Injection moulding tolerances to DIN ISO 20457: fundamentals, planning and practice

Tolerances in plastic injection moulding determine whether parts function reliably later, can be assembled securely and are produced economically. For development managers, designers and buyers, it is therefore essential to understand the specific constraints of injection moulding and to plan tolerances realistically from the outset.

This article explains how tolerances arise in injection moulding, what role the standard DIN ISO 20457 “Plastics moulded parts – Tolerances and acceptance conditions” plays, and how to align your application requirements with manufacturing capabilities. This helps you avoid costly iterations, unnecessary tool modifications and complaints while creating transparency in collaboration with your injection moulding partner.

Fundamentals: what are tolerances in injection moulding?

Tolerances in injection moulding describe the permissible deviation range of a dimension from its nominal value. They define how far a part in series production may deviate from the drawing without impairing function, safety or appearance.

In plastic injection moulding, tolerances are particularly important because plastics shrink more than metals, behave more strongly with temperature and show greater process-related variation. In addition, part geometry, wall thicknesses, tool concept and process control influence dimensional accuracy.

Typical tolerance types include:

• dimensional tolerances (e.g. ±0.1 mm for linear dimensions)
• form and position tolerances (e.g. flatness, parallelism, position)
• function-related tolerances (e.g. for fits, snap connections, sealing surfaces)

DIN ISO 20457 provides general tolerances and acceptance conditions for many of these cases, serving as a common reference framework between design, quality and injection moulding production.

DIN ISO 20457: standard for plastic moulded parts

DIN ISO 20457:2021-06 is entitled “Plastics moulded parts – Tolerances and acceptance conditions” and defines achievable tolerances and inspection criteria for non-porous plastic moulded parts made from thermoplastics, thermoplastic elastomers and thermosets. It is based on the international ISO 20457:2018 and is tailored specifically to injection-moulded parts.

The standard pursues two central objectives:

• providing uniform, realistic tolerance bands for plastic moulded parts
• establishing clear acceptance conditions and inspection conditions between customer and supplier

Important: DIN ISO 20457 does not replace product-specific standards but complements them. Function-critical characteristics can – and should – be toleranced more tightly where necessary and combined with geometrical product specifications (GPS) such as form and position tolerances.

Tolerance groups to DIN ISO 20457

A particular feature of DIN ISO 20457 is the use of tolerance groups (TG). These define how tightly a part is to be manufactured and can be selected depending on material, process and part requirements.

In practice, the following approach has become established:

• TG6 as the standard tolerance group for many injection moulded parts
• tighter groups (e.g. TG4) for function-critical dimensions and CTQs (Critical To Quality)

The standard provides tables in which general dimensional tolerances are given for each tolerance group and nominal dimension range. This makes it transparent which tolerances are economically achievable at typical nominal dimensions such as 10 mm, 50 mm or 100 mm. Determining the tolerance group depends on various factors, for example material, tool and process.

For your drawings, you can formulate for example:

• “General tolerances for untoleranced dimensions to DIN ISO 20457, tolerance group TG6”
• “Function-critical dimensions according to individual GPS specification, supplemented by DIN ISO 20457 TG4”

This clearly regulates which tolerance basis applies to standard dimensions and where project-specific, tighter requirements exist.

How tolerances arise in injection moulding

Material shrinkage and material influence

Plastics shrink as they cool from melt temperature to part temperature. Shrinkage varies depending on material, filler content and flow direction – and these effects are reflected in the tolerance groups of DIN ISO 20457.

Some typical material trends:

• unfilled standard plastics (e.g. PP, PE) show higher shrinkage and greater dimensional variation
• glass-fibre reinforced plastics tend to allow tighter tolerances but require targeted flow path design
• high-performance plastics (e.g. PEEK) place higher demands on tool and process control, which must be considered when selecting the tolerance group

The better the material is understood and the more stable the process control, the more likely tighter tolerance groups (e.g. TG4) can be maintained reliably.

Geometry, wall thicknesses and tool concept

Part geometry and wall thickness distribution strongly influence how stably a part can be reproduced in series. Thick-walled areas cool more slowly, tend towards differential shrinkage and can lead to warpage or local dimensional deviations. Thin-walled areas react sensitively to even minor process fluctuations.

The tool concept supports or limits the achievable tolerance situation:

• symmetrical injection and well thought-out gating concepts
• adequately dimensioned, uniform cooling
• reproducible multi-cavity concepts for production tools

When selecting the tolerance group, you should therefore always consider the combination of material, part geometry and tool design.

Process parameters and reproducibility

Even with identical tool and material, process control determines how well tolerances are maintained. Tool temperature, melt temperature, injection speed, holding pressure and cooling time influence dimensional accuracy and warpage.

In practice, this means:

• defined process windows secure compliance with tolerance groups
• statistical process control (SPC) shows trends and deviations at an early stage
• documented set-up data serve as a reference for complaints or series start-ups

DIN ISO 20457 requires and supports this systematic view by linking tolerances with clearly described acceptance and inspection conditions.

