economical means of production. There are, however, a few caveats here.

Firstly, everything hinges on the mould. Get the mould wrong and your entire production run will go wrong. Secondly, the mould needs to be kept scrupulously clean. If it gets at all dirty, this may show on the finished parts, especially if they are clear or light-coloured.

Secondly, the plastic needs to be handled with care. If it isn’t it can warp (especially if it’s thick) or bubble (especially if there’s excess moisture). There may be ways to recover from this, at least to some extent. For example, damaged or excess plastic can often be reused in future production. Your production run will, however, almost certainly take some kind of hit.

You also need to remember that many vacuum-formed products need some extra work done to finish them. This may not be hugely expensive. You do, however, need to keep it in mind when comparing vacuum forming with other production technologies.

Complexity

Vacuum forming still calls for relatively simple designs. Firstly, there’s a limit to how much you can realistically expect from the moulding process. Secondly, vacuum forming doesn’t apply the same degree of force as pressure forming or injection moulding.

Consistency

On the other hand, vacuum forming is great for ensuring consistency. If you use the same mould, you should almost certainly get the same results. The only exception to this is if the plastic is mishandled. Bluntly, however, that is a reflection of your manufacturer’s skill (and equipment) not on vacuum forming itself.

Robustness

In simple terms, the fewer parts an item has, the harder it is to break. Vacuum forming creates parts as a single piece. This makes them inherently more robust than similar items made from more than one component part.

The caveat here is that the robustness of the part depends, in part, on the robustness of the plastic used. Firstly, some plastics are generally more sensitive than others. Secondly, some plastics have strong resistance to some forces but weak resistance to others. The onus is therefore on the designer to choose the right plastic for the right situation.

Hygiene/sterility

Hygiene and sterility were important considerations long before COVID19. They were (and are) particularly important for food and medical packaging. They also apply, at least to an extent, in many other areas. It seems reasonable to assume that they will be considered even more important in a post-pandemic environment.

Vacuum forming is definitely not the only option for manufacturing sterile products. It is, however, definitely one of the quickest and most cost-effective options.

Is vacuum forming still an important technology?

Vacuum forming has been in use for over 80 years now. This means that you might reasonably expect it to be at least close to obsolete. In actual fact, vacuum forming is still very much going strong. What’s more, its speed, versatility and affordability mean that it is still in huge demand.

Admittedly, the future of vacuum forming is very much tied to the future of plastics. This may, however, not be the drawback it seems. Given the usefulness of plastics, it seems likely that the way forward is to make them more sustainable, rather than to try to eliminate them.

This means that, in the long term, vacuum forming is likely to sit alongside pressure forming/injection moulding and 3D-printing as one of the world’s most important technologies.

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Whatever your product or industry, our plastic forming production specialists can advise on the best manufacturing process to give you the best solution for your budget. If you are interested in learning more about our vacuum forming products or would like to arrange a free consultation, please contact Ansini today on 01623 812333 or email info@ansini.co.uk.

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