Polyimide (Kapton) Capillary Tubing: Thin-Wall Sheathing for Sensors and Micro Parts
When the wall has to disappear but the insulation still has to hold, polyimide is usually the only tube that fits. The two things that decide success are wall thickness control and moisture handling.
The short answer
- Polyimide is the thin-wall answer. Walls down to 0.013 mm and micro bores from 0.05 mm, which no fluoropolymer tube matches at the same dielectric strength.
- Continuous service 240 to 260 °C, short-term peaks to 400 °C. Dielectric strength is quoted around 160 to 200 kV/mm, well above PTFE at the same wall.
- Polyimide is hygroscopic. Unless it is conditioned or coated, it takes on moisture and the dielectric performance moves with humidity. State the environment before you specify.
- The wall is the whole point and also the whole risk. A thin wall protects the sensor and keeps response time short, but it also gives heat and mechanical damage a short path. Send the duty, not just the diameter.
Polyimide capillary tube, sold under the Kapton trade name, is a seamless, non-porous high-performance polymer tube. The reason engineers reach for it is a combination nothing else covers: an extremely thin wall at high dielectric strength, stable from cryogenic temperatures up to the 240 to 260 °C band, good radiation resistance, and dimensional stability that holds a micro bore in place. That puts it in thermocouples, RTDs, high-density wiring, micro coax insulation, coil forms, and precision guides in analytical instruments.
The trade is moisture and mechanical fragility. Polyimide absorbs water unless it is conditioned or coated, so an unprotected thin wall in a humid environment will not hold the dielectric figure printed in a data sheet. And a wall measured in hundredths of a millimetre will not survive a burr, a rough bore, or a careless assembly. Both of those are reasons to talk to us before you fix the drawing, not after the parts arrive.
Where polyimide sits against the alternatives
| Property | Polyimide (PI / Kapton) | PTFE tube | FEP / PFA tube |
|---|---|---|---|
| Practical wall floor | From ~0.013 mm on micro bores | ~0.25 mm typical | ~0.25 mm typical |
| Practical bore floor | From ~0.05 mm | ~1 mm | ~0.5 mm |
| Continuous service temperature | 240 to 260 °C | +260 °C | +200 to +260 °C |
| Short-term peak | Up to 400 °C | ~300 °C | ~300 °C |
| Dielectric strength (quoted) | 160 to 200 kV/mm | ≥ 25 kV/mm | ≥ 25 kV/mm |
| Moisture uptake | Hygroscopic, conditioning or coating advised | Negligible | Negligible |
| Radiation resistance | Very good | Fair | Fair |
| Abrasion and cut-through | Good at normal wall, weak at micro wall | Excellent | Very good |
| Toxic decomposition on overheating | None reported | Forms HF, limits use in enclosed hot spaces | Forms HF, same limitation |
| Stiffness | Stiff but springs back to small radii | Soft and flexible | Flexible |
Quoted figures vary by grade and wall. We confirm the actual bore, wall, tolerance and conditioning against your drawing rather than quoting a blanket number.
Pick this, skip this
Polyimide capillary is the right call when
- A thermocouple, RTD or thermistor needs a sheath that does not slow the thermal response
- The sensor assembly has almost no radial space and the wall has to be measured in hundredths of a millimetre
- The part sees radiation: nuclear, X-ray or high-energy instrument duty
- Overheating is a real risk and you cannot have toxic decomposition products in a sealed enclosure
- You need a seamless, non-porous micro bore for fluid or gas transfer in an analyser
- High-density wiring needs thin insulation between micro cables or micro coax
- The operating window spans both cryogenic cold and high heat in the same assembly
Polyimide is the wrong call when
- The environment is humid, condensing or immersion. Polyimide absorbs moisture and the dielectric figure drops. Use PTFE, or specify a coated or conditioned polyimide and state the test method
- The surface is abrasive, or the assembly involves sliding contact. Choose PTFE or a filled grade with better cut-through resistance
- You need a thick, mechanically robust wall. Above roughly 0.5 mm, PTFE and PEEK usually give better value
- The duty is continuous above 260 °C. Polyimide is not a 300 °C continuous material
- You need to bond the tube. Polyimide needs a primer or surface treatment; untreated PI is difficult to adhere
- The budget leads. Polyimide costs more per metre than PTFE for the same bore, and the premium is only justified by the thin wall or the temperature
Specifications we hold
| Parameter | Value |
|---|---|
| Material | Polyimide (PI), Kapton film basis, seamless drawn tube |
| Inner diameter range | From ~0.05 mm up to ~10 mm |
| Wall thickness range | From ~0.013 mm; 0.02 to 0.05 mm common |
| Continuous service temperature | -269 to +240 °C (grade dependent, up to ~260 °C) |
| Short-term peak | ~400 °C |
| Dielectric strength | 160 to 200 kV/mm (dry, wall dependent) |
| Tensile strength | 135 to 210 MPa (grade dependent) |
| Elongation at break | 50 to 70% |
| Moisture behaviour | Hygroscopic; conditioning or coating recommended for humid duty |
| Colour | Natural amber, clear, black, green; other on request |
| Standards cited | IEC 60672, ASTM D5487, IPC film basis, RoHS, REACH |
What goes wrong in the field
Specifying a dry dielectric figure for a wet environment
Quoted dielectric strength assumes dry polyimide. In humid or condensing service the material takes on water and the number moves. If the environment is wet, say so, and we quote a coated or conditioned tube against a stated test method.
