Rubber Shrinkage: Why the Hot Part Doesn’t Fit the Cold Mold

A rubber part doesn’t stop changing when it comes out of the mold. It keeps shrinking — for hours.

You measured the hot part straight out of the cavity. It matched the drawing. You approved the tool. Then the parts sat overnight, cooled down, and shrank below spec. Now nothing fits.

If you designed the cavity based on the hot part, the cold part won‘t work.

It’s not your measurement. It‘s the rubber.

Shrinkage isn’t a fixed number. Compound, part geometry, mold design, and process conditions all pull it in different directions. And what works for one rubber won‘t work for another.

Here’s what moves the number — and how to get closer to the right answer before you cut steel.

Factors That Increase Shrinkage

• Higher vulcanization temperature – Every 10°C above optimum adds 0.1–0.2% shrinkage.

• Flow direction – Rubber shrinks more along the flow direction than across it.

• Longer flow distance – More travel in the mold means more shrinkage.

• Higher rubber content – More polymer, more shrink.

• Thin‑wall parts (under 3 mm) – Shrinkage runs 0.2–0.6% higher than thick parts (over 10 mm).

Factors That Reduce Shrinkage

• Higher filler content – Fillers reduce shrink.

• Higher compound plasticity – Easier‑flowing compounds shrink less.

• Higher compound hardness – Generally less shrink (but above 90 Shore A, it may creep back up).

• Injection molding vs. compression molding – Injection gives you less shrink.

• Center cavities in multi‑cavity molds – Shrink slightly less than edge cavities.

• Larger parts – Shrinkage decreases as size goes up.

• Fabric reinforcement – More layers, less shrink.

Shrinkage by Rubber Type

From highest to lowest: fluororubber (FKM) → silicone (VMQ) → EPDM → natural rubber (NR) → nitrile rubber (NBR) → chloroprene rubber (CR), as shown in Figure 5-1.

These are general trends — the actual number depends on your specific compound. Always verify with a test shot before cutting steel for production cavities.

Reinforced and Fabric‑Inserted Parts

• Fabric‑laminated parts: 0–0.4%

• Polyester cord‑inserted: 0.4–1.5%

• Nylon fabric‑inserted: 0.8–1.8%

• More layers → less shrink

• Metal inserts: 0–0.4% (shrinks toward the metal)

• Single‑side bonded: 0.4–1.0% (shrinks less near the bond)

• Hard rubber (>90 Shore A, ~20% rubber content): ~1.5%

• Rubber/plastic blends: 1.1–1.6% (0.1–0.3% lower than pure rubber)

Geometric Effects

For grooved square parts, shrinkage runs 0.2–0.4% higher in the compression direction than perpendicular to it — because the rubber flows differently in each direction.

Two Ways to Calculate Shrinkage

Method 1: Compare part to cavity (after molding)

C = (L₂ – L₁) / L₁ × 100%

Where:

• C = Shrinkage rate of the compound (%)

• L₁ = Dimension of the molded rubber part at room temperature

• L₂ = Corresponding cavity dimension at room temperature

This is the most reliable method — but it requires a molded part to measure. Good for tryout verification.

Method 2: Estimate from thermal expansion (before molding)

C = (α – β) × ΔT × R × 100%

Where:

• α = Linear expansion coefficient of the rubber compound

• β = Linear expansion coefficient of the mold material

• ΔT = Temperature difference between vulcanization and measurement

• R = Total volume fraction of rubber, sulfur, and organic curatives in the compound

This method helps during the design stage, before any parts exist — but it’s an estimate, not a guarantee.