When There's No Cure

The ability to measure the UV energy output from your lamps can provide valuable information when dealing with curing problems.

by Tom Polischuk, Editor-in-Chief

UV FLEXOGRAPHIC PRINTING is well established in the package printing industry and its impact has been significant. The use of UV inks is at the top of many lists when discussing the reasons for the significant quality improvements of flexo printing over the past several years.

Many converters are multi-year veterans in the application of UV technologies, but many others have recently made the jump into this new technology arena. Routine processing problems come with the territory, but they are so much more prevalent and frustrating when applying new technologies and working through a learning curve.

What this means is that more often, converters are facing the problem of what to do when the UV ink is just not curing properly. For starters, it's critically important for converters to have close working relationships with their ink, substrate, and UV curing system suppliers to deal with both routine problems and the more difficult challenges that crop up at the most inopportune times.

One of the first approaches that converters can take is to check out and verify that the UV curing system is working as it should. There is an array of tools available, and it's a necessary and much simpler proposition, than jumping into analytical chemical analysis of what are fairly complex UV inking systems.

First, some of the chemistry

In my initial discussion with Mark Hahn, VP of sales and marketing for AAA Press International (Arlington Heights, Ill.), he gave an overview of the ink chemistry, and therefore, what the UV curing equipment is targeted to accomplish. According to Hahn, inks and coatings contain three key elements for proper curing—monomers, oligomers, and photoinitiators. Monomers and oligomers are the reactive components, while photoinitiators are the components activated by the UV energy to set off the curing process for the inks/coatings. "Typically, ink manufacturers will put specific photoinitiators into the inks for surface cure, depth of cure, bonding, and other ink features," explains Hahn. "A standard medium-pressure, mercury vapor lamp will produce a specific intensity curve with peak irradiance being at 365 nanometers (nm). Ink manufacturers try to match their ink chemistry to this curve for optimum curing performance."

Both Hahn and Dan Naughton, director of sales and marketing for UV Research (Brea, Calif.), made the point that the photoinitiators are typically the most expensive components making up the UV ink, and also have a major affect on curability and therefore, the speed at which the press can run. Therefore, any modifications to the photoinitiator components by your ink supplier should be something you are very interested in knowing.

Although mercury vapor lamps with wavelengths at 365 nm are most common, other lamps may be required if your ink chemistry cures at a different wavelength. "If a UV chemistry cures at a wavelength other than 365 nm, a specialty (doped) bulb will be needed that matches the wavelength the chemistry's photoinitiators react to," says Elinor Midlik, president of Prime UV Systems (Carol Stream, Ill.). "Doped bulbs are available that create additional peak wavelengths."

James McCusker, president of Honle UV America (Marlboro, Mass.), also cautions that not all lamps are identical with respect to their electrical characteristics. "UV lamps that are not electrically matched to the system can accelerate UV ageing and can over tax components within the power supply, causing premature failure." Further (and this is one of my personal favorites for any manufacturing component), "Inexpensive lamps can provide purchasing agents relief, but create nightmares for pressmen."

One other factor affecting UV curing pointed out by Hahn is that differences in pigments and chemistry can cause certain colors to be more difficult to cure than others. This is a common problem according to TAPP Technologies' (Langley, B.C.) Pressroom Supervisor Dave Swartz. He has found that different colors need different UV intensity to cure properly.

Swartz also mentions another typical problem: inadequate cure depth in the ink. In these cases, the surface cure is acceptable, but the ink hasn't cured throughout its entire thickness. Usually, this problem becomes apparent when foil stamping, and is usually caused by the normal deterioration of the UV lamps over time.

Checking your UV

curing system

When you're faced with ink curing problems, how can you make sure that your UV curing system is working properly before you contact your ink supplier? There are several methods, including off-line tests, UV intensity tapes, various types of radiometers, and what may be the latest approach, spectroradiometers. These different approaches run the gamut from being "subjective at best, to providing concrete spectral data to verify optimum UV output," says Hahn.

Off-line testing is simple and repeatable, says Mark Tausch, R&D engineer for UVTechnology (Cincinnati, Ohio). "The best method of monitoring quality of cure is real-time burnish, through-cure, and tape peel testing," he explains. "A burnish test will ensure surface cure; solvent double rubs will ensure through-cure; tape peel tests are a measure of adhesion and can serve as a secondary test for through-cure." Although these tests are simple and check the finished product itself, they do require that the printing process be stopped to get the samples for testing.

If off-line testing or other factors indicate an incomplete cure of the ink, the next approach is to check the curing system's UV output. A first step for converters could be the use of tape indicators, says Jeff Bade, sales manager for Primarc UV Technology (Phillipsburg, N.J.). "Tape indicators can be a fast and effective method for comparing setups. Tapes can be easier to use in web and sheet-fed applications."

