Silicone (LSR) Injection Molding Guide
Process
What silicone molding actually is
Liquid silicone rubber (LSR) is not a thermoplastic. It arrives as two liquid components (base + catalyst) that are metered, mixed and pumped into a hot mold where they cross-link into a solid elastomer. There is no pellet melting, no sprue to reclaim, and no crystallization to manage — instead you fight dosing accuracy, cure time and flash at the parting line.
This guide covers what a buyer or designer needs to specify an LSR part: the process flow, the tool demands, the venting rules that make or break flash, and the numeric windows that keep cure consistent.
The Snapshot
- LSR is dosed 1:1 by a A/B pump (typical 10–60 g/s shot) and cured at mold temps of 170–200 °C in 10–120 s depending on wall thickness.
- Real program: medical ECG button in medical-grade LSR, held to ±0.03 mm, 300K units/year, first parts in 7 weeks rapid tooling.
- Shrinkage runs 2.0–3.0 % — roughly 3× a thermoplastic — so the mold is cut oversize and the cavity steel tolerance is held to ±0.02 mm.
- Flash is the #1 defect: LSR viscosity (2,000–100,000 mPa·s at shear) leaks through any >0.01 mm parting-line gap.
- Steel tools are mandatory — 420 stainless or H13; aluminum softens under LSR’s sustained 170–200 °C cure heat.
The LSR process flow
Unlike thermoplastic molding, LSR runs as a closed dosing loop:
- Metering — A (vinyl polymer) and B (platinic catalyst + cross-linker) components are pumped at a fixed 1:1 ratio; a ratio error of even 0.5 % shifts cure and leaves uncured tack.
- Static mixing — a 12–24 element static mixer blends the two; insufficient mixing leaves streaky, under-cured zones.
- Injection — the mixed liquid is pushed into the hot cavity at low pressure (30–120 bar), filling thin sections the way water fills a mold.
- Cure — the mold at 170–200 °C cross-links the rubber in 10–120 s; thick sections need the long end of that window.
- Demold — the part springs out clean; no cooling-channel chill needed beyond holding the mold at cure temp.
A typical small LSR part cycles in 30–90 s; the cure step — not injection — is the rate limiter.
Tooling: why steel, not aluminum
LSR cures hot and stays hot. Aluminum molds (fine for thermoplastic rapid tooling at 40–120 °C mold temp) degrade under LSR’s sustained 170–200 °C, losing dimension and surface. So:
- Mold steel: 420 stainless (corrosion-resistant, common for medical LSR) or H13 (hardened, long-run). Cavity tolerance held to ±0.02 mm.
- Parting line: ground and fitted to ≤0.01 mm gap; a visible hairline there becomes a flash fin on every part.
- Venting: LSR displaces air it cannot compress; vents at the parting line and ejector pins must be cut to 0.005–0.01 mm depth so air escapes but liquid does not.
- Nozzle / cold runner: LSR is kept cool in the barrel (20–40 °C) until the mold; a cold-runner system recovers the sprue automatically in high-volume runs.
Venting and flash control — the real battle
LSR’s low viscosity is its blessing and its curse. It fills 0.1–0.2 mm walls that thermoplastics would short, but it also finds every gap.
| Risk | Cause | Control |
|---|---|---|
| Flash fin | Parting-line gap > 0.01 mm | Grind/fit line to ≤ 0.01 mm |
| Air trap / burn | No vent path | Vents at parting + ejector, 0.005–0.01 mm |
| Short fill | Under-dosing | Verify A/B ratio ±0.5 % |
| Uncured tack | Cure < 10 s on thick wall | Hold 170–200 °C, extend cure |
| Streaks | Poor mix | 12–24 element static mixer |
The ECG button program held ±0.03 mm across 300K units/year precisely because the parting line and vents were cut and verified before the production run — not corrected after.
Material and process windows
LSR grades span soft to firm; the grade sets hardness and cure behavior.
| Property | Typical LSR range | Notes |
|---|---|---|
| Hardness | Shore A 10–70 | Medical buttons ~ Shore A 40–60 |
| Cure temp | 170–200 °C | Mold, not barrel |
| Cure time | 10–120 s | Scales with wall thickness |
| Shrinkage | 2.0–3.0 % | Mold cut oversize |
| Viscosity | 2,000–100,000 mPa·s | Low → thin walls, flash risk |
| Service temp | -50 to 200 °C | Stable across thermal cycle |
Medical-grade LSR is platinum-cured (no peroxide residues), biocompatible and autoclave-tolerant — why the ECG wearable button runs in it. Food- and skin-contact grades follow the same cure chemistry.
Tolerances and shrinkage reality
LSR shrinks 2.0–3.0 %, about triple a thermoplastic’s 0.4–0.8 %. That alone changes the mold math:
- Mold (cavity) tolerance: ±0.02 mm on the steel, cut oversize by the shrink factor so the cooled part lands in spec.
- Part tolerance: ±0.03 mm achieved on the medical ECG button; ±0.1–0.2 mm is the general band for non-critical LSR features.
- Shrink direction: LSR shrinks isotropically more than filled thermoplastics, so round features stay round — a plus for seals and gaskets.
- Post-cure: some grades get a +1–4 h @ 150–200 °C post-bake to finish cross-linking; design for that if the spec demands full mechanicals.
Even LSR can start on a faster (steel) tool — a 420 stainless prototype tool lands first parts in 7 weeks (as on the ECG button) and supports bridge runs before the hardened production tool is cut. Aluminum is off the table for LSR regardless of volume.
Where LSR molding goes wrong
- Aluminum tool → dimension drift and surface loss under 170–200 °C cure.
- Loose parting line → flash on every part; the fin then needs a deflash operation.
- A/B ratio drift → uncured tack or over-cure brittleness; monitor the pump ratio continuously.
- No vent path → burned, trapped-air voids on the last-filled feature.
- Ignoring 2–3 % shrink → mold cut to nominal and the part lands undersize.
Grade selection and secondary operations
LSR is graded by Shore A hardness, and the grade drives both feel and cure behavior. Picking the right one avoids an over-firm part that cracks or an over-soft part that tears.
| Shore A | Grade | Application |
|---|---|---|
| 10–20 | Very soft | Skin-contact pads, thin seals |
| 30–50 | Soft | Buttons, gaskets, wearable parts |
| 40–60 | Medical-soft | ECG / medical buttons (our program) |
| 50–70 | Firm | Industrial seals, high-tear parts |
Medical-grade LSR is platinum-cured and biocompatible, which is why the ECG wearable button runs in the 40–60 Shore A band. Self-leveling (low-viscosity) grades fill 0.1–0.2 mm walls; high-tear grades trade some flow for tear strength at demold. The hardness also shifts shrinkage slightly — softer LSR trends toward the top of the 2.0–3.0 % band, so the mold is cut accordingly.
Most LSR parts need a light deflash (the parting-line fin is thin, <0.1 mm) and, for full-mechanical grades, a post-cure bake of +1–4 h @ 150–200 °C to finish cross-linking. Plan these into the cycle cost: a 300K units/year medical run still budgets the deflash and bake per part.
Compliance pass
Send the part geometry, target hardness (Shore A 10–70) and whether it needs medical/food grade. We return a steel-tool plan (420 or H13), a vent/parting-line spec and a cure window — held to ±0.03 mm on real programs — with first parts in 7 weeks via rapid tooling.