Core Question: What is LSR Injection Molding? Why Can It Produce Precision Products?

Liquid Silicone Rubber Injection Molding

I. What is Liquid Silicone Rubber (LSR)?

The materials for baby silicone pacifiers, medical infusion catheters, and smartphone waterproof gaskets you use daily are all Liquid Silicone Rubber (LSR) — a thermosetting elastomer composed of two components (Part A and Part B). Part A contains a platinum-based catalyst, while Part B includes crosslinking agents. They are mixed in specific ratios, commonly 1:1 or 10:1 (adjustable based on material grades).
Key advantages of LSR:
  • Excellent temperature resistance: Maintains elasticity between -50℃ and 250℃, withstanding microwave heating and outdoor exposure;
  • Safety and durability: Chemically stable (resistant to acids, alkalis, and UV rays) and biocompatible, making it suitable for human contact;
  • High precision: Its liquid state allows it to fill micro-scale mold details easily, with a molding shrinkage rate of only 2%-3% (lower than ordinary plastics);
  • Permanent curing: Curing involves irreversible chemical changes, ensuring stable structure and long service life.

II. LSR Injection Molding: Complete Process and Key Technologies for Precision Molding

LSR molding involves 8 core steps, and detailed control of each step directly determines product quality:

(1) Detailed Explanation of Three Basic Steps

  1. Precise Mixing
Parts A and B are delivered via metering pumps in preset ratios (1:1 or 10:1) and mixed thoroughly in a static mixer. A 5% ratio deviation will cause curing defects. After mixing, vacuum degassing is mandatory to remove air and prevent bubbles in the final product. The barrel temperature is controlled at 40-80℃ for low-temperature delivery to avoid premature vulcanization.
  1. High-Temperature Curing
The mixture is injected into molds (160-200℃) at 50-150bar pressure, and crosslinking curing occurs within seconds to tens of seconds. An oil temperature controller is used for precise temperature control, ensuring the temperature difference across the mold is ≤5℃ to prevent uneven curing.
  1. Intelligent Demolding
Special cold runner molds reduce material waste. Ejector pins or air-blow systems enable “flash-free production,” with flash controlled within 0.01mm for precision parts.

(2) Easily Overlooked Key Links

  • Raw material pretreatment: Check humidity after opening new materials (ambient humidity must be <60%); damp materials require drying before use;
  • Equipment debugging: Clamping force is calculated based on the product’s projected area (5-8 tons of clamping force per cm²) to prevent mold expansion during high-pressure injection;
  • Post-processing: Medical-grade products require ultrasonic cleaning + ethylene oxide sterilization. Some products need secondary vulcanization at 150-200℃ for 2-4 hours to remove low-molecular by-products and improve aging resistance.

III. LSR Material Selection: Practical Guide for Scenario Matching

The “how to select materials accurately” is a core unmet need. Practical selection follows the “scenario-performance-process” three-layer matching principle:

(1) Targeted Selection by Industry Needs

Industry
Core Requirement Indicators
Recommended Material Properties
Medical/Maternal-Infant
FDA/ISO 10993 certification, sterilization resistance
Cytotoxicity ≤ Grade 1, performance retention ≥95% after gamma ray sterilization
Automotive
Resistance to high/low temperatures (-60℃~200℃) and oil
Compression set ≤15% (150℃×22h)
Consumer Electronics
High light transmittance (>90%), antistatic
Surface resistivity 10⁶-10⁹Ω·cm
Industrial Sealing
Chemical corrosion resistance, high hardness
Performance retention ≥95% after 24h immersion in 10% NaOH

(2) Matching Key Performance Parameters

  • Hardness (Shore A): Soft rubber (10-40A) is suitable for tactile parts (e.g., phone cases); hard rubber (50-80A) for seals (e.g., O-rings). Overmolded parts with a hardness difference >50A require stress relief structures;
  • Tensile strength: ≥6MPa for medical-grade, ≥8MPa recommended for industrial-grade;
  • Flowability: High-flow grades (MFR >150g/10min) for thin-walled parts (<0.5mm).

