Why Some Silicone Overmolds Peel Apart — and How Primer-Free Bonding Works

A silicone-over-steel valve seat that peels in the field is a warranty disaster. Yet overmolding failure remains one of the most common quality issues we see when auditing competitor parts. The good news: bonding failures are almost always predictable and preventable at three stages.

1. Substrate chemistry

Not all metals bond equally. Stainless steel, aluminum and brass bond well with standard LSR grades. Anodized surfaces, PTFE coatings and some nickel platings actively resist adhesion. If your insert carries a plating for corrosion resistance, tell your molder early — the bonding strategy changes.

2. Surface preparation

Inserts arrive with stamping oils, anti-rust films and fingerprints — all bond killers. We degrease ultrasonically, then heat-activate surfaces immediately before molding. For primer-free food-contact and medical parts, plasma treatment replaces solvent-based primers entirely, keeping the part compliant with EU 1935/2004 without a bonded "interlayer" of unlisted substances.

3. Thermal management

LSR cures in seconds at 170–200°C, but a massive metal insert acts as a heat sink, cooling the contact zone below cure temperature. The fix is mold-side: local heating near the insert, longer cure windows, or pre-heating inserts. Skip this and you get a bond that passes peel tests on day one but fails after thermal cycling.

Our validation standard for every overmolded part: peel strength testing plus 100 thermal cycles from -40°C to +150°C before the part is approved for production. If your current supplier does not cycle-test bonds, ask why.