In overmolding of liquid silicone rubber (LSR) with substrates such as metals and plastics, poor interfacial bonding is one of the most common defects. Debonding or delamination after the product is subjected to stress or prolonged use directly affects sealing performance and structural strength.

Substrate Surface Treatment: The Primary Factor Determining Bonding Success

Due to its low surface energy and chemical inertness, LSR exhibits poor adhesion to most substrates. If oil contamination, oxide layers, or mold release agent residues are present on the substrate surface, they will directly hinder the physical and chemical bonding between LSR molecules and the substrate.

For metal substrates, a combined treatment of "plasma cleaning + sandblasting" is recommended. Plasma cleaning removes surface oxide layers, while sandblasting creates a micron-level rough surface that enhances physical anchoring. In terms of chemical treatment, metal parts can use silane coupling agent-containing primers (such as KH-550), which require curing in an 80°C oven for 30 minutes after application, with thickness controlled at 5–10 μm.

For plastic substrates, substrates containing mold release agents must be avoided. For low-polarity plastics such as PP, flame treatment can be used to raise surface energy above 40 mN/m. Polar plastics such as ABS/PC can use dedicated primers containing silane coupling agents.

Vulcanization Process Parameters: The Key Window for Interfacial Wetting

Vulcanization temperature and time directly affect the wetting effect at the LSR-substrate interface. Insufficient mold temperature leads to incomplete crosslinking reactions, preventing LSR from effectively penetrating the substrate's microporous structure; excessively short holding time results in "false bonding" between the adhesive layer and the substrate.

Industry practice shows that increasing mold temperature to 160–180°C can promote rapid leveling and wetting of LSR on the substrate surface. For precision overmolded parts, clamping pressure is recommended to be controlled at 8–12 MPa to ensure tight conformity between LSR and substrate, avoiding micro-gaps.

Mold Design and Material Selection

If the overmolding area lacks undercut structures (such as annular grooves or serrated patterns), mechanical anchoring force will be insufficient. When irrational runner design generates turbulence, air may be entrained, forming an isolation layer.

In terms of material selection, if high adhesion is required, self-adhesive LSR products containing adhesion-promoting components can be prioritized. These products incorporate internal adhesives directly into the silicone polymer, enabling reliable bonding with rigid components in the injection molding machine without additional surface treatment steps.

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