Silicone Rubber Manufacturing: LSR, HCR and the Five Processes

Silicone is not rubber

Silicone gets grouped with rubber because it is elastic, but the chemistry has nothing in common. Natural rubber, EPDM and nitrile have a carbon backbone; silicone has a silicon-oxygen backbone with organic side groups. That backbone is why silicone holds properties from −60 °C to +200 °C, resists ozone and UV indefinitely, and stays flexible where organic rubbers harden and crack.

It is also why silicone is expensive, has poor tear strength and is difficult to bond. The same molecular structure that makes it stable makes it weak mechanically and hard to stick to anything.

Table 1 — Silicone against the two materials it most often competes with. The high gas permeability is a real limitation for sealing applications and is often overlooked.

Three processes, three geometries

Silicone parts are made by liquid injection moulding, by compression or transfer moulding, or by extrusion. Each process covers a different shape space, and the overlap is smaller than people expect.

Table 2 — Silicone processes and the geometry each covers. Extrusion is the only one that makes a continuous length; everything else makes discrete parts.

LSR injection moulding

Liquid silicone rubber is supplied as two components — A and B — that are pumped separately, mixed in a static mixer at a 1:1 ratio and injected directly into the mould. The material cures in the tool by platinum addition, typically in 30–90 seconds at 170–200 °C.

The metering system is the heart of the process. Two drum pumps feed a static mixer, and the ratio has to hold within about 1% or the cure is wrong — too little catalyst and the part is under-cured and sticky, too much and it cures in the mixer. Most systems monitor pressure on both lines and alarm on ratio drift.

Table 3 — LSR against high consistency rubber. LSR wins on cycle, labour and flash; HCR wins on tool cost and material price.

Cold runner systems are what make LSR economical at high volume. A cold runner holds the material in the runner at a temperature below cure — typically 20–30 °C — and injects through needle valves that open only for the shot. Without it, every shot wastes a cured runner that has to be thrown away, and on a small part the runner can be several times the part weight.

The material is shear thinning, which is a useful property. At the high shear rates in a small gate it flows easily, and once it stops it thickens and stays where it was placed. That is why LSR fills thin walls and fine detail better than its nominal viscosity suggests.

Compression and transfer moulding

Compression moulding is the oldest and simplest route. A weighed preform of uncured silicone is placed in the heated cavity, the press closes, and the material flows to fill the shape and cures under pressure.

It is labour intensive and it produces flash — the excess material has to escape somewhere, and it escapes into the parting line. Every part then needs deflashing, by hand, by cryogenic tumbling or by a second operation. That cost is invisible in the tool quote and very visible in the piece price.

Table 4 — Compression against transfer. Transfer moulding costs more in tooling and buys better material distribution and insert capability.

Transfer moulding is worth considering when the part has inserts or when the geometry makes a compression preform awkward. The material is pushed from a pot through runners into closed cavities, so it fills more evenly and the flash is thinner. It is still a batch process with manual loading, so the labour cost stays.

Silicone extrusion

Silicone profiles, tube, cord and sheet are extruded from HCR through a die and then cured continuously. The line is a cold-feed extruder, a die, and a curing tunnel — either hot air, infrared or a liquid salt bath at 200–250 °C.

The complication is that uncured silicone has almost no green strength. A hot-air tunnel supports the profile on a conveyor, but a soft or large section will deform under its own weight before it cures. Salt bath curing supports the profile by buoyancy and is the standard answer for large or complex sections.

Table 5 — Continuous curing methods for extruded silicone. Salt bath is the one that allows large hollow sections to hold shape.

Tolerances on extruded silicone are looser than on plastics, because the material shrinks 2–4% on cure and continues to move for a day afterwards. A ±0.3 mm tolerance on a 10 mm section is achievable; ±0.1 mm is not, and should be designed out or moved to a moulded part.

Curing: platinum or peroxide

Both cure systems are in use and they are not interchangeable. The choice affects cost, what the part can touch, and whether a post-cure is needed.

Table 6 — The two cure systems. Platinum cure is why LSR can be used in medical and food contact applications; the same chemistry makes it sensitive to contamination.

Platinum inhibition is the practical trap. A mould that previously ran a sulphur-cured rubber, or a glove contaminated with a sulphur-based release agent, will stop platinum silicone from curing — the part stays tacky no matter how long it stays in the tool. The rule is that platinum-cure silicone gets its own tools, its own handling gloves and its own clean area.

Tooling and cost

Tool cost is where the three processes diverge most, and it drives the volume at which each one makes sense.

Table 7 — Tooling comparison. An LSR cold runner tool costs several times a compression tool and pays for itself through cycle time and the absence of deflashing.

The crossover is usually somewhere between 2,000 and 10,000 parts a year, depending on part size and how much deflashing the compression route needs. Below that, compression or transfer is cheaper; above it, LSR's cycle time and zero-flash output dominate. Extrusion sits outside the comparison entirely because it makes a different product.

How to choose

Four questions settle it. Is the shape a constant cross-section? Then it is extrusion. Is it a discrete part with fine detail and a volume above a few thousand a year? Then LSR. Is it thick, simple and low volume? Then compression. Does it need inserts or a complex multi-cavity layout at moderate volume? Then transfer.

Two design rules apply to all of them. Radius every internal corner — silicone tears at sharp transitions, and a 0.5 mm radius can double the tear life of a feature. And do not design a thin flexible web and expect it to hold tolerance; silicone moves, and a 0.3 mm web will vary by more than its own thickness.