Liquid silicone rubber injection molding (LSR Molding) is a process used to produce pliable, durable parts and products out of silicone. This type of molding uses specialized injection molding machines to precisely inject liquid silicone into metal molds. As the LSR cools inside the mold, it cures and hardens into the final molded shape.
The LSR Molding Process
The LSR molding process involves several key steps:
1. Material preparation – The two main LSR component liquids are thoroughly mixed together. This creates a chemical reaction that starts the curing process.
2. Injection – The mixed material is loaded into the injection barrel and injected into the mold under high pressure. This allows the liquid to completely fill the mold cavity.
3. Curing – Inside the mold, the LSR cures from a liquid to a solid rubber state. Curing times vary from a few seconds to a few minutes depending on the material chemistry and part design.
4. Ejection – After curing, the solid molded part is ejected from the mold. The part will be an exact replica of the mold cavity.
5. Post-curing – Parts may undergo an additional heating process to ensure full curing. This removes any remaining volatiles in the material.
6. Finishing – Flash and other imperfections are removed from the molded parts. Additional assembly or inserts may be added.
Benefits of LSR Injection Molding
LSR injection molding provides numerous benefits compared to conventional rubber molding:
- High production speed – LSR can be injected and cured rapidly, allowing for very high volume production. Parts can be molded in a matter of seconds.
- Intricate geometries – The liquid material flows easily into complex mold cavities, making it possible to produce parts with precise details.
- No flash – LSR does not produce any excess flash around the molded part, minimizing secondary operations.
- Excellent tear strength – Exceptional material strength and flexibility compared to other rubbers.
- Thermal stability – LSR retains its properties and shape even when exposed to extreme high or low temperatures.
- Chemical resistance – Highly resistant to oils, solvents, and environmental conditions. Does not degrade over time.
- Biocompatibility – LSR grades are available for manufacturing medical devices and implants.
LSR Material Considerations
There are a few key factors to consider when selecting an LSR material:
- Hardness – Shore hardness ranges from very soft and flexible to more rigid. Harder grades provide higher tear and abrasion resistance.
- Transparency – Clear, translucent, and opaque colors are available. Transparency is affected by fillers.
- Flame Resistance – Flame retardant LSR materials meet strict flammability standards for electronics and vehicles.
- FDA Compliance – Grades certified as FDA compliant for repeated skin contact or food contact.
- Custom Formulations – Properties like viscosity and cure rate can be adjusted to suit specific application needs.
LSR Mold Design Guide
Molds for LSR injection molding require precision engineering to account for the unique liquid material and fast cycle times. Below are some key mold design considerations:
- Uniform wall thickness – Maintain an even thickness wherever possible to prevent cosmetic issues.
- Draft angles – Include a slight taper on walls to ensure proper part ejection. 5-7° is common.
- Shut-offs – Close-fitting shut-offs are critical to prevent flash and leakage.
- Venting – Vent channels allow trapped air to escape the mold as it fills with LSR.
- Surface finishes – Polished cavities reduce friction and sticking. Textures can be added if a matte finish is desired.
- Hot runner systems – Heated manifold systems keep the LSR at a constant temperature as it flows into the cavities.
Proper LSR mold design combined with precision processing ensures flawless production of silicone parts. Expert mold makers work closely with LSR material suppliers to create molds optimized for the material and application.
LSR Injection Molding Applications
The unique properties of LSR make it an ideal choice for products requiring flexibility, heat resistance, and extreme durability. Typical applications include:
Medical Industry
- Seals for medical devices and instrument housings
- Stoppers for vials and syringes
- Masks, tubing, and other components
Automotive Industry
- Airbag covers
- Connectors
- HVAC flaps and valves
- Exhaust hangers
Electrical Industry
- High voltage insulators
- Jacket material for cables
- Waterproof connectors and gaskets
Baby Silicone
- Nipples, pacifiers, teethers
- Soft grips on toys
Food Production
- Seals, gaskets, tubing for food processing equipment
- One-way valves in milk and juice containers
In summary, LSR molding uses specialized equipment to precisely mold liquid silicone into durable, heat resistant rubber parts. With fast cycle times and the ability to produce complex geometries, LSR is advantageous for high volume production across many industries.
