Nobody should guess a UV exposure time. It can be derived from three numbers: the dose the material needs, the irradiance the lamp delivers at the part, and the thickness being cured.
Q: How Long Should a UV Curing System Expose the Part?
A: Required Dose Divided by Irradiance at the Part
Exposure time in seconds equals required dose in mJ/cm² divided by irradiance in mW/cm² at the part surface. A material needing 2,000 mJ/cm² under a lamp that delivers 500 mW/cm² needs 4 seconds. The harder questions are where the dose figure comes from and whether it reaches the full depth. For the system-selection side, see Incure’s guide to benchtop UV curing systems.
Step 1: Find the Dose the Material Requires
The adhesive or coating data sheet states a cure condition, often as irradiance and time at a named wavelength. Convert it to dose. If the data sheet only gives a time, treat it as valid only for the irradiance it names. Never carry a published time to a weaker lamp without recalculating.
Step 2: Measure Irradiance at the Working Distance
Lamp ratings are measured at a fixed point. The same lamp reads far less farther away. Incure’s L-Series™ flood lamps show the range across the line: L44 at 3,100 mW/cm² at 365 nm, L1212 at 1,900 mW/cm², and the water-cooled W44 at 8,100 mW/cm² over the same area as the L44. Use a radiometer at the real part position rather than the catalog figure.
Step 3: Calculate and Add a Margin
Divide dose by irradiance, then extend by a safety margin sized from your own validation data, to cover lamp aging, lens fouling, and batch variation. Overexposure wastes cycle time and can heat thin substrates, so check the material’s datasheet for any upper exposure limit.
Worked Scenarios (Illustrative)
Consider three situations, with figures chosen for explanation only.
| Case | Dose needed | Irradiance | Base time | With 30% margin |
|---|---|---|---|---|
| Thin tack-cure | 1,000 mJ/cm² | 1,000 mW/cm² | 1.0 s | 1.3 s |
| Standard bond | 3,000 mJ/cm² | 600 mW/cm² | 5.0 s | 6.5 s |
| Thick filled layer | 6,000 mJ/cm² | 300 mW/cm² | 20 s | 26 s |
In the third case, doubling irradiance to 600 mW/cm² roughly halves the time, which is one reason a higher-intensity lamp pays off on thick sections.
Cure Depth Is Not Dose
A dose calculated at the surface says nothing about the bottom of the layer. Light is absorbed as it travels, so intensity declines with depth. Thicker or pigmented material needs either a longer wavelength that penetrates farther, higher irradiance, or staged exposure. A depth test, curing a wedge or stacked samples and measuring the solid thickness, replaces assumption with data.
Email Us with your material, layer thickness, and lamp, and Incure’s team can help set a starting exposure.
Distance, Time and Speed on Conveyors
On a conveyor, exposure time equals the lamp’s footprint length divided by belt speed. A 100 mm footprint at 10 mm/s gives 10 seconds. Incure’s CDM™ conveyor accepts several lamp heads, so changing the head changes irradiance while belt speed sets time. Dose then follows from both.
Verifying the Result
After setting time, cure a sample and test: no surface tack, adequate hardness, and bond strength to the specification. If the surface stays tacky while the bulk is solid, increase irradiance rather than time, since oxygen inhibition responds to intensity. Record the setting alongside the radiometer reading so the process can be reproduced.
Reciprocity and Its Limits
The dose formula assumes that intensity and time trade evenly, but real chemistry only follows this roughly. At very low irradiance, oxygen dissolved in the resin quenches radicals faster than the lamp creates them, leaving a tacky surface even with a long exposure. At very high irradiance, polymerization races ahead and builds internal stress. Treat the calculated time as a starting point near the conditions the data sheet used, and test any large departure from them.
Pulse, Ramp and Shutter Effects
Short exposures of a few tenths of a second are sensitive to lamp ramp time and to any shutter opening delay. A mercury arc source with a shutter, such as the lamps in Incure’s F-Series™, reaches full output before the shutter opens, so its exposure timing is set by the shutter. LED sources switch on at full intensity within moments, which makes short, repeatable exposures easier to hold.
Keeping Settings Current
Lamp output declines with age and lenses collect residue, so a time set once will slowly under-deliver. Re-measure irradiance on a schedule, compare it to the original reading, and lengthen exposure when the margin shrinks. Logging each reading builds a trend line that shows when to clean or replace parts before cure quality is affected.
Final Thoughts
Exposure time is the output of a calculation, not an operator preference. Find the dose, measure irradiance where the part sits, add a margin, and confirm depth with a test. Contact Our Team to review the exposure settings for your curing system.
Visit www.incurelab.com for more information.