Bond-line pressure, read before cure
A press reporting the correct total force can still leave half a bond line starved. The number that matters is pressure along the line — and it has to be read while the adhesive is still workable, or the reading cannot change the outcome.
Force is not pressure, and pressure is not uniform
On a spar cap the adhesive layer is the load path between shell and spar. If it is starved in one stretch and over-thick in the next, the joint does not fail evenly — it starts where the gap was never controlled.
A press applying the correct total force can still leave part of the line under-filled, because the force divides according to local stiffness, not according to intent. The usual correction is a shim routine: press, open, look at the squeeze-out, add or remove a shim, press again. That routine does not remove the problem, it moves it along the line — and every iteration costs a cure cycle.
Four decisions that make the reading usable
Film under the adhesive
The sheet is laid on the bonding flange and the adhesive applied over it, so each point records the pressure the adhesive actually sees — not the force the press reports.
Calibrate per sheet
Applied pressure is calibrated sheet by sheet and the values stored with the project. That is what keeps runs comparable months apart, which is the whole point of a baseline.
Close at production speed
A slow, museum-speed closure flattens exactly the gradient the check is looking for. The press closes at the rate used in production, otherwise the measurement is of a process you do not run.
Read before cure
The map is captured while the adhesive is still workable. After cure the data is an autopsy, not a correction.
The step that is usually skipped is the second one. Without per-sheet calibration a map is a picture; with it, two campaigns a year apart can be compared as numbers, and a process change can be judged instead of argued about.
Why a strip-shaped measurement wants Config C
| Build | Points | Sheets | Collectors | Why you would pick it |
|---|---|---|---|---|
| Config A | 40,960 | 10 × 490 × 520 mm | 10 × PXS064mini | Finest granularity over the full area. Best when the gradient you are hunting is shorter than 10 mm and you cannot predict where it sits. |
| Config B | 20,480 | 5 × 490 × 1040 mm | 10 × PXS064mini | Half the seams of A, collectors small enough to mount off the part. The usual choice for a full-field panel test. |
| Config C | 20,480 | 5 × 490 × 1040 mm | 5 × PXS128 | One collector per long sheet, so fewer boxes and cable runs on the fixture. Suits strip-shaped measurements such as a bond line. |
All three builds cover the same 1040 × 2480 mm surface. A long sheet still carries 4096 points (64 × 64), so its pitch along the long axis is roughly 16 mm against 7.6 mm across — a trade that suits a narrow band and not a full field.
A bond line is a narrow band. The 4096-point grid per sheet is more than enough across its width, and the coarser pitch along the long axis is the right trade — which is why Config C, with one PXS128 per long sheet, is the usual build here. Five collectors instead of ten means five cable runs on a fixture that is already crowded, and fewer boxes that can be knocked during closure.
What the map showed
The bond line was not uniformly loaded. Pressure fell away over specific stretches while other areas sat well above the average. Because the whole line was covered in one session, the low regions could be named and treated individually instead of adding shims along the entire cap.
The workflow difference is the point: an iterative press–open–shim loop became a measurement taken before cure, on which a correction could be decided in a single pass.
The same question, already measured
Configuration, method and finding are published as delivered; the client is not named.
Bond-line pressure during spar cap bonding
Adhesive pressure mapped along the bond line before cure, replacing a trial-and-error shim routine.
Where this setup stops working
Every one of these has cost somebody a re-test. They are listed here rather than discovered on site.
One reading is not the whole cure
Adhesive rheology changes with temperature and time. The map is a snapshot of the moment it was taken, not an integral over the cure cycle.
0.3 mm is part of the joint
In a thin bond line the film thickness itself matters. It has to be designed for, or the film removed after the check — there is no third option.
Pressure is not thickness
Squeeze-out carries adhesive away from the measured point, so the map shows loading, not the final bond-line geometry. Reading it as a thickness measurement is a common and expensive mistake.
Quasi-static only
20 Hz suits closure and hold. It does not resolve impact, snap-through or a rapid press fault.
Re-use needs inspection
A sheet that has been through a full cure cycle is inspected and recalibrated before the next run. Adhesive residue is a measurement error, not a cosmetic problem.
The parts this measurement is built from
Film, collectors and software are quoted as one chain — the sheet count follows the surface.
Config C — 20,480 points
Sensing sheets: 5 × 490 × 1040 mm (active 487 × 1034 mm) · Collectors: 5 × PXS128 (one per long sheet)
Large-format collector
Model: PXS128 · Dimensions: 180 × 180 mm
4096-point sensing film
Sensing principle: Piezoresistive · Thickness: 0.3 mm
One session, all sheets
Platform: Windows desktop application · Input: Multiple collectors via docking hub
Frequently asked questions
It is rated for over 10,000 cycles and is used through the closure and hold, then removed or left in place depending on the joint design. Where it stays in the joint, its 0.3 mm thickness has to be part of the stack-up. Most projects remove it after the check.
No. It measures pressure. Thickness has to be inferred from the process, or measured separately — squeeze-out carries adhesive away from the point being measured, so the two are not interchangeable.
Per-sheet calibration is done before the run, on the same sheet, and the values are stored with the project. The adhesive is applied over the calibrated sheet; what it changes is the load path, not the calibration curve.
It complements it. The map shows whether the process loaded the line evenly; destructive or ultrasonic inspection shows what the bond actually looks like. A map taken before cure is the only one of the two that can still change the outcome.
Coverage is built from 490 × 1040 mm sheets; the standard builds cover 1040 × 2480 mm and larger areas are covered by adding sheets, at the cost of more seams — and a seam is exactly where a reading is least trustworthy. Seam positions are agreed in the layout drawing, not on site.
Different surface, same method
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Send us the surface and the decision it has to support
Area, peak pressure, the feature you are looking for, and what happens if you miss it. That is enough for a first assessment.