Liquid Silicone Rubber (LSR) Molding

Liquid Silicone Rubber (LSR) molding injects 2-part silicone into a cold mold for flexible, durable, high-precision elastomer parts at moderate-to-high volumes.

Overview

Liquid Silicone Rubber (LSR) molding is a closed-mold injection process that meters and mixes a two-part silicone system, then injects it into a cooled mold where it cures in place. It produces flexible, temperature- and chemical-resistant elastomer parts with excellent repeatability and fine detail, including thin features, living hinges, and complex geometries. Typical parts are flash-minimized, biocompatible, and stable over a wide temperature range.

You should consider LSR molding for medium to high production volumes where you need consistent, soft-touch or elastomeric components: seals, gaskets, diaphragms, medical parts, or consumer contact surfaces. Startup costs are higher than soft tooling or casting due to steel or aluminum molds and specialized metering equipment, but part cost drops quickly with volume. Tradeoffs include less aggressive dimensional tolerances than rigid thermoplastics, limited post-machining, and the need to design around parting lines and venting to control flash. For bonded assemblies, LSR can be molded directly onto compatible inserts to create integrated, leak-tight components.

Common Materials

  • General-purpose LSR
  • Medical-grade LSR
  • High-transparency LSR
  • Flame-retardant LSR
  • Oil-resistant LSR

Tolerances

±0.005" to ±0.010"

Applications

  • Medical seals and diaphragms
  • Baby bottle nipples and pacifiers
  • Keypads and switch boots
  • Automotive connector seals and grommets
  • Wearable device bands and cushions
  • Valve seats and pump components

When to Choose Liquid Silicone Rubber (LSR) Molding

Choose Liquid Silicone Rubber (LSR) molding when you need flexible, temperature-stable, or biocompatible elastomer parts at medium to high volumes with consistent properties and fine detail. It fits best for seals, soft-touch interfaces, and complex rubber geometries where tooling cost is justified by part count and performance requirements. It is especially suitable when parts must withstand sterilization, weathering, or long-term compression without losing function.

vs Standard Injection Molding

Pick LSR molding instead of standard thermoplastic injection molding when the part must be elastomeric, soft, or rubber-like with high temperature or chemical resistance. Standard molding fits rigid components; LSR excels for seals, cushions, and flexible interfaces where hardness, elongation, and compression set matter more than stiffness.

vs Overmolding

Use LSR molding over conventional overmolding when you need a true silicone elastomer bonded to a compatible substrate for improved sealing, softness, or biocompatibility. It’s ideal for handles, keypads, and medical components where a durable silicone layer must permanently adhere to plastic or metal without secondary assembly.

vs Insert Molding

Choose LSR insert molding when you want silicone permanently bonded around metal or plastic inserts such as terminals, bushings, or rigid carriers. Compared to post-assembled O-rings or boots, LSR insert molding reduces leak paths, simplifies assembly, and improves alignment for high-reliability seals and electrical connectors.

vs Compression Molding

Select LSR injection molding over compression molding when you need higher production volumes, better repeatability, and more complex geometries with fine details or thin walls. Compression can be cost-effective for simple, low-volume parts, but LSR injection provides better automation, cycle times, and process control for tight-tolerance seals and functional components.

vs Transfer Molding

Use LSR molding instead of transfer molding when you require automated, high-volume production with consistent shot control and minimized labor. Transfer molding suits some legacy or low-volume rubber programs, while LSR injection supports more consistent part quality, cleaner processing, and easier integration into medical or cleanroom environments.

Design Considerations

  • Target uniform wall thickness (0.020"–0.120") to control cure time, reduce sink, and minimize flash and voids
  • Add generous radii and avoid sharp internal corners to improve flow, reduce stress concentrations, and extend tool life
  • Design parting lines intentionally on non-critical surfaces and away from sealing interfaces to manage flash and simplify deflashing
  • Limit very tight tolerances to critical features only and specify realistic ranges for elastomeric behavior and post-cure growth
  • Use undercuts and flexible features judiciously, ensuring the part can stretch for demolding without tearing or permanent deformation
  • Clearly define bonding areas, compatible insert materials, and surface finishes when overmolding or insert molding with LSR to ensure reliable adhesion