What Is Liquid Silicone Rubber? Properties,Types, and Applications

Liquid silicone rubber, often called LSR, is a two-part liquid silicone rubber based on polysiloxane chemistry. It is usually supplied as Part A and Part B. Part A contains the base polymer and platinum catalyst. Part B contains the crosslinker. After the two parts are mixed in the correct ratio, the material cures quickly under heat and forms an elastic silicone rubber part.

LSR can be divided by curing system, application grade, and functional property. By curing system, it is mainly divided into addition-cure LSR and condensation-cure liquid silicone. By application grade, it can be supplied as general industrial grade, food-grade, medical-grade, conductive, thermally conductive, flame-retardant, self-bonding, or optical-grade material.

The main value of LSR is its combination of heat resistance, cold resistance, aging resistance, biocompatibility, electrical insulation, high transparency, and high molding efficiency. Because it is processed by LSR injection molding, it supports automated production, short cycle times, stable dimensions, and repeatable quality.

Over the past decade, liquid silicone rubber has become one of the fastest-growing segments in the silicone rubber industry. Compared with traditional high-consistency rubber, LSR is replacing rubber and engineering plastics in many applications. It is now used in electric vehicles, medical consumables, consumer electronics, baby care products, food-contact parts, and precision sealing components.

Many engineers and buyers still see LSR mainly as an expensive material. In practice, the right LSR grade and process can reduce labor, improve consistency, lower scrap, and support large-scale production. A clear understanding of LSR types, properties, processing methods, and selection rules helps avoid wrong material choices and unnecessary cost.

Main Types of Liquid Silicone Rubber

Addition-Cure LSR

Addition-cure LSR is also called platinum-cured LSR. It is the mainstream material in the current market and accounts for more than 90 percent of many LSR applications.

It cures through an addition reaction between silicon-hydrogen groups and vinyl groups under platinum catalysis. The curing process does not create by-products.

Key features include fast curing, deep curing, very low shrinkage, low odor, good biocompatibility, and high transparency. It is widely used when product cleanliness, safety, precision, and repeatability matter.

The main advantages are high molding efficiency, good dimensional accuracy, and easier access to food-grade and medical-grade certifications. The main limitation is cost. Platinum-cured LSR can also be affected by sulfur-, nitrogen-, phosphorus-, or tin-containing substances. These substances may poison the catalyst and cause incomplete curing.

Typical applications include medical consumables, baby care products, food-contact parts, and precision electronic components. For deeper material background, see our article on platinum-cured silicone.

Condensation-Cure Liquid Silicone

Condensation-cure liquid silicone is an earlier type of liquid silicone material. Its use has gradually decreased in high-end applications.

It usually uses organotin catalysts. Crosslinking occurs through dealcoholization or dehydrogenation reactions, and the process produces small-molecule by-products.

This material is lower in cost and less sensitive to catalyst poisoning. It can also cure at room temperature. However, it cures more slowly, has higher shrinkage, may release small volatile molecules, and has weaker biocompatibility. It is generally not suitable for food-grade or medical-grade certification.

Typical applications include potting compounds, mold-making silicone, and general industrial sealing parts where regulatory requirements are lower.

LSR Grades by Application

General Industrial-Grade LSR

General industrial-grade LSR is the basic LSR category for common industrial use. It offers balanced performance, moderate cost, and many hardness options.

Typical hardness ranges from Shore A 0 to Shore A 80. Common applications include industrial seals, O-rings, silicone gaskets, silicone keypads, and vibration-damping pads.

Food-Grade LSR

Food-grade LSR is designed for food-contact safety. It is non-toxic, low odor, heat resistant, and suitable for direct or indirect food contact when the correct certified grade is selected.

Common standards include FDA 21 CFR 177.2600, GB 4806.11, and LFGB. Typical applications include baby bottle nipples, silicone kitchenware, baking molds, food sealing rings, and drinking-water components. For production service details, see our food-grade silicone molding page.

Medical-Grade LSR

Medical-grade LSR is used for products that need strong biocompatibility and high material purity. It is designed to be non-sensitizing, non-irritating, and suitable for sterilization when the correct grade is used.

Common references include ISO 10993, USP Class VI, and medical device registration requirements. Typical applications include medical tubing, respiratory masks, syringe seals, and selected implant-related components. For related manufacturing support, see medical silicone molding.

Functional LSR

Functional LSR uses modified formulations to meet special performance needs.

Conductive LSR contains conductive fillers such as nickel-coated graphite or silver-coated copper. It can be used for EMI shielding, silicone rubber keypads, and electrical contact parts. You can also read our guide to conductive silicone rubber.

Thermally conductive LSR contains fillers such as aluminum oxide or boron nitride. It is used for heat transfer in electronic assemblies. For related products, see custom thermal pads.

Flame-retardant LSR contains flame-retardant additives and can be formulated to meet UL 94 V-0 requirements for electrical and electronic parts.

Self-bonding LSR can bond directly to selected plastics or metals without primer. It is useful for two-shot molding and silicone overmolding.

Optical-grade LSR can reach more than 95 percent light transmission in suitable formulations. It is used for optical lenses, light guides, and transparent components. For more detail, see optically clear silicone.

Key Properties of Liquid Silicone Rubber

Wide Temperature Range

LSR can usually be used for long periods from about -60 C to 200 C. Some grades can tolerate short-term exposure above 250 C. It also remains flexible at low temperatures, which is difficult for many organic rubbers and plastics.

