Polyimide Film (Kapton type) is chosen for standard polyimide film heaters before anything else about the part is decided. An 0.05 to 0.125 mm film laminated over an etched copper or Inconel foil circuit. Total thickness 0.2 to 0.4 mm, very low thermal mass, and dimensionally stable enough to be bonded to optics. The consequences — thickness, flexibility, what it survives and what it does not — follow below.
Polyimide Film (Kapton type) as an Insulation System
An 0.05 to 0.125 mm film laminated over an etched copper or Inconel foil circuit. Total thickness 0.2 to 0.4 mm, very low thermal mass, and dimensionally stable enough to be bonded to optics.
| Working range | -200 °C to +200 °C continuous, +220 °C short term |
|---|---|
| Total thickness in this construction | 0.25 mm |
| Dielectric withstand | 1500 V AC, 1 minute (2500 V AC on the 0.125 mm film build) |
| Insulation resistance | greater than 100 Mu03a9 |
| Maximum size, one piece | 300 x 500 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: -200 °C to +200 °C continuous, +220 °C short term. The ceiling belongs to the insulation. Nichrome will run red; polyimide film (kapton type) 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 Polyimide Film (Kapton type)
Polyimide absorbs moisture and hydrolyses in continuous steam; it also tears from a nick, so the outline needs radiused corners rather than sharp ones.
Treat moisture as a design case rather than an environment. Continuous steam, repeated autoclave cycles and standing condensation are three different duties, and the material only tolerates the mildest of them; where the heater cannot be kept dry, the edge seal and the lead exit are the two places to specify.
What a Standard Polyimide Film Heaters Is
The baseline PI heater: an etched foil circuit between two polyimide films, 0.25 mm total, flexible enough to wrap a 5 mm radius and thin enough to disappear into an assembly.
A photo-etched copper or Inconel foil circuit laminated between polyimide films, 0.2 to 0.4 mm thick in total. It is the thinnest and fastest flexible heater made: almost no thermal mass of its own, so what it heats is the part rather than itself. It will wrap a cylinder; it will not take a compound curve, and a nick in a 0.05 mm film propagates from the edge, so outlines are drawn with radiused corners.
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 | photo-etched copper or Inconel foil |
|---|---|
| Insulation system | polyimide film |
| Total thickness | 0.25 mm |
| Maximum size, one piece | 300 x 500 mm (one piece) |
| Continuous working temperature | 200 °C |
The continuous limit of 200 °C is set by polyimide film. The element alloy would run far hotter; it is never the element that decides.
What the Insulation Changes About the Heater
The insulation decides the thickness (0.25 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 wraps a cylinder but will not take a compound curve.
Attachment and Thermal Contact
Attachment options are a factory-applied pressure-sensitive adhesive, vulcanising or bonding to the customer's part, or mechanical clamping with a plate or a strap.
A PSA bond wants a clean, dry, degreased surface, continuous pressure while it wets out, and no trapped air: a bubble under the element is a spot with no thermal path, and it will be the first place the heater fails. Acrylic adhesives reach full strength over 24 to 72 hours, so the assembly is not at its rated watt density the moment it is pressed down.
On a 0.2 mm film the adhesive is a meaningful fraction of the total stack and of the thermal resistance. It is also the temperature limit of the assembly in most cases: acrylic PSA is comfortable to about 150 °C, well short of what the polyimide itself will take, so a 200 °C duty needs a silicone adhesive or a bond made by the customer.
From the capability table: Free, 3M PSA backing, or bonded with a two-part thermally conductive adhesive.
The bond line is part of the thermal circuit and should be specified like one. A 0.05 mm acrylic PSA layer conducts roughly 0.2 W/m·K; a filled silicone adhesive is two to five times better, and a bolted joint with an interface pad better again. Whatever is chosen, it has to be continuous — the heater cannot spread heat sideways to cover a gap, and the temperature under a void rises until something in the stack gives way.
Watt Density: the Two Figures
This construction is rated at 0.5 W/cm² in free air and 5 W/cm² bonded over its whole area to metal. The second figure is 10 times the first, and the difference is not a safety margin — it is the heat sink. On a 100 x 150 mm outline, which is 150 cm², that is about 75 W hanging in air against about 75 W bonded down.
Specify at the figure that matches how the heater will actually run. A heater ordered at the bonded density and then switched on before it is attached — on the bench, during a test, after it has lifted off in service — reaches its insulation limit in minutes, because there is nowhere for the heat to go but through the insulation.
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.
T
h
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 specify polyimide for continuous steam or repeated autoclave cycles — it hydrolyses. Do not fold it to a tight radius, and do not put a sharp internal corner on the outline.
- Do not rely on the insulation as a seal in a wet installation. Where washdown or condensation is expected, the assembly is sealed at the edge and at the lead exit, and both are drawing items.
Leads and Terminations
Lead wire as supplied: PTFE or silicone insulated 24 AWG to 20 AWG, or an FFC tail etched as part of the circuit. Length, gauge, exit position and termination are drawing items — the exit position in particular, because it decides where the heater can be routed and it cannot be changed after the tool is cut.
The lead exit is a thermal discontinuity as well as an electrical one. The joint between a fine element and a stranded lead is the hottest point of many heaters, so it is kept out of the highest watt density zone, strain relieved, and — where the exit sits in a hot area — insulated in PTFE rather than in silicone.
On a 0.2 mm film the lead joint is thicker than the heater. Where the assembly has to stay flat, the leads are brought out to a tail or a flat flexible tab rather than soldered onto the heated area.
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.
Standard Polyimide Film Heaters — 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 | Photo-etched copper or Inconel foil circuit laminated between polyimide films with a high temperature acrylic or FEP adhesive | Decided at quotation from the outline and the wattage |
| Insulation system | Polyimide film, 0.05 mm or 0.125 mm per face | This is what sets the temperature limit, not the element |
| Total thickness | 0.25 mm | Nominal, before adhesive; add 0.2 mm for PSA backing |
| Minimum size | 10 x 10 mm | Below this the element cannot be routed with a safe edge margin |
| Maximum size, one piece | 300 x 500 mm (one piece) | Larger areas are supplied as tiled zones with separate circuits |
| Dimensional tolerance | u00b10.5 mm on dimensions up to 200 mm | 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 | 0.5 W/cmu00b2 typical, 0.8 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 | -200 u00b0C to +200 u00b0C continuous (+220 u00b0C short term) | Of the heater itself, measured at the element, not the ambient |
| Dielectric strength | 1500 V AC, 1 minute (2500 V AC on the 0.125 mm film build) | 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 | PTFE or silicone insulated 24 AWG to 20 AWG, or an FFC tail etched as part of the circuit | Length, gauge and termination to your drawing |
| Attachment | Free, 3M PSA backing, or bonded with a two-part thermally conductive adhesive | 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, RoHS, REACH, CE, ISO 13485 QMS | Constructions are built to these; the certificate covers the construction, not every outline |
| Minimum bend radius | 5 mm, static; 25 mm for repeated flexing | Repeated bending across the element is what cracks a foil circuit |
| Thermal response | Full temperature within 5 to 20 seconds | Very low thermal mass u2014 the reason PI is chosen for anything that cycles |
Capability figures are this factory's process limits for etched-foil polyimide construction. The 200 °C continuous limit and the watt density ceiling (to about 7.75 W/cm² with heat sinking) are properties of the polyimide film and the adhesive system, and are published consistently across the industry.