A micro wall ruined by a rough bore
A 0.02 mm wall will not survive being pushed over a burr or a sharp shoulder. In practice the failure is a split that only appears after assembly. Deburr the substrate, chamfer the entry, or let us make the tube with a flared entry.
Treating polyimide like PTFE for flexing
Polyimide is stiff. It will spring back to a small radius, but it is not a continuously flexing sleeve. If the run has to flex in service, FEP or a braided sleeve is the right family, and PTFE is better than polyimide on chemical duty anyway.
Assuming the tube can be bonded as supplied
Untreated polyimide is hard to adhere to. Assemblies that pot, glue or over-mould around a PI capillary need a primer or a surface treatment step. Plan that into the process or the bond fails at qualification.
Under-specifying the tolerance, then blaming the tube
At micro bores the tolerance is a larger share of the dimension. If the assembly is an interference fit, a bore tolerance that looked generous on a 5 mm tube is not generous on a 0.5 mm one. Send the fit and the gage method.
Overlooking the temperature at the sensor tip
The tube may be rated to 240 °C continuous, but the sensor tip can see a spike well above that during a process upset. Tell us the peak, not just the setpoint, and we will confirm whether PI still fits or whether the duty has moved to a different class.
Related Engineering Guides
Components for this solution
Polyimide Capillary Tube for Temperature Sensors
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Polyimide Tubing
General-purpose PI tube for high-temperature insulation and micro assembly duty.
PTFE Tubes
Choose PTFE instead when the environment is wet, chemically aggressive or mechanically rough.
PTFE Tubing for Medical Devices
Medical-grade liner and shaft tubing with tight ID control.
Filament Wound Epoxy Fiberglass Tube
The thicker-wall structural alternative when the part also carries mechanical load.
Phenolic Kraft Paper Tubes
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Related solutions
Common questions
What is polyimide capillary tubing used for?
The main uses are thermocouple and RTD sheathing, high-temperature probe wire insulation, thin-wall insulation over fine sensor wires, micro coax and micro-cable insulation, coil forms, and precision guides or flow channels in analytical and biomedical instruments. The common thread is a very thin wall at high dielectric strength in a small space.
How thin can the wall go?
Walls down to roughly 0.013 mm are producible, with 0.02 to 0.05 mm being the common range for sensor sheathing. The practical floor depends on the bore, the drawing tolerance and how the part is handled during assembly, so we confirm the wall against your specific bore rather than agreeing to a number in isolation.
What is the smallest bore you can make?
Micro bores from around 0.05 mm are available, and standard micro capillary lines start near 0.07 to 0.10 mm ID. At those sizes the tolerance becomes a significant share of the dimension, so send the fit and the gage method as well as the nominal bore.
Is polyimide the same as Kapton?
Kapton is a DuPont trade name for polyimide film. Polyimide tube is made from the same polymer family and is often called Kapton tube in the trade. If your drawing names a specific film grade or manufacturer, tell us and we will confirm equivalency rather than substituting silently.
Why is polyimide better than PTFE for sensor sheathing?
Two reasons. The wall can be far thinner at much higher dielectric strength, so the sensor response time stays short, and polyimide is stable to a higher short-term peak. PTFE still wins on moisture, chemical resistance and mechanical toughness, so the choice depends on which of those dominates your duty.
Does polyimide absorb moisture?
Yes. Polyimide is hygroscopic and will take on water unless it is conditioned or coated. That is why a dry dielectric figure should not be applied directly to a humid environment. Tell us the humidity, condensing behaviour and any immersion, and we will state what conditioning or coating the duty needs.
Is polyimide tubing suitable for radiation environments?
Yes, radiation resistance is one of its distinguishing properties, along with dimensional stability under high-energy exposure. It is used in nuclear-instrument, X-ray and high-density electronics packaging where other polymers degrade.
Can polyimide tubing be bonded or potted?
Only with preparation. Untreated polyimide resists adhesion, so assemblies that glue, pot or over-mould around it need a primer or a surface treatment. Plan that step into the process before qualification, because retrofitting it after a bond failure is expensive.
How does polyimide behave at cryogenic temperature?
It stays dimensionally stable and does not embrittle in the way many polymers do, which is why it appears in assemblies that span cryogenic cold and elevated heat. If your duty crosses that range, say so, because it narrows the grade choices.
What tolerances can you hold on micro capillary?
Tight ID and OD control is the point of the process, but the achievable number depends on bore, wall and length. We confirm capability against your drawing and state the measurement method, rather than publishing a blanket tolerance that does not apply at every size.
Can the ends be machined, flared or cut square?
Yes. We cut to length, square the ends and can flare or chamfer the entry to protect a thin wall during assembly. Burred or skewed ends are the most common cause of a micro tube splitting on installation, so this step is worth specifying.
Do you supply test data for dielectric performance?
Yes. Dielectric strength and insulation resistance testing can be run on request, with batch traceability to the incoming material certificate. State the test method you want; dry and conditioned results are not interchangeable.
What do you need to quote a polyimide capillary job?
ID, OD, wall, length, tolerance, the fit at both ends, continuous and peak temperature, humidity and chemical exposure, whether the tube will be coated or conditioned, the assembly method, and annual volume plus prototype quantity.