Liquid Silicone Rubber Molding (LSR) is a high-performance two-part thermoset elastomer with organic and inorganic properties (watch this video). Silicone elastomers have typical viscosities of 500,000 – 2,000,000 cps. They are used due to their process versatility, high-temperature stability, low-temperature flexibility, excellent weather resistance, long service life, excellent sealing properties, ease of coloration, and inertness. Due to its proven application performance, LSR is commonly used in aerospace, automotive, consumer goods, healthcare, and textiles.
Made of silicon, a nonmetal found in the earth’s crust that does not occur freely in nature but is a major component of clay, quartz, rock, and sand. Manufactured in an internal planetary mixer with methyl vinyl polymer, silica filler, and additives. The orbital motion creates high shear forces to provide a homogeneous mixture of the polyvinylsiloxane matrix. After a relatively long mixing time, the base is divided into equal amounts of different additives to form “A” and “B” components:
hydride crosslinking agent, the inhibitor
Material volumes are then filled into batch-matched containers, 200L (55gal) or 20L (5gal)
Advantages of LSRs:
Automated process, fast cure cycle – excellent productivity, multi-cavity molds, very low flash tolerance, high strength product, low viscosity = low injection pressure. Overmolding and two-shot capability make liquid silicone rubber one of the fastest-growing technologies in the industry.
LSR molding process:
The LSR molding process is always called the “opposite case” of the thermoplastic molding process, where the mold is hot, and the screw/barrel is cooled. When mixed in a 1:1 ratio, LSR will begin to cure at room temperature. Adding heat to the equation produces a faster curing profile, so a proper chilled water system is required to delay the curing cycle after introducing these two components. The approximate cooling temperature for the unit is 70°F. This is a 30,000 ft view, as many differences from the LSR process must be correctly specified to succeed.