Liquid Silicone Rubber (LSR) Injection Moulding: Process, Design Rules and Applications
Liquid silicone rubber (LSR) is the material behind baby bottle teats, medical seals, wearable gaskets and the soft valves inside coffee machines. It moulds on injection machines, but the process is thermoplastic injection moulding turned inside out: cold liquid in, hot mould, chemical cure instead of cooling. This guide explains how LSR processing works, what design rules follow, and when it beats thermoplastic elastomers.
How does the LSR process work?
LSR arrives as two liquid components (A and B) in drums or cartridges. A metering unit pumps them at a precise 1:1 ratio through a static mixer into a cooled injection unit — the material must stay cold until the last moment. The mixed liquid injects into a mould heated to 160-200 °C, where platinum-catalysed addition curing cross-links the silicone in typically 10-60 seconds depending on wall thickness. The part ejects fully cured: no post-cure is needed for modern grades, and no cooling phase exists at all. Cycle time is governed by cure, not cooling — the mirror image of the thermoplastic cycle.
Why do moulds run hot and runners run cold?
Because the material cures wherever it is hot. The mould cavity is heated, but the runner system must keep material cold and uncured between shots, which is why LSR moulds use cold-runner decks: water-cooled needle-valve nozzles that inject directly into the cavity with zero waste. A well-built cold deck means no sprue, no regrind, and material usage of essentially 100% — important when LSR costs €10-25/kg. The heated mould and cold deck also make mould flow behaviour unusual: filling is easy (LSR is very low viscosity when sheared), but venting and flash control dominate tool design.
What are the design rules for LSR parts?
- Wall thickness: 0.5-3 mm typical; down to 0.3 mm possible. Keep walls uniform, as thick sections extend cure time and risk undercure at the core
- Flash control: liquid silicone flashes into any gap above ~0.005 mm, so parting lines, shut-offs and ejector fits must be machined to tight precision — budget toolmaking, not standard
- Venting: vacuum venting is standard practice; trapped air causes burns and voids because the cavity fills fast and the mould is hot
- Draft: LSR is flexible and demoulds against undercuts that would destroy a rigid part, but soft grades also tear — design ejection with air poppets or stripper plates rather than pins on thin membranes
- Gates: needle-valve cold drops directly onto the part; gate marks are small but must be positioned where they do not affect sealing surfaces
- Parting lines on seals: keep them off the sealing face; a 0.05 mm flash line on a gasket face is a leak path
LSR versus TPE: which soft material?
| Property | LSR | TPE/TPU |
|---|---|---|
| Service temperature | -50 to +200 °C | typically -40 to +120 °C |
| Compression set | Excellent (seals for decades) | Moderate to poor |
| Chemical/UV resistance | Excellent | Varies widely by type |
| Biocompatibility | Medical grades, implant history | Limited |
| Processing | Specialised machines and moulds | Standard thermoplastic machines |
| Material cost | €10-25/kg | €3-10/kg |
| Overmoulding | Onto PC/PA/PBT with self-adhesive grades | Broad, easy on standard two-shot machines |
The short version: choose LSR for heat, sealing longevity, medical contact and outdoor life; choose TPE for cost-driven soft-touch on standard equipment, including two-shot moulding.
Where is LSR used?
- Medical and infant care: teats, valves, respiratory masks, seals — biocompatibility plus sterilisation resistance
- Automotive: connector gaskets, sensor membranes, spark-plug boots (heat plus compression set)
- Consumer and kitchen: baking moulds, spatula heads, coffee-machine valves
- Electronics: keypad membranes, waterproof seals on wearables, often overmoulded onto rigid substrates
What does LSR cost?
Material at €10-25/kg and specialised cold-deck tooling at a 20-50% premium over thermoplastic moulds make LSR a deliberate choice, not a default. Part prices typically run 2-5× the thermoplastic equivalent. The payback comes from part consolidation (a moulded seal replaces an assembled gasket), lifetime (silicone seals outlive the product), and compliance (one medical-grade material instead of a qualified assembly).
How to start an LSR project
LSR rewards early process involvement more than most materials: gate position, parting-line placement and venting are fixed in steel, and soft-material handling (automatic demoulding is standard — parts are blown or brushed out) must be designed in. Nordmould reviews LSR part designs with our silicone-capable partners before quoting, so tooling, cure cycles and demoulding method are settled before commitment.
Related guides
- Two-shot and multi-shot injection moulding
- Overmoulding and insert moulding guide
- Injection moulding cycle time and clamp force
- Injection moulding materials: which plastic to choose
- Injection moulding for medical devices
- Mould flow simulation explained
Designing a silicone seal or soft-rigid part? Send Nordmould your drawings for an LSR feasibility review.
Frequently asked questions
How is LSR moulding different from thermoplastic injection moulding?
Everything is inverted. LSR is pumped as two cold liquid components into a heated mould (160-200 °C) where it cures chemically. Thermoplastics are melted hot and injected into a cold mould to solidify. LSR machines, moulds and process logic are therefore built the opposite way around.
What wall thickness can LSR parts have?
LSR flows extremely well before curing, filling walls down to 0.3-0.5 mm over surprising flow lengths. Typical design walls are 0.5-3 mm. Very thin flash-prone features need excellent mould venting and parting-line precision because liquid silicone flashes into gaps above ~0.005 mm.
What hardness range does LSR cover?
Commercial LSR grades span roughly Shore A 10 to 80. Soft grades (Shore A 10-30) suit sealing and skin-contact parts; mid grades (40-60) are the general-purpose range; harder grades approach thermoplastic-like handling for membranes and valves.
Can LSR be overmoulded onto thermoplastics?
Yes, and it is one of LSR's biggest uses: two-shot or insert moulding onto PC, PA or PBT substrates creates soft-rigid parts like seals on connectors and soft-touch medical grips. Self-adhesive LSR grades bond chemically without primers on qualified substrates.
What does LSR tooling cost compared with thermoplastic tooling?
LSR moulds typically cost 20-50% more than equivalent thermoplastic tools because they need heated plates, cold-runner needle-valve systems, vacuum venting and very precise parting lines. Part prices are higher too, driven by material cost and cure-limited cycles.
Last reviewed: 2026-08