The cost-effective nature of vacuum forming (thermoforming) is what makes this process the production method of choice for many plastics component manufacturers. However, as with any complex processes, it’s essential to follow each stage accordingly to ensure the quality of the final product.

In this guide we aim to offer advice on design issues that need to be considered when designing plastic parts that are to be vacuum formed.

1. Choosing the right draft angle

This term refers to the extent of a taper exists on a vertical sidewall. You should aim to provide the highest draft angle for the vertical feature, as its role is to help remove the part from the tool.

A female feature should have a minimum of 1.5 to 2-degree draft angle. However, be aware that this will need to increase for a deep texture. In the case of male features, aim for a 4 to 6-degree range.

As a general rule, if you have a rougher surface texture or deep feature, you will need larger draft angle.

The draft angle also plays a role in keeping down the cost of vacuum forming. It can help prevent issues with de-moulding, which can be costly to rectify.

2. Ensure material distribution with radii

Corners with radii is another area to consider in a bid to reduce costs. In the majority of instances, the greater the radii, the better the material distribution. You’re also likely to find the radius at the bottom of the draw to be the most critical.

For male parts, strive for the thickness of the material and add between 1mm and 2mm.

3. Set the right draw ratio

This is term used to describe the relationship between the depth and width/length of the part to be moulded. The deeper or taller the part to be moulded, the thicker the starting thickness of the sheet you will need.

For the best possible finish, make sure you avoid creating several tall features that are too close to one another.

4. Creating quality undercuts

These are features that project from or into the surface of the tool. In some cases, this may stop you from removing the part from the tool, which is why you should strive for an undercut no greater than 15mm in depth.

You always need to bear the draw ratio in mind whenever you’re creating designs for undercuts.

5. Include sufficient reference points

Parts should include reference parts, as this will allow you to measure from a controlled surface or point to one of its important features. Examples include cut-outs and drilled hole centres.

6. Attach ribs and bosses

Ribs are generally used to support a flat surface and can be created separately and glued onto the original part. The same goes for internal bosses, which it’s possible to machine separately and attach with an adhesive. This often comes at an additional cost.

7. Achieving texture

Texture can be achieved either by using a raw material that already has a shaped surface, or by adding texture to the face of the tool. This means you will need a larger draft angle and is likely to incur added cost.

8. Merge parts with joining lines

There might be occasions when you need to join two components together. A witness line of around 2mm is necessary to cope with the joint’s tolerance variation.

The preferred method is to use a lap joint. This lead to an undercut, therefore increasing project costs.

9. Machining cut-outs, holes, trim and vents

You should always reference your trimming dimensions from the moulded side of the part. Any excess can be machined off to create a higher quality finish when creating cut-outs, holes, trim and vents.

10. Insert moulding hardware

Hardware can be insert moulded into the part and is often a key component of secondary operations.

11. Aim for a high tolerance

It’s a good idea to aim for the greatest tolerance possible, as this will bring down costs and lead time. If you’re using parts from a machined aluminium mould, use a tolerance of 0.25mm for the first 25mm and 0.25mm for every 250mm thereafter.

We can we help with your product 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.

Expert advice

If you need any advice or have any specific questions about designing plastic parts for vacuum forming then please feel free to call our team on 01482 646464.