Silicone Extrusion: Cold Feed, Hot Cure and the Tunnel Behind It

What a silicone extrusion line looks like

A silicone extrusion line is shorter than a thermoplastic one and stranger. There is no long water tank, no vacuum calibration, and no haul-off pulling against a solidifying melt. Instead there is a cold-feed extruder, a shaping die, a hot-air tunnel several metres long, and a haul-off that has to pull a soft, uncured profile without stretching it.

The material arrives as a soft, uncured compound with the consistency of putty. It is pushed through the die by a screw that is cooled, because silicone cures with heat and any heat in the barrel would start the reaction before the material reached the die. The die shapes the profile, and the profile then enters the tunnel, where hot air at 200 to 250 °C vulcanises it in flight.

The result is a continuous process with no mould and no cycle time, which is why silicone extrusion is the cheapest way to make a long silicone profile. The constraints are different from thermoplastic extrusion: the profile is soft until it is cured, so it cannot be calibrated, and it distorts under its own weight in the tunnel.

Cold feed and the screw

The screw runs cold, usually with water through the barrel, and its job is to convey and compress rather than to melt. A silicone extruder is therefore short: a length-to-diameter ratio of 10 to 15 rather than the 25 to 30 of a thermoplastic machine, because there is no melting to do and a long barrel would only add heat and residence time.

The compression ratio is low, typically 1.2 to 1.5, because the material is already soft and a high compression would overheat it. The screw is often a simple single-flight design with a deep channel, and the die pressure is low compared with thermoplastic extrusion.

Temperature control on the barrel is critical and it is a cooling function, not a heating one. The mechanical work of the screw generates heat, and if the barrel cannot remove it the compound starts to scorch or cure on the screw. A silicone extruder that is running hot will show it as lumps in the profile or as a rough surface, and the fix is more cooling water rather than a different screw.

The tunnel sets the line speed

Vulcanisation is a time-at-temperature reaction, and the tunnel is where the time comes from. The line speed and the tunnel length together set the residence time, and the tunnel temperature sets how much time is needed. At 180 °C a typical HCR compound needs about 180 seconds, at 200 it needs 110, at 220 it needs 70 and at 250 it needs 40.

That is a factor of four and a half across the practical range, which means the tunnel temperature is the main lever on output. A 6 metre tunnel at 180 °C running at 2 metres a minute produces 120 metres an hour; the same tunnel at 250 °C runs at 9 metres a minute and produces 540.

The limit is not the cure chemistry but the material. Above about 250 °C the surface of the profile can degrade before the core is cured, and a profile with a hard skin and a soft core is worse than one that is uniformly under-cured because it looks acceptable and fails later. The practical ceiling for most compounds is 230 to 250 °C.

Table 1 — Tunnel temperature against cure time, line speed and output.

Section size limits the line

A thin profile cures quickly and a thick one does not, for the same reason as in moulding: heat has to conduct to the centre. A 5 mm² cross-section runs at about 30 metres a minute, a 10 mm² at 22, a 20 mm² at 15 and a 40 mm² at 9.

The relationship is not the square law of a moulded part, because the profile is passing through a tunnel rather than sitting in a hot mould, and because the surface is being heated continuously. But the trend is the same, and it means that a line running a thin profile and a line running a thick one are effectively different machines even if the hardware is identical.

This is why silicone extrusion lines tend to specialise. A line tooled for 6 mm cord runs at 30 metres a minute and needs a short tunnel; a line tooled for a 40 mm² window gasket runs at a third of that and needs a longer tunnel or a hotter one. Trying to run both on one line means running the thin profile far below its capability.

Distortion: the problem thermoplastic extrusion does not have

A thermoplastic profile is calibrated and cooled while it is stiff enough to hold its shape. A silicone profile is soft when it leaves the die and stays soft until it is cured, so it sags, flattens and twists in the tunnel. There is no vacuum calibration to hold it, because the material would be pulled into the vacuum slots.

The industry answers are support, speed and formulation. Support means a conveyor or a belt that carries the profile through the tunnel so it does not hang. Speed means getting it through the tunnel before it sags. Formulation means a compound with enough green strength to hold its shape, which is a property of the base polymer and the filler loading rather than something the line can change.

• Support the profile on a moving belt or a bed of rollers through the whole tunnel.
• Keep the die close to the tunnel entry, so there is minimal unsupported span.
• Use a profile design with a thick base or a flat back, so it can be supported on a stable face.
• Avoid thin, unsupported legs and long horizontal flanges, which will droop regardless.
• Cure fast at the entry to the tunnel, so the surface sets before the profile has time to sag.

Die design and the die swell

A silicone die is simpler than a thermoplastic die because there is no melt to balance and no shrinkage to compensate for in the same way. But it has its own problem: die swell. The compound is compressed in the die and expands when it leaves, so the die opening is smaller than the profile and the amount of swell depends on the compound, the screw speed and the die land length.

In practice the die is cut by trial. A first die is made at 80 to 90 percent of the target dimension, a profile is run, and the die is opened up in stages until the profile is right. That is why silicone tooling is cheap but slow to develop, and why a compound change mid-project means a die change.

Table 2 — The die features that decide whether a silicone profile runs.

Where silicone extrusion wins

Silicone extrusion is chosen for continuous profiles where the temperature resistance, the flexibility or the electrical properties matter, and where the length makes moulding impossible. Cord, tube, window gaskets, oven seals, cable insulation, and the bulb seals on doors and hatches are the classic applications.

It loses to moulding whenever the part is short, complex in three dimensions, or needs tight tolerances. A silicone extrusion can hold plus or minus 0.1 mm on a cross-section and a few millimetres on a cut length, which is fine for a gasket and not fine for a precision part.