Many buyers expect molded silicone rubber parts to fit perfectly, but sometimes they are shocked to find the parts are smaller than designed. I have seen entire batches scrapped simply because no one planned for silicone’s unique shrinkage rate.
Silicone rubber typically has a shrinkage rate of 2% to 3% during molding. The exact rate depends on the type of silicone, its formulation, and processing conditions. Always account for this shrinkage when designing molds and parts.
Understanding silicone shrinkage is essential for achieving tight tolerances and reducing costly production mistakes. Let’s take a closer look at why silicone shrinks and how to manage it.
Why does silicone rubber shrink more than other rubbers?
Manufacturers often ask why silicone parts seem to “shrink away” from the mold more than EPDM or NBR. The answer lies in the polymer chemistry and curing method.
Silicone rubber has a highly flexible molecular structure, and its crosslinking (vulcanization) leads to more pronounced shrinkage as the material cools and sets. The curing process—whether compression, injection, or transfer—also impacts the final shrinkage rate.
Main Factors Affecting Silicone Shrinkage
| Factor | Effect on Shrinkage |
|---|---|
| Type of Silicone | LSR often shrinks more than HTV |
| Filler Content | More filler, less shrinkage |
| Curing Method1 | Injection > Compression |
| Mold Temperature2 | Higher temp, higher shrinkage |
| Part Thickness | Thick parts shrink more |
I always advise mold makers to adjust cavity sizes upward based on the expected shrinkage, especially for Liquid Silicone Rubber (LSR)3, which is widely used for medical and baby products.
How do you calculate and compensate for silicone rubber shrinkage?
I have worked on projects where a 1 mm gap meant the difference between success and failure. Shrinkage must be calculated early in the mold design stage.
To compensate, multiply the final part size by (1 + shrinkage rate4). For example, a finished part of 100 mm with 2.5% shrinkage should have a mold cavity size5 of 102.5 mm.
Silicone Shrinkage Calculation Example
| Final Part Size (mm) | Shrinkage Rate (%) | Mold Cavity Size (mm) |
|---|---|---|
| 100 | 2.5 | 102.5 |
Formula:
Mold Size = Final Size × (1 + Shrinkage Rate)
I always test sample runs when using a new silicone formulation to confirm the actual shrinkage rate4, since even small changes in the process can affect the result.
How does silicone shrinkage compare to other rubbers like EPDM or NBR?
Many customers ask me why their silicone parts shrink more than their EPDM or nitrile parts. The answer is in the numbers.
Silicone rubber usually shrinks more than EPDM, NBR, and SBR. For example, silicone is typically 2–3%, while EPDM and NBR are about 1.2–2%.
Shrinkage Rate Comparison Table
| Rubber Type | Typical Shrinkage Rate (%) |
|---|---|
| Silicone6 | 2.0 – 3.0 |
| EPDM7 | 1.2 – 2.0 |
| NBR8 | 1.0 – 1.8 |
| SBR9 | 1.2 – 1.8 |
| FKM (Viton®)10 | 1.5 – 2.2 |
For high-precision parts11, I always recommend consulting with the compound supplier and running a few test samples.
What can influence the final shrinkage of silicone rubber parts12?
Getting the right part size means thinking beyond just the shrinkage rate on paper. Real-world production introduces many variables.
Factors like curing temperature, mold design, part thickness, filler type, and post-curing can all affect silicone shrinkage. The production process must be tightly controlled to ensure consistent part size.
Influencing Factors Table
| Factor | Impact on Shrinkage |
|---|---|
| Cure Temperature | Higher temp, more shrinkage |
| Filler Type | More filler, less shrinkage |
| Mold Design | Venting, geometry, release |
| Part Thickness | Thick parts, higher shrinkage |
| Post-curing | May cause additional shrinkage |
In my factory, we always document the exact molding conditions and run first-article inspections to confirm that the shrinkage is within acceptable limits.
How do I ensure correct dimensions for silicone molded parts?
Precision and consistency are crucial for medical, automotive, and food-grade silicone parts. I help customers avoid costly remakes by following a few best practices.
Work with your supplier to test the actual compound, design the mold for the expected shrinkage, and validate with sample parts before mass production. Communicate all tolerance needs up front.
Best Practice Checklist
- Confirm shrinkage rate with material supplier.
- Calculate mold cavity size to include shrinkage.
- Run sample tests with production compound.
- Monitor process parameters for each batch.
- Adjust mold or process if measured parts are out of spec.
By following these steps, I have helped many customers get their silicone parts right the first time—saving time, money, and frustration.
Conclusion
Silicone rubber’s shrinkage rate is higher than most other rubbers, typically 2–3%. Always factor this into your mold and part design for perfect fit and reliable performance.
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Explore this resource to understand how various curing methods impact silicone shrinkage, ensuring optimal mold design. ↩
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Understanding mold temperature's effect on shrinkage is crucial for precise mold design and production quality. ↩
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Explore the versatility of LSR in various industries, especially in medical and baby products. ↩
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Understanding shrinkage rates is crucial for accurate mold design and production quality. ↩ ↩
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Learn the best practices for adjusting mold sizes to compensate for shrinkage in silicone parts. ↩
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Explore this resource to understand silicone's unique properties and its diverse applications in various industries. ↩
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Explore this resource to understand EPDM's unique properties and how it compares to silicone, enhancing your material selection process. ↩
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Exploring this resource will provide insights into NBR's unique properties and its applications, enhancing your understanding of rubber materials. ↩
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Explore this link to understand how SBR compares to silicone in shrinkage, crucial for material selection in manufacturing. ↩
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Explore this link to understand FKM's unique properties and its advantages in various applications, enhancing your material selection. ↩
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Find expert tips on achieving precision in silicone manufacturing for critical applications. ↩
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Explore this resource to learn effective strategies for controlling shrinkage in silicone rubber parts, ensuring precision and quality. ↩
