What Is LSR Injection Molding? Process, Benefits & Uses

What Is Liquid Silicone Rubber (LSR)?

Liquid Silicone Rubber (LSR) is a two-part silicone elastomer supplied in liquid form and typically cured in the mold using a platinum-catalyzed system. The two components are metered and combined shortly before molding. Heat then initiates the cure, turning the flowing material into a flexible, durable rubber part.

If you are asking, “What does LSR stand for in silicone?” the answer is Liquid Silicone Rubber. It differs from solid silicone rubber, often called HCR or high-consistency rubber, because LSR is processed as a pumpable liquid rather than a firm, gum-like compound. It also differs from a thermoplastic elastomer (TPE): LSR chemically cures, while most TPEs soften when heated and can be processed again as thermoplastics.

LSR Material Composition and Key Properties

An LSR system generally uses two components—commonly identified as A and B—that are blended in a controlled ratio. The formulation and cure package determine how the material behaves during processing and in service. Colorants or other approved additives may be incorporated when the application calls for them.

What Is LSR Injection Molding?

LSR injection molding is a manufacturing process in which liquid silicone is accurately metered, mixed, and injected into a heated mold. The heat cures the material inside the cavity, producing a finished elastomer part that can be removed once it has developed the required shape and properties.

Although both processes use injection equipment and a mold, LSR molding is not simply thermoplastic injection molding with a different resin. LSR is a reactive two-part liquid that cures under heat; thermoplastic resin is melted and then cools to solidify. LSR therefore calls for different material delivery, temperature control, mold design, venting, and process settings.

How Does the LSR Injection Molding Process Work?

The basic sequence is straightforward, but repeatable results depend on carefully controlled material handling, injection, cure, and demolding. In our work, the process plan starts with the component requirements and is refined through sampling and validation.

1. 1

   Prepare and condition the material

   The A and B components are supplied to the metering system. Material handling is controlled to limit contamination and preserve the intended mix and flow behavior.

2. 2

   Meter and mix the components

   Precision equipment meters the components at the specified ratio and blends them consistently. Color may be incorporated when the selected material system supports it.

3. 3

   Inject into the mold

   The mixed liquid silicone is delivered through the feed system and fills the mold cavities. Injection settings and venting must support complete filling without introducing avoidable air or flash.

4. 4

   Cure and stabilize the part

   The heated mold drives the cure. Mold temperature, cycle settings, and material characteristics all influence cure consistency and cycle time.

5. 5

   Demold, inspect, and finish

   The cured part is removed and checked against the agreed requirements. Depending on the design, secondary work may include post-curing, trimming, marking, bonding, or assembly.

Precision equipment is only one part of repeatability. We also monitor process settings, mold condition, material handling, and part results. These controls help keep cavity filling and cure consistent across production runs.

LSR Tooling and Mold Design

LSR tools are built to control material flow, cure, and part release. Mold construction and cavity layout are selected around the part geometry, target volume, acceptable parting-line location, and inspection requirements. Gates, runners, and vents must be considered together: a flow path that fills easily in one geometry may create air traps or excess flash in another.

Cold-runner systems can help manage material delivery to cavities and reduce material in the runner, but the right arrangement depends on the part and tool design. Cavity count, shutoff details, venting, and maintenance access also affect tool cost, cycle time, and long-run consistency.

Overmolding and Multi-Component Molding

LSR overmolding combines silicone with a substrate such as a compatible plastic, metal insert, or electronic component. It can create a seal, grip, flexible interface, or protective feature directly on a rigid component, reducing the need to handle separate parts during assembly.

Reliable overmolding starts with material compatibility and substrate preparation. Bonding may depend on chemical adhesion, mechanical retention, or a combination of both. The substrate’s surface, geometry, temperature tolerance, and position in the mold all need review. We support overmolding on plastic substrates including PC, PA, PBT, PPSU, PPA, and PEI, as well as metals such as aluminum, stainless steel, brass, and copper; suitability remains application-dependent.

For a connector seal, for example, the team should define both the sealing requirement and how the silicone is retained on the connector body. Bond testing and functional validation should reflect the actual use conditions, not just the appearance of the molded assembly.

What Is LSR Used For?

LSR is used for custom parts that need the flexibility of rubber and dependable performance in repeated use or demanding environments. Common examples include seals, gaskets, valves, diaphragms, buttons, wearable components, tubing-related parts, and soft-touch features. The right application depends on matching the grade and geometry to the required function.

For an OEM, the material choice is often tied to a specific problem: a seal must retain its function through temperature changes, a valve must flex predictably, or an overmolded grip must stay attached to its substrate. We use those functional requirements—not a generic material label—as the starting point for material and process discussions.

Medical and Life Science Applications

Medical and life science components can include seals, tubing, diaphragms, valves, septa, plunger tips, and other device parts. These components may need to flex repeatedly, seal fluids, or remain functional through defined cleaning or sterilization cycles.

We select medical-grade material only in the context of the specific device and intended use. Biocompatibility evaluation, sterilization compatibility, process controls, and applicable regulatory requirements must be addressed for the grade and finished component. A material designation by itself does not establish that a finished part is compliant or suitable for a particular patient-contact application.

