Silicone Rubber Shrinkage Guide: LSR vs HCR & Mold Compensation

Silicone rubber does not come out of a production mold at exactly the same dimensions as the mold cavity. As the material cures and cools, dimensional change occurs. For precision silicone components, this silicone rubber shrinkage must be considered before the production mold is manufactured.

Ignoring shrinkage can produce parts that are consistently undersized even when the mold itself has been machined correctly. But applying one generic shrinkage percentage to every silicone material can create the opposite problem.

The actual result depends on the silicone grade, hardness, filler system, molding process, curing conditions, part geometry and post-curing requirements.

This guide explains how silicone shrinkage works, how LSR and HCR differ, how mold compensation is calculated, and why precision parts often require dimensional correction after initial sampling.

What Is Silicone Rubber Shrinkage?

Silicone shrinkage is the dimensional difference between the mold cavity and the final molded component after curing and cooling.

If a mold cavity is exactly 100 mm long, the silicone part produced from that cavity will usually not remain exactly 100 mm after the complete manufacturing process.

Instead, the finished component may contract. The mold therefore needs to be made slightly larger than the desired finished dimension.

Shrinkage is one of the reasons silicone dimensions and silicone molding tolerances should be reviewed before tooling rather than after mass production begins.

Why Does Silicone Shrink?

Silicone rubber changes from an uncured or partially prepared material into a crosslinked elastomer during molding. This curing process changes the material structure.

At the same time, molded silicone is exposed to elevated temperatures and then cools after leaving the mold. The combined effects of crosslinking, formulation and temperature create dimensional change.

The result is usually expressed as a percentage of linear shrinkage.

However, silicone shrinkage should not be treated as a single material constant comparable to the density listed on a data sheet. It can be strongly influenced by processing.

Typical Silicone Shrinkage: Why 2–4% Is Only a Starting Point

Technical guidance from major silicone raw-material suppliers commonly shows linear shrinkage in the low-single-digit percentage range. A general silicone rubber reference of approximately 2–4% is useful during early product design.

But actual commercial grades can have their own values. For example, one LSR formulation may publish approximately 2.2% linear shrinkage while another may specify approximately 2.5%.

The material selected during final tooling should ideally remain stable throughout production. Changing the silicone grade after mold completion may alter the final dimensions.

LSR vs HCR Shrinkage

Both liquid silicone rubber (LSR) and high-consistency rubber (HCR) experience dimensional change during curing.

There is no reliable universal rule such as:

“LSR always shrinks 2% and HCR always shrinks 3%.”

The actual percentage depends on the specific material and manufacturing conditions.

Therefore, process selection should not be made only according to an assumed shrinkage percentage.

For a broader comparison of the two material systems, see LSR vs HCR Silicone: Which Molding Process Fits Your Part?.

How Is Silicone Shrinkage Calculated?

One common way to express linear molding shrinkage is:

Using this definition, the required mold dimension can be estimated by:

Example

Suppose the required finished dimension is:

100.00 mm

and the expected linear shrinkage is:

2.5%

The theoretical mold dimension would be:

This does not mean the toolmaker should automatically machine the mold to exactly 102.56 mm.

The calculation provides a starting point. Actual mold compensation must also consider material supplier data, geometry, process history and sampling results.

Why Mold Compensation Is Not Just “Add 2.5%”

A common shortcut is:

Target dimension × 1.025

This can be useful as an early approximation, especially when discussing concepts. But precision tooling should not be designed solely from a simplified percentage calculation.

Consider a silicone component containing:

• a thick center section
• two thin sealing lips
• a locating hole
• a long outer dimension
• an overmolded insert

These features may respond differently during curing, cooling and demolding.

The moldmaker may need to consider individual feature behavior rather than globally scaling the entire CAD model by one percentage.

Factor 1: Silicone Material Grade

The exact silicone formulation is one of the most important shrinkage variables.

Two materials that are both described as:

50 Shore A LSR

may still contain different polymer systems, fillers and additives. Their published shrinkage and processing behavior may therefore differ.

This is why defining only hardness on a product drawing is not enough for dimensional-critical applications.

For more detail on material hardness and why it should not be used as the only material specification, see our Silicone Rubber Hardness Guide.

