Complete Guide to LSR Injection Molding: From Raw Material to Finished Product

Liquid Silicone Rubber (LSR) injection molding is an efficient and precise manufacturing process for silicone products, widely used in medical, baby care, electronics, and other industries. This article provides a comprehensive guide to the LSR injection molding process.

1. LSR Material Properties

Liquid silicone rubber consists of two components (A and B). Component A contains vinyl-terminated polydimethylsiloxane, while Component B contains cross-linking agent and catalyst. When the two components are mixed in a 1:1 ratio, addition-cure reaction occurs at elevated temperature to form elastomeric products.

Key Characteristics:

• Low viscosity: Enables precision injection into complex molds
• Fast cure cycle: Typically 10-60 seconds at 150-200°C
• Excellent thermal stability: Operating range from -60°C to 200°C
• Biocompatibility: Medical-grade LSR passes ISO 10993 testing
• High transparency: Suitable for optical and aesthetic applications

2. Injection Molding Process Flow

2.1 Material Preparation

LSR A/B components are loaded into the dosing unit of the injection molding machine. The material must be stored at room temperature (20-25°C) and kept sealed to prevent premature curing.

2.2 Metering & Mixing

The precision metering pump delivers A and B components in a strict 1:1 ratio to the static mixer. Pigments or additives can be introduced at this stage for colored or modified products.

2.3 Injection

The mixed LSR is injected into the heated mold cavity. Injection pressure typically ranges from 50-150 MPa depending on part geometry and material viscosity.

2.4 Curing

Inside the heated mold (typically 150-200°C), the LSR cures rapidly. Cure time depends on wall thickness: thin-wall parts (< 1mm) cure in 10-20 seconds, while thicker sections may require 60-120 seconds.

2.5 Demolding

After curing, the mold opens and the finished part is ejected. LSR parts typically have excellent release properties due to the material's low surface energy.

2.6 Post-Processing

Secondary operations may include:

• Deflashing: Removal of flash or parting line remnants
• Post-curing: Optional additional curing for optimal properties
• Inspection: Dimensional, visual, and functional quality checks

3. Equipment Requirements

LSR injection molding requires specialized equipment:

3.1 Injection Molding Machine

• Vertical or horizontal LSR-specific machines
• Precision metering system for A/B components
• Closed-loop temperature control

3.2 Mold Design

• Cold runner system to prevent premature curing
• Proper venting to avoid air entrapment
• Heating elements for uniform mold temperature

3.3 Dosing Unit

• Dual-cylinder design for A/B components
• Precision gear pump or progressive cavity pump
• Integrated mixing head

4. Process Advantages

5. Common Applications

• Medical devices: Seals, valves, respiratory masks
• Baby products: Nipples, pacifiers, teething toys
• Electronics: Keypads, connectors, waterproof seals
• Automotive: Connector seals, grommets, dampers
• Wearables: Watch bands, ear tips, fitness trackers

6. Quality Considerations

Key factors affecting product quality:

• Material quality: Use certified medical-grade or food-grade LSR
• Mixing ratio: Precise 1:1 ratio is critical for complete curing
• Mold temperature: Uniform heating prevents incomplete cure
• Cleanliness: Class 100,000 cleanroom for medical products
• Process monitoring: Real-time tracking of pressure, temperature, and cure time

Conclusion

LSR injection molding offers significant advantages in precision, efficiency, and product consistency compared to traditional solid silicone processing methods. As the technology continues to mature, its applications will expand further into high-precision and high-reliability industries.