Tolerance management in elastomer processing

Elastomers must be able to deform and then return to their original shape as reliably as possible. It is precisely this property that makes them interesting for seals, damping elements, membranes and numerous technical moulded parts. At the same time, it makes it more difficult to comply with tight dimensional tolerances.

Effective tolerance management therefore does not only take into account the nominal dimension of a technical drawing. Material, component geometry, tooling, manufacturing process, vulcanisation, storage and measuring conditions must be considered together.

For dimensional tolerances of products made of rubber or vulcanised elastomers, DIN ISO 3302-1 is particularly relevant. It defines different tolerance classes and thereby creates a common basis for design, production and quality assurance.

Why are tolerances for elastomers particularly demanding?

Elastomers react far more strongly to mechanical and thermal influences than many metallic or thermoplastic materials. Even measuring itself can influence the result if a soft component is deformed by the measuring force.

In addition, there are several influencing variables related to production and material:

• shrinkage during and after vulcanisation
• temperature during processing
• pressure and mould filling
• material formulation and hardness
• component geometry and wall thicknesses
• demoulding of the component
• post-shrinkage and relaxation
• storage and test conditions

Tolerances should therefore be specified during the design stage. If unnecessarily tight requirements are defined only after the mould design has been completed, additional tooling costs, rework or unnecessarily high reject rates can result.

Further fundamentals on the influence of temperature, moisture and material behaviour can be found in the article on dimensional stability of plastics.

DIN ISO 3302-1: dimensional tolerances for rubber moulded parts

DIN ISO 3302-1 specifies classes for dimensional tolerances of moulded parts as well as extruded and calendered products made of solid rubber. The specifications are primarily intended for vulcanised rubber, but may also be suitable for products made of thermoplastic rubber.

Four tolerance classes are distinguished for moulded parts:

Tolerance class	Classification	Typical significance for production
M1	very fine	particularly high requirements for tooling and process control
M2	fine	increased requirements for dimensional accuracy
M3	medium	practical tolerance class for many technical moulded parts
M4	coarse	larger permissible dimensional deviations

The following does not automatically apply: the tighter the tolerance, the better the component. Unnecessarily tight tolerancing can increase tooling and inspection effort without improving the subsequent function.

For designers, the more important question is therefore which dimension is actually critical for the function.

Mould-bound and interlock-bound dimensions

For elastomer moulded parts, how a dimension is created within the mould also plays a role.

DIN ISO 3302-1 distinguishes between mould-bound dimensions (F) and interlock-bound dimensions (C).

Mould-bound dimensions (F) are created within the same mould part. As a rule, they can be controlled more precisely.

Interlock-bound dimensions (C) are determined by the interaction of different mould parts or by the interlock. Additional influences from mould movement and mould parting can cause larger deviations here.

When creating drawings, therefore, not only the nominal dimension should be considered. The position of the component in the mould can also influence which tolerance can be reproduced economically.

Distinguish dimensional tolerances and geometrical tolerances

Dimensional tolerances and geometrical tolerances should be considered separately for elastomer components.

DIN ISO 3302-1 deals with dimensional tolerances.

For geometrical requirements such as flatness, parallelism, perpendicularity, coaxiality or position, ISO 3302-2 is relevant.

This distinction is particularly important for more complex moulded parts. An outer diameter, for example, can be within its permissible dimensional tolerance while a geometrical deviation still impairs assembly or function.

In technical drawings, therefore, only the functionally relevant dimensions and geometries should be given correspondingly tight requirements.

Which factors influence the dimensional accuracy of elastomers?

Material and formulation

Elastomers are not fully rigid materials. The rubber base, fillers, plasticisers, crosslinking system and other formulation constituents influence hardness, elasticity, shrinkage and recovery behaviour, among other things.

A change in the compound can therefore also affect dimensional accuracy. When selecting materials, therefore, not only mechanical properties but also temperature behaviour, media resistance and long-term dimensional stability should be taken into account.

An overview of the available technical materials can be found in the Materials section. We also explain how aggressive media can affect engineering plastics and component properties in the article on chemical resistance of plastics.

Component geometry

Different wall thicknesses, solid accumulations of material, delicate areas and long flow paths can lead to different cooling and shrinkage conditions.

A tolerance-compliant design therefore begins with a geometry that is as production-oriented as possible. In subsequent plastic machining, too, geometry, clamping and machining strategy influence the achievable dimensional accuracy.

Mould design

Mould parting, venting, cavities, gating and demoulding influence the reproducibility of a moulded part.

Particularly tight tolerances can therefore require a more precise mould design and additional coordination effort during sampling.

For extruded components, process parameters such as mould geometry, material flow and cooling are also added. You can find out more about the basic process under Extrusion.

Temperature and vulcanisation

Temperature, time and pressure influence the crosslinking of an elastomer. Fluctuating process conditions can therefore also cause dimensional deviations.

For tolerance-critical components, reproducible process control is therefore just as important as a precise mould.

Demoulding and post-shrinkage

After demoulding, an elastomer component is not necessarily immediately in its final dimensional state.

Cooling, relaxation and material-dependent post-shrinkage can still change the dimensions. The time and conditions of measurement must therefore also be defined.

How to specify tolerances sensibly in design

Good tolerance management begins with the function of the component.

Instead of tolerancing every dimension as tightly as possible, designers should distinguish between:

• function-critical dimensions
• assembly-critical dimensions
• sealing-relevant surfaces and geometries
• reference dimensions
• dimensions that are not function-critical

Tight tolerances should be used where they are needed for sealing performance, assembly, guidance, force transmission or service life.

