Liquid Silicone Rubber (LSR) Molding
LSR molding injects two-part liquid silicone into a heated mold where it cures, producing flexible parts with a very wide service temperature range.
- Part
- Forming
- Revised
- 2026-08-11
At a glance
- Family
- Plastics and Rubber
- Typical tolerances
- About ±0.002–0.004 in (±0.05–0.10 mm) on small precision features; ISO 3302-1 class M1 is achievable on tightly controlled dimensions. Larger dimensions scale with the 2–3.5% shrinkage allowance.
- Surface finish
- Reproduces the mold finish from polished gloss to matte; a parting-line flash witness is inherent to the material's low viscosity
- Typical volumes
- 1,000–1,000,000+ parts; below about 500, compression molding avoids the tooling cost
- Lead time
- 6–12 weeks for a production LSR tool with cold runner and vacuum sealing; cycles of 20–60 seconds thereafter, plus any post-cure
- Materials
- Plastic
What it is
Liquid silicone rubber molding injects a two-part, platinum-cured silicone into a heated mold, where it cures in seconds into a flexible elastomer. Unlike thermoplastic injection molding, the barrel runs cold and the mold runs hot: the material is a low-viscosity liquid at room temperature and crosslinks only once it reaches the cavity.
LSR is chosen for what silicone does that organic elastomers do not — continuous service from roughly -58 °F to 392 °F (-50 °C to 200 °C), excellent resistance to UV, ozone, and steam sterilization, and biocompatibility grades qualified to USP Class VI and ISO 10993. Baby bottle nipples, medical seals and valves, respirator masks, automotive connector seals, and LED optics are all LSR.
Hardnesses run 5–80 Shore A, most commonly 30–70. The material's very low viscosity means it will flash through any gap the tool leaves, so LSR tooling is built to tighter shutoffs than thermoplastic tooling.
How it works
- Meter and mix. Two components — one carrying a platinum catalyst, the other a hydride crosslinker — are pumped from drums at a 1:1 ratio through a static mixer, with pigment and any additive dosed in line. Ratio control matters: an off-ratio shot undercures and stays tacky.
- Feed cold. The mixed material passes through a temperature-controlled barrel held near ambient, roughly 60–77 °F (15–25 °C). Crosslinking is heat-activated, so keeping the barrel cold is what preserves working life. Any heat leak from the mold back into the nozzle will cure material in the barrel and shut the machine down.
- Inject into a hot mold. The mold runs at roughly 300–400 °F (150–205 °C). Because LSR has a viscosity closer to honey than to polymer melt, injection pressure is far lower than for thermoplastics and the cavity must be evacuated — production LSR tools pull vacuum before the shot to avoid trapping air the material is too thin to push out.
- Cure. Crosslinking begins on contact with the hot cavity. Cure time depends on section thickness and catalyst level; typical cycles run 20–60 seconds for parts under about 0.2 in (5 mm) section. Cure is chemical, so unlike thermoplastic molding, cooling is not part of the cycle at all.
- Demold. LSR elongates 300–700% before break, so parts are usually stripped, air-blown, or peeled off cores rather than pushed off by ejector pins — which is why LSR parts can carry undercuts that would require a side action in a rigid material.
- Post-cure if required. Food-contact and medical parts are typically post-cured about 4 hours at 392 °F (200 °C) to drive off residual volatiles and stabilize properties. Post-cure is not needed for most industrial parts.
Why a cold runner?
LSR is a thermoset — runners and sprues cannot be reground. A cold runner block keeps the feed system below cure temperature so no material is wasted between shots, and it is standard on any LSR tool running meaningful volume.
Design guidelines
Wall thickness
LSR handles a far wider range than thermoplastics: from about 0.010 in (0.25 mm) in membranes and diaphragms up to 0.400 in (10 mm) or more in solid seals. Thick and thin sections in one part are acceptable, because cure is chemical rather than heat-conduction limited, though very thick sections do lengthen the cycle. Uniformity, the governing rule for thermoplastics, is much less important here.
Draft — often zero
Parts strip off tooling on their own elongation, so 0–0.5° is workable and many LSR parts carry no draft at all. Where a part must be pulled over a deep core, a small draft still eases automation.
Undercuts
Moderate undercuts, snap features, and internal grooves release directly because the material stretches. This removes the side actions that the same geometry would require in a rigid molded part and is one of the strongest reasons to choose LSR over a molded thermoplastic elastomer.
Shutoffs, flash, and vents
The design consequence of low viscosity: LSR flashes through clearances that would be perfectly sealed for a thermoplastic. Parting lines must land where a thin flash witness is acceptable and where deflashing is practical. Plan the parting line as a functional design decision, not a tooling afterthought.
Shrinkage
Budget 2–3.5% linear shrinkage, higher than most thermoplastics and dependent on the specific grade, hardness, and whether the part is post-cured. Confirm the number for the actual compound before the tool is cut, because the entire cavity is scaled by it.
