Solid-to-Liquid Composite Process: Perfect Bonding of Metal and Silicone

The solid-to-liquid composite process is an advanced manufacturing technology that achieves chemical bonding between pre-formed solid substrates (metal/plastic) and liquid silicone rubber (LSR) during a single molding cycle.

1. Process Principles

1.1 Basic Concept

The solid-to-liquid process involves placing a pre-formed solid substrate into a mold, then injecting LSR. Chemical bonding occurs between silicone and substrate during high-temperature curing.

1.2 Bonding Mechanisms

• Chemical bonding: Si-H bonds in silicone react with functional groups in primer, forming covalent bonds
• Mechanical interlocking: Micro-roughness on substrate surface provides physical anchoring
• Van der Waals forces: Intermolecular forces provide basic adhesion

2. Key Process Steps

2.1 Substrate Preparation

1. Stamping/injection molding for solid substrate
2. Surface treatment (sandblasting/plasma/chemical etching)
3. Primer application (dipping/spraying/brushing)
4. Primer drying (80-120 degrees, 5-15 minutes)

2.2 Insert Placement

• Manual placement: for small batch, multiple varieties
• Robotic placement: for high volume, high cycle rates
• Visual positioning: ensuring placement accuracy

2.3 LSR Injection and Curing

• Close mold and inject LSR
• Cure at 150-200 degrees for 30-90 seconds
• Silicone and primer cross-link simultaneously

3. Primer Selection

3.1 Metal Primers

• Silane coupling agents: KH-550, KH-560
• Titanate coupling agents: suitable for stainless steel and aluminum
• Silicone primers: specialized silicone-metal adhesion promoters

3.2 Plastic Primers

• For PA: amino coupling agents
• For PC: epoxy silane types
• For ABS: acrylate coupling agents

4. Mold Design Considerations

• Precision locating pins/slots
• Magnetic positioning for metal inserts
• Anti-float design: prevent insert displacement during injection
• Submarine gate: clean, mark-free
• Film gate: for large-area overmolding

5. Typical Applications

• Power tool grip overmolding: PA6+GF30% substrate + 50 Shore A LSR
• Medical device handle: 304 stainless steel + 40 Shore A medical-grade LSR
• Smartwatch shell: PC+ABS + 55 Shore A LSR

6. Bond Quality Assessment

• Cohesive failure (optimal): Silicone body fractures, bond surface intact
• Adhesive failure (worst): Silicone peels from substrate surface
• Mixed failure: Partial cohesive + partial adhesive
• Reliability testing: Temperature-humidity, thermal cycling, salt spray

Conclusion

The solid-to-liquid composite process achieves perfect bonding between metal/plastic and silicone through chemical bonding, providing key technical support for multi-functional integrated product design.