What Is LSR Injection Molding and When Should You Use It Instead of Standard Rubber Molding?

Most buyers who need a rubber part know roughly what they want: something that seals, cushions, or insulates. The harder question is often which rubber process and which material will hold up in the actual application. LSR injection molding is a process that comes up frequently in medical, defense, and high-performance industrial work, and it is one that is easy to overlook if you are more familiar with standard rubber molding.

This post explains what LSR is, how the molding process works, and when it is the right choice over more conventional rubber molding methods.

What Exactly Is LSR?

LSR stands for Liquid Silicone Rubber. It is a two-part thermoset elastomer, meaning it starts as a liquid and cures into a permanently solid, flexible rubber through a chemical reaction. The two components are mixed in a precise 1:1 ratio just before injection, then catalyzed by heat in the mold. The platinum-based curing system is what gives LSR its distinctive properties: excellent flexibility, high tear strength, and a broad operating temperature range.

Unlike thermoplastic elastomers, which can be remelted and reprocessed, LSR is permanently set once it cures. That characteristic is actually a feature for applications where dimensional stability and consistent performance over time are priorities.

How Does the LSR Injection Molding Process Work?

The process starts with two separate liquid components stored in sealed containers. A precision metering system combines them in equal parts and moves the mixed material to the injection molding press. The mold is heated, which triggers the platinum-catalyzed curing reaction when the liquid silicone is injected into the cavity.

The mold itself is typically machined from aluminum or hardened steel, depending on production volume and part complexity. Because LSR is flexible when cured, parts are typically removed manually rather than using ejector pins, which could damage the soft material.

Cycle times for LSR molding are relatively fast, and the process scales well to high-volume production. Part dimensions are highly consistent across large runs because the liquid material fills the mold cavity completely and the curing reaction is tightly controlled.

What Are the Key Properties of LSR Parts?

LSR produces parts with a specific combination of properties that make it distinctly valuable in certain applications. The most important ones to understand are:

• Biocompatibility. LSR is hypoallergenic, odorless, tasteless, and can be formulated to meet USP Class VI and ISO 10993 biocompatibility requirements. It does not support bacterial growth and is compatible with sterilization processes including autoclave and gamma radiation.
• Temperature range. LSR maintains its flexibility and physical properties from around -60 degrees Celsius to above 200 degrees Celsius, which is a much wider range than most standard rubber compounds.
• Chemical resistance. LSR resists a wide range of chemicals, solvents, and cleaning agents without degrading or swelling, which matters in medical, laboratory, and industrial environments.
• Dimensional consistency. Because LSR is injected as a liquid and cures in a controlled mold environment, part dimensions are highly repeatable across production runs.
• Transparency options. LSR can be produced in translucent and clear grades, which is useful in optical, medical, and lighting applications.

When Should You Choose LSR Over Standard Rubber Molding?

Standard rubber molding, including compression and transfer molding with conventional rubber compounds like EPDM, neoprene, or nitrile, is a proven and cost-effective process for a wide range of applications. It is not the right answer for every job, but it handles general-purpose sealing, gasketing, and cushioning work reliably and at a lower tooling cost than LSR.

LSR is the better choice when one or more of these factors apply to your application:

• The part will contact human tissue, body fluids, food, or beverages and requires biocompatibility documentation
• The part must perform reliably at temperatures below -40 or above 150 degrees Celsius
• The part will be subjected to repeated steam sterilization cycles
• Tight dimensional tolerances are required across a high-volume production run
• The part requires chemical or fluid resistance beyond what conventional rubber compounds can provide
• The design includes thin walls, complex geometries, or features that benefit from a liquid injection process

What Industries Use LSR Injection Molded Parts?

LSR shows up across a range of demanding industries because its property profile matches their requirements well. Medical device manufacturers use it for seals, diaphragms, catheter components, and patient-contact parts where biocompatibility is non-negotiable. Defense and aerospace applications often require the temperature range and chemical stability that LSR provides. Industrial manufacturers use it for sealing components in harsh fluid environments.

Consumer applications also benefit from LSR’s combination of flexibility, clarity, and food-safety potential, with products ranging from infant care items to wearable electronics seals seeing regular use of the material.

Can LSR Be Overmolded onto Other Materials?

Yes, and this is one of the more valuable things you can do with LSR. Overmolding allows liquid silicone to be molded directly onto a plastic substrate, a metal insert, or an existing component, forming a permanent bond without adhesives or secondary assembly steps. This capability is what makes LSR overmolded cable assemblies possible, for example, where silicone is molded around a cable termination to create a sealed, flexible strain relief.

At Mid-Atlantic Rubber, our injection molding equipment handles LSR parts as well as overmolded applications where silicone needs to bond directly to another substrate. If your part involves LSR overmolding, we can discuss the design requirements and process options when you reach out.

How Do You Get Started with an LSR Part at MAR?

The starting point is the same as any custom part: a drawing. Submit your part geometry, the required material properties or grade, your target quantity, and any relevant compliance requirements for your industry. Our team will review the design and provide a quote that covers tooling and per-part production cost.

If you are not sure whether LSR or a standard rubber compound is the right call for your application, that is a conversation we are happy to have before you commit to anything. Getting the material decision right at the beginning saves time and money down the road.

Ready to Discuss Your LSR or Rubber Molding Project?

Mid-Atlantic Rubber has the equipment and experience to handle LSR injection molding alongside our full range of custom rubber, foam, plastic, and metal fabrication services. If you have a part that needs silicone, reach out and let us help you find the right path forward.

Contact MAR to Discuss Your LSR Molding Project

Frequently Asked Questions About LSR Injection Molding

What is LSR injection molding?

LSR injection molding is a process that injects a two-part liquid silicone rubber compound into a heated mold. The material cures through a platinum-catalyzed chemical reaction, producing flexible, durable, and biocompatible silicone parts. Once cured, LSR parts are permanently set and cannot be remelted.

When should you use LSR instead of standard rubber molding?

LSR is the better choice when parts require biocompatibility for medical or food-contact use, resistance to extreme temperatures, sterilization compatibility, or high dimensional consistency across large production runs. Standard rubber molding handles general-purpose sealing and cushioning where those requirements do not apply.

Does Mid-Atlantic Rubber manufacture LSR injection molded parts?

Yes. Mid-Atlantic Rubber in Fredericksburg, Virginia, produces LSR injection molded parts for defense, medical, industrial, and commercial applications. Contact MAR at marmfg.com to discuss your LSR part requirements.

Can LSR be overmolded onto metal or plastic?

Yes. LSR can be overmolded directly onto metal inserts, plastic substrates, or existing components, forming a permanent bond without adhesives. Mid-Atlantic Rubber produces overmolded LSR assemblies including overmolded cable assemblies for defense and industrial applications.