Typical tolerance ranges in injection moulding to DIN ISO 20457

DIN ISO 20457 provides general dimensional tolerances in tabular form, graded by nominal dimension ranges and tolerance groups. For a practical understanding, you can work with guideline values that occur in many projects.

For example, for standard applications with tolerance group TG6 and typical thermoplastics, tolerance ranges often arise of approximately:

• up to approx. 10 mm nominal dimension: tolerances in the range of a few hundredths to around ±0.1 mm
• 10 mm to 100 mm nominal dimension: tolerances in the range of around ±0.1 to ±0.3 mm
• larger dimensions over 100 mm: tolerances in the range of a few tenths of a millimetre

For function-critical dimensions, a tighter tolerance group such as TG4 can be selected under defined framework conditions (e.g. stable process, suitable material, optimised tool). However, effort in toolmaking, process control and quality assurance increases – which affects tool and unit costs.

Important: the specific limit deviations are to be taken from the standard or tolerance tables and assessed together with your injection moulding partner.

Acceptance conditions and inspection strategy to DIN ISO 20457

A key benefit of DIN ISO 20457 lies in the clearly described acceptance conditions. The standard regulates how and under which conditions plastic moulded parts are to be inspected and assessed.

These include in particular:

• reference temperature and climatic conditions (e.g. 23 °C, defined humidity)
• reference and measurement conditions (storage time after demoulding, measuring equipment)
• definition of reference systems and references for dimensional inspection

For you as the customer, this means:

• measurement and inspection results are more easily comparable between different partners
• releases (PPAP, PPF) are based on traceable, normatively described criteria
• discussions about measurement conditions or inspection set-ups are reduced

It is advisable to supplement these points on the drawing or in specification sheets, for example through references to the standard, reference temperature and special inspection requirements.

Benefits of realistic tolerances to DIN ISO 20457

Realistic, standard-based tolerances are an important lever for cost, quality and project security. Excessive, unaligned tolerance requirements frequently lead to:

• unnecessary tool corrections and longer sampling phases
• increased scrap rates and rework effort
• difficult discussions in complaint cases

With tolerance groups to DIN ISO 20457, you can:

• synchronise expectations between design, purchasing and manufacturing at an early stage
• make quotations more comparable (e.g. “quotation applies to DIN ISO 20457 TG6”)
• base quality and inspection concepts on a uniform foundation

The standard thus helps avoid “fear tolerances” and focus on genuinely function-critical dimensions.

Approach to defining tolerances in a project

Step 1: function analysis and prioritisation

First analyse the functions of your part:

• which dimensions are critical for function (e.g. fit, sealing surface, latch geometry)?
• where are there special visual requirements (visible surfaces, design)?
• which areas are non-critical and can be toleranced generously?

Classify characteristics into categories such as “critical”, “important” and “non-critical”. This makes it possible to define later where TG4 is appropriate and where TG6 or even more generous tolerance groups are sufficient.

Step 2: selecting the tolerance group

Choose a tolerance group as the basis together with your injection moulding partner – in many cases TG6 offers an economical standard. For function-critical dimensions, TG4 or a project-specific GPS specification can be agreed individually.

When choosing the tolerance group, consider:

• material (modulus of elasticity, shrinkage, data availability)
• process (injection moulding machine, series batch size, automation)
• part geometry and tool design

Document the chosen tolerance groups and their scope of validity in drawings, specifications and, where applicable, in your requirements specification.

Step 3: sampling, measurement strategy and release

During tool sampling, the agreed tolerances are verified in practice. Procedure:

• define a measurement plan with focus on function-critical dimensions
• produce sample parts under production-near conditions
• document measurement results and process parameters

On this basis, process windows are defined, Cpk values are assessed and tool corrections are initiated where necessary. The aim is to maintain the chosen tolerance group reliably under series conditions.

Step 4: series support and adjustments

Even after release, you should keep tolerances and process capability in view:

• use SPC to detect deviations early
• review tolerances when material, tool or machine changes
• adjust inspection plans or tolerance requirements where necessary

This ensures your part remains within the intended tolerances over its entire life cycle.

Practical tips and best practices

For designers

• plan tolerances from the outset with DIN ISO 20457 as the reference.
• avoid unnecessarily tight tolerances on non-critical characteristics.
• clearly mark function-critical dimensions and supplement them with GPS specifications where necessary.

For buyers and project managers

• ask your suppliers specifically about the tolerance group used.
• always evaluate quotations in the context of the tolerances required (e.g. TG6 vs. TG4).
• agree clear quality and acceptance conditions to DIN ISO 20457.

For quality and series production

• define measurement and inspection concepts on a normative basis.
• document process windows and monitor them continuously.
• use the standard as a reference to objectify discussions and clarify complaint cases factually.

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