Much established knowledge about tolerances in construction applies to metals. How to think about details in engineering plastics?
The simple answer is that it depends... Plastic is an umbrella term for many materials that do not behave in the same way. A general tip is to set tolerances as generously as possible. It will be cheaper to manufacture and less likely that the material will change after manufacture.
Here are some factors to consider when it comes to tolerances for plastic parts:
Expansion is a function of the operating environment
What environment are your components in and how does it change when the part is used? Temperature and humidity, in particular, have the ability to affect plastics where changes over time also change the material. Tolerance requirements are very much about choosing a material from the start that is adapted to the operating environment. Polyamide (PA), for example, is a plastic that is affected to a greater extent by moisture, while PET is not suitable for lower temperatures. So higher/lower temperatures or varying humidity require thought.
Dimensional stability
Plastic blanks may contain stresses after manufacture. They are greatest in the centre closest to the 'core'. In addition, different plastics have different stiffness and ability to resist stresses. This means that the design of the part, such as milling out a lot of material in the centre of a profile, can affect the tolerances when the stresses are released and the material can move. Stiffer materials such as POM and PEEK resist stresses better than softer materials such as HDPE.
Inner and outer radii of the design
Avoid making unnecessary demands on sharp radii. As well as making machining more expensive because it requires small tools, it can also contribute to fracture indications or the release of stresses.
Surface roughness
Different plastics are suited differently if there is a requirement for a fine surface. It is always unnecessary to set finer requirements than absolutely necessary because a fine surface takes longer to process and thus increases the retail price. A fine surface is easiest to achieve on stiffer plastics such as POM, PET and PEEK. For other plastics, it may be better not to set finer requirements than Ra 3.2.
The method (turning/milling etc) and the tools we can choose for fast material removal etc always have a big impact on the final price so avoid unnecessarily tight tolerances.
When tolerances are really important, we have some "tricks" to use. We can machine in batches, let the material rest etc and choose the method and tools that give the best result.
But a lot is already in the choice of materials and construction. Please contact us at the design stage and we will be happy to discuss tolerance requirements, material selection and choices for the best manufacturing economy.
