HCR Solid Silicone Rubber: Mixing, Moulding and the Cure Curve That Decides Everything

What HCR is, and what arrives at the mixer

High-consistency rubber is silicone gum with a filler, a structure control additive and a curative. The gum itself is a polydimethylsiloxane with a molecular weight high enough that the material behaves like a very stiff putty rather than a liquid. Uncured viscosity is in the millions of pascal-seconds.

The filler is almost always fumed silica at 20 to 45 parts per hundred of rubber. It is not there to cheapen the compound, as calcium carbonate is in PVC. Fumed silica forms a network of hydrogen bonds through the gum that gives uncured silicone its green strength and cured silicone its tear strength. Remove it and the material will not hold shape before cure and will tear after.

The structure control additive — usually a hydroxyl-terminated siloxane — stops that same network from continuing to build during storage. Without it, a compound that was workable on Monday is crumbly by Friday. This is why HCR has a shelf life and why the date code matters more than most buyers expect.

Table 1 — What is in an HCR compound. phr is parts per hundred of rubber, the standard way rubber formulations are written.

Mixing on a two-roll mill

The compound is made on a two-roll mill or in an internal mixer. The gum is banded on the hot roll, the filler is added gradually, and the sheet is cut and re-fed repeatedly until the filler is dispersed. It is a craft process with a measurable endpoint, and the endpoint is dispersion, not time.

Poor dispersion shows up as white specks in the cured part, and more importantly as weak points. An undispersed silica agglomerate is a stress concentration in a material that has no other way to stop a tear. A part that tears at a location with no geometric reason is usually a dispersion problem, not a design problem.

The mill also adds heat history. Every pass raises the compound temperature, and above about 100 °C the curative starts to work prematurely. The rolls are water-cooled for a reason, and a mill running too fast will produce compound that is partly scorched before it ever reaches the press.

The cure curve: scorch and t90

A curemeter heats a small sample at a fixed temperature and records torque as the compound crosslinks. The trace gives two numbers that between them describe the whole process.

Scorch time is how long the compound stays workable. It sets how long you have to fill the cavity and close the press. A compound with a 30-second scorch time in a mould that takes 45 seconds to close will produce parts with flow marks and weak knit lines, because the material started to cure before it finished moving.

t90 is the time to reach 90 percent of full torque. It is the minimum cure time, and it is a minimum in the strict sense: a part demoulded at t90 has 90 percent of its properties, which for a seal means a compression set 30 to 50 percent worse than a fully cured one.

Both numbers move with temperature, and they move in opposite directions from what an operator wants. Raising the mould temperature shortens t90 and shortens scorch time by more, so the process gets faster and less forgiving at the same time. This is why silicone moulding has a practical temperature ceiling around 180 °C even though the material would tolerate more.

Table 2 — Cure parameters for a typical 50 Shore A HCR compound. Practical cure is t90 plus 20 percent, which is what most shops actually run.

Compression moulding: the four-step cycle

Compression moulding is the default for HCR because it is simple, tolerant of geometry and cheap to tool. The cycle has four steps and each has a failure mode.

1. Preform. A weighed blank of compound is placed in the cavity. Too little and the part is short; too much and the flash is thick enough to distort the parting line. Preform weight tolerance should be ±2 percent.
2. Close and breathe. The press closes slowly, stops just short, and opens a millimetre to let trapped air escape. Skipping the breathe produces parts with voids at the last point to fill, and the void is always in the same place.
3. Cure. Held at temperature and pressure for the practical cure time. Pressure must be high enough — 5 to 15 MPa on the cavity area — or the compound will not fill thin sections before it scorches.
4. Demould. The part comes out hot and soft. It must be handled flat and allowed to cool unrestrained, or it will set in whatever shape the handler gave it.

The breathe step is where inexperienced shops lose the most parts. A silicone part with a 3 mm section and a deep core needs two breathes, not one, because the air has to travel further. The symptom is a void or a weld line at the bottom of the deepest feature, and it appears even when every other parameter is correct.

Transfer moulding, and when it beats compression

Transfer moulding puts the compound in a pot above the cavities and forces it through runners by a plunger. The material enters the cavity already under pressure and already hot, which solves three problems at once: air is pushed out rather than trapped, thin sections fill, and inserts are held in place by the flow rather than by the operator.

The cost is a more expensive tool and more scrap, because the runner and the pot caddy are cured with the part. For a part with a 1 mm wall, a deep core, or a metal insert that must not move, transfer is usually the only process that holds tolerance. For a simple flat gasket, compression is cheaper and just as good.

Table 3 — Compression against transfer moulding for HCR. The decision is usually made by the part, not by the volume.

Post-cure: the step that decides compression set

A demoulded silicone part is not finished. Peroxide-cured compounds leave acidic residues and volatile fragments that continue to affect the part for weeks. Platinum-cured compounds have fewer residues but still contain volatiles from the filler treatment.

Post-curing at 200 °C for 2 to 4 hours drives those volatiles out. The visible effect is a slight shrinkage and a change in hardness of one or two points. The functional effect is on compression set: a 22-hour test at 175 °C gives 24 percent on a post-cured part against 46 percent on one that was only press-cured.

For a static gasket that never moves, that difference is a longer service life. For a dynamic seal or a part that must hold force at temperature, it is the difference between passing and failing the specification. Post-cure is cheap — an oven and time — and skipping it is the most common false economy in silicone moulding.

Reading defects from the cure curve

Table 4 — HCR defects mapped to the process step that causes them. The cure curve explains more than half of them.

The one that is misread most often is a tear that appears in the same place every time but has no geometric reason. It is nearly always a knit line: two flow fronts met after the compound had already started to scorch, and the joint between them never formed. The fix is not a stronger compound; it is a faster fill or a cooler mould, so that the fronts meet while the material can still bond.