Silicone Sheet Manufacturing: Calendered, Moulded and the Difference

Three processes, one material

Silicone sheet is sold by thickness, hardness, colour and temperature rating, and buyers treat those four as the specification. They are not. Two sheets that match on all four can behave completely differently in a gasket press, because one was calendered, one was moulded and one was extruded, and the process decides the things that are not on the data sheet: thickness distribution, surface, and how much the sheet moves when it is compressed.

Calendering passes compounded silicone between rolls, with the gap setting the thickness. Moulding presses a measured charge in a heated cavity. Extrusion pushes compound through a die and cures it in a tunnel, and sheet made this way is really a very wide profile. Each has a natural thickness range, a natural width, and a natural tolerance.

Achievable thickness tolerance: plus or minus 0.05 mm calendered, 0.15 mm moulded and 0.25 mm extruded, on a nominal 2 mm sheet. That is a five-fold spread, and it is not a quality difference. It is what each process physically controls.

Calendering wins on thickness and loses on everything else

A calender controls thickness with a roll gap, and a roll gap is a machine setting that holds to microns. That is why calendered sheet is the only route to a tight thickness tolerance over a large area, and why it dominates anything where the sheet has to be uniform: diaphragms, thin gaskets, sheet that will be die-cut into thousands of identical parts.

What calendering does not give you is a wide sheet with a good surface on both faces. The sheet is squeezed between two rolls, and it carries the roll finish on both sides, including any mark, and it carries the grain direction of the calendering operation. Calendered silicone is slightly anisotropic: it is a little stronger along the machine direction than across it, and a gasket cut from it will show that if the application is sensitive.

It also needs a compound that is stiff enough to stay on the roll, which means fillers. A highly filled calendered sheet is cheap and dimensionally excellent and has mediocre compression set. That trade is invisible on a data sheet that only lists hardness and temperature.

Why thick sheet cures slower than it looks

Cure time against sheet thickness: one minute at 0.5 mm, two at 1 mm, four at 2 mm, seven at 3 mm and twelve at 5 mm. The curve is not linear, and it is not proportional to thickness either, because heat has to travel through a material that conducts it poorly and that is also reacting as it heats.

The practical consequence is that thin sheet is a continuous process and thick sheet is a batch process. A 0.5 mm sheet can be cured in a tunnel in a minute and coiled at the end. A 5 mm sheet cannot: it needs a press, a long cycle, and a flat platen, because if it is cured under tension in a tunnel it will be cured unevenly through the thickness and will curl.

Uneven cure through the thickness is the defect that shows up weeks later. The surface is fully crosslinked and the core is not, so the sheet feels correct, measures correct, and then takes a permanent set under compression that is much larger than the data sheet value. A 5 mm sheet with a 30 second shortfall in cure time can show twice the specified compression set.

Moulded sheet: the only route to very thick

Above about 6 mm, calendering and extrusion both stop being practical, and sheet is moulded. A moulded sheet is a platen-sized part: a measured charge in a heated cavity, pressed, cured and demoulded. It is slow, it is limited to the press size, and it is the only way to make 12 mm silicone sheet that is cured all the way through.

Moulded sheet has the best compression set of the three, because the compound can be formulated without the flow aids that calendering needs and because the cure is uniform and can be long. It has the worst thickness tolerance, because the thickness is set by the closing of a press on a charge whose volume varies, not by a roll gap.

Table 1 — The three routes to silicone sheet, and what each one is actually good at

Reading a sheet specification

Most silicone sheet specifications are copied from a supplier catalogue and do not describe the sheet that will arrive. Four things are usually missing, and each of them decides whether the sheet works.

• The process, because it sets the tolerance the supplier can actually hold rather than the one they quoted.
• Thickness measured where: at the edge, at the centre, and with what pressure. A durometer and a caliper disagree by 0.1 mm on soft sheet.
• Compression set with the test conditions, because 22 hours at 175 C and 70 hours at 100 C give different numbers for the same sheet.
• Whether the sheet is post-cured, since a sheet that is not post-cured will continue to crosslink and to outgas in service.

The thickness point is the one that causes the most argument. Soft silicone sheet compresses under a caliper foot, so a 2 mm sheet measures 1.92 mm with a light touch and 1.85 mm with a firm one. If the drawing says 2.00 plus or minus 0.10 and the measurement method is not stated, the sheet will pass on one bench and fail on another.

Matching the sheet to the job

Table 2 — Choosing the process from the part rather than from the catalogue

The one thing not to do is to specify a tolerance tighter than the process can hold and then accept whatever arrives. A calendered tolerance quoted on an extruded sheet is a promise the supplier cannot keep, and the sheet that turns up will be at the edge of a distribution that was never centred on the drawing.

What to specify

A silicone sheet specification that can be enforced names the process and the measurement.

• Process: calendered, moulded or extruded, stated explicitly.
• Thickness with tolerance, and the measurement method, including the pressure on the foot.
• Hardness in Shore A, with the standard used, because 50 Shore A is not the same number on every scale.
• Compression set with the test conditions, not just a percentage.
• Post-cure, if the sheet will see temperatures above 150 C or must not outgas.
• Whether the sheet has a grain direction and whether it matters for the cut parts.

Then buy a sample and measure it before the production order. Silicone sheet is cheap enough that a sample roll is a rounding error, and the thickness distribution across a roll is the single number that predicts whether the die-cut parts will seal.