This wide temperature range allows LSR parts to keep sealing performance and elasticity in both cold and high-temperature environments.

Excellent Aging and Weather Resistance

The silicone-oxygen backbone has high bond energy. It resists damage from UV, ozone, heat, moisture, and oxidation. In outdoor use, LSR can maintain stable performance for many years.

Its weathering resistance is much better than many common organic rubbers. In some outdoor sealing applications, service life can be several times longer than EPDM, depending on the environment and design.

Good Biocompatibility and Safety

After curing, addition-cure LSR is low odor, non-toxic, non-irritating, and suitable for many food-contact and medical applications when the correct grade is selected. It can meet standards such as FDA, LFGB, and ISO 10993.

This makes LSR a preferred material for baby care, medical, and food-contact products.

Strong Electrical Insulation

LSR is an excellent electrical insulating material. It has high dielectric strength, stable dielectric properties, arc resistance, and corona resistance. It is widely used in electrical insulation, high-voltage sealing, cable accessories, and electronic assemblies.

High Flow and High Precision

Before curing, LSR has very low viscosity and flows easily into fine mold details. It can fill small cavities, thin walls, and complex structures. With the right mold and process control, LSR parts can reach very tight dimensional tolerances.

This makes LSR suitable for precision silicone products that are difficult to produce with traditional solid silicone rubber.

Fast Curing and High Production Efficiency

LSR cures very quickly in heated molds. Typical curing time can range from seconds to a few minutes, depending on part size and wall thickness. This is much faster than many compression-molded silicone processes.

When paired with automated liquid injection molding equipment, LSR can support very high daily output and stable mass production.

High Material Utilization

LSR injection molding uses a closed feeding and metering system. Material utilization can be close to 100 percent, with very little flash or trimming waste. The curing process also produces no by-products in addition-cure systems, which helps keep the production environment clean.

How Liquid Silicone Rubber Is Processed

LSR is a two-part material. Correct handling and processing are important for stable product quality.

Step 1: Material Preparation and Storage

Confirm that Part A and Part B are the correct matching grades. Check shelf life, which is usually 6 to 12 months depending on the supplier.

Store the material at room temperature, around 25 C, in a dry, sealed, and dark environment. Avoid high temperature and direct sunlight.

If the material was stored at low temperature, bring it back to room temperature before use. In many cases, it should be conditioned for about 24 hours before processing.

Step 2: Mixing

For addition-cure LSR, the standard mixing ratio is usually A:B = 1:1 by weight or volume. Always follow the supplier’s technical data sheet.

In mass production, an LSR injection molding machine uses a metering and mixing system to combine the two parts automatically. This gives high accuracy and good batch-to-batch consistency.

For small trial runs or sampling, the two parts can be weighed and mixed manually. Mix for about 3 to 5 minutes and scrape the container wall and bottom to avoid unmixed material.

Mixing must be complete because poor mixing can cause weak curing and poor part performance. The mixing speed should not be too high, or air bubbles may be trapped. After mixing, the material has a limited pot life, often about 1 to 4 hours at room temperature.

Step 3: Degassing

Degassing may be needed when mixed material contains air bubbles. Bubbles can affect both appearance and mechanical performance.

A common method is vacuum degassing at about -0.09 MPa or better for 3 to 5 minutes. In injection molding, the machine or mold may include a vacuum system, so separate degassing is not always required.

Step 4: Curing and Molding

In liquid injection molding, the mixed material is fed into the LSR injection unit and injected into a heated mold by screw or plunger. It then cures quickly under heat.

Typical curing temperature is about 150 C to 200 C. Curing time depends on wall thickness and part geometry, but it often ranges from 10 seconds to 2 minutes. Mold temperature should be controlled evenly, with the upper and lower mold temperature difference usually kept within about +/-5 C.

For some non-injection applications, mixed liquid silicone can be poured directly into a mold or part cavity and cured at room temperature or with heat, depending on the formulation.

Step 5: Post-Curing

Freshly molded parts may contain a small amount of low-molecular volatile material and may have a slight odor. Food-grade and medical-grade parts often require post-curing.

Typical post-curing conditions are 200 C for 4 hours or 180 C for 6 hours in an oven, depending on the material and product requirement.

Post-curing helps remove volatiles, stabilize dimensions, improve mechanical properties, and reduce odor.

Important Processing Notes

Addition-cure LSR is sensitive to catalyst poisoning. Contact with sulfur, nitrogen, phosphorus, or tin compounds may deactivate the platinum catalyst and cause no cure or incomplete cure.

During production, avoid contact with latex gloves, sulfur-containing materials, organotin compounds, and some plastics or rubbers. Molds, containers, and tools should be clean before use.

When Should You Choose Liquid Silicone Rubber?

LSR is usually a good fit when the product needs high precision, high repeatability, clean production, tight sealing, food-contact safety, medical-grade performance, or automated high-volume manufacturing.

It is especially useful for parts with complex geometry, thin walls, small features, high transparency, soft touch, or stable performance across a wide temperature range.

However, LSR may not be the best choice for every project. If the part is very large, low-volume, simple in shape, and not highly precision-sensitive, HCR or compression molding may offer a lower-cost route. The right choice depends on geometry, annual volume, material grade, tolerance, tooling budget, and regulatory requirements.

Fecision supports LSR material review, mold design, production molding, overmolding, secondary operations, and quality inspection for custom silicone components.