DFM GUIDE — DESIGN FOR MANUFACTURABILITY

Shrinkage & Tolerance
Guide for Silicone & Rubber

Shrinkage is the silent killer of part accuracy. Understanding material shrink rate and designing tolerances correctly is the difference between a functional part and expensive scrap.

What Is Mold Shrinkage?

When silicone or rubber cools from cure temperature to room temperature, it contracts (shrinks). The mold cavity must be cut larger than the final part to compensate for this shrinkage. Shrinkage rate is expressed as a percentage of the linear dimension (e.g., 2% shrinkage on a 100mm part = 2mm smaller than the cavity).

Getting shrinkage wrong during mold design means the first parts are out of specification. Mold correction (re-cutting) adds time and cost. Our engineers factor in the correct shrinkage rate before we cut steel.

Typical Shrinkage Rates

Material	Shrinkage Rate	Notes
HTV Silicone (Compression)	1.5 – 3.0%	Higher for soft grades (Shore A 10–30)
LSR (Injection)	2.0 – 4.0%	Higher than HTV — critical to account for in mold design
Rubber (NBR/EPDM/CR)	1.5 – 2.5%	Varies with compound and cure temperature
FKM (Viton)	1.0 – 2.0%	Lower shrink rate; high-temperature dimensional stability
TPE / TPU (Overmolding)	1.0 – 2.5%	Depends on Shore hardness and formulation

Factors That Affect Shrinkage

Durometer (Hardness)

Softer silicones (Shore A 10–30) shrink more than harder grades (Shore A 60–80).

Mold Temperature

Higher mold temp = faster cure but can increase shrinkage. Controlled within ±5°C.

Part Geometry

Thick sections shrink more than thin walls. Non-uniform walls = non-uniform shrink = warpage.

Post-Cure (HTV)

Post-curing (200°C, 4–8 hrs) causes additional 0.2–0.5% shrink.

Fillers & Additives

Carbon, flame-retardant agents, and color pigments all change shrink rate.

Overmolding Inserts

Metal/plastic inserts restrict shrinkage, causing internal stress.

Mold Cavity vs. Finished Part

The image shows the steel mold cavity alongside the finished silicone parts, demonstrating the dimensional difference caused by shrinkage. The mold is intentionally cut larger to compensate.

Shrinkage is not a single number — it varies with material hardness, mold temperature, part geometry, post-cure, fillers, and overmolding inserts. Our engineers use empirical data from production to predict shrinkage accurately for each compound and geometry.

Standard Tolerances — Silicone & Rubber Molding

Dimension Range (mm)	Compression Molding	LSR Injection	Note
0 – 10 mm	±0.15	±0.05	Tight control on small features
10 – 50 mm	±0.20	±0.10	Typical for most parts
50 – 100 mm	±0.30	±0.15	Larger parts have more variation
100 – 200 mm	±0.50	±0.25	Discuss with DFM team
>200 mm	±0.8–1.0	±0.5	Custom agreement required

Design Guidelines for Tolerances

Avoid tight tolerances unless functionally necessary. Every ±0.05mm added costs money. Standard ±0.20mm is sufficient for 80% of features.
Specify tolerances on drawing. Do NOT apply ISO 2768 machined tolerances to rubber/silicone. Use ISO 3302-1.
Shrink compensation is iterative. First mold may need 1–2 rounds of re-cutting to hit dimensional targets.
O-ring cross-section tolerance is critical. Use ISO 3601-1 — do NOT use generic molding tolerances for O-ring seal grooves.
Operating temperature matters. Parts that operate at high temp will shrink further in service. Specify operating temperature range.

Recommended Drawing Note

DIMENSIONS PER ISO 3302-1, CLASS M2, UNLESS OTHERWISE SPECIFIED.

SHRINKAGE ALLOWANCE IN MOLD DESIGN PER THOR-SOFT ENGINEERING DISCRETION.

CRITICAL DIMENSIONS: [LIST THEM] ±0.10 MM.

O-RING CROSS-SECTION TOLERANCES PER ISO 3601-1.

Other DFM Guides: