What Tolerances Are Achievable With Silicone Rubber Extrusions?

Silicone rubber extrusion tolerances depend on the size and shape of the profile, the silicone compound, the manufacturing process and which dimensions are critical to the finished component. Unlike machined metal or rigid plastic parts, flexible silicone extrusions naturally undergo dimensional changes as they leave the extrusion die and pass through curing.

For this reason, specifying silicone rubber extrusion tolerances requires an understanding of what the extrusion process can realistically control. Asking for every dimension to be held to an unnecessarily tight tolerance can make a component more difficult and expensive to manufacture without providing any practical improvement in performance.

A recognised starting point is ISO 3302-1, the international standard covering dimensional tolerances for moulded, extruded and calendared solid rubber products. The current ISO 3302-1:2014 edition was confirmed as current in 2024.

For engineers and OEM manufacturers, the most effective approach is usually to establish a suitable general tolerance for the profile and then identify the dimensions that genuinely affect sealing, fitting or assembly.

Why do silicone rubber extrusion tolerances vary?

Rubber extrusion differs considerably from machining a rigid material.

During extrusion, uncured silicone is pushed through a shaped die to produce a continuous profile. The material then undergoes vulcanisation or curing so that it develops its finished elastomeric properties.

At several stages, the material can change dimension slightly.

One important effect is die swell. Rubber can expand after it exits the extrusion die, meaning the dimensions of the die itself are not necessarily identical to those of the finished profile.

The material can then experience further shrinkage or deformation during curing.

ISO guidance specifically recognises that extruded rubber requires greater manufacturing tolerances than moulded products because die swell, subsequent vulcanisation, shrinkage and deformation can affect the finished dimensions.

This does not mean silicone extrusion is an imprecise process. It means that dimensional control needs to reflect the behaviour of a flexible elastomer rather than applying expectations developed for rigid machined components.

Silicone is flexible by design

Another factor is the way the finished component is measured.

A soft rubber profile can deform under relatively small forces. Measuring equipment, handling technique and the way a profile is supported can therefore influence the reading.

A thin lip on a sealing profile, for example, may naturally move or deflect more easily than a thick central section.

This is why the functional purpose of each dimension matters.

An overall dimension controlling how the extrusion fits into a channel may require closer control than a flexible sealing fin that is specifically intended to compress during installation.

What does ISO 3302-1 specify for rubber extrusions?

ISO 3302-1 provides recognised tolerance classes for rubber products rather than treating every extrusion as though the same dimensional accuracy can be achieved regardless of its construction.

For unsupported extrusions, the standard uses three principal cross-sectional tolerance classes:

• E1, high quality
• E2, good quality
• E3, non-critical

The permitted dimensional variation increases with the nominal size of the feature being measured.

For example, published ISO 3302-1 tolerance tables show that for an unsupported extruded dimension above 2.5 mm and up to 4 mm, the tolerance values are ±0.25 mm for E1, ±0.40 mm for E2 and ±0.70 mm for E3.

For a dimension above 10 mm and up to 16 mm, the corresponding values are ±0.50 mm, ±0.80 mm and ±1.30 mm. Above 100 mm, the standard expresses these tolerances as percentages of the nominal dimension.

These figures are useful reference points, but they should not be interpreted as a guarantee that every silicone extrusion can automatically be produced to E1 tolerances.

Profile design, dimensions, material and manufacturing method all need to be considered first.

Different types of extrusion use different tolerance classes

ISO 3302-1 also recognises that not every extruded rubber component is produced or controlled in the same way.

Alongside the E classes for unsupported extrusion dimensions, the standard includes separate classifications for features such as mandrel-supported extrusions, surface-ground tubing, wall thickness and cut lengths.

This distinction is particularly relevant for silicone tubing.

A tube may have an outside diameter, inside diameter and wall thickness, but these dimensions are interrelated. ISO guidance states that in an extruded cross-section, only two of the three variables of inside dimension, outside dimension and wall thickness can be toleranced to control the cross-section.

