Liquid silicone rubber (LSR) is mixed with a foaming agent (physical or chemical), and during the heating and vulcanization molding process in a mold, the foaming agent generates gas, thereby forming a uniform and fine microcellular foaming structure.
1. Raw Material Preparation:
Use two-component liquid silicone rubber (A&B components). The base rubber compound is usually pre-prepared by the supplier. Component A typically contains a catalyst, and Component B contains a crosslinking agent. Depending on the performance requirements of the final product (such as density, hardness, and color), it may be necessary to add color pastes, thermal conductive fillers (such as alumina), or flame retardants and other additives to the base rubber compound. These additives are usually mixed evenly with Component A or Component B in advance.
2. Foaming Agent Introduction (Foaming agent introduction) this is the most crucial link. According to the type of foaming agent, it is mainly divided into two technical routes:
Process: Generating supercritical CO₂ : Liquid CO₂ is heated and pressurized to a supercritical state through dedicated equipment. Metering and injection: Through A precise high-pressure metering pump, a certain amount of supercritical CO₂ is injected into the A component flow path of liquid silicone rubber. Static mixing: Component A (already containing scCO₂) and Component B are thoroughly mixed when passing through the static mixer. Meanwhile, scCO₂ is uniformly dissolved in the rubber compound under high pressure, forming a single-phase homogeneous mixture. The entire process is maintained under high pressure to prevent CO₂ from escaping prematurely.
b) Chemical foaming foaming agent: Use thermally decomposable chemical foaming agents, such as azodicarbonamide (AC foaming agent). Process: Pre-mix the foaming agent powder evenly with Component A or Component B in a stirring device. To ensure the product is defect-free, vacuum deaeration is usually required after mixing to remove the air introduced during the stirring process.
3.Vulcanization and foaming:
Physical foaming: After the rubber compound is injected into the mold cavity, the pressure is released, and the supercritical CO₂ dissolved in the rubber compound rapidly vaporizes, forming bubble nuclei. Meanwhile, high temperatures cause vulcanization and cross-linking reactions in silicone rubber, locking the bubble structure and forming cells.
Chemical foaming: The high temperature of the mold causes the chemical foaming agent to decompose, generating gases (such as N₂, CO₂). At the same time, the silicone rubber is vulcanized, and the gases are encapsulated in the formed rubber network, creating cells.
4. Demolding: After the vulcanization foaming cycle is completed (usually within tens of seconds to several minutes), open the mold and remove the product. Due to the low viscosity of LSR, the products usually do not stick to the mold and are relatively easy to demold.
5. Post-curing (secondary vulcanization) (Post-Curing) the foamed products after demolding usually need to undergo post-curing in an oven at a temperature of approximately 150°C - 200°C for 1-2 hours.