In silicone heaters with integrated thermostat, Fibreglass-Reinforced Silicone Rubber is the insulation system — and on a heater the insulation system is what sets the temperature limit, the flexibility and the dielectric strength. Its range is -50 °C to +200 °C continuous, +240 °C intermittent. What that means in service, what attacks it, and what to use instead above its ceiling are below.
Fibreglass-Reinforced Silicone Rubber as an Insulation System
Vulcanised silicone sheet with a woven glass carrier. Conformable, tear resistant, unaffected by moisture and ozone, and the default flexible insulation wherever the surface is not flat.
| Working range | -50 °C to +200 °C continuous, +240 °C intermittent |
|---|---|
| Total thickness in this construction | 1.5 mm |
| Dielectric withstand | 2500 V AC, 1 minute |
| Insulation resistance | greater than 100 Mu03a9 |
| Maximum size, one piece | 500 x 1000 mm (one piece) |
A flexible insulation has to do three jobs at once: hold off the supply voltage, survive the element temperature, and bend without cracking. Thickness trades against all three — a thinner build responds faster and conforms better, and has less material between the element and whatever the heater is touching.
The Temperature Ceiling, and What Sets It
Working range: -50 °C to +200 °C continuous, +240 °C intermittent. The ceiling belongs to the insulation. Nichrome will run red; fibreglass-reinforced silicone rubber will not, and it is the material around the element that decides what the part is rated at.
This construction is rated 200 °C continuous. That is a continuous figure at the element, not an ambient and not a surface temperature measured somewhere convenient — and it is not a figure that derating extends downwards from something higher. Past the ceiling the insulation degrades, the dielectric strength falls with it, and the failure is electrical rather than thermal.
The boundaries across this catalogue are all material boundaries: PET stops at 120 °C, filled epoxy at 150 °C, silicone rubber and polyimide at 200 °C, and above that only fired and inorganic systems — thick film on stainless or alumina, and mica — are candidates at all.
What Attacks Fibreglass-Reinforced Silicone Rubber
Silicone is permeable to water vapour and is attacked by concentrated acids and by some solvents; it is not a barrier layer.
Resistant is not sealed. The material shrugs off splash, ozone and humidity, and it is still permeable to water vapour over time; a heater that will be immersed or steam-cleaned needs a sealed edge, not just a resistant one.
What a Silicone Heaters with Integrated Thermostat Is
Silicone heaters with a bimetal thermostat or thermal cutoff vulcanised inside the same insulation, so the protection cannot be omitted during assembly.
Nichrome resistance wire, or an etched Inconel foil circuit, vulcanised between two sheets of fibreglass-reinforced silicone rubber. The construction is about 1.2 to 2 mm thick, conforms to a curved surface, and holds its dielectric strength from -50 °C up to 200 °C continuously. The sensor or thermostat is fitted under the same insulation and the assembly is tested as one part, so the temperature that is controlled is the one at the element rather than one measured somewhere downstream.
The heat leaves through the bonded face. Everything that is not in contact with the load radiates and convects from the back of the heater instead, which at these thicknesses is a poor path — so an air gap under part of the outline is not a small loss of efficiency, it is a hot spot at that spot.
| Element | nichrome wire or etched Inconel foil |
|---|---|
| Insulation system | fibreglass-reinforced silicone rubber |
| Total thickness | 1.5 mm |
| Maximum size, one piece | 500 x 1000 mm (one piece) |
| Continuous working temperature | 200 °C |
The continuous limit of 200 °C is set by fibreglass-reinforced silicone rubber. The element alloy would run far hotter; it is never the element that decides.
Above Its Ceiling: What to Use Instead
Silicone runs to 200 °C continuously and 240 °C intermittently; past that no polymer is a candidate. The next constructions up are thick film on stainless or alumina, which reaches 400 °C and higher watt densities, and mica-insulated elements where a rigid, clamped, high-temperature part is acceptable. Downwards, polyimide is thinner and faster where the surface is flat, and PET is cheaper where 120 °C is enough.