Bonding to a substrate
Self-bonding LSR grades adhere directly to some thermoplastics and to primed metals during cure; standard grades do not. If you are overmolding LSR onto a rigid part, either specify a self-bonding grade, prime the substrate, or design a mechanical interlock — a through-hole or undercut groove — and do not rely on the chemistry alone.
Material comparison
Where the requirement is only flexibility rather than temperature or biocompatibility, a thermoplastic elastomer molded conventionally is usually cheaper and faster. Compare against the production polymers on /charts/material-properties before committing to silicone.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Wall thickness | 0.040–0.200 in (1–5 mm) | 0.010–0.400 in (0.25–10 mm) | Chemical cure tolerates thick and thin in one part |
| Draft | 0–0.5° per side | 0° | Elongation of 300–700% lets parts strip |
| Undercuts | Moderate, released by stretch | Deep sharp undercuts | No side action needed for most snap geometry |
| Shrinkage allowance | 2–3.5% linear | — | Grade and post-cure dependent; confirm before cutting steel |
| Hardness | 30–70 Shore A | 5–80 Shore A | Standard commercial compound range |
| Service temperature | -58 to 392 °F (-50 to 200 °C) | — | The main reason to choose silicone at all |
| Shutoff clearance | As tight as the tool can hold | — | Low viscosity flashes through any gap |
Cost drivers
LSR tooling costs more than an equivalent thermoplastic tool. It is heated rather than cooled, it needs vacuum sealing, it needs a cold runner block to avoid wasting a thermoset that cannot be reground, and its shutoffs are cut to tighter clearances to control flash. The raw material is also several times the cost per pound of commodity thermoplastics.
Against that, cycles are short — 20–60 seconds for most parts — the process automates well, and the finished part often eliminates an assembly that would otherwise need a separate seal, gasket, or grip.
Volume breakpoints:
- Under 500 parts: compression molding of high-consistency rubber, or machining from silicone sheet, avoids the tooling investment.
- 1,000–100,000: single or low-cavity LSR tool with a cold runner.
- Over 100,000: multi-cavity tooling with automated demolding, valve-gated cold runners, and in-line vision inspection.
Cost reduction:
- Specify post-cure only where it is required. Four hours at 392 °F is a real oven cost and is unnecessary for most industrial parts.
- Design the parting line for flash you can live with. Deflashing is manual labor on every part unless the geometry allows cryogenic tumbling.
- Use the elongation. Designing undercuts that strip out eliminates side actions and their maintenance entirely.
- Choose the softest durometer that works. Softer compounds generally flow and fill more easily, which shortens the cycle.
- Check whether a TPE will do. If the part does not need silicone's temperature range, chemical resistance, or biocompatibility, a thermoplastic elastomer molded conventionally is usually cheaper on both tooling and material.
Questions
7 questionsWhat temperature range can LSR parts survive?
Roughly -58 °F to 392 °F (-50 °C to 200 °C) in continuous service, with short excursions higher. That span, combined with UV, ozone, and steam-sterilization resistance, is the main reason to choose silicone over a thermoplastic elastomer, which typically tops out far lower.
Why is the LSR mold hot and the barrel cold?
It is the inverse of thermoplastic molding. LSR crosslinks on heating, so the barrel is held near ambient — roughly 60–77 °F (15–25 °C) — to preserve working life, and the mold runs at 300–400 °F (150–205 °C) to trigger cure. Heat leaking back from the mold into the nozzle will cure material in the barrel and stop production.
What draft angle does LSR molding need?
Often none. LSR elongates 300–700% before break, so parts strip off cores on their own flexibility; 0–0.5° per side is typical and many parts carry zero draft. The same elongation lets moderate undercuts and snap features release without any side action in the tool.
How much does LSR shrink?
Budget 2–3.5% linear shrinkage, which is higher than most thermoplastics. The exact figure depends on the grade, the durometer, and whether the part is post-cured, so confirm it against the specific compound before the cavity is cut — the entire tool is scaled by that number.
Does LSR need to be post-cured?
Only for food-contact and medical applications, where a post-cure of about 4 hours at 392 °F (200 °C) drives off residual volatiles and stabilizes properties. Most industrial parts ship as molded, and specifying an unnecessary post-cure adds real oven cost to every batch.
Will LSR bond to a plastic substrate when overmolded?
Only with a self-bonding grade, or with a primed substrate. Standard LSR grades do not adhere to thermoplastics during cure. Where the joint is structural, design a mechanical interlock — a through-hole or undercut groove the silicone fills — rather than relying on chemistry alone.
Why does LSR flash more than thermoplastic molding?
Its viscosity before cure is closer to honey than to polymer melt, so it penetrates clearances that a thermoplastic would seal against. LSR tooling is cut to tighter shutoffs and usually pulls vacuum on the cavity, but a thin parting-line witness is inherent. Place the parting line where that witness is cosmetically and functionally acceptable.