Tool (or mould) design is a very important part of vacuum forming as it can mean the difference between a defective part, and one that is high quality and works well. No matter how good the vacuum forming process and how robust the material, if the tool itself isn’t properly designed the part simply won’t come out right.
In the plastic product design stage we will work with you to decide exactly how the finished part will look and what its exact dimensions are; these two important factors will determine the type of tool that is made.
When designing a tool it is also important to know the vacuum forming sheet material you will be using and determine what tolerances are expected.
As specialists in vacuum forming tool manufacture we know all of the different factors that need to be considered when designing and manufacturing the perfect mould for your plastic product. See below for an overview of the most important of these factors.
Types of moulds
There are basically two different types of vacuum forming moulds: male (positive) and female (negative). So how do you decide which type is right for your tool? Essentially the top surface of a moulding is the better finished surface as it doesn’t come into contact with the mould, and therefore can’t pick up any marks such as dust particles from the tool.
Another consideration when choosing they type of vacuum forming mould would be the desired shape of the finished plastic part. Often a male mould is used for single deep-draw objects, whereas a female mould would be used for a compartment tray with multiple divisions or a sink for example.
Different materials for making moulds
A wide range of materials can be used for the mould and they each have characteristics that are suited to different applications.
There are 4 major deciding factors when choosing the right material for a project:
- The severity and length of service required. For example if only a few parts are required and material temperatures are going to be fairly low, then wood would be a good option. However if the quantities required are 1000+ and high temperature plastics are going to be used, then we would more than likely recommend an aluminium mould.
- Accuracy / tolerance of finished parts- most accurate is machined aluminium then cast aluminium, resin & finally wooden tools.
- Basis of design- digital model versus basic drawing. If a part is modelled in 3D then machined tooling options are available.
- Type of materials- specifically polyolefins – HDPE / PP- these types of materials can only be moulded (barring small scale prototyping) from temperature controlled aluminium tooling.
Popular mould materials & construction methods
Here are some essential facts about five popular mould materials / construction methods.
- Cheap and easy to fabricate, with a longer life than plaster. Hardwoods are recommended to reduce the chance of any cracking and splitting during forming. Deterioration through use is inevitable but can be reduced by using an enamel or varnish sealant.
- Used for Prototyping & where there is a minimum of information to manufacture the tool.
Polyester / Fibreglass
- Cost effective and considered permanent tooling for lower volume ( < 2000) projects. Difficult to match accuracy achieved by using a temperature controlled tool but is still better than wood. Will eventually show fine cracks.
- Lower volume, less dimensionally accurate applications.
- Polyurethane tooling block or modelling board CNC routed to produce dimensionally accurate patterns of a composite part. The filled nature of tooling block means that the shaped pattern can already have a high quality surface finish which needs little surface preparation before it can be used to produce a mould.
- Ideal for prototyping where a 3D model is available.
- Popular due to its heat conductive properties, good surface hardness and low wear. It is lightweight and has a great strength to weight ratio. Sand blasted surface often chosen as it prevents any trapped air between the mould and sheet.
- Cast to size is cheaper than machined aluminium and is used for larger parts and where dimensional accuracy is less important. Particularly suitable for polyolefins. Medium to high volume applications. Allows for temperature control of tools.
- Popular due to its heat conductive properties, good surface hardness and low wear. It is lightweight and has a great strength to weight ratio. Sand blasted surface often chosen as it prevents any trapped air between the mould and sheet. Requires 3D modelling.
- Good dimensionally accuracy. Temperature control of tools. Medium to high volume applications.
Draught angles / tapers
Draught angles are an important element in tool design as they must be added to all sides of the mould, in order to ease removal of the plastic part from the tool after forming.
If forming over a male mould, the draught angles must be at least 3 degrees, but if forming into a female mould, the draught angles need to be at least 5 degrees. For trays that are pocketed the same angles should be added to all sides of the product.
For vacuum formed trays or plastic parts with pockets, it is important to make sure that each pocket depth is no more than 75% of the smallest width at the top of the pocket. If this rule isn’t adhered to it will result in weak pockets and excessive thinning due to the material stretching too far over the mould.
Suitably positioned vent holes facilitate the evacuation of air trapped between the mould and the plastic sheet and ensure good definition. These need to be factored into the mould design and located in parts where the plastic sheet last makes contact: such as edges, internal corners and cavities.
Parts with undercuts are either difficult or impossible to remove from a standard vacuum forming tool after they’ve been formed; it is for this reason that we recommend avoiding undercuts where possible.
However, if your part has an undercut at one end, but an equal angle at the other it can sometimes be possible; as long as the the direction of removal of the part from the mould tool can be done at an angle that is equal to or greater than the undercut.
There is another solution for parts with undercuts which involves using a split tool with side entrant tool that is removed first after forming. This method of course can increase tooling costs so it is generally something we will try to avoid and look to do a redesign instead to keep your costs low.
Plug assist design
For a particularly complicated mould or one which requires deep draws in the plastic, it is necessary to consider if you would need to use the plug assist method as this would require designing an additional plug tool. This involves using a plug to push the plastic sheet fully into any deep draws in the mould to ensure an even thickness and avoid webbing.
We can we help with your tooling design
Bridgewood UK are leading vacuum forming specialists. For over 30 years we have used our experience and technical capability to produce plastic products that are cost effective and designed to specification for businesses across the UK.
If you need any advice or have any specific questions about vacuum forming and tool / mould design then please feel free to call our team on 01482 646464.