What the Insulation Changes About the Heater
The insulation decides the thickness (1.5 mm here), and the thickness decides the response: less material between the element and the load means less to heat before anything useful happens, and less thermal resistance in the way of the heat that follows. It also decides what the heater will wrap around — this construction takes a compound curve, not just a cylinder.
Watt Density: the Two Figures
Watt density, not wattage, is what the construction is limited by. 0.8 W/cm² unbonded and 5 W/cm² bonded to metal are the limits for this build — so if the wattage is fixed by the duty, the area is what has to move.
Where the required power will not fit into the available area at the free-air figure, the answer is a better thermal path rather than a thinner margin: bond the whole face down, or add a spreader plate. On 150 cm² the difference is roughly 12 W against 75 W, which is usually the whole argument.
Control and Protection
This heater has no way of limiting its own temperature. Resistance heating draws roughly the same power whatever the temperature, so with the load removed, the fluid gone, the fan stopped or the sensor open circuit, the element keeps producing watts until something stops it.
The design must therefore contain a control device and, separately, a protective one: a controller with a sensor to hold the set point, and a bimetal thermostat or a one-shot thermal cutoff wired in series with the element to open the circuit if the loop ever fails. The two are not interchangeable — a controller that has lost its sensor cannot protect anything, which is exactly the fault the cutoff exists for.
Sensors can be built into the part: NTC thermistor, PT100 or PT1000 RTD, or a K-type thermocouple, fitted under the same insulation and tested with the heater as one assembly. A sensor laminated into the heater reads the heater; a sensor on the load reads the load. Both are legitimate, and the drawing has to say which is wanted.
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How This Construction Fails
- Run at the bonded watt density in free air. The insulation reaches its limit within minutes and the element burns through where the local density is highest. Specify at 5 W/cm² only where the heater is genuinely bonded to metal over its whole area; unbonded, the figure is 0.8 W/cm².
- Voids under the bond. Air trapped during application, a contaminated surface or an adhesive that never reached full cure all produce the same result: a patch with no thermal path, running far above the rest of the heater, which is where the failure starts.
- Loss of the control loop. An open-circuit sensor, a welded relay contact or a controller in manual leaves the element at full power with nothing watching it — the case the series thermal cutoff exists for, and the reason it must not be omitted to save a part.
- Mechanical damage at the edge. Silicone is tough but it tears from a cut, and a heater trimmed to fit on assembly has had its edge seal opened; moisture then tracks along the glass carrier to the element.
Where This Construction Is the Wrong Answer
- Do not plan on running above 200 °C. That ceiling belongs to the insulation system, not to the element alloy, and it is not a figure that derating extends — past it the insulation degrades and the dielectric strength goes with it.
- Do not treat silicone as a moisture barrier. It is permeable to water vapour and is attacked by concentrated acids and some solvents; where the heater is immersed or splashed continuously, the sealing is a design item and not a property of the material.
How to Specify One
This is a made-to-drawing part, so there is no catalogue number to quote. A quotation is built from the following, and the more of it that arrives with the enquiry the fewer rounds it takes:
- An outline drawing, or a sketch with dimensions: overall size, holes, cut-outs, radii and the lead exit position.
- The supply voltage, and whether it is AC or DC. The element is wound for one voltage; it is not a switchable range.
- The power required, or — better — the temperature to be reached and the mass or flow it has to be reached in. Given the second, the wattage can be calculated instead of guessed.
- What the heater is attached to, and how. This is what decides which watt density figure applies, and it is the question a quotation cannot proceed without.
- The ambient, and the lowest temperature the assembly must start from. A heater sized for a 20 °C start is undersized for a -30 °C one.
- Duty cycle: continuous, intermittent, or a warm-up to a deadline. Time to temperature is a specification, not a by-product.