Automotive, Consumer, and Industrial Applications

In automotive and EV programs, LSR components may include connector, wire, battery, sensor, and housing seals. Electronics and sensor products may use waterproof seals, keypads, or overmolded interfaces. Consumer applications include infant-care products, bottle valves, kitchenware, and soft-touch controls, while industrial applications often include seals and components for pumps, valves, and fluid-control equipment.

We evaluate the real service conditions before recommending a grade: operating temperature, contact media, exposure duration, compression or movement, expected service life, and any applicable food-contact or electrical requirements. The same part shape can call for a different compound when its environment changes.

Benefits and Limitations of LSR Injection Molding

LSR injection molding can deliver repeatable, flexible components at production scale, including complex geometries that benefit from liquid material delivery and a controlled cure. It can be a strong option for programs where dimensional consistency, sealing performance, durability, and repeat production are important.

• Repeatable production: Controlled metering, tooling, and process monitoring support consistent runs when the program is properly established.
• Design and application flexibility: Multiple hardnesses, colors, part details, and overmolding strategies can be considered for a custom component.
• Durable performance: Selected LSR grades can address demanding thermal, chemical, electrical, or repeated-flex requirements.
• Production scale: Single- and multi-cavity tooling, automation, and process monitoring may support high-volume programs.

There are trade-offs. Tooling requires upfront investment, LSR needs dedicated material delivery and process control, and very thin or highly intricate features may constrain filling, cure, or demolding. Quality-control requirements can also be significant when a seal, medical component, or safety-related feature has critical performance characteristics.

LSR vs. Thermoplastic Injection Molding, HCR, and TPE

LSR may be preferable when a part needs silicone’s selected service properties, flexibility, or cure-based performance. A TPE or another molding method may be a better choice when its material behavior, production route, or part economics better match the design. We recommend deciding from the part requirements and life-cycle conditions rather than from process labels alone.

LSR Injection Molding Design Guidelines

Good LSR design makes the part easier to fill, cure, release, inspect, and produce consistently. Geometry that looks simple in CAD can still create a manufacturing risk if it traps air, creates a difficult shutoff, or concentrates material in one region.

Quality Control, Tolerances, and Common Defects

Quality control for molded LSR parts should connect the drawing to the process and the application. We consider material verification, process monitoring, dimensional inspection, visual criteria, and any functional testing needed for the component. A sealing part, for instance, may require more than a dimensional check to show that it performs as intended.

Tolerances depend on part geometry, material, tooling, process capability, and the measurement method. As a guide to our stated capability, LSRParts lists typical part tolerance at ±0.05 mm and critical tolerance down to ±0.02 mm where application-dependent; neither should be assumed for every feature without review. We agree critical dimensions and inspection methods for the specific drawing.

Common LSR Molding Defects and Troubleshooting

Defects can stem from the mold, material handling, machine settings, or an interaction between them. Troubleshooting is most effective when we check the complete process rather than changing one setting without confirming the cause.

Materials, Standards, and Testing

Start material selection with the part’s job and environment. Relevant factors include Shore A hardness, operating temperature, color, contact media, electrical or thermal requirements, and expected service life. Depending on the application, candidate materials may include general-purpose, medical-grade, food-grade, optical, fluorosilicone, high- or low-temperature, electrically or thermally conductive, flame-retardant, or custom compounds.

We compare those requirements with available grade information and the intended molding process. A fluorosilicone candidate, for example, may be explored for particular fuel or chemical exposures, while other compounds may be considered for optical, thermal, or electrical needs. The exact formulation and its documented performance must be checked for the specified operating conditions.

Quality-system expectations, testing, and regulatory obligations vary by industry and product. Medical applications may require biocompatibility evaluation; food-contact applications may require evidence appropriate to the intended market and use; sterilization claims need validation for the chosen method and number of cycles. Certifications or compliance statements must be verified for the specific material and, where required, the finished part and its manufacturing process.

Cost, Lead Time, and Production Considerations

LSR program cost is shaped by part size and complexity, material grade, cavity count, mold construction, production volume, secondary operations, and inspection requirements. A simple part with stable demand and a clear drawing presents a different tooling and quality plan from a complex overmolded component with tight functional criteria.

A typical development route includes DFM review, material selection, mold design and manufacture, T0/T1 sampling, validation, production approval, mass production, inspection, and any assembly or packaging. Prototype tooling can help test fit and function before committing to a production tool. Sampling feedback may lead to design or process updates, so the schedule depends on the scope and number of validation steps.

Lead time depends on tooling complexity, material availability, sample iterations, and approval requirements. As a planning benchmark, LSRParts lists prototype tooling at approximately 10–12 days and production tooling at approximately 3–6 weeks; actual timing is project-specific and should be confirmed during review. Production cycle time likewise depends on the part, material, tool, and process settings.

Automation and process monitoring may improve consistency and throughput when the part and volume justify them. For programs with repeat demand, we discuss cavity strategy, secondary operations, inspection, and production ramp-up early so the plan supports both the initial validation build and ongoing supply.

Frequently Asked Questions About LSR Injection Molding