Understanding Extruded Rubber Tolerances

A rubber extrusion tolerance is a measure of how much dimensional flexibility is allowed when manufacturing an extruded rubber part. Some extruded rubber parts must match extremely strict dimensions to ensure appropriate performance in the field. Other parts can serve their function well even if there is some variation in dimensions. The level of precision required depends on the application and the service environment of the part.

Because the extrusion process is inherently variable, due to the nature of rubber manufacturing, there are several trade-offs to consider when determining the dimensional tolerances of an extruded part or shape. This includes tolerances outlined by industry standards, the resources required to achieve difference tolerance levels, and the differences between the rubber extrusion and rubber molding processes.

ISO Tolerance Classes in Rubber Extrusion

ISO 3302-1 details different classes of dimensional tolerances for rubber and rubber sponge components. The spec details three classes of tolerance for nominal, cross-sectional dimensions of unsupported extrusions: E1, E2, and E3. Other classes detailing extruded rubber tolerances for mandrel-support extrusions, surface-ground extrusions, and cut sections of extrusions are also included.

As a rule of thumb, higher degrees of accuracy demand tighter controls during rubber manufacturing, resulting in higher costs. 

Class E1

Class E1 of ISO 3302-1 refers to high-precision, high-quality extrusion, often using precision machining. Extrusions compliant with Class E1 are often destined for applications that require an extremely close fit while in service. Class E1 requires more expensive controls and inspection procedures, and molds must be kept in good repair.

Class E1 extruded rubber tolerances cannot be achieved with some synthetic polymers due to the nature of those materials.

Class E2

Class E2 of ISO 3302-1 refers to good-quality, precision extrusions for general grade or commercial products. The controls and inspection procedures are more moderate for Class E2, which keeps costs manageable while still maintaining good quality. Class E2 is the most commonly used class and is sufficient for many applications. 

Class E3

Class E3 of ISO 3302-1 refers to extrusions destined for non-critical applications. Class E3 is often chosen for situations in which cost savings is more important than dimensional control of extruded rubber tolerances.

Tight vs. Wide Tolerances: Cost and Manufacturing Impact

Dimensional tolerances refer to the level of accuracy required when manufacturing a given rubber or sponge product or component. Dimensional controls are often defined by a specific margin of error, measured in inches or millimeters, that is tolerable in the finished product. The more precision required, the smaller the margin of error. Extruded rubber tolerances can have a significant impact on both the cost of the part and the rubber manufacturing process.

Some dimensional tolerances are outlined in standardized specifications, such as ISO, ASTM, or the Rubber Manufacturers Association (RMA). Others may come directly from OEMs or other private sources.

Tight Tolerances

A tight tolerance, which refers to a very strict dimensional tolerance with little margin for error, is generally more challenging to achieve. Class E1 of ISO 3302-1 is a perfect example of a spec with tight extruded rubber tolerances. Medical equipment, aerospace components, and other sensitive assemblies are most likely to require tight tolerances.

Tight extruded rubber tolerances require precision tooling and manufacturing, meticulous cleaning and calibration of machinery, and, in some cases, strategic compounding. These extensive requirements often lead to more material waste and longer lead times. Given the extra time and resources required to achieve the desired outcome, parts with tight tolerances are often more expensive.

Wide Tolerances

A wide tolerance refers to a more flexible dimensional tolerance with room for variation. For many applications, wider extruded rubber tolerances or sponge tolerances, such as those outlined in ISO 3302-1’s Class E2 and E3, are more than sufficient.

Because wider tolerances allow for more variability in the rubber manufacturing process, these parts don’t require as much intense, hands-on time from rubber manufacturers. Additionally, wider extruded rubber tolerances often translate to shorter lead times and less scrap waste. The result is a lower cost per part when compared to parts requiring tighter tolerances.

In some applications, cost savings may be of equal or greater importance than dimensional control. Class E3, described in ISO 3302-1, is ideal for these situations, since it allows for considerable flexibility in final product dimensions and cuts down on costs by minimizing controls and other time-intensive manufacturing practices.

Why Extrusion Has Wider Tolerances Than Molding

Extruded rubber parts and sponge parts often allow for wider tolerances than molded rubber parts. This can be attributed to a few factors:

• Rubber undergoes die swell during the extrusion process, which changes the dimensions of the extruded part.
• Shrinkage and deformation often occur during the vulcanization process.

These phenomena can be controlled with supports, material selection, and process parameters, but some fluctuation is difficult to avoid. In fact, the tight extruded rubber tolerances required of Class E1 are all but impossible to achieve with certain synthetic rubbers, given their tendency toward change during the extrusion and curing processes.

The rubber molding process, on the other hand, is less vulnerable to these types of fluctuations, allowing for more control over the final product. For this reason, molding usually has tighter tolerances than extrusion. 

Choose UP&R for Quality Rubber Extrusions

Understanding the many variables of the rubber extrusion process, such as extruded rubber tolerances and sponge extrusion, is key to achieve the outcome you want. The experts at Universal Polymer & Rubber (UP&R) have years of experience helping our customers balance their priorities related to quality, control, and cost and delivering finished products that meet their requirements.

You can count on UP&R to help you determine the appropriate tolerance for your rubber or sponge project and the appropriate strategy for meeting any relevant specifications, such as ISO 3302-1 or RMA. With UP&R as your sponge and rubber extruder, you can breathe easy and trust that your extruded rubber product will meet your needs, from performance expectations to budget and timeline.

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