In less critical areas, larger tolerance ranges can be more economical.

The subsequent field of application also plays a role here. In mechanical and plant engineering, for example, different requirements for dimensional accuracy and load capacity may apply than in medical technology or in the food industry.

Measurement and quality assurance for elastomer components

Even a correctly manufactured elastomer component can be assessed incorrectly if unsuitable measuring methods are used.

With soft materials, tactile measuring devices can deform the component simply through the measuring force applied. Non-contact optical measuring methods are therefore suitable for certain geometries.

Depending on the component and requirement, the following may be considered, for example:

• optical measuring systems
• profile projectors and contour measuring devices
• 3D measuring systems
• tactile measuring methods with controlled measuring force
• automated image processing
• computed tomography measuring methods for complex or internal geometries

Reproducible test conditions are important. Temperature, storage duration, positioning and measuring method should be defined so that results remain comparable between production, quality assurance, supplier and customer.

In addition to dimensions, hardness can also be a relevant quality parameter for elastomers. Further information can be found in our article on hardness testing methods for plastics and elastomers.

Standards-compliant processes and documented quality are particularly important for tolerance-critical components. An overview of the relevant evidence can be found under Certificates and quality.

Tolerance management from development to series production

In stable series production, tolerance management should not begin only at final inspection.

A sensible sequence is:

1. Define functional requirements: which dimensions and geometries influence the function?
2. Specify the material: which shrinkage, hardness and ageing properties are to be expected?
3. Specify tolerances: which tolerance class is technically required and economically sensible?
4. Align the mould to this: take into account mould parting, cavities and shrinkage.
5. Sample components: compare real production results with the requirements.
6. Define measuring conditions: specify the measuring method and test time.
7. Monitor the series process: detect process changes and trends at an early stage.

For higher volumes, statistical methods can be used to assess process capability and detect changes in the production process at an early stage.

For components that must additionally be precisely machined after shaping, processes such as CNC milling or CNC turning may be relevant depending on the material and geometry.

Typical errors in the tolerancing of elastomer parts

Tolerances are adopted from metal components

Tolerances of a metal part cannot automatically be transferred to an elastomer component. Material behaviour and manufacturing processes differ fundamentally.

All dimensions are toleranced particularly tightly

More precision often means more tooling, production and inspection effort. Therefore, above all the functional characteristics should be specified tightly.

Measuring conditions are missing

For elastic components, measuring force, temperature and test time can have a significant influence on the result.

Mould and component design are considered separately

The achievable accuracy depends substantially on how a dimension is created in the mould. Design and mould concept should therefore be coordinated with each other at an early stage.

Material properties are not taken into account

Hardness, formulation, temperature behaviour or media contact can change the dimensions and properties of an elastomer component. In addition, depending on the application, regulatory and documentation-related requirements may exist. Further background can be found in the article Material compliance and product approval.

Economic advantages of systematic tolerance management

Correctly specified tolerances not only improve technical quality. They can also increase the economic efficiency of production.

Realistic specifications help to

• avoid unnecessarily complex moulds,
• reduce inspection effort,
• lower rejects and rework,
• design production processes more stably and
• avoid complaints due to different interpretations of measurements or tolerances.

At the same time, stable processes can reduce material and energy losses. Systematic tolerance management thus also contributes to resource-efficient production. You can find out more in the Sustainability at Martan Plastics section and in the technical article Sustainability with elastomers.

Coordinate tolerances with material and production at an early stage

For elastomer components, precision arises from the interaction of design, material, mould, production process and quality assurance.

DIN ISO 3302-1 provides an important basis for defining dimensional tolerances. For geometrical requirements, ISO 3302-2 supplements the consideration.

In practice, the following applies: the smallest possible tolerance is not automatically the best solution. What makes sense is a specification that reliably safeguards the function of the component and at the same time can be manufactured reproducibly and economically.

If you are developing technical components for demanding industrial applications, Martan supports you in selecting suitable materials, manufacturing processes and design solutions. An overview of the various solutions from Martan Plastics can be found on the corresponding overview page.

FAQ

Which tolerances apply to elastomer parts?

For dimensional tolerances of moulded parts as well as extruded and calendered products made of solid rubber, DIN ISO 3302-1 is particularly relevant. Which tolerance should be applied depends on the manufacturing process, nominal dimension and the required tolerance class.

What do M1, M2, M3 and M4 mean for rubber moulded parts?

M1 to M4 are tolerance classes for moulded parts. M1 stands for particularly tight or very fine requirements, while M4 permits larger dimensional deviations. Which class is sensible should be determined on the basis of the component function and the production process.

What is the difference between F and C dimensions?

F denotes mould-bound dimensions, which are determined by the same mould area. C denotes interlock-bound dimensions, the creation of which depends on the interaction of different mould areas. This can result in different achievable tolerances.

Which standard applies to form and position tolerances for elastomers?

For geometrical tolerances such as flatness, parallelism, perpendicularity, coaxiality and position, ISO 3302-2 is relevant. DIN ISO 3302-1, by contrast, deals with dimensional tolerances.

Why are tight tolerances more complex for elastomers?

Elastomers can deform during production, demoulding, cooling, storage and measurement. The tighter the required tolerance, the more strictly the material, mould, process control and test conditions must be controlled.

How can dimensional deviations in elastomer parts be reduced?

A production-oriented component geometry, a suitable mould concept, stable vulcanisation conditions, controlled material quality and clearly defined measuring conditions are helpful. Tolerance-critical characteristics should be identified during development.