What Is the LSR Injection Molding Process?

Liquid silicone rubber (LSR) injection molding is a manufacturing process used to produce precision silicone parts

Liquid silicone rubber (LSR) injection molding is a manufacturing process used to produce precision silicone parts from a two-part liquid material. Unlike traditional solid silicone rubber, which is shaped through compression or transfer molding, LSR is pumped, mixed, and injected directly into a heated mold where it cures into a solid elastomer within seconds. The process is designed for speed, repeatability, and the ability to mold complex geometries with tight tolerances.

The Two-Part Material

LSR arrives as two separate components, typically called part A and part B. One contains the platinum catalyst, and the other contains the crosslinker and inhibitor. Kept apart, each component is stable and has a long shelf life. When mixed, the curing reaction begins. This two-part system is what allows LSR to remain a low-viscosity liquid until it enters the mold, rather than curing prematurely in storage.

Because the material is platinum-cured, it contains no peroxide byproducts. This gives molded LSR high purity, excellent clarity, and good biocompatibility, which is why it is widely used in medical, food-contact, and optical applications.

Step 1: Metering and Mixing

The process starts with the two components held in separate drums or pails. Metering pumps draw each component in a precise ratio—often 1:1, though some grades use different ratios—and deliver them to a static mixing unit. The mixer blends the two streams into a homogeneous liquid without introducing air. Accurate metering is critical: an off-ratio mix can lead to incomplete cure, inconsistent hardness, or surface defects.

Step 2: Injection

The mixed liquid is injected into a heated mold cavity. LSR has a low viscosity, similar to motor oil, so it flows easily and fills fine details, thin walls, and complex undercuts. Injection pressures are relatively low compared to thermoplastic injection molding, which reduces stress on mold components and allows for delicate part features.

Cold-runner systems are common in LSR molding. In a cold runner, the material stays cool and liquid right up to the mold cavity, minimizing waste and eliminating the need to trim a runner after curing. This is a significant advantage over solid silicone processing, where excess material must be removed manually.

Step 3: Curing

Once inside the heated mold, the LSR begins to cure. Mold temperatures typically range from 150°C to 200°C. The platinum-catalyzed addition reaction crosslinks the polymer chains, transforming the liquid into a solid elastomer. Cure times are short—often 10 to 90 seconds, depending on part thickness and geometry. Fast curing is one of the main reasons LSR injection molding supports high-volume production.

Step 4: Demolding and Post-Processing

After curing, the part is removed from the mold. Because LSR is flexible and release-friendly, demolding is usually straightforward, though automated handling is common in high-volume operations. Parts may then undergo secondary steps such as deflashing, inspection, or assembly. Some applications require post-curing to remove residual volatiles or to stabilize properties, particularly for medical or optical components.

Key Advantages of the Process

LSR injection molding offers several benefits that explain its growing use:

• Precision: Tight tolerances and consistent part-to-part dimensions.

• Complex geometry: Thin walls, fine details, and undercuts are achievable.

• Speed: Short cure times enable high-volume output.

• Purity: Platinum curing avoids peroxide residues.

• Automation: The process lends itself to automated metering, injection, and demolding.

• Material efficiency: Cold-runner systems reduce waste.

Challenges to Consider

The process is not without its demands. Tooling costs are higher than for solid silicone compression molding, so LSR injection molding makes economic sense mainly at higher production volumes. Mold design must account for the material's low viscosity and thermal expansion, and precise temperature control is essential to avoid premature cure or incomplete filling. Additionally, the two-part system requires careful handling to prevent contamination and ratio errors.

Summary

LSR injection molding is a precision process that combines two liquid components, injects them into a heated mold, and cures them rapidly into a solid silicone part. It is distinguished from solid silicone processing by its flowable material, platinum cure system, and injection-based workflow. For applications demanding intricate geometry, high purity, and consistent quality at scale, it is often the manufacturing method of choice.