What are the steps involved in the liquid silicone encapsulation metal injection molding process?

Liquid silicone-coated metal injection molding (integrated molding of liquid silicone and metal inserts) is a process in which liquid silicone (LSR) is injected into a mold under high temperature and pressure using an injection molding machine, where it combines with a pre-placed metal insert and solidifies to form a composite product. The core of this process is ensuring a strong bond between the metal and silicone, dimensional accuracy, and appearance quality. The specific process steps are as follows:

Automated Production of Liquid Silicone-Coated Metal Injection Molding
I. Preliminary Preparation Stage

1. Metal Insert Design and Selection

Based on product functional requirements, determine the material (e.g., stainless steel, aluminum alloy, iron), shape (reserving space for silicone encapsulation to avoid stress concentration caused by sharp corners), and dimensional tolerances (typically controlled within ±0.02mm to ensure mold compatibility).

The insert surface needs to have pre-reserved "bonding reinforcement structures," such as shallow grooves, bumps, or threads (to assist mechanical anchoring and improve silicone adhesion).

2. Liquid Silicone Resin (LSR) Selection and Preparation

Selection: Choose a suitable LSR based on product performance requirements (e.g., hardness, temperature resistance, corrosion resistance). For example, medical-grade LSR must meet FDA standards, while industrial-grade LSR must be oil-resistant and aging-resistant.

Raw Material Processing:

Weigh the two-component LSR (component A contains base adhesive + catalyst, component B contains base adhesive + crosslinking agent) according to the ratio (commonly 1:1 or 10:1). The error should be ≤0.5% (imbalance in the ratio will lead to incomplete vulcanization or excessive hardness/softness).

Mixing and Stirring: Use a dedicated mixer to stir at low speed for 3-5 minutes, then stir at high speed for 5-10 minutes to ensure uniformity (avoid local performance differences).

Vacuum Degassing: Degas under a vacuum of -0.095MPa for 5-15 minutes to remove air introduced during mixing (air bubbles will cause insufficient adhesive and reduced strength in the product).

II. Pre-treatment of Metal Inserts (Critical Step)

The surface condition of the metal directly affects the adhesion of the silicone adhesive. A multi-step process of "cleaning + activation + roughening" is required:

1. Degreasing and Oil Removal

Purpose: To remove cutting oil, rust-preventive oil, fingerprints, and other organic matter from the metal surface (grease can isolate the silicone from the metal, causing delamination).

Method:

Ultrasonic Cleaning: Use a neutral cleaning agent (such as an alkaline degreaser) for ultrasonic cleaning at 60-80℃ for 10-20 minutes (suitable for complex structural parts; high-frequency vibration removes tiny oil stains).

Solvent Wiping: For precision small parts, soak in alcohol or isopropyl alcohol and then wipe with a lint-free cloth (avoid scratching the surface).

Acceptance Criteria: A continuous water film on the surface (water droplets do not bead up), i.e., "Water film test passed".

2. Surface Roughening

Purpose: To increase surface roughness (Ra value controlled between 1.6-6.3μm) and enhance mechanical adhesion through "physical interlocking" (silicone fills pits to form an anchoring structure).

Methods:

Sandblasting: Use 80-120 mesh alumina abrasive particles at a pressure of 0.3-0.5MPa to blast the surface, creating a uniform rough surface (care should be taken to protect non-encapsulated areas to avoid dimensional deviations).

Chemical Etching: For inert metals such as stainless steel, use an acidic etching solution (such as a mixture of hydrofluoric acid and nitric acid) to etch the surface, creating microscopic irregularities (suitable for precision parts; etching depth must be controlled to ≤0.05mm).

3. Surface Activation (Optional but Recommended)

Purpose: To form a "transition layer" on the metal surface through chemical action, enhancing the chemical affinity between silicone and metal (especially for inert metals such as stainless steel and aluminum alloys).

Methods:

Coating with silane coupling agent (e.g., KH-550, KH-560): Dilute the coupling agent (2%-5% concentration), cover the metal surface by immersion, spraying, or wiping, and dry at 60-80℃ for 15-30 minutes to form a molecular-level bonding layer.

Plasma treatment: Bombard the surface with oxygen or argon plasma (500-1000W power) for 30-60 seconds to introduce active groups such as hydroxyl and carboxyl groups, increasing surface energy (environmentally friendly and residue-free, suitable for high-end products).

