Polyimide Tube Wall Thickness and ID Tolerance: Reading a PI Spec Sheet
Engineer's Guide · 2026-09-16 · 5 min read
A polyimide tube quotation tells you the outside diameter and a tolerance, and that is often where the information stops. For probe-card guides, test-socket liners and sensor sheaths, the outside diameter is the least important of the four dimensions. Bore, wall, concentricity and cut quality are what decide whether the part works, and only one of them appears on a typical catalogue line. This is how to read the sheet properly.
Four dimensions, and only one of them is usually quoted
| Dimension | How it is set | Why it decides the application |
|---|---|---|
| Inner diameter | Mandrel size for a wound tube, die and draw control for an extruded tube | Sets whether the part slides over a probe, a wire or a pin, and how much clearance the assembly has |
| Outer diameter | Bore plus twice the wall | Sets whether the part fits into the bore it has to sit in. It is a derived number, not an independent one |
| Wall thickness | Bore and OD together, or the winding build | Sets dielectric strength, stiffness and how much the part resists buckling during insertion |
| Concentricity | Process control, not a dimension you can specify away | An off-centre bore gives a thin side. The part measures correct and insulates unevenly |
The consequence is simple. A tube quoted only by outside diameter cannot be manufactured to the bore your socket needs, because the supplier has no idea what the bore has to be. Specify ID, OD and wall together, and let the supplier tell you which two of the three are controlled and which one floats.
The tolerance trade you cannot avoid
On a thin wall, tolerance spending is a zero-sum exercise. Tightening the bore while holding the outside diameter fixed tightens the wall by the same amount. Tightening all three at once is what drives the part into a different process.
- Tight bore, free outside diameter: the usual choice for a liner or a guide, where the inside is the functional surface.
- Tight outside diameter, free bore: the choice for a part that presses into a housing, where the outer surface governs.
- Tight wall: needed where dielectric strength is the point. It usually comes with a wider bore or OD band, because the process controls the wall, not the diameters.
- Everything tight: possible, but it moves the part to a controlled mandrel process and a longer qualification, and the price reflects it.
Extruded or wound, and what it changes
Polyimide tube reaches the market by two routes. Extruded tube gives the more consistent thin wall in micro sizes, because the wall is formed uniformly around the die rather than built up in layers. A wound tube is built on a mandrel from film, which suits larger diameters and thicker sections and can hold a bore accurately if the mandrel is right.
| Process | Where it wins | Where it gives ground |
|---|---|---|
| Extruded | Micro bores, thin walls, consistent wall around the circumference, higher output on a fixed size | Large diameters, and wall thickness beyond what the die and draw can hold |
| Wound | Larger diameters, thicker walls, accurate bore from the mandrel, small batches | Wall uniformity on very thin sections, and the seam has to be controlled |
Cut quality is a functional specification
On a 0.1 mm wall, the cut is a feature. A burr that would be invisible on a 3 mm pipe becomes a raised edge that fouls a probe or a pin, and a ragged cut sheds particulate into a cleanroom or a semiconductor tool. A sheared cut leaves a lifted lip on one side of the tube and nothing on the other.
- Ask what the cut method is, and ask for the burr height, not a statement that the part is deburred.
- Specify end squareness on any tube that seats against a shoulder or a ferrule.
- For cleanroom or semiconductor use, ask for a particulate and cleanliness statement on the cut parts, not on the extruded length.
- Agree the inspection method for the cut in advance, because a visual check means different things to different inspectors.
What to put on the drawing
- State the functional surface and its tolerance, then set the other two dimensions to sensible bands.
- State concentricity as a maximum wall variation, which is the form an inspector can measure.
- State the cut method, burr limit and end squareness.
- State the temperature the part sees in service, because polyimide tube is quoted at around 260 C continuous and that figure is grade dependent.
- State any cleanliness or particulate requirement, and the certificate you want with the batch.
- Name the process if you have a reason to, and leave it open if you do not. Pushing the wrong process on a supplier usually raises the price.
Where it fits in the wider build: our polyimide capillary tubing route gathers every part that goes into it.
Products for this job
These are the polyimide tubing routes we run, from sub-millimetre bore to sensor-sheath wall. Send the drawing and the duty, and we will confirm the grade and the wall against both.
Related reading
- Polyimide Capillary Tube for Thermocouples and RTDs
- Polyimide Tube in Radiation and Semiconductor
- PEEK vs PI (Polyimide) vs PTFE
- How to Specify a Custom Fuse Tube
Common questions
What tolerance can a polyimide tube hold?
It depends on the bore, the wall and the process. Micro bores with thin walls are held tighter than large diameters with heavy walls. State the dimension that matters functionally, and expect the other two to float. A supplier will confirm the achievable band on your specific size rather than quote one number for the range.
Why can I only get a tolerance on the outside diameter?
Because the tube was made to a stock outside diameter. If the bore is your functional surface, ask for it to be specified and quoted directly. A tube quoted only by OD cannot be made to the bore you need.
What is a reasonable concentricity figure?
It is expressed as a maximum wall variation, and it is set by the process rather than requested. For probe-card and sensor work it is the dimension that decides whether the part insulates evenly, so state the maximum wall variation you can accept and ask what the process delivers on your size.
Extruded or wound polyimide tube?
Extruded for micro bores and thin walls with consistent wall around the circumference. Wound for larger diameters and thicker sections, where the mandrel sets an accurate bore. We advise by application rather than by preference.
Does cut quality matter that much?
On a thin wall, yes. A burr or a lifted lip becomes a functional defect: it fouls a probe, changes the fit and sheds particulate. Specify the cut method, a burr limit and end squareness, and agree how the cut is inspected.
What temperature is polyimide tube rated for?
Around 260 C continuous, grade dependent. The material's full span is much wider, and our sensor capillary is quoted from minus 269 C to plus 400 C. Tell us the service temperature and the duration at the top of the range and we confirm the grade.
Can you supply small quantities?
Yes. Small-batch and mixed-size orders are normal for polyimide work, which is one reason the material suits probe-card and instrument builds. Send the drawing and the quantity and we confirm the route.
How do I get a quotation against a drawing?
Send the print with ID, OD, wall, the tolerance on your functional surface, the cut specification and the quantity. WELLELE makes polyimide tubing for probe cards, test sockets, motor insulation and precision sensor work to a stated ID and OD tolerance with controlled concentricity. We confirm capability against the drawing rather than a catalogue size.
Specifying this part?
Send the drawing for a manufacturability review.