Why does an opaque white run so much slower than a clear coat?

Because pigment scatters and absorbs the UV before it reaches the bottom of the film, and press speed scales directly with the fraction that gets through. An ink passing a third of the incident UV runs at a third of the speed, all else equal. It is not a small correction. On heavily pigmented whites it is the dominant term in the whole calculation.

How do I find my ink's transmission?

The practical route is a speed trial. Slow the press until the ink genuinely through-cures by a rub or adhesion test, then run this page backwards from that speed: the transmission it implies is grounded in your ink, your film weight and your lamp. Published tables vary enormously with formulation and lay-down, which is why this page asks for your figure rather than supplying one.

My ink cures on top but stays soft underneath. What is happening?

Exactly what this page is about. The surface received its dose and gelled; the base did not. That is the signature of setting press speed from a surface measurement on a pigmented film, and the usual consequences are adhesion failure, blocking on the rewind and ink lifting downstream, all after the job looks finished at the press.

Will a more powerful lamp let me run faster?

Proportionally, yes. Dose is irradiance times time, so doubling irradiance halves the dwell needed. But transmission multiplies the whole thing, so on a heavily pigmented ink you are buying speed at a discount set by the pigment. Changing to a photoinitiator that absorbs at a wavelength the pigment passes changes the transmission term itself, which is usually the better buy.

Does slowing down fix a tacky surface?

Not if the cause is oxygen inhibition, which is the usual cause. Atmospheric oxygen quenches the radicals at the very top of the film, and the reaction only wins where the instantaneous irradiance is high enough to consume oxygen faster than it diffuses back in. That is a peak-intensity problem: more time at the same intensity does not help, while more intensity, an inert blanket or a surface-active photoinitiator does.

Does the substrate matter?

For cure, only where you are curing through it. For speed, often decisively: thin films and heat-sensitive stocks can reach their thermal limit under a mercury lamp well before the ink reaches its cure limit, in which case the press speed on this page is unreachable and the real constraint is the infrared coming off the lamp, not the UV.

UV Curing calculator

UV Ink Cure Speed Calculator: Through-Cure, Not Surface Cure

Work out how fast a UV press can run and have the ink cure all the way through. Enter the dose the ink needs at the base of its film, the length of the cure zone, the irradiance measured at the web, and how much of that UV actually reaches the bottom of the film. The calculator returns the press speed, the dwell it needs, the dose the surface has to see to deliver it, and, for comparison: the speed a perfectly clear film would allow. That last pair is the point: pigment does not slow the press a little, it scales the speed directly.

What this calculator does

Find the press speed a UV ink can run at, based on the dose reaching the BOTTOM of the film rather than the dose a radiometer sees on top of it.
Use it for setting press speed for a pigmented ink rather than a clear coat, explaining why opaque whites will not run at rated line speed, deciding whether a lamp upgrade will actually buy speed, deriving an ink's effective transmission from a speed trial, diagnosing soft-under-cured ink that passed a surface check.
Find the press speed a UV ink can run at, based on the dose reaching the BOTTOM of the film rather than the dose a radiometer sees on top of it.

Formula used

Irradiance at the base of the film = measured irradiance × transmission
Required dwell = target dose ÷ irradiance at the base
Press speed = (cure-zone length ÷ 12) ÷ required dwell × 60
Dose needed at the surface = target dose ÷ transmission
Speed given up to pigment (%) = (1 − transmission) × 100

Inputs explained

Ink target cure dose: Dose the ink needs at the base of the film to through-cure, in the band the lamp is measured in.
Lamp cure-zone length: Length of web genuinely under useful irradiance, along the direction of travel. Measure the illuminated band, not the housing.
Measured irradiance at the web: Radiometer reading at web height with the lamp at its running power: the irradiance arriving at the TOP of the ink film.
UV reaching the base of the film: Fraction of incident UV that gets through the ink to its bottom surface. Clear varnishes pass most of it; opaque whites and heavy blacks very little. Set to 100% to reproduce a surface-dose calculation.

How to use the result

Best suited to setting press speed for a pigmented ink rather than a clear coat, explaining why opaque whites will not run at rated line speed, deciding whether a lamp upgrade will actually buy speed, deriving an ink's effective transmission from a speed trial, diagnosing soft-under-cured ink that passed a surface check.
Transmission is treated as one number. It is really wavelength-dependent, and the whole point of a long-wavelength photoinitiator is that pigment leaves a window open at one end of the spectrum. Ignores oxygen inhibition at the film surface, which is a peak-irradiance problem and is not solved by slowing down. Assumes uniform film weight. A heavier lay-down transmits less, so a press running at this speed can through-cure in one area and not another. Says nothing about substrate heating, which is often what really limits speed on a thin film or a heat-sensitive stock. Treats the cure zone as uniformly illuminated, when in reality irradiance falls off at both ends of the band.

Common questions

Why does an opaque white run so much slower than a clear coat? Because pigment scatters and absorbs the UV before it reaches the bottom of the film, and press speed scales directly with the fraction that gets through. An ink passing a third of the incident UV runs at a third of the speed, all else equal. It is not a small correction. On heavily pigmented whites it is the dominant term in the whole calculation.
How do I find my ink's transmission? The practical route is a speed trial. Slow the press until the ink genuinely through-cures by a rub or adhesion test, then run this page backwards from that speed: the transmission it implies is grounded in your ink, your film weight and your lamp. Published tables vary enormously with formulation and lay-down, which is why this page asks for your figure rather than supplying one.
My ink cures on top but stays soft underneath. What is happening? Exactly what this page is about. The surface received its dose and gelled; the base did not. That is the signature of setting press speed from a surface measurement on a pigmented film, and the usual consequences are adhesion failure, blocking on the rewind and ink lifting downstream, all after the job looks finished at the press.
Will a more powerful lamp let me run faster? Proportionally, yes. Dose is irradiance times time, so doubling irradiance halves the dwell needed. But transmission multiplies the whole thing, so on a heavily pigmented ink you are buying speed at a discount set by the pigment. Changing to a photoinitiator that absorbs at a wavelength the pigment passes changes the transmission term itself, which is usually the better buy.
Does slowing down fix a tacky surface? Not if the cause is oxygen inhibition, which is the usual cause. Atmospheric oxygen quenches the radicals at the very top of the film, and the reaction only wins where the instantaneous irradiance is high enough to consume oxygen faster than it diffuses back in. That is a peak-intensity problem: more time at the same intensity does not help, while more intensity, an inert blanket or a surface-active photoinitiator does.

Last reviewed 2026-08-25.