For buyers specifying silicone rubber tubing, it is therefore important to identify which dimensions are genuinely critical rather than independently applying demanding tolerances to every measurement.

What factors affect dimensional accuracy during extrusion?

Several factors determine how tightly a silicone profile can be controlled.

Profile size

Very small dimensions can present different manufacturing challenges from large cross-sections.

The relationship between the feature size and the permitted tolerance therefore needs to be considered rather than applying one fixed value to every dimension.

This principle is reflected directly in ISO 3302-1, where permissible variation changes according to nominal dimension.

Profile geometry

Simple cross-sections are generally easier to control than highly complex shapes with multiple thin walls, hollow chambers, delicate fins or large differences in material thickness.

A rectangular silicone rubber strip may have relatively straightforward dimensional requirements.

A bespoke sealing profile containing a hollow bulb, retaining leg and thin flexible sealing lips has considerably more geometry to control.

The design itself can therefore influence what tolerances are practical.

Material hardness

Silicone compounds can be produced in different hardnesses depending on the application.

Softer materials deform more readily, which can affect both manufacturing behaviour and dimensional measurement.

Hardness should consequently be considered alongside tolerance rather than treated as an unrelated specification.

Curing behaviour

The extrusion must be cured after leaving the die.

Changes occurring during this stage can influence final dimensions. How the profile is supported during curing can also affect the degree of deformation, particularly with shapes that are naturally prone to sagging or movement.

Cross-sectional balance

Profiles with very uneven material distribution can behave differently during manufacture.

For example, a design with one heavy section and several extremely thin features may be more difficult to keep dimensionally stable than a relatively balanced profile.

This is one reason early discussion between the component designer and extrusion manufacturer can be useful before tooling is finalised.

Can tighter tolerances be applied to critical dimensions?

Often, yes, but tighter tolerances should be applied selectively and agreed with the manufacturer.

Not every dimension on an extrusion drawing normally needs the same degree of control.

Consider a profile designed to push into a metal channel.

The width of the retaining section could be critical because it determines whether the profile fits securely. A flexible sealing bulb positioned above it may be deliberately designed to compress substantially during use, making an extremely tight free-state dimension less important.

Giving both features the same demanding tolerance may therefore add manufacturing difficulty without improving the way the product works.

A more practical specification identifies which dimensions influence:

• Assembly or insertion
• Sealing compression
• Interface with surrounding components
• Position within a housing or channel
• Finished product function

Other non-critical dimensions can then be controlled using an appropriate general extrusion tolerance.

This approach allows engineering effort to focus on the measurements that genuinely affect the performance of the assembly.

Why can excessively tight tolerances cause problems?

It can be tempting to specify the smallest tolerance possible because greater numerical precision appears to imply better component quality.

With flexible rubber products, this can be misleading.

A dimension only needs to be as precise as the application requires.

Requiring extremely tight tolerances across every feature may increase tooling development, process control requirements, inspection time and potentially manufacturing costs.

It can also lead to unnecessary rejection of parts that would perform perfectly well in their intended application.

The better question is not “What is the tightest tolerance available?” but “What tolerance does this feature need for the component to function reliably?”

This functional approach is particularly valuable in OEM production, where repeatability and ease of assembly can be more important than achieving an arbitrary dimensional figure.

How does profile design affect achievable tolerances?

The earlier tolerance requirements are considered during product development, the easier it becomes to create a profile that combines manufacturability with functional performance.

Certain design choices can make dimensional control more challenging.

Very thin unsupported sections may move easily during curing or measurement. Large hollow profiles can respond differently to external pressure from solid sections. Sharp dimensional transitions can also create manufacturing challenges.

None of these necessarily means that the profile cannot be produced.

Instead, it means the geometry should be assessed as a complete extrusion rather than as a collection of isolated dimensions.

For custom silicone components, discussing the intended function can sometimes allow a dimension or feature to be adjusted without changing the performance of the final assembly.

For instance, increasing the thickness of a non-critical feature or adjusting a transition between sections could potentially make production more stable while retaining the required sealing behaviour.

What should engineers include on a silicone extrusion drawing?