Factor 2: Hardness and Filler Content

Hardness can correlate with changes in the formulation, particularly filler loading and crosslink density.

Supplier technical guidance indicates that increasing hardness and filler content can reduce shrinkage in some silicone systems.

However, designers should not turn this into a rule such as:

“Harder silicone always has exactly X% less shrinkage.”

The specific material data remains more useful than a general hardness-based assumption.

Factor 3: Mold and Cure Temperature

Silicone molding typically uses heated tooling to drive curing. The selected mold temperature affects:

• cure speed
• cycle time
• material behavior
• final dimensional response

Changing molding conditions after the tool has been dimensionally tuned can change finished-part dimensions.

For this reason, precision validation should be performed using conditions that represent the intended production process.

Factor 4: Post-Curing

Some silicone products require post-curing after molding. Post-curing can be used for reasons such as:

• removing residual volatile components
• meeting selected regulatory or application requirements
• stabilizing material properties

Additional heat exposure can also produce further dimensional change.

Therefore, if a production part is specified to be post-cured, final dimensional inspection should generally be based on the agreed final process state rather than only on the freshly molded component.

Factor 5: Part Thickness and Geometry

The geometry of the component affects how material flows, cures, cools and releases from the mold.

Features that deserve particular attention include:

• large changes in wall thickness
• thick solid sections
• long unsupported dimensions
• thin sealing lips
• deep ribs
• holes
• large flat areas
• asymmetrical geometry

Good silicone DFM therefore tries to create predictable material behavior rather than relying on tooling correction alone.

For more information on wall design, see our Silicone Injection Molding Design Guide.

Factor 6: Molded Part Direction and Geometry

For complex molded components, dimensions in different directions may not always respond identically.

Material flow, cavity shape, gating, restraint and local geometry can all influence final dimensions.

This matters when a part contains several critical dimensions oriented differently within the same cavity.

For precision projects, individual critical dimensions should be validated during sampling rather than assuming perfectly uniform shrinkage throughout the entire part.

Factor 7: Demolding and Part Deformation

Silicone is flexible. A freshly molded component may stretch or deform while it is removed from the mold.

This is particularly important for:

• deep undercuts
• thin walls
• soft silicone
• large flexible components
• complex cores
• long narrow features

What appears to be a shrinkage problem may sometimes be related partly to demolding deformation or measurement conditions.

Shrinkage vs Tolerance: They Are Not the Same Thing

These two terms are closely related but describe different concepts.

A mold can compensate accurately for average shrinkage but the finished parts will still have a production tolerance.

For a detailed explanation of dimensional tolerance classes, read Silicone Molding Tolerances: ISO 3302-1 Guide.

Shrinkage vs Compression Set

Shrinkage should also not be confused with compression set.

Shrinkage describes dimensional change associated with molding and curing.

Compression set describes how well an elastomer recovers after being compressed for a defined time and temperature.

Compression set is particularly important for long-term sealing applications, while mold shrinkage is primarily a tooling and dimensional-control issue.

How Shrinkage Affects Silicone Tooling

The production CAD model represents the desired finished component. The mold cavity must then be created using appropriate compensation.

A typical engineering workflow is:

1. Confirm the target finished dimensions.
2. Select the intended production silicone grade.
3. Review the supplier's shrinkage data.
4. Apply initial mold compensation.
5. Manufacture the tool.
6. Mold samples using production-representative conditions.
7. Condition or post-cure the parts if required.
8. Measure critical dimensions.
9. Compare actual results with drawing requirements.
10. Correct selected mold dimensions if necessary.

This is why first tooling samples are not merely color or appearance samples. They are also an important dimensional validation stage.

Why Precision Silicone Molds May Need Correction After T1

Even with good raw-material data and experienced tool design, the actual molded result can differ slightly from the theoretical prediction.

This does not necessarily mean the original tool was designed incorrectly.

Material data represents controlled testing conditions, while a real product has its own:

• geometry
• wall transitions
• gate layout
• cure profile
• demolding behavior
• measurement method

For dimensional-critical silicone parts, T1 sampling should therefore be treated as a data collection step.

Example: Compensating a Silicone Gasket

Suppose a gasket must have a finished outside diameter of:

80.00 mm

and the selected material's initial expected shrinkage is:

2.5%

Using the theoretical formula:

The engineering team can use approximately 82.05 mm as an initial cavity reference.

