Tolerances in plastics: what you need to know before placing an order

Plastics do not adhere to the same tolerances as metal. Here’s what you need to bear in mind to get the right dimensions from the outset. By taking the properties of the plastic into account, you can avoid costly errors and ensure you end up with a component that works in practice.

Plastic is not metal

When ordering a machined plastic component, it is important to remember that plastic behaves differently to metal. Plastic expands and contracts more in response to temperature changes, absorbs moisture to varying degrees and has lower rigidity.

Therefore, the same tolerances cannot always be used for plastic as for metal. A tolerance that is reasonable for aluminium may be difficult or expensive to achieve in plastic. It is better to take the properties of the plastic into account and adapt the tolerances to the intended function.

A common mistake is to design a component to metal tolerances and then order it in plastic. The result is often costly machining, a high scrap rate and a component that still does not work in service.

Temperature and humidity affect dimensions

Most plastics have a linear coefficient of expansion that is several times higher than that of steel. A component that fits perfectly at 20 °C may be too large or too small at 60 °C. This is particularly important for fits between plastic and metal.

PA6 absorbs moisture and can swell. If you require high dimensional stability in a humid environment, POM-C or PEEK may be better alternatives. For outdoor use, you should also take UV light and temperature fluctuations into account.

When recording measurements, you should always specify the temperature at which the component is being measured. A component measured immediately after machining may still retain heat and therefore give incorrect dimensions.

Get it right from the start

Always define dimensions and tolerances based on how the component is to be used. Specify the measurement temperature if precision is important. Be clear about the required surface finish and machining requirements.

A component that is to be pressed into a metal holder does not require the same absolute precision as a component that is to rotate with a small amount of play. By adapting the tolerances to the function, you will achieve the right quality at the right price.

Talk to your supplier at an early stage. An experienced plastics supplier can advise on what tolerances are reasonable for the chosen material and the planned machining process.

Common pitfalls

A common problem is failing to take moisture adjustment into account. Nylon that has not been moisture-adjusted can be brittle and have tighter tolerances than expected. Another pitfall is assuming that a plastic component can withstand the same load as a metal counterpart.

Also avoid specifying unnecessarily fine surface finishes. A high-quality surface finish increases machining time and cost without always providing any functional benefit.

Next steps

Would you like to know more about tolerances and dimensional adaptation? Read our tolerance guide, use VisualCutter for precise dimensions, or contact us for advice.

Examples of reasonable tolerances

For a turned plastic component with a diameter of 50 mm, a reasonable general tolerance might be ±0.2 mm. For a press fit between plastic and metal, the tolerance may need to be tighter, but in that case, humidity and temperature must be taken into account.

For large-dimension moulded PA6, tolerances may need to be wider due to internal stresses. Discuss with your supplier which tolerances are realistic for your specific material and dimensions.

Tools and cutting parameters

The right tools and cutting parameters affect both surface quality and dimensional accuracy. Sharp tools with the correct geometry produce cleaner cuts and generate less heat. For soft plastics such as PE-HD, it is important not to use excessively high cutting speeds, as the material may melt.

For hard plastics such as POM and PA6, sharp tools and good cooling are required. When threading, you should select a suitable thread profile and tolerance class to ensure a proper fit.

How to specify tolerances

To achieve the correct results, you should always specify tolerances clearly on the drawing or in the order. State which standard applies, for example ISO 2768, and whether the tolerances refer to individual dimensions or the entire component.

Bear in mind that plastics expand and contract more than metal in response to temperature changes and humidity. It may therefore be wise to specify different tolerances for different directions or to specify measurement conditions. A dimension measured at 23 degrees and 50 per cent relative humidity may differ from the same dimension in the production hall.

If you do not have technical drawings, you can send us a component or a sample for us to measure. This ensures that the tolerances are correct from the outset.

Summary

Tolerances in plastics require an understanding of the material, the process and the environment. Be realistic about precision, specify clear dimensions and discuss any uncertainties with the manufacturer.

With the right preparation, you’ll receive a part that fits together with the other components without the need for post-processing.

Frequently asked questions

Can plastic maintain the same tolerances as metal? Generally speaking, no. Plastic is more susceptible to movement due to temperature and humidity, which makes tight tolerances more difficult to achieve.

Which standard applies to plastic tolerances? ISO 2768 is common, but industry-specific standards exist for certain applications.

How are tolerances measured in plastic? Measure at a constant temperature and humidity, preferably after the component has acclimatised to the room environment.