When producing custom rubber seals—such as O-rings, flat gaskets, grommets, bellows, or complex rubber molded components—one factor influences mold design more than anything else: the rubber material itself.

Whether the part is made from NBR, FKM, EPDM, HNBR, or silicone, each material behaves differently during molding.
These differences directly impact:

• mold structure

• shrinkage compensation

• venting design

• curing parameters

• cavity pressure

• tolerance control

In this article, DXTSEALS explains why rubber material selection must be finalized before mold making, and how material properties influence every aspect of mold engineering.

1️⃣ Different Rubber Materials Have Different Shrinkage Rates

Shrinkage rate is the most important factor in rubber mold design.
Every type of rubber shrinks differently during vulcanization.

Typical shrinkage rates:

If a mold is designed using NBR shrinkage but later uses silicone, the final product may be:

• oversized

• undersized

• out of tolerance

• unable to seal properly

Accurate mold dimensions depend entirely on correct material shrinkage data.

2️⃣ Material Hardness (Shore A) Influences Cavity Structure

Rubber hardness dramatically affects mold engineering.

Soft materials (30–50 Shore A)

• deform easily

• need more venting

• require larger supporting areas

• are prone to flashing if mold is not tight enough

Hard materials (70–90 Shore A)

• require stronger molds

• need precise cavity gap control

• create higher internal pressure during molding

Therefore, mold structure—including parting lines, flash control, and tolerance compensation—must be optimized according to material hardness.

3️⃣ Curing Characteristics Differ Between Rubber Types

Different rubber materials require different curing temperatures, times, and pressures:

Incorrect assumptions can cause:

• bubbles

• incomplete vulcanization

• surface defects

• part deformation

A mold designed for NBR curing may not work correctly for FKM or silicone.

4️⃣ Rubber Flow Behavior Affects Gate and Runner Design

Rubber flow behavior determines how the rubber fills the mold cavity.

Flow characteristics by material:

• Silicone: flows very easily → requires more gate control

• EPDM: moderate flow → stable filling

• NBR & HNBR: medium viscosity → uniform filling

• FKM: high viscosity → needs higher pressure and careful gate design

If flow behavior is mismatched:

• the part may not fill completely

• excessive internal stress may occur

• flash lines may increase

• cavity wear may accelerate

Thus, runner size, gate type, and injection pattern depend directly on rubber material.

5️⃣ Temperature Resistance Dictates Mold Steel Selection

Some rubber materials require very high curing temperatures:

• FKM: 170–200°C

• Silicone: 160–190°C

• HNBR: 160–180°C

This affects:

• steel hardness

• thermal expansion

• mold lifetime

• deformation risk

Molds for high-temperature rubbers must use more durable steels and optimized cooling channels.

6️⃣ Material Compression Set Determines Tolerance Strategy

Different materials respond differently to compression and recovery.

• Silicone: high compression set → needs larger dimensional compensation

• FKM & NBR: low compression set → more stable dimensions

• EPDM: moderate compression set → depends on formulation

This affects the mold’s tolerance design.
A single mold cannot ensure proper tolerance for different rubber types.

7️⃣ Material Influences Venting Structure

Venting is essential to remove trapped air during molding.

• Silicone releases large amounts of gas → requires many micro-vents

• FKM has less gas release → fewer vents required

• NBR/EPDM → medium venting required

Improper venting causes:

• air pockets

• surface burns

• incomplete filling

• mold damage over time

Thus, the mold’s venting layout must reflect the rubber’s behavior.

8️⃣ Changing Material After Mold Production Causes Major Problems

If the rubber material changes mid-project:

• the shrinkage rate becomes wrong

• the mold’s tolerance will be inaccurate

• curing will not match mold design

• venting may be insufficient

• flashing increases

• the mold may need modifications

• or a completely new mold may be required

This results in extra cost, delays, and production inconsistencies.

⭐ Conclusion

Different rubber materials—whether NBR, FKM, EPDM, silicone, or HNBR—have completely different behaviors during molding.
These differences affect:

• mold structure

• shrinkage rate

• venting

• curing parameters

• runner and gate design

• tolerance compensation

• material flow and pressure distribution

This is why rubber material must be confirmed before mold making begins.

For custom rubber seals, precision mold design, and material selection guidance, DXTSEALS provides complete technical support and manufacturing solutions.