Last year I helped a wood-coating customer in Daliao evaluate switching from mercury lamps to UV LED, and the thing they cared about most was one question: “Can I use my current ink as-is?” The answer is usually no. This is also the most common misconception when adopting UV LED curing — assuming you just swap the lamp head, only to end up with incomplete cure and tacky surfaces, wasting weeks. Phoseon is a pioneer in UV LED curing (it merged into Excelitas in February 2023) and the technology is mature, but a smooth rollout means first getting clear on how wavelength, energy and formulation relate to one another. Below is a rundown of the key points from the evaluations and troubleshooting we have done alongside customers.
Choosing a Wavelength: 365, 385, 395, 405 nm
Most Phoseon products are concentrated at four peak wavelengths — 365, 385, 395 and 405 nm — and they are narrow-band, with the spectrum falling roughly within ±10 to 15 nm of the peak. How does wavelength affect curing? As a rule, longer wavelengths (395, 405 nm) penetrate better; they pass more readily through thick films and colored, pigmented systems, achieving through-cure at the bottom layer and improving adhesion, while shorter wavelengths lean toward surface cure. For that reason, the industry mainstream is 395 nm: first because it matches common free-radical photoinitiators well, and second because LED chips in this band deliver high usable output and are mature and cheap to manufacture. On the same unit, the peak irradiance at 395 nm is often noticeably higher than at 365 nm — per Phoseon’s data, the usable peak at 365 nm is only about half that at 395 nm.
One nuance worth flagging: the maxim “short wave for the surface, long wave for deep penetration” holds at the level of pigmented inks, but it can reverse in an optimized clear formulation, because with little pigment scattering the short wave can actually reach deeper. So penetration depth is really determined by wavelength together with the pigment and photoinitiator system — do not just look at the wavelength number. Which band to actually choose still comes down to running trial cures with your own ink formulation and measuring the results.
UV LED Versus the Traditional Mercury Lamp
The traditional medium-pressure mercury lamp is broad-spectrum: on top of the UV you want, it also emits unwanted UVC and a great deal of infrared heat, and in operation it produces ozone, needs exhaust ventilation, needs warm-up and cooling, cannot switch on and off instantly, and has a tube life of roughly one to two thousand hours. UV LED, by contrast, is narrow-band and monochromatic — ozone-free, mercury-free, low-heat and capable of millisecond instant switching — drawing power only while curing, with a rated life commonly quoted at 20,000 hours or more (higher with proper maintenance). On efficiency, Phoseon’s figure is that roughly 20 to 40% of the input electrical energy is converted into usable UV, saving on the order of 80% of the electricity and heat compared with a mercury lamp. On the numbers alone, UV LED wins almost across the board.
But there is one critical difference that bites the process directly: the mercury lamp’s spectrum contains UVC, which is high-energy, readily absorbed by surface photoinitiators, and able to counter oxygen inhibition; most UV LEDs are UVA with no UVC, so the surface is more easily inhibited by atmospheric oxygen, giving you the situation where the lower layer has cured but the surface stays tacky. Excelitas’s literature even notes that some formulations remain tacky at the surface even after 30 J/cm² of 395 nm UVA. So a mercury-lamp formulation cannot be transferred straight to UV LED — the photoinitiator often has to be changed. That is the real answer to that customer’s opening question, “Can I use my ink as-is?”
Irradiance, Dose and Thermal Management
Cure performance hinges on two quantities: irradiance (W/cm², the instantaneous power per unit area — the striking force that initiates polymerization) and energy dose (J/cm², the integral of irradiance over time — the key to ensuring through-cure and adhesion). In practice you first have to reach the minimum irradiance threshold that activates the photoinitiator; once past that threshold, dose is what governs cure and sets the ceiling on line speed. A common misconception is “the higher the peak irradiance, the faster the cure,” when total dose actually matters more — a slightly wider window with a slightly lower peak can deliver more dose thanks to the longer dwell time. Dwell time on the conveyor is determined by line speed and the width of the emitting window; if the line runs too fast and the dose falls short, the cure will be incomplete.
Thermal management is UV LED’s Achilles’ heel. An LED converts only about 10 to 20% of the incident electrical energy into light; the remaining 70 to 80% becomes heat that must be removed. The lower the junction temperature, the higher the UV output and the longer the life; conversely, poor heat removal both cuts power and shortens life. Phoseon offers air cooling and water cooling: air cooling is simple and economical overall, while water cooling is compact and delivers higher peak irradiance. For water cooling, coolant of around 30 to 35 °C is recommended to reduce the risk of condensation, while for air cooling you will need to change filters often in dusty, oil-mist environments. Planning your cooling before rollout is, in effect, securing your output and lamp life up front.
Common Pitfalls in Rollout, and How to Address Them
Let us boil the above down into a checklist you can use on the floor. For incomplete cure and tacky surfaces, look at four things first: one, whether the wavelength and photoinitiator match; two, whether the dose is sufficient (is the line speed too fast?); three, whether there is oxygen inhibition (especially to watch for in UVA-only systems); and four, the working distance. A UV LED has no focal point — irradiance is greatest at the lamp-head exit and falls off with distance close to an inverse-square law, so doubling the distance leaves roughly a quarter of the irradiance; this is a very common but easily overlooked cause of cure failure. There are several fixes for oxygen inhibition: add a UVC LED for surface post-cure, raise the irradiance, use nitrogen inerting to purge the oxygen at the surface, or adjust the formulation.
On the commercial side, UV LED does cost more up front than a mercury lamp, but thanks to lower energy use, long life and no lamp-replacement consumables, the long-term cost of ownership is usually lower; just do not cut corners on the transition, since the process has to be re-validated and the formulation reworked during rollout. Phoseon’s product lines split into air-cooled (FireJet, FireEdge, FireFly) and water-cooled (FireLine, FirePower, FireFlex), with top-end peak irradiance landing around 25 W/cm² for water cooling and roughly 16 to 20 W/cm² for air cooling; size the system to your line speed, cure window and cooling conditions.
Success or failure in UV LED curing is 80% about the wavelength, dose and formulation planning done before rollout — it is not just a matter of bolting the lamp on. TingHao, based in Kaohsiung, helps printing, coating and electronics production lines evaluate Phoseon and other UV LED curing systems, looking at wavelength selection, mercury-lamp conversion, thermal management and formulation validation together. If you are considering switching from mercury lamps to UV LED, or your cure keeps coming out tacky, call us at 07-363-0802 and let us validate what needs validating before you spend the money.
This article shares UV-curing know-how for reference. TingHao currently focuses on infrared heating and indoor air disinfection and does not supply UV-curing equipment; for related needs, feel free to talk to us about a suitable solution.