Liquid Silicone Rubber (LSR) Injection Molding Guide

Liquid silicone rubber is not a soft thermoplastic. It arrives as two low-viscosity liquids that cure by platinum-catalyzed addition reaction, and once cured it cannot be re-melted. Every design rule that works for PP or ABS has to be revisited: the material flows like water, it cures rather than freezes, and it flashes through gaps a thermoplastic would never penetrate.

Figure 1. Liquid Silicone Rubber (LSR) Injection Molding Guide.

This guide covers the machine, the mold and the process decisions that determine whether an LSR tool runs at 95% yield or fights flash for its whole life.

1. Why LSR Behaves Differently

The single most important sentence in LSR tooling: you are not cooling a melt, you are curing a liquid.

2. Machine and Metering

LSR is supplied in two drums — component A with the platinum catalyst, component B with the crosslinker. They are metered in a 1:1 ratio, mixed in a static mixer, and fed to the injection unit.

Requirements:

• Positive-displacement metering pumps with pressure feedback. A typical tolerance is ±0.5% on the mix ratio; beyond that, cure state and physical properties drift.
• Static mixer sized for the shot volume, replaced on a defined schedule. A partially blocked mixer gives an unmixed streak that shows up as a soft, tacky region in the part.
• Cold runner block cooled to roughly 15–25 °C, holding the mixed material below cure temperature until it enters the hot cavity. This block is the heart of the system.
• No residence time in the hot zone. Mixed LSR begins to cure at room temperature slowly and at mold temperature in seconds. Any dead volume in the hot half will cure and become a "silicone plug" that blocks the flow path.
• Shot size control. Because LSR is incompressible and does not shrink into a cushion like thermoplastics, injection is usually volumetric — the shot is metered by volume, then injected. There is no meaningful packing phase.

3. Cold Runner and Valve Gate Design

A cold runner block uses a needle valve gate to open and close each drop. This is not a luxury; it is required.

• Each drop has an independently actuated needle, typically pneumatic or hydraulic.
• Open time and needle stroke control the shot; the needle closes positively to shut off the flow.
• The needle seat must be a metal-to-metal shut-off lapped to a very high standard — a leaking needle gate produces a cured silicone stringer in the cold runner that will eventually block it.
• Thermal separation is critical. The needle must be cooled so the LSR in the gate does not cure; the cavity must be hot. The gate area is where this thermal conflict has to be resolved, usually with a cooled needle plus a thermally isolated nozzle tip.
• Water cooling channels in the cold runner block must be generous and close to the flow channels — typically within 10–15 mm.

Gate location guidance for LSR:

• LSR fills easily (low viscosity), so gates can be small and flow lengths long.
• Place gates to avoid air entrapment; LSR traps air readily and the entrapment becomes a visible bubble.
• For multi-cavity tools, each cavity gets its own balanced drop — balance matters, but the low viscosity makes LSR more forgiving of minor imbalance than thermoplastics.

4. Mold Design Essentials

Steel and surface.

• Cavity and core steel should be hardened, polished and corrosion-resistant. LSR releases well from a highly polished surface; a rough surface creates mechanical keying and flash.
• Typical practice: harden to 48–54 HRC and polish to a mirror finish on molding surfaces.
• Avoid porous steels and avoid EDM recast layers on shut-off faces — both create leak paths.

Shut-offs and flash control. This is where LSR tools are won or lost.

• Target shut-off gaps of 0.002–0.005 mm or less. In practice this means lapped, matched shut-off faces, not simply machined ones.
• Use shear-edge shut-offs wherever possible, so the closing action creates a metal-to-metal seal under load.
• Design for uniform clamping pressure across the shut-off. A slightly flexible plate will open a gap in the middle of the tool and flash there.
• Vacuum venting is nearly universal in LSR tools. Because the material flashes through vents, you cannot vent conventionally; instead the cavity is evacuated before injection. A vacuum of about 100–200 mbar is typical, and the seal must be designed as carefully as the shut-offs.

Ejection.

• Cured silicone is soft and flexible, so ejection forces are low — but the part is easily deformed and marked.
• Use stripper plates or large-area ejectors rather than small pins.
• Provide generous draft (1.5°–3° minimum) even though the material is compliant; silicone will tear if dragged.

Thermal design.

• Cavity temperature is typically 150–200 °C, achieved with electric cartridge heaters or hot oil.
• Uniformity is critical. A 10 °C variation across the cavity changes cure rate and shrinkage visibly. Target ±3 °C across the molding surface.
• Insulate the hot half from the cold runner block thoroughly; a thermal bridge will cure material in the runner.

5. Process Parameters

Cure time estimation. As a planning figure, cure time scales roughly with the square of the thickest wall:

> t ≈ k × d² > > where d is the thickest section in mm and k is a material-and-temperature constant. Doubling the wall thickness quadruples the cure time. This is why LSR parts are usually designed with uniform, thin walls.

6. Common Defects and Fixes

Figure 2. Quick reference: the key checks and reference values from this guide.

7. Design Checklist

• [ ] Part wall thickness uniform; thick sections cored where possible
• [ ] Draft 1.5°–3° minimum on all surfaces
• [ ] Cold runner block with independently actuated needle valve gates
• [ ] Cold runner water cooling within 10–15 mm of flow channels
• [ ] Metal-to-metal lapped needle seats, verified for shut-off
• [ ] Cavity and core steel hardened 48–54 HRC, mirror polished
• [ ] Shut-off faces lapped to ≤ 0.005 mm; tested with a shim
• [ ] Plate stiffness reviewed so clamping pressure is uniform
• [ ] Vacuum venting designed with a proper perimeter seal
• [ ] Cavity temperature uniform to ±3 °C; sensors placed to verify
• [ ] Hot half thermally isolated from the cold runner block
• [ ] Large-area ejection (stripper or plate), not small pins
• [ ] Post-cure specified if the part is medical or food-contact
• [ ] Tool cleaning procedure defined to avoid catalyst contamination

Conclusion

LSR molding rewards a different mindset. The material is a curing liquid, not a freezing melt, so the tool's job is to control heat and seal against an exceptionally penetrating fluid. Get the cold runner, the thermal separation and the shut-offs right and LSR runs at very high yield with short cycles. Treat the tool like a thermoplastic tool and you will spend your life chasing flash and cured plugs.

Send us your LSR part and we will review the wall sections, shut-off strategy and cold runner layout before the tool is cut.