Out-of-Round Tubes and End Delamination: Thin-Wall Wound Tube Defects
Engineer's Guide · 2026-09-16 · 5 min read
A wound tube that arrives out of round, or with a lifted edge at the end, is usually reported as a material fault. It almost never is. The six defects below account for most thin-wall wound tube rejections we see, and each one traces to a specific stage: winding tension, cure, mandrel removal, or machining. Knowing which stage produced the symptom is what turns a rejected lot into a corrected process.
The six defects and where they come from
| Symptom | How it presents | Root cause | Fix |
|---|---|---|---|
| Out of round, oval across the diameter | The tube will not pass a ring gauge, or rattles in a close bore | Cure shrinkage against an insufficiently supported mandrel, or handling before the part has cooled and stabilised | Cure under support, control the thermal ramp, and keep the part on the mandrel until it is dimensionally stable |
| Bore eccentric to the outside diameter | Wall measures thin on one side and thick on the other | Mandrel run-out, or the outside diameter machined from a centre that does not follow the bore | Machine from the bore, not the outside diameter, and verify mandrel straightness before each run |
| Lifted or frayed edge at the cut | A visible lip or loose glass fibres at the end face | A worn or dull tool rubbing rather than shearing, with no bore support behind the cut | Sharp tooling on a change schedule, bore support during the cut, and a controlled deburr that removes lifted fibre |
| Delamination between layers at the end | The wall separates into layers when the end is machined or when a fitting is pressed on | Machining introduced a crack at the free edge and it propagated along the interface | Reduce the depth of cut at the end, support the bore, and seal or radius the free edge so the crack has no start point |
| Void or dry patch in the wall | Visible porosity or a light patch on a sectioned part | Wetting failure during winding, from resin viscosity, tension or a contaminated roving | Control resin viscosity and roving tension, and verify the fibre is not contaminated before winding |
| Spiral crack following the winding angle | A crack that runs along the fibre direction rather than across it | Thermal mismatch on a rapid cooldown, or the part loaded in the weak direction during handling | Slow the cool-down ramp and support the part through every handling step |
Why thе bore runs eccentric
On a wound tube the mandrel sets the bore directly, so the bore is as straight and as round as the mandrel. The outside diameter is then either left as wound or machined concentric to the bore. If the outside diameter is machined from the outside in, referenced on the outside rather than on the bore, the wall thickness follows whatever run-out was in the mandrel. That is how a part ends up with a thin side that no one saw at incoming inspection.
The machining rule for thin walls
Composite delamination starts when the cutting edge rubs instead of shearing. On a thin wall there is very little material to absorb that mistake, and the damage does not always show until the part is in service.
- Support the bore before the tool engages. An unsupported thin wall deflects away from the cutting edge and the cut goes out of tolerance.
- Take a lighter depth of cut at the free end than in the body of the part, because the free edge has the least support.
- Run sharp tooling on a change schedule rather than on failure. A worn edge rubs.
- Deburr deliberately and inspect for lifted glass under magnification, not by eye at arm's length.
- Measure the bore and the end-squareness after machining. Those two dimensions are what the assembly depends on.
Separating a process fault from a design fault
| Observation | Likely a process fault | Likely a design fault |
|---|---|---|
| Every part in the batch is out of round by a similar amount | Yes, look at mandrel support and the cure ramp | No |
| Some parts are fine and some are not | Yes, look at handling and cooldown, or at mandrel condition changing across the run | No |
| The wall is thin on one side in every part, in the same orientation | Yes, mandrel run-out or a machining datum problem | No |
| The part fails only after assembly, and only at one station | No | Yes, the assembly load or the support in the fixture is the problem |
| The part meets dimension and fails electrically | Possibly a void or dry patch in the wall | Possibly the wall is too thin for the dielectric duty |
| The end delaminates only on parts that are pressed into a housing | No | Yes, the interference is too high or the lead-in is missing |
What to do with a rejected lot
- Section one part and measure the wall at several points around the circumference. The pattern tells you whether the cause is the mandrel or the machining.
- Check the batch rather than the part. A single bad part is handling; a whole batch is process.
- Check the mandrel before blaming the material. Run-out and straightness are the usual source of an eccentric bore.
- Review the cooldown ramp if roundness is the issue, since thermal shrinkage against support is what moves a section.
- Once the cause is fixed, verify on a sample lot before releasing the balance of the order, and section one of the new parts.
Seen in context: the thin-wall filament wound micro tube route brings together every part that goes into this job.
Products for this job
These are the wound tubes we run, from thin-wall micro tube up to larger load cases. Describe the failure and the process that produced it, and we will point at the variable responsible.
Related reading
- Thin-Wall Filament Wound Tubes Below 0.5 mm
- Filament Wound vs Pultruded Fiberglass Tube
- G10 vs FR-4 Epoxy Fiberglass
- How to Specify a Custom Fuse Tube
Common questions
Why is my filament wound tube out of round?
Cure shrinkage against insufficient mandrel support is the usual cause, followed by handling the part before it has cooled and stabilised. The fix is a controlled cure under support and keeping the part on the mandrel until it is dimensionally stable. A whole batch showing the same ovality points at the process rather than at handling.
Why is the wall thin on one side?
The bore is eccentric to the outside diameter. Either the mandrel had run-out, or the outside was machined from a centre that did not follow the bore. Machine the outside diameter from the bore as the datum, and verify mandrel straightness before each run.
What causes end delamination?
A cutting edge that rubbed instead of shearing, with insufficient support behind the cut. That starts a crack at the free edge, and it propagates along the interface between layers. Lighter depth of cut at the end, bore support and sharp tooling on a change schedule prevent it.
How do I specify roundness and concentricity?
As a maximum wall variation around the circumference, which an inspector can measure. A concentricity ratio is open to interpretation, and on a thin wall the interpretation is where the argument happens. State the number you can accept and ask what the process delivers at your size.
Can a delaminated end be repaired?
No. A delamination at the free edge has already separated the layers, and any fill or seal hides the defect without restoring the load path. Scrap the part and fix the machining operation.
Does the winding angle affect roundness?
It affects the stiffness in each direction, and a wall that is stiff in one direction and soft in another behaves differently during cure and during handling. The winding angle should follow the load case, and roundness is then a cure and support question.
What should I check on incoming parts?
Bore and outside diameter at several points, wall variation around the circumference, end-squareness, and a visual check on the end faces for lifted fibre. If the part is critical, section one from each lot and look at the wall for voids and at the ends for separation.
Who do I talk to about a problem on a running part?
Send the drawing, the measured dimensions, a photograph of the end face and the assembly step where it fails. WELLELE makes filament wound epoxy fiberglass tube with controlled concentricity and machined, deburred ends, and we check bore, wall, end-squareness and edge quality before the lot ships.
Specifying this part?
Send the drawing for a manufacturability review.