Liquid Silicone Rubber Overmolding on Metal (LSR Overmolding on Metal) is an advanced composite manufacturing technology that tightly integrates liquid silicone rubber (LSR) with metal substrates through injection molding. This process leverages LSR's excellent biocompatibility, high and low temperature resistance, electrical insulation, and chemical stability to form a dense, uniform silicone coating on metal surfaces, achieving multiple objectives including functional enhancement, sealing protection, and tactile optimization.

Unlike traditional thermoset silicone compression molding, LSR overmolding on metal uses injection molding, offering significant advantages such as high production efficiency, excellent dimensional accuracy, and high automation. It is particularly suitable for industries with stringent quality requirements including medical devices, consumer electronics, and automotive components.

2. Core Process Principles and Technical Challenges

2.1 Bonding Mechanism Analysis

The bond between liquid silicone and metal is not simple physical adhesion but involves multiple mechanisms. Mechanical interlocking creates micron-level roughness (Ra 3-8μm) through sandblasting, wire drawing, or chemical etching, allowing LSR to penetrate microscopic grooves during injection. Chemical bonding uses specialized primers to introduce active functional groups on the metal surface, forming cross-linked covalent bonds with LSR molecular chains. The interface diffusion layer forms a 5-20μm transition zone during high-temperature vulcanization, effectively relieving stress concentration caused by thermal expansion coefficient differences.

2.2 Key Technical Challenges

Challenge 1: Insufficient Bond Strength.This is the most common failure mode, caused by incomplete surface treatment, uneven primer application, mismatched LSR formulation, or poor mold venting. Solutions include standardized pretreatment processes and regular peel testing to ensure strength ≥8N/mm.

Challenge 2: Uneven Coating Thickness and Flash.LSR's extremely low viscosity makes it prone to flash formation. Control strategies include zero-clearance mold design, injection pressure of 80-120MPa, optimized gate location, and proper holding time.

Challenge 3: Warpage from Thermal Expansion Mismatch.Measures include symmetrical design, mold temperature uniformity within ±2°C, annealing processes for stress relief, and two-shot molding for high-precision applications.

3. Typical Applications

Medical Devices: Ultrasonic probe encapsulation and pacemaker sealing components using USP Class VI certified medical-grade LSR on stainless steel housings, providing excellent biocompatibility and sterilization resistance.

Consumer Electronics: Smartwatch waterproof buttons and TWS earphone charging contact protection, achieving IP68 waterproof sealing with soft tactile feedback and ≥100,000 cycle button life.

Automotive Industry: New energy vehicle battery pack sensor encapsulation with LSR-coated aluminum alloy housings operating reliably from -40°C to 125°C, meeting UL94 V-0 flame retardant rating.

4. Material Selection Guide

Metal substrate options: Stainless steel 304/316 for medical devices and food contact; Aluminum alloy 6061/7075 for consumer electronics and automotive; Copper alloys for electrical connectors; Titanium alloys for high-end medical implants.

LSR material recommendations: General-purpose Wacker ELASTOSIL® LR 3003/50 for most applications; High-bonding Dow Corning SE 1700 optimized for metal overmolding; Medical-grade Wacker ELASTOSIL® LR 5040 certified to USP Class VI and ISO 10993.

5. Quality Testing and Acceptance Standards

Recommended testing systems include: Visual inspection for flash, voids, and impurities; Dimensional measurement within ±0.05mm tolerance; Peel strength testing ≥8N/mm; Seal testing at 0.5MPa with leakage<5mL/min; Aging testing at 85°C/85%RH for 1000h.

6. Future Development Trends

With Industry 4.0 and smart manufacturing advancement, LSR overmolding on metal is trending toward intelligent production, green transformation, miniaturization, and multi-functional integration. IoT sensors enable real-time monitoring of key parameters with AI algorithms predicting product quality. Solvent-free primers reduce VOC emissions. Ultra-thin coating technology (layer thickness<0.3mm) enables micro-structure replication. Functional fillers for conductivity, thermal management, and EMI shielding are being directly incorporated into LSR formulations for "one-coating-multiple-functions" capability.

Keywords: LSR Overmolding on Metal, Liquid Silicone Rubber Metal Coating, Silicone over Stainless Steel, LSR Injection Molding, Medical-grade Silicone Encapsulation