Aluminium or Steel? Tooling for Silicone Compression Moulds

What a silicone mould actually has to do

A compression mould for silicone is a heated press tool. It is not a precision injection tool with a cavity pressure of 800 bar; it closes on a weighed blank of uncured rubber and squeezes it into shape at 5 to 20 MPa while the platens hold it at 165 to 175 degrees Celsius.

That changes the requirements. Wear is not the main enemy, because the compound is soft and filled rather than abrasive. What matters is heat transfer, dimensional stability at temperature, and a surface that releases without tearing the part.

It also changes the economics. Silicone parts are often low volume with many variants, so a tool that costs a third as much and can be re-cut when the design changes is worth more than a tool that lasts a million shots.

The result is that aluminium is far more common in silicone tooling than in thermoplastic injection tooling, and for good reasons that are mostly thermal.

Thermal expansion decides the tolerance

Aluminium expands at roughly 23 parts per million per degree Celsius. Steel expands at about 11 to 12. Over a 150 degree rise, that is a factor of two, and on a 200 mm cavity it is the difference between 0.72 mm and 0.33 mm of growth.

Both numbers are large compared with a typical silicone part tolerance. The point is not that aluminium is worse; it is that the cavity has to be cut for the temperature it runs at, not for room temperature, and the correction is twice as large for aluminium.

That correction is manageable when the tool runs at one temperature and one material. It becomes a problem when a tool is used for several compounds at different cure temperatures, because the same cavity is now being asked to be right at two different expansions.

Steel also has a lower and more predictable expansion, which is why a tight-tolerance part with a flatness or a bore callout usually ends up in steel even when the volume does not justify it.

Where aluminium wins

Heat-up time is the first win. Aluminium conducts heat roughly four times better than steel, so a 200 mm plate reaches 170 degrees in about 22 minutes where a steel plate takes 45. On a low-volume part with many short runs, that difference is paid back every time the press is started.

Cycle time is the second. Faster heat transfer through the plate means the cure is more even across the part, and the platen does not have to overshoot to keep the cavity face at temperature. Even cure matters more for silicone than fast cure.

Machinability is the third. An aluminium cavity can be cut, modified and re-cut in a fraction of the time, and design changes on a silicone part are common because the part is often a seal or a cover that gets revised.

The limit is pressure and repetition. Aluminium is softer, so parting lines and shut-offs wear and dent. At high closing pressures, or with a filled compound that is abrasive, an aluminium tool will lose its shut-off within tens of thousands of shots.

Where steel wins

Steel wins on shut-offs. A silicone mould seals on a thin land at the parting line, and that land has to survive repeated closing under pressure. Hardened steel holds a sharp land for a million shots; aluminium rounds it over and starts flashing.

Steel wins on thin walls and deep cores. A core that is 30 mm tall and 3 mm wide in aluminium will deflect under closing pressure, and the part comes out with a wall thickness that varies along its length.

Steel wins on surface finish retention. Silicone releases from a polished steel surface very predictably. Aluminium picks up dents and scratches that transfer into the part as small raised marks, and those are hard to polish out without losing the cavity form.

The cost of that durability is heat-up time and machining time. A steel tool for the same part costs more, takes twice as long to reach temperature, and is slower to modify when the design changes.

Table 1 — Tool material compared on the properties that matter for silicone.

Surface finish and release

Silicone releases from a smooth, hard surface. A polished steel cavity at Ra 0.2 to 0.4 micrometre releases cleanly and repeatedly. The same finish on aluminium is achievable but does not last, because the surface work-hardens and then scratches.

Texture is the opposite problem. A textured silicone part needs the texture in the tool, and texturing aluminium is possible but the texture wears faster. On a part where the texture is functional, such as a grip surface, steel is the safer choice.

Coatings help both materials. A thin hard coating on aluminium improves release and shut-off life at modest cost, and it is often the compromise that makes an aluminium tool viable for a mid-volume part.

Whatever the material, the parting line has to be vented. Silicone traps air in the same way thermoplastics do, and a tool with no vent shows it as short moulding in the last-filled corner rather than as a burn.

Choosing between them

Start with the tolerance. If the part has a bore or a flatness callout tighter than about 0.15 mm over 100 mm, steel is the default, because the expansion correction is smaller and more repeatable.

Then the volume. Under about 5,000 parts, aluminium with a coating is usually the right answer: it heats fast, it can be modified, and it will last the programme. Above 50,000 parts, steel pays for itself on shut-off life alone.

Then the shape. Thin upstanding cores, deep narrow slots and long unsupported sections all push toward steel regardless of volume, because deflection under closing pressure is a stiffness problem and aluminium has a third of the modulus.

Finally, the number of variants. A family of five similar seals with small differences is a natural aluminium programme, because five aluminium tools cost less than two steel ones and each can be re-cut when a variant changes.

Table 2 — Which tool material fits which programme.