When parts leave a UV conveyor soft, tacky, or weak, the reflex is to slow the belt. Sometimes that works. Often it hides the real cause, costs throughput, and returns the defect the next time the lamp ages. A structured check sequence finds the actual fault faster.
Q: Why are parts under-cured coming off a UV conveyor?
A: Usually one of five things — wrong spectrum, lost intensity, wrong dose, wrong distance, or light that never reaches the adhesive
Under-cure means the photoinitiator did not receive enough usable energy. That can happen because the wavelength does not match the chemistry, because the lamp has lost output, because the belt speed delivers too little dose, because the part sits outside the head’s working height or width, or because the adhesive is shadowed. Check them in that order: the early checks are quick and rule out the causes that no amount of belt-speed adjustment would fix.
Check 1: Does the Spectrum Match the Adhesive?
On the Incure CDM™, LED heads are factory-configured to one wavelength — 365, 385, 395, or 405 nm. If a formulation designed for 365 nm is run under a 405 nm head, slowing the belt adds time but not the right light. Arc configurations raise a related trap: an x2AC assembly includes a 254 nm UVC tube, while an x2A configuration is UVA-only. A process qualified on x2AC broadband exposure can under-cure after a switch to x2A even though x2A delivers double the UVA dose. Confirm the head’s part number against the adhesive’s qualification record first.
Check 2: Has the Lamp Lost Output?
Measure irradiance at the working height with a radiometer and compare it with the value recorded at qualification. Rated decay gives a sense of scale: LED heads are rated to 20% or less intensity loss over more than 20,000 hours; arc heads are rated beyond 1,000 hours with up to 45% decay. A 45% loss on an F400x2AC would take its 490 mW/cm² down to about 270 mW/cm². On an x2A configuration, check both heads — if one has been replaced and the other has not, the two exposure zones deliver unequal energy, and a single combined reading may not reveal it.
Check 3: Is the Belt Delivering the Dose You Think?
Dose scales inversely with speed, so a speed setting that drifted, or was changed for a different product and never reset, changes everything. The CDM™ F200x2AC delivers 2,250 mJ/cm² UVA at 6 ft/min but only 1,130 mJ/cm² at 12 ft/min. Verify the actual belt speed against the recipe and confirm the dose at that speed still clears the data sheet requirement with margin.
Email Us with your head configuration, belt speed, and radiometer readings and Incure can help narrow down which check is failing.
Check 4: Is the Part Inside the Working Envelope?
Irradiance figures are published at a stated working distance — 2 inches for the LED heads. Curing height adjusts from 0.5 to 2.0 inches on the LED heads and from 3.0 to 6.5 inches on the F400/F500 heads. A taller part than the one the line was set up for puts its top surface closer to the lamp and its base farther away. Width matters too: maximum part width is fixed by head, from 1 inch on the M51 to 8 inches on the L88. Parts that under-cure only along one edge often extend past the head’s rated width.
Check 5: Can the Light Reach the Adhesive?
If the exposed bead is hard but adhesive under a component, inside a recess, or behind a tall feature stays liquid, the cause is geometry, not dose. More energy from the same direction will not reach a shadow. Options include reorienting the part, a second pass in a different orientation, or a formulation with a secondary cure path.
Symptom-to-Cause Quick Reference
| Symptom | Check first |
|---|---|
| Every part under-cured after a lamp change | Check 1 (wrong head or wavelength) |
| Gradual decline over weeks | Check 2 (lamp output) |
| Under-cure began after a product changeover | Check 3 (belt speed recipe) |
| Only tall parts, or only one edge, affected | Check 4 (distance and width) |
| Exposed adhesive cured, hidden adhesive liquid | Check 5 (shadowing) |
| One zone of an x2A line weaker than the other | Check 2 (mismatched head hours) |
Fixing the Process, Not Just the Symptom
Once the cause is found, write the fix into the recipe: the correct head part number, the qualified irradiance and the threshold that triggers a lamp change, the belt speed, and the curing height. Then the next shift inherits a controlled process instead of a workaround. Full configuration data is in the Incure CDM™ UV conveyor guide, the general overview of conveyor curing is in the curing conveyor industrial guide, and the background on arc spectrum is in the Incure F-Series™ flood lamp guide.
Contact Our Team to work through a persistent under-cure problem on your line.
Visit www.incurelab.com for more information.