Liquid Silicone Rubber (LSR) Molding

LSR molding injection-molds two-part liquid silicone into flexible, clean parts with excellent sealing and biocompatibility, suited to high-volume production and automated demolding.

Overview

Liquid Silicone Rubber (LSR) molding is an injection molding process that meters and mixes two-part liquid silicone, then injects it into a heated mold where it rapidly cures. Tooling is typically hardened steel with cold-runner/valve-gate systems to control flash and enable high cavitation. The result is elastic, temperature-stable parts with strong chemical resistance, low compression set, and options for medical/food-grade compliance.

Choose LSR when you need repeatable seals, diaphragms, grips, or soft-touch components in medium to high volumes, especially where cleanliness, transparency, or biocompatibility matter. Typical strengths include thin features, fine detail, and automated production.

Tradeoffs: molds and process development cost more than standard thermoplastics, and LSR is sensitive to flash, venting, and parting-line design. Cosmetic surfaces can show gate/parting artifacts, and post-cure, degassing, or cleanroom controls may be required for regulated applications.

Common Materials

  • LSR 40A
  • LSR 60A
  • LSR medical-grade (platinum-cure)
  • LSR food-grade
  • Fluorosilicone LSR

Tolerances

±0.003" to ±0.005"

Applications

  • O-rings and custom seals
  • Medical device valves and diaphragms
  • Baby bottle nipples and pacifiers
  • Gaskets for enclosures and connectors
  • Keypads and button membranes
  • Soft grips and protective boots

When to Choose Liquid Silicone Rubber (LSR) Molding

Choose LSR molding for elastic parts that must seal, flex, or recover shape over many cycles, with stable performance across wide temperatures. It fits best at medium to high volumes where automation and multi-cavity tools offset higher tooling and process setup cost. It’s a strong choice when you need clean, low-odor, low-extractable silicone for consumer, food-contact, or medical applications.

vs Standard Injection Molding

Choose LSR when the part must remain elastic, seal under compression, or survive heat/chemicals that degrade many thermoplastics. LSR also works well for soft, skin-contact parts and applications needing low compression set and long-term flexibility.

vs Overmolding

Choose LSR molding when the silicone part can be molded as a standalone component and assembled, or when you need the best control of flash and seal geometry. LSR can be overmolded, but a dedicated LSR tool is often simpler and more robust when adhesion and interface geometry are not the primary requirements.

vs Insert Molding

Choose LSR molding when the silicone component does not require encapsulating metal/plastic inserts, or when insert handling would drive cycle time and contamination risk. If inserts are necessary, LSR can work, but the process needs careful fixturing, venting, and thermal control to avoid flash and knit defects around the insert.

vs Compression Molding

Choose LSR molding when you need high repeatability, short cycle times, and automation for higher volumes. Injection-style filling and controlled gating typically produce more consistent dimensions and less manual labor than compression for complex or thin features.

vs Transfer Molding

Choose LSR molding when you want fully automated metering/mixing and low material waste with cold-runner systems. LSR injection molding generally supports higher cavitation and better process control for tight sealing features than transfer molding at scale.

Design Considerations

  • Specify durometer, cure system (platinum vs peroxide), and any post-cure requirement up front to avoid rework and re-qualification
  • Design parting lines and shutoffs to avoid critical sealing surfaces; LSR flash control is a tooling and geometry problem
  • Use uniform wall thickness where possible and avoid abrupt thick-to-thin transitions that trap air and drive venting/flash issues
  • Add draft on all non-sealing surfaces (often 1–3°) and plan ejection/stripper features early for reliable demolding
  • Call out surface finish and cosmetic side; gate vestige, vent marks, and parting lines are unavoidable without clear expectations
  • Include realistic tolerances for elastomers and define how dimensions are measured (free state vs under load/fixture)