How Is LSR Silicone Rubber Cured?

Liquid Silicone Rubber (LSR) is generally supplied as a two-component system, commonly referred to as Part A and Part B.

Typically:

• Part A contains the platinum catalyst.
• Part B contains the silicone crosslinker containing Si-H groups.
• Both components contain silicone polymer and other formulation ingredients.

When the two components are accurately metered and mixed, the curing reaction begins. The material is then injected into a heated mold, where it rapidly crosslinks and forms the final silicone elastomer. This process is commonly known as Liquid Injection Molding (LIM-LSR)

LSR is normally cured through a platinum-catalyzed addition reaction.

The basic reaction can be simplified as:

Vinyl-functional silicone polymer + Si-H crosslinker → Crosslinked silicone elastomer

The platinum catalyst promotes the reaction between vinyl groups on the silicone polymer and Si-H groups on the crosslinker. This process is called hydrosilylation.

As the reaction proceeds, individual silicone polymer chains become connected through crosslinks, creating a three-dimensional network. This network gives cured LSR its characteristic elasticity, strength, flexibility, and thermal stability.

Platinum catalysts enable LSR to cure rapidly and efficiently through addition curing. A major advantage is that platinum-catalyzed addition curing does not generate the typical volatile by-products associated with peroxide curing.

This characteristic is particularly relevant for applications where low volatile content and efficient processing are important. For selected applications, post-cure-free liquid silicone rubber can further simplify the production process by reducing or eliminating a separate post-curing step, depending on the material specification.

This can be useful for applications where cleanliness, appearance, and material purity are important.

Platinum-cured LSR is widely used for medical and healthcare components, baby-care products, food-contact components, automotive seals, electrical components, and precision molded silicone parts. For medical applications, manufacturers may select medical grade LSR for injection molding according to the required material specifications and regulatory requirements

For food-contact applications in the United States, applicable requirements should be reviewed according to the intended use and finished article. FDA regulations include requirements for certain rubber articles intended for repeated use under 21 CFR 177.2600.

The curing temperature depends on the LSR formulation, mold design, part thickness, and required cycle time.

For conventional LSR injection molding, mold temperatures are commonly in the range of approximately 130–200°C, although individual formulations may require different conditions.

Some specialized LSR grades are designed for low-temperature curing. For example, certain commercial systems are designed to cure around 90–120°C for applications involving temperature-sensitive substrates.

Manufacturers should not select a curing temperature based only on a general range. The supplier's Technical Data Sheet (TDS) and recommended processing conditions should be used as the starting point.

LSR is known for relatively fast curing. In injection molding, curing can occur within seconds to a few minutes depending on mold temperature, part thickness, formulation, injection conditions, mold design, catalyst/inhibitor system, and required physical properties.

Higher mold temperatures generally accelerate curing and can reduce molding cycle time.

There is no single curing time that applies to every LSR material. Production conditions should be established through the supplier's TDS and actual molding trials.