What is the shrinkage rate of rubber?

Anyone who has ordered custom-molded rubber parts knows the pain of receiving pieces that do not fit. I have seen projects delayed or rejected because no one considered rubber’s shrinkage rate in the mold design.

The shrinkage rate of rubber typically ranges from 1% to 3%, depending on the rubber type, formulation, and molding conditions. Accurate shrinkage prediction is crucial for achieving the correct final dimensions of molded rubber parts.

Ignoring shrinkage can result in parts that are too small or too large, causing problems in assembly or sealing. I always recommend understanding this property before starting any rubber tooling project.

Why does rubber shrink after molding?

Engineers often find their finished rubber parts are smaller than expected. This is not a mistake but a natural property of how rubber behaves during processing.

Rubber shrinks after molding because the material contracts as it cools from vulcanization temperature to room temperature, and as internal stresses relax. The polymer structure and filler content also impact the final shrinkage.

Main Factors Affecting Shrinkage

For example, I have worked on both EPDM and silicone rubber parts. Silicone usually shrinks a bit more than EPDM, so I always adjust the mold dimensions to account for this.

What are typical shrinkage rates for common rubber materials?

I often help customers estimate final part sizes based on the type of rubber used. Here is a simple reference table I use in daily work.

Most rubber materials have shrinkage rates between 1% and 3%, but some special formulations can vary. Always confirm with your supplier or test samples.

Rubber Shrinkage Rates⁶ Table

Mold makers at my factory always add this shrinkage value to the mold drawing so that the final rubber product matches the required size after demolding and cooling.

How do you calculate and compensate for shrinkage in molded rubber parts?

Precision is everything, especially for O-rings, gaskets, and custom rubber seals. Getting the final part size right means accounting for shrinkage at the design stage.

To compensate, multiply the finished part dimension by (1 + shrinkage rate). For example, a required 100 mm part with 2% shrinkage means making the mold cavity 102 mm.

Shrinkage Calculation Example

1. Desired final size: 100 mm
2. Shrinkage rate: 2% (0.02)
3. Mold size = 100 mm × (1 + 0.02) = 102 mm

Simple Formula

Mold Size = Final Size × (1 + Shrinkage Rate)

At Julong Rubber, we use this formula every day. We also test sample parts from each new mold to fine-tune the actual shrinkage rate, since mixing, temperature, and part design can cause small variations.

What else can influence rubber shrinkage during molding?

Even with the best design, real production sometimes produces surprises. I always remind clients that real shrinkage may change slightly due to multiple factors.

Rubber shrinkage is also affected by part thickness, cure time, post-cure treatment, and the cooling rate. Environmental humidity and storage time can also have minor effects.

Detailed Factors Table

I encourage clients to always confirm real shrinkage with test runs, especially for tight-tolerance parts.

What is the shrinkage rate of EPDM compared to other rubbers?

Many buyers in HVAC and automotive industries ask me about EPDM, since it’s one of the most popular rubbers.

EPDM rubber typically has a shrinkage rate between 1.2% and 2.0%, which is lower than silicone and about the same as NBR and SBR. Its low and stable shrinkage makes it a favorite for precise sealing applications.

EPDM vs Other Rubbers Table

At my factory, we always confirm the shrinkage of the EPDM batch by trial runs, especially when making new or tight-tolerance products for clients in Europe.

Conclusion

The shrinkage rate of rubber is a critical factor in mold design and quality control. By knowing and compensating for typical shrinkage (usually 1–3%), you can achieve reliable, precise molded parts every time.