After molding and completing the specified final cure process, actual samples are measured.

Imagine the average finished result is:

79.75 mm

If the drawing requires tighter dimensional control, the toolmaker can then use the measured result to calculate a targeted mold adjustment.

This feedback-based correction is much more reliable than changing the entire mold according to an assumed generic silicone shrinkage percentage.

Should Every Mold Dimension Use the Same Shrinkage Factor?

Not always.

For relatively simple parts, a uniform initial compensation factor may work well.

For complex precision components, however, different features can respond differently because of:

• geometry
• material flow
• local wall thickness
• tool restraint
• parting-line configuration
• insert molding

In these cases, dimensional correction may need to be feature-specific.

Silicone Shrinkage in Overmolding

Overmolding introduces additional constraints because silicone cures around a rigid insert or substrate.

The rigid component can restrict free dimensional movement in some areas.

Engineers therefore need to consider:

• substrate geometry
• mechanical locking features
• chemical bonding
• silicone thickness
• material compatibility
• thermal expansion differences

Shrinkage behavior in an overmolded component should not automatically be assumed to match a free-standing silicone test specimen.

How Color and Additives Can Affect the Final Material

Production silicone often contains pigments or other approved additives.

While color matching is usually considered a visual requirement, the complete production formulation should remain controlled for precision applications.

Changing the compound formulation after mold validation can affect more than appearance.

For projects requiring precise brand colors, see How Pantone Color Matching Works for Silicone Products.

How to Control Silicone Shrinkage in Production

Shrinkage cannot simply be “eliminated.” The goal is to make the process predictable and compensate for it correctly.

1. Lock the Material Grade

Do not change suppliers or silicone grades casually after dimensional validation.

2. Control Mixing and Material Preparation

Use stable material handling procedures appropriate to the selected silicone system.

3. Control Mold Temperature

Maintain the validated processing window instead of treating mold temperature as an unrestricted production variable.

4. Control Cure Conditions

Cycle time and curing conditions should remain consistent with the validated process.

5. Standardize Post-Curing

If post-curing is required, use a defined time and temperature process.

6. Measure After a Defined Conditioning Period

Do not compare dimensions taken at inconsistent times or process stages.

7. Use Production Data

For precision parts, dimensional data from actual production runs is more useful than relying indefinitely on a theoretical shrinkage percentage.

Common Silicone Shrinkage Mistakes

Mistake 1: Using One Internet Value for Every Silicone Material

“Silicone shrinks 3%” is not a sufficient mold specification. Different grades can behave differently.

Mistake 2: Selecting Material After the Mold Is Finished

If dimensional accuracy matters, the intended material should ideally be established before final mold compensation.

Mistake 3: Assuming Hardness Alone Predicts Shrinkage

Hardness can correlate with formulation changes, but Shore A alone cannot determine the exact shrinkage rate.

Mistake 4: Ignoring Post-Cure Dimensional Change

If the final production part is post-cured, final dimensions should be validated after the specified post-cure process.

Mistake 5: Scaling the Entire CAD Model Without DFM Review

Uniform scaling can be useful as an initial tooling method for simple geometry, but complex precision components may require feature-specific consideration.

Mistake 6: Confusing Measurement Error With Shrinkage

Soft silicone can deform under calipers or fixtures. A dimensional discrepancy may therefore come partly from the measurement method.

Silicone Shrinkage Checklist Before Tooling

• Has the production silicone grade been selected?
• Do you have the material supplier's shrinkage data?
• Is the hardness finalized?
• Is post-curing required?
• Are critical dimensions identified?
• Are there large variations in wall thickness?
• Are any dimensions especially sensitive to shrinkage?
• Does the part contain rigid inserts?
• Has the measurement method been defined?
• Is there enough tooling steel for dimensional correction after sampling?
• Will T1 samples be measured before final approval?

These questions should be reviewed as part of DFM before production tooling is finalized.

Frequently Asked Questions

Final Takeaway

Silicone rubber shrinkage is predictable enough to engineer around, but not simple enough to reduce to one universal percentage.

For dimensional-critical parts, good results come from combining material data, mold design, controlled processing and actual measurement rather than relying on a generic shrinkage factor.

Technical References