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When we receive a drawing for a new plastic component, one of the first things we look at is the tolerances.

A tolerance tells us how much a particular dimension is allowed to vary from the figure shown on the drawing. That sounds straightforward enough, but with plastic components, specifying tolerances is rarely a case of simply making everything as tight as possible.

The size and mass of the component will affect what can realistically be achieved, while the number of tolerances specified across the drawing can make the difference between a practical design and one that becomes very difficult to manufacture.

Size and Mass Make a Difference

A small plastic component and a much larger moulding cannot necessarily be produced to the same tolerances.

As the overall dimensions and mass of a component increase, maintaining very tight tolerances across the whole part can become more challenging. This needs to be considered when the drawing is being produced, rather than assuming the same tolerance can simply be applied regardless of the size of the moulding.

This is why we prefer to look at the component as a whole. We need to understand its dimensions, its shape and, most importantly, what it actually needs to do once it has been manufactured.

There may be one particular diameter, thickness or fitting point where accuracy is critical, while another dimension has much more room for variation.

Be Careful How Many Tolerances You Specify

The other thing we regularly see is drawings where tight tolerances have been added to a large number of dimensions.

Each individual requirement may look achievable when considered on its own. The difficulty comes when the component has to meet all of them at the same time.

The more dimensions that are tightly controlled, the more restrictive the design becomes. Eventually, you can reach a point where the combination of tolerances makes the component impractical or, in some cases, impossible to manufacture as drawn.

That does not mean tolerances should be avoided. They are essential where dimensions affect how a component fits, functions or interacts with another part. The key is deciding which dimensions genuinely need close control.

We have seen this in our own projects. For one transport industry component, for example, simplifying the original design included reducing unnecessary tolerances to produce a more practical and cost-effective solution.

Specify What the Component Needs to Do

Our advice when producing a drawing is to start with the function of the part.

Which dimensions are genuinely critical? Where does the component have to fit another part? Which measurements could vary slightly without affecting its performance?

If you can identify those areas, it gives us much more scope to manufacture the component successfully, rather than working with tight tolerances that provide no practical benefit.

You also do not need to have every detail resolved before speaking to us. We regularly work with customers who already have detailed drawings, but we can also start with an initial sketch and help develop the component from there.

If we think a tolerance could cause problems, we can discuss it before tooling or production begins. Likewise, if injection moulding is not the most suitable manufacturing process for the accuracy or quantity required, we can consider alternatives such as CNC machining.

The important thing is to get us involved early. Send us the drawing and explain what the component needs to achieve. We can then look at the size, mass and tolerances together and advise what is realistic before you commit to manufacture.