What is ISO 20457?

ISO 20457 specifies geometrical and dimensional tolerances for plastic moulded parts, together with acceptance conditions and guidance on technically achievable tolerances.

The standard addresses an important difference between plastics and metals. Plastic moulded parts can experience significantly greater variation in dimensions, form and location because of material behaviour, moulding shrinkage, processing conditions, part geometry and factors such as warpage and non-uniform cooling. As a result, tolerance approaches developed for metal parts cannot generally be applied directly to plastics.

ISO 20457 therefore provides a plastics-specific framework covering general tolerances, direct tolerances and limits of technical manufacturability. It is intended to help designers specify functionally necessary tolerances while enabling manufacturers to confirm that those requirements are realistically achievable with the selected moulding process.

The standard applies to non-porous moulded parts made from thermoplastics, thermoplastic elastomers and thermosets using processes including injection moulding, injection compression moulding, transfer moulding, compression moulding and rotational moulding.

Why is ISO 20457 important?

Overly tight tolerances on plastic moulded parts can create unnecessary production difficulty, higher scrap rates, longer processing cycles and increased costs. At the same time, tolerances that are too broad can compromise fit, assembly or product function.

ISO 20457 helps designers, moulded part manufacturers, toolmakers and material suppliers establish a more realistic basis for dimensional requirements. By considering the specific behaviour of plastics and the capability of the manufacturing process, it supports better alignment between functional requirements and what can be produced economically.

The standard also provides a common basis for evaluating deviations and agreeing acceptance conditions, helping reduce misunderstandings between design and manufacturing teams.

Benefits

• Helps designers specify realistic geometrical and dimensional tolerances for plastic moulded parts
• Aligns functional tolerance requirements with the technical capabilities of moulding processes
• Supports more efficient production by reducing impractical tolerances, excessive scrap and avoidable manufacturing costs
• Provides a common basis for general tolerances, direct tolerances and acceptance conditions
• Accounts for plastics-specific effects such as moulding shrinkage, warpage, material behaviour and processing conditions
• Improves coordination between part designers, moulders, toolmakers and material suppliers during product development

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