(3) Substrate Adhesion Matching Solutions

Substrate Type
Recommended LSR Properties
Auxiliary Process
ABS/PC
Contains acrylate functional groups
Silane coupling agent (e.g., TC-5985)
Nylon/PP
High surface energy modified LSR
Plasma treatment + titanate primer
Stainless Steel
Contains carboxylic acid groups
Chromium-free passivation + silane primer

IV. 4 Core Application Scenarios of LSR Injection Molding

Thanks to its “safety, precision, and durability,” LSR injection molding is widely used in high-demand fields:
  1. Medical & Healthcare
Medical-grade LSR must pass ISO 10993 biocompatibility tests. It is used to produce laryngeal masks and urinary catheters (resistant to high-temperature sterilization). A manufacturer’s silicone valves reach a precision of ±0.05mm, meeting surgical zero-defect requirements.
  1. Maternal-Infant Products
Food-grade LSR complies with FDA/LFGB standards (heavy metal content <0.1ppm). It is used for pacifiers and teething toys (resistant to 120℃ boiling water sterilization, no harmful substance release).
  1. Consumer Electronics
Relying on LSR’s sealing and insulation properties, it is used for smartphone waterproof overmolding and watch gaskets. A brand’s waterproof parts passed 1,000 water immersion tests without leakage.
  1. Automotive Industry
It is used for high-transparency lampshades for automotive LED lights and spark plug seals. These parts maintain sealing performance at 150℃ and have an aging resistance lifespan matching that of automobiles.

V. Common Defects and Solutions

The defect rate in actual production is as high as 20%. Key issues and countermeasures are as follows:
Defect Type
Main Causes
Solutions
Surface Bubbles
Poor mold venting, air in materials
Add 0.015-0.025mm vent grooves, conduct vacuum degassing
Poor Adhesion
Oil contamination on substrates, poor material compatibility
Clean with alcohol + plasma treatment, select dedicated LSR
Dimensional Deviation
Fluctuations in process parameters, mold deformation
Stabilize temperature and pressure, calibrate molds regularly
Flow Marks
Insufficient flowability, uneven mold temperature
Increase material temperature, optimize conformal cooling system

VI. Selecting Services/Equipment: Dual Guide for Avoiding Pitfalls & Selection

(1) 3 Criteria for Selecting Service Providers

  1. Certifications: Food-grade products require FDA/LFGB reports; medical-grade products need ISO 13485 certificates + Material Test Certificates (MTC). Avoid providers without certifications.
  1. Equipment Check: Prioritize manufacturers with cold runner molds and automatic mixing systems (flash control <0.01mm). For micro-parts (e.g., 10mg electronic components), confirm they have needle injection and CCD visual inspection equipment.
  1. Process Inquiry: For complex parts, confirm “mold compensation solutions” (e.g., segmental temperature control for parts with uneven wall thickness to ensure uniform curing); for multi-material parts (silicone overmolded plastics), confirm they have 90-110℃ low-temperature vulcanization technology to prevent substrate deformation.

(2) Core Parameters for Injection Molding Machine Selection

  • Clamping force: Calculated as (product projected area × injection pressure × 1.2 safety factor). Example: 100cm² projected area requires 500-800 tons of clamping force;
  • Injection volume: 10%-20% larger than the product weight to avoid insufficient filling;
  • Temperature control accuracy: Mold temperature fluctuation ≤±1℃, barrel temperature control range 20-80℃.

VII. Clarifying Misconceptions

  1. “Can LSR only be mixed at 1:1?”
No. Ratios like 1:1 or 10:1 exist depending on material grades. Metering pumps must be calibrated according to material instructions.
  1. “Can any release agent be used?”
No. Special release agents for LSR are required; ordinary release agents cause vulcanization failure or poor adhesion.
  1. “Higher hardness means better sealing performance?”
No. Seals need a balance of hardness and rebound rate. Generally, grades with 50-70A hardness and >60% rebound rate are selected.

Conclusion: Core Technology of LSR Injection Molding

LSR injection molding combines “material properties + precision equipment + process control.” With accurate ratios (1:1/10:1), second-level curing, and micro-scale molding capabilities, it addresses needs for safe contact, extreme environment resistance, and precision sealing. Full-process management — from material selection to defect troubleshooting — is the core competitiveness of this “elastic black technology.”

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