III. Liquid Silicone Mold Preparation and Debugging

1. Mold Design and Processing

The mold must meet the following requirements:

Precise insert positioning: Fix the insert with positioning pins, magnets, or clamps to ensure no displacement during injection molding (positioning tolerance ≤ 0.01mm).

Runner and Venting Design: The runner should be smooth with a gradually changing cross-sectional area (to prevent silicone buildup). The venting groove depth should be 0.02-0.05mm (to prevent trapped air from causing scorching or insufficient silicone).

Temperature Resistance: The mold material should be S136 or H13 hot work mold steel (resistant to temperatures above 180℃, corrosion resistant), and the surface should be polished to Ra0.02-0.05μm (to ensure a smooth silicone surface).

2. Mold Pretreatment

Cleaning: Wipe the mold cavity, runner, and positioning components with alcohol to remove oil and metal filings (to avoid contaminating the silicone or scratching the product).

Applying Release Agent (Optional): For complex cavities, a small amount of silicone-specific release agent (such as silicone-based) can be sprayed, but the amount must be controlled (excessive amounts will reduce the adhesion between the silicone and metal).

Preheating: Raise the mold temperature to 120-180℃ (controlled by a mold temperature controller, with a temperature difference ≤±5℃ to ensure uniform vulcanization).

IV. Injection Molding and Vulcanization

The injection molding and vulcanization process is completed using a liquid silicone injection molding machine:

1. Insert Positioning

Manual or automated equipment places the pre-treated metal inserts into the mold positioning slots, ensuring no skewing or loosening (positioning deviation must be ≤0.1mm, otherwise uneven silicone coating will occur).

Large metal inserts can be preheated to 60-80℃ (to reduce mold temperature fluctuations and prevent premature curing of silicone upon contact with cold metal).

2. Mold Closure and Injection

The mold closes (clamping force is calculated based on the product's projected area, typically 50-200kN), and the injection molding machine screw injects the degassed LSR into the mold cavity through the runner.

Parameter Control:

Injection Pressure: 5-30MPa (High pressure for thin-walled parts, low pressure for thick-walled parts to prevent insert displacement).

Injection Speed: 10-50mm/s (Too fast a speed can cause air entrapment, too slow a speed can cause premature curing of the silicone in the runner).

Injection Amount: Precise control is crucial (too much will result in overflow, too little will lead to insufficient material), typically 5%-10% more than the cavity volume (to compensate for vulcanization shrinkage).

3. Vulcanization and Curing

The silicone undergoes a cross-linking reaction at the high temperature (120-180℃) of the mold, gradually curing (vulcanization time is adjusted according to product thickness: approximately 10-15 seconds for 1mm thickness, increasing by 5-10 seconds for each additional 1mm).

Holding Pressure: Maintain a holding pressure of 5-10MPa during vulcanization (to prevent indentations caused by silicone shrinkage).

V. Post-processing and Inspection

1. Demolding and Trimming

After mold opening, remove the product using ejector pins or a robotic arm (avoid pulling force to prevent silicone tearing or metal insert detachment).

Trimming: Remove excess silicone using a blade or a cryogenic trimming machine (-30℃ low-temperature embrittlement of flash) (flash thickness must be ≤0.1mm to ensure acceptable appearance).

2. Secondary Vulcanization (Optional)

For products requiring high temperature resistance and low volatile matter (e.g., medical, food grade), secondary vulcanization in an oven is required: baking at 180-200℃ for 2-4 hours (to remove residual small molecules and improve performance stability).

3. Quality Inspection

Appearance: Inspect the silicone surface for bubbles, scratches, missing adhesive, and exposed metal inserts.

Dimensions: Use a coordinate measuring machine to check key dimensions (e.g., silicone thickness, total length; tolerances must meet drawing requirements).

Adhesion: Perform a peel test (use a tensile testing machine to vertically pull the silicone and metal interface; peel strength must be ≥1.5 N/mm, with no delamination for acceptance).

Performance: Test temperature resistance (-60℃~200℃ cycling), water resistance (IP67/IP68), and aging resistance (UV aging test) as needed.

Key Summary

Metal pretreatment is crucial for ensuring adhesion (cleaning, roughening, and activation are all essential).

Mold temperature, injection parameters, and vulcanization time must be precisely matched (affecting product quality and production efficiency).

Automated equipment (such as robotic insert placement and automatic inspection) can improve the stability of mass production.

This process is widely used in electronic components (such as connector waterproof sleeves), medical devices (such as infusion needle protective sleeves), and automotive parts (such as sensor sealing rings), among other fields.