Understanding the Material Properties and Molding Process of Liquid Silicone Rubber (LSR)

                                        Liquid silicone rubber (LSR) is a robust and flexible material that can retain its shape memory very well.      

Liquid silicone rubber is a non-toxic, heat-resistant, highly elastic soft thermosetting material. Its rheological behavior is characterized by low viscosity, rapid curing, shear thinning phenomenon, and high coefficient of thermal expansion. Liquid silicone rubber is a two-component fast-curing material with platinum (Pt) as a catalyst, which can be molded by injection molding, achieving mass production, rapid cross-linking curing, and stable production repeatability.

The injection products of liquid silicone rubber have good thermal stability, cold resistance, excellent electrical insulation, and do not produce toxic substances during combustion. Liquid silicone rubber has a wide range of applications, including health products, automotive, baby products, medical devices, diving equipment, kitchen utensils, and sealing applications, making it an irreplaceable material in current production designs.

01｜The molding process of liquid silicone rubber

Crosslinking determines many properties of thermosetting materials, such as strength, stability, and heat resistance.

The molding process of liquid silicone rubber only requires three steps: "metering mixing," "molding," and "curing forming." Liquid silicone rubber has excellent flowability and solidifies in the mold, exhibiting strength and flexibility after curing, allowing for unconventional design features such as intricate structures and undercuts, which cannot be achieved with other injection materials such as plastics or thermoplastic elastomers. Liquid silicone rubber is typically injection molded, with molding equipment similar to that commonly used for thermoplastic plastics, but with somewhat different requirements for detail.

Liquid silicone rubber is a liquid raw material composed of two components or two-component compositions, consisting of Component A and Component B, most commonly packaged in 20 kilograms (approximately 5 gallons) or 200 kilograms (approximately 55 gallons) drums. The drum components are placed in high-pressure pumps equipped with precise follower plates, which push the viscous components out of the drum with precise high-pressure control pumps, accurately mixing Components A and B in a 1:1 ratio using mixing devices. Additionally, because some products are designed with color, colorant pumps and colorant metering devices can be installed, with Components A, B, additives, and colorants fully mixed in a static mixer before being introduced into the plasticizing system. In the mixing system, the cooling medium must ensure that the mixture remains at a controlled cooling temperature (10~20℃) to suppress premature cross-linking or curing reactions, allowing the cooled liquid silicone rubber mixture to remain stable for several days.

The special plasticizing screw for liquid silicone rubber has the functions of homogenization and mixing. The mixed material is injected into a high-temperature mold for molding, triggering the initial cross-linking reaction of the silicone rubber material to form the final product under mold temperatures of 170~200℃. When using a cold runner feed system for liquid silicone rubber molds, it is important to note that the runner must be sufficiently low temperature and cool. Additionally, to prevent leakage or spillage, needle valve devices are installed on the surface of mold components, which immediately close the nozzle after injection is complete.

02｜Considerations for Liquid Silicone Rubber Molding Equipment

In some cases, liquid silicone rubber is prone to flashing, with gaps as small as 0.005mm.

Key Components of Liquid Silicone Rubber Injection Molding Machine

• Due to the low viscosity of liquid silicone rubber, the sealing of the injection screw is crucial due to material reflux and leakage considerations during processing.

• To prevent premature curing of liquid silicone rubber due to temperature sensitivity, proper insulation design between the injection machine and mold is crucial, preferably using needle valve nozzles that close immediately after injection.

• Metering and mixing devices for Components A and B.

Metering Feeding System

The metering feeding system comes in several forms to choose from:

• Bidirectional pumps with movable feeders can achieve good pressure stability and synchronize the delivery of Components A and B through hydraulic pneumatic mechanisms, allowing for precise and reliable metering.

• Unidirectional pumps are general-purpose and can only be selected for single-direction feeding.

• Synchronized unidirectional pumps with check valves.

• Metering cylinder systems are mainly used in conjunction with unidirectional pumps.

Mold Design

In terms of design considerations, there are several forms to consider:

• Hot runner forms are more material-wasting, with simple designs and lower costs, often used for large parts.

• Cold runner systems with needle valves can achieve automation and shorter molding cycles.

• Cold runner systems without needle valves may experience expansion during heating due to the high expansion coefficient of liquid silicone rubber, resulting in minimal shrinkage during cooling. Therefore, finished products cannot maintain precise dimensional tolerances in the mold, so it is recommended to use needle valve cold runner systems for processing.

• Liquid silicone rubber should be kept at low temperatures and flowable. Closed-loop systems are used in cold runner systems, where each runner is equipped with "closure needles" or "needle valves" to control the accurate metering of liquid silicone rubber. Due to the significant heat expansion characteristics of silicone rubber, the shrinkage rate is 2-4% (reaction curing temperature is 150℃), and silicone rubber has a compressible deformation characteristic. The analysis of liquid silicone rubber flow/reaction curing is as follows:

• The reaction curing chemical reaction of liquid silicone rubber requires a certain reaction time.

• Ideal flow fields (length-to-thickness ratio) can exceed 100cm in a flow length of 170cm in a 2mm diameter flow path (L/t >500). 

• Minimum thickness of 1/1000mm.

• Excessive mold temperatures can cause premature reaction cross-linking, leading to flow blockages.

• Laminar flow forms can prevent bubble formation.

• High-speed injection causes turbulent flow.

• Viscosity changes affect flow patterns.

• Turbulent or disturbed flow forms are prone to white spots.

Liquid silicone rubber parts are located at the top of a pair of aluminum molds.

Based on the above analysis and statements, the following key points should be noted in mold design:

• To avoid turbulent and jet flows, smaller needle points or wing-shaped small sprues should be used.

• Use T-shaped guide pins instead of columnar guide pins to increase mold positioning accuracy.

• Thermal insulation layers or plates should be added.

• Avoid using rust inhibitors/oils containing polymer inhibitors. Generally, toluene, xylene, etc., can be used to wipe the mold. Additionally, cured liquid silicone rubber is prone to stick to metal surfaces, and the most common demolding techniques include top-out needle and air push demolding. From the above information, it can be seen that silicone injection molding technology mainly addresses issues related to its mixing, metering parts, screw sealing, and mold design.

Application of Liquid Silicone Rubber in Sprinklers

The application range of liquid silicone rubber is very wide, and its application can be seen in products in many fields.

03｜Conclusion

Liquid silicone rubber injection molding requires a basic understanding of liquid silicone rubber, and proper and correct design and planning should be given to product design, mold design, and production processing to fully understand the principles of liquid silicone rubber injection molding and production process design to avoid product problems and produce high-quality, efficient liquid silicone rubber products.

Liquid silicone rubber injection molding is currently considered a market with good economic benefits, and there is an increasing demand for liquid silicone rubber products in the industry. New application products will also continue to emerge, which will inevitably lead to more injection molding companies investing in liquid silicone rubber product production, making the liquid silicone rubber injection molding industry chain and market more robust.