A clear engineering drawing can prevent considerable uncertainty during quotation, tooling and production.

The drawing should show the nominal dimensions needed to define the profile, but it should also distinguish genuinely critical measurements from those that can follow an agreed general tolerance.

Useful information can include the required silicone hardness, profile dimensions, critical tolerances, cut length and any particular application requirements relevant to material selection.

The operating environment should also be communicated where appropriate.

Temperature, compression, exposure to weather and contact with other substances may all influence whether the proposed compound and profile design are suitable.

Where dimensions are based on an established standard such as ISO 3302-1, the required tolerance class should be identified clearly rather than leaving the manufacturer to infer the intended accuracy.

Silco Products can discuss bespoke silicone manufacturing requirements and component specifications before production. For a new profile, providing a drawing and details of the intended application helps establish whether the requested dimensions and tolerances are practical.

Are cut-length tolerances different from profile tolerances?

Yes.

Cross-sectional tolerances describe the dimensions of the extruded profile itself, while cut-length tolerances deal with the length of individual pieces produced from the continuous extrusion.

ISO 3302-1 uses separate L1, L2 and L3 classes for cut lengths, with permitted variation increasing as the nominal cut length grows.

This distinction can be important in assembly operations.

A continuous sealing extrusion supplied in coils may not require a precise factory-cut length because the material is trimmed during installation.

Pre-cut components fitted directly onto a production line may require much more controlled lengths.

Once again, the specification should therefore reflect what the component actually needs to do.

How should OEMs approach tolerance specification?

For OEM manufacturing, repeatability is often the principal concern.

A component may be installed hundreds or thousands of times, so even a small issue with fitting can create considerable disruption to an assembly process.

The most useful tolerance strategy is usually to identify the features that determine assembly and product performance.

Those dimensions can receive specific attention while other measurements are allowed to follow appropriate general rubber extrusion tolerances.

Prototype samples can also provide valuable information before volume production begins.

They allow the extrusion to be fitted to the actual assembly and help confirm whether the dimensions shown on the drawing correspond with the practical requirements of the finished product.

This is particularly helpful when the silicone element needs to compress, stretch or deform during installation.

The free-state dimensions shown on a drawing do not always tell the complete story of how an elastomeric component will behave once fitted.

Frequently Asked Questions

Can silicone extrusions be made to ±0.1 mm?

That should not be assumed as a general tolerance for silicone extrusion. Achievable tolerances depend on the dimension, geometry, material and manufacturing process. ISO 3302-1 provides dimension-dependent tolerance classes, so any unusually tight requirement should be discussed with the manufacturer rather than applied universally.

What is the standard tolerance for rubber extrusions?

ISO 3302-1:2014 provides recognised dimensional tolerance classes for extruded rubber products. Unsupported extrusion cross-sections are classified as E1, E2 or E3, while other classifications are used for specific extrusion types and cut lengths.

Do all dimensions on a silicone extrusion need a tolerance?

Dimensions should be controlled sufficiently to define and manufacture the component, but not every feature necessarily needs an individually specified tight tolerance. Identifying critical sealing, fitting and assembly dimensions can produce a more practical specification.

Setting realistic silicone rubber extrusion tolerances

The tolerances achievable with silicone rubber extrusions are determined by more than the capability of the extrusion equipment. Material behaviour, profile geometry, curing, hardness, nominal dimension and measurement method all contribute to the final dimensional variation.

ISO 3302-1 provides a useful framework for specifying rubber extrusion tolerances, including E1, E2 and E3 classes for unsupported profiles and separate categories for other extrusion characteristics.

For most projects, the objective should not be to demand the tightest possible tolerance throughout the profile. It should be to identify the dimensions that determine whether the component fits, seals and functions correctly, then establish realistic tolerances around those features.

Silco Products supplies silicone rubber products for industrial and OEM applications and can discuss bespoke profile requirements, drawings and dimensional considerations. To discuss a custom silicone component, contact Silco Products.

Phone: 01903 732088
Email: sales@silcoproducts.co.uk
Find out more: https://siliconerubberproducts.co.uk/