A metal-parts coating customer in Fengshan could not get their white coating to cure properly no matter how they baked it — “dry on top, wet underneath” — and a scrape of the lower layer still came up soft, with complaints piling up. For a while they thought the lamp was old; they fitted a new one and got the same result. Once we were on-site and asked about the formulation and looked at the lamp type, it was clear the problem was not an old lamp but the wrong lamp — white coating paired with a standard mercury lamp simply will not cure to the bottom. This is a textbook situation with Dr. Hönle and IST METZ medium-pressure mercury curing systems. Below I will lay out the key selection points for these systems, how to measure UV dose, how to set conveyor speed, and the common problems, all in one go.
Power Density and Tube Length of Medium-Pressure Mercury Lamps
The output of a medium-pressure mercury lamp is conventionally expressed in W/cm — power per centimeter of arc length. On the market you will commonly see anywhere from about 80 up to 240 W/cm, graded by model. Per the manufacturers’ specs, Dr. Hönle’s UVAPrint series goes up to about 240 W/cm (up to 330 W/cm in intermittent operation), with arc lengths of roughly 100 to 1,055 mm and power continuously adjustable from 30% to 100%; IST METZ’s in-house lamp tubes cover an even wider output range, roughly 40 to 330 W/cm, with tube lengths from 200 to 3,400 mm and a rated life longer than a typical medium-pressure mercury lamp. Higher power density is not automatically better — it has to match your coating thickness, line speed and cure requirements; it only means something when the power is adequate and the band is right.
Doped Lamps: Iron, Gallium and Their Relationship to White Coatings
Back to that opening case of white coating that would not cure through — the crux is doping. A standard undoped mercury lamp has its main spectral lines at 254, 365 and 405 nm and so on: strong in UVC, well suited to quickly curing ordinary pigmented inks and clear varnishes. But once the pigment gets dark and the coating gets thick, the short wave cannot reach the bottom. That is when you use doping to push the energy toward longer wavelengths: iron doping strengthens the UVA around 380 nm, suited to through-cure of thick films and dark pigments; gallium doping introduces strong spectral lines at 403 and 417 nm, pushing the energy to even longer wavelengths — precisely the answer for white, titanium-dioxide-containing coatings. Why? Because a white pigment like titanium dioxide absorbs and scatters away almost all the ultraviolet below 380 nm, so the short wave never reaches the photoinitiator at the base of the coating; only longer wavelengths above roughly 380 nm (the 400 to 417 nm reinforced by a gallium lamp) can get through and carry the energy all the way down, achieving through-cure. IST METZ divides its lamps into several spectra — standard mercury, Type A gallium-indium, Type B iron, and Type C lead — of which the lead type is especially suited to white clear coats; Dr. Hönle likewise offers interchangeable iron, gallium and mercury spectra. Nine times out of ten, white coating that will not cure through means you should switch to a gallium lamp, not to a new lamp.
Measuring UV Dose: mJ/cm² and Four Bands
Whether the cure is any good cannot be judged by eye — you measure the UV dose, in mJ/cm². The formula is simple: dose equals irradiance (mW/cm²) times exposure time. Measurement calls for a UV radiometer; common ones include Hönle’s own UV-Meter, IST’s UMS and UV Analyzer mobile instruments, and third-party devices like the EIT Power Puck that ride along on the conveyor and measure with the part. The key point is that you should measure across four bands: UVA, UVB, UVC and UVV. Why? Because with different formulations and different doped lamps, the proportions across the bands are completely different; for the same total dose, surface cure depends on UVC and UVB, while deep or through-pigment penetration depends on UVA and UVV. Only by measuring band by band do you learn whether the lamp type is right, which band is degrading first, and whether the shortfall is in surface dry or bottom dry. Reporting a single total figure often leads to a misdiagnosis.
Conveyor Speed, Lamp Life and Other Common Situations
Conveyor speed and dose are inversely related: the faster the belt, the less time spent passing through the cure zone, and the lower the dose. Exposure time equals the length of the irradiated zone divided by belt speed, so you have three levers for adjusting dose — change belt speed to control exposure time, change lamp power or lamp height to control irradiance, and add lamp heads to increase total dose. In practice you first work back from the target dose to a belt speed, then fine-tune with power and lamp height, and finally verify with the radiometer.
Most common problems circle around “not enough dose,” and the causes need to be ruled out one by one: lamp aging and output decline (a medium-pressure mercury lamp typically lasts one to two thousand hours, and many industries replace it at 1,000 to 1,500 hours to avoid too much intensity loss); a dirty or degraded reflector (a substantial fraction of the UV reaching the substrate arrives by reflection, so a dirty reflector sharply drops the energy at the part — clean it regularly with isopropyl alcohol); quartz devitrification showing up as white spots; excessive belt speed; and, as covered above, choosing the wrong doped lamp. Temperature matters too: a medium-pressure mercury lamp needs a hot start, and the lamp-shoulder temperature has to stay high enough to sustain the mercury vapor pressure, yet not so hot that the tube warps and deforms — the cooling system is critical. Also, wavelengths below 240 nm generate ozone, so provide adequate exhaust ventilation. Dr. Hönle’s UVACUBE, UVAHAND and UVAPrint, and IST METZ’s BLK and MBS series, are all common system options; when configuring them, plan lamp type, cooling and measurement together so the cure stays stable.
Medium-pressure mercury curing looks simple, but choose the wrong lamp type, skip the dose measurement or neglect the reflector, and problems keep recurring — and they are easily misdiagnosed as “the lamp is just old.” TingHao, based in Kaohsiung, helps coating and printing lines with the selection, dose measurement and cure-fault diagnosis of Dr. Hönle, IST METZ and other UV curing systems. If you are also stuck on white coating that will not cure through, tacky surfaces or drifting dose, call us at 07-363-0802, bring your coating and your lamp type, and we will measure it and get the parameters dialed in.
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.