- Sensor and protection: which sensor, where it goes, and which limit device is fitted in series with the element.
- The certification the end product is built to, named on the enquiry rather than after approval.
- Quantity and target price, because the construction that is right for fifty pieces is often not the one that is right for fifty thousand.
- Whether adhesive backing is wanted, and what surface it goes onto — a low-energy plastic needs a different adhesive from bare aluminium.
Lead time: Samples 7-10 days from approved drawing; production 15-20 days.
Silicone Heaters with Integrated Thermostat — Capability Table
These are process limits for the construction, not the specification of one stocked item. Every line is what the factory can hold; what your part holds is fixed on your drawing.
| Parameter | Specification | Limit / note |
|---|---|---|
| Construction | Nichrome resistance wire wound on a fibreglass carrier, or etched Inconel foil, vulcanised between two sheets of fibreglass-reinforced silicone rubber | Decided at quotation from the outline and the wattage |
| Insulation system | Fibreglass-reinforced silicone rubber, both faces | This is what sets the temperature limit, not the element |
| Total thickness | 1.5 mm | Nominal, before adhesive; add 0.2 mm for PSA backing |
| Minimum size | 25 x 25 mm | Below this the element cannot be routed with a safe edge margin |
| Maximum size, one piece | 500 x 1000 mm (one piece) | Larger areas are supplied as tiled zones with separate circuits |
| Dimensional tolerance | u00b11.5 mm on dimensions up to 300 mm, u00b10.5%% above | On the die-cut outline; hole positions are held tighter |
| Supply voltage | 5 V DC to 240 V AC, wound to order | Wound to the ordered voltage u2014 it is not a switchable range |
| Resistance tolerance | u00b110% standard, u00b15% on request | Measured at 20 u00b0C; hot resistance differs with the element alloy |
| Watt density, free air | 0.8 W/cmu00b2 typical, 1.2 W/cmu00b2 maximum | Nothing conducting heat away u2014 the honest figure for an unbonded heater |
| Watt density, bonded to heat sink | 5.0 W/cmu00b2 maximum | Only valid with full-area contact to metal; specifying at this figure and running in free air destroys the heater |
| Continuous working temperature | -50 u00b0C to +200 u00b0C continuous (+240 u00b0C intermittent) | Of the heater itself, measured at the element, not the ambient |
| Dielectric strength | 2500 V AC, 1 minute | One minute, element to any exposed surface, no breakdown |
| Insulation resistance | greater than 100 Mu03a9 | At 500 V DC, measured after the dielectric test |
| Lead wire | Silicone-insulated stranded copper, 22 AWG to 16 AWG, PTFE where the exit is hot | Length, gauge and termination to your drawing |
| Attachment | Free, 3M 468MP or 9077 PSA backing, vulcanised to the part, or mechanically clamped | Adhesive-backed heaters need continuous contact pressure while curing |
| Sensor and control options | NTC 10K B3950, PT100/PT1000, K-type thermocouple, KSD9700 bimetal thermostat, thermal cutoff | Fitted under the same insulation and tested as one assembly |
| Certifications available | UL 499 construction, IEC/EN 60335, UL 94 V-0 materials, RoHS, REACH, CE | Constructions are built to these; the certificate covers the construction, not every outline |
| Thermostat | KSD9700 bimetal, 40 u00b0C to 150 u00b0C open point, u00b15 u00b0C | Self-resetting; a one-shot thermal fuse is fitted where reset must not happen |
| Wiring | Thermostat in series with the element, or on separate leads | In series is fail-safe and gives you no diagnostic; separate leads give the opposite |
Capability figures are this factory's process limits for the construction described. Watt density limits follow the published maxima for fibreglass-reinforced silicone rubber heaters (to about 12 W/cm² bonded to a heat sink, far less in free air); continuous temperature limits are those of the silicone insulation system, not of the element alloy.