Using these tapes may be the simplest way to check for spectral output, says Naughton. "The tape can be placed on the substrate and run through the curing system, preferably at the same rate of speed you are intending to cure at. If the tape changes color, it means that the proper wavelength of light is being produced for curing; if the color does not change it could mean any number of problems may exist." He cautions, though, that standard tapes only measure spectral range, not actual intensity.

UV Process Supply (Chicago, Ill.) has taken the tape approach a step further with its CON-TROL-CURE® Rad Check™ UV Measurement System. This disposable test strip can be placed on a substrate and passed between rollers and around cylindrical objects. It uses a UV-sensitive compound that can be measured by a Rad Check dosimeter to produce a numerical value that reflects the UV energy dose.

The use of radiometric tools to measure UV energy output "was a huge leap" when it was first applied several years ago, says Hahn. He explains that there are four bandwidths that should be considered in UV measurement—UV-A (320-390 nm), UV-B (280-320 nm), UV-C (250-260 nm), and UV-V (395-445 nm). "A typical radiometer will have a probe that will measure one particular bandwidth. If you require readings from a different bandwidth, then the instrument may require a probe change to the desired bandwidth."

One type of radiometer is designed to be placed on the moving web and run through the UV curing system. As Tausch points outs, this device must be removed before coming in contact with any rollers, and the test is typically done at slower speeds. In these cases, he says that the radiometric dose data can be extrapolated for actual run speeds.

If this approach is acceptable in your operation, EIT Inc. (Sterling, Va.) offers a unit with the ability to measure and display peak UV intensity and total UV energy, monitoring peak intensity in watts/cm2 in each bandwidth. This instrument, the UV Power Puck®, is 4.6˝ in diameter and 0.5˝ high, allowing it to be used in most curing environments, says EIT.

Handheld radiometers can be used to measure the actual intensity of UV-A, UV-B, UV-C, and UV-V. As mentioned earlier, they typically require a separate probe attachment for each of these bandwidths. Midlik points out the advantage of these portable units: "A mobile measurement unit can be easily moved between multiple UV irradiators. Most converters choose a mobile unit since it allows them the flexibility to monitor numerous UV systems without having to invest in a measurement unit for each."

It must be emphasized that the proper and repeatable placement of the probe is critical to using handheld measurement of UV energy. Naughton suggests "a trap door or opening of some sort into the cure area so the probe can be inserted into the optical path in such a way as to have the sensor right at the point of curing." To be able to meaningfully compare repeated measurements over time, the probe must be placed in the same location each time. "The converter who is diligent about taking measurements in a standard fashion will be able to establish benchmarks that can be used as a quality tool in their operations," emphasizes Bade.

Another approach that Naughton says is available from EIT and is in the moderately priced range is the use of relative intensity monitoring. With these systems, relative intensity is measured starting with the installation of a new bulb. The sensor is set at 100 per cent and as the bulb degrades, the system continuously monitors the intensity as it decreases. Typically, the bulb should be changed when it reaches 60 per cent of its original intensity.

For continuous monitoring, UV Research has joined with Full Spectrum Technology (Brea, Calif.) to develop a fixed, real-time approach. A sensor is mounted inside the lamp housing and can be monitored by a display. The display can be switched from station to station to read real-time UV-A for multiple UV systems.

Prime UV also offers an integrated device built into its Prime Smart 2100 Control Platform, with sensors that allow the continuous monitoring of UV output. Midlik says the information can be used to allow a PLC to adjust the radiation intensity. This approach, however, might not be necessary for everyone. "Typically, only especially sensitive converting applications, such as silicone release coatings, require an integrated device to ensure that the intensity of the UV system fits the exact process needs of the chemistry and substrate," she says.

The use of spectroradiometers is a relatively new approach and expands the capabilities of radiometric measurement. Hahn points out that a spectroradiometer, like the Sola-Check and Sola-Scope handheld units manufactured by Solatell (Arlington Heights, Ill.), will measure the full spectrum of UV energy from 230-470 nm, covering the UV-A, UV-B, UV-C, and UV-V bandwidths. AAA Press has been integrating this test equipment into its LIGHTouch™ UV curing equipment for the past several months.

Hahn reiterates the importance of precise location of the probe into the lamp assembly when measuring with either a radiometer or spectroradiometer. With this as a requirement, "it is most typical to take a baseline reading with a fresh bulb, clean reflectors, and optimum running electronics package," says Hahn. "After entering the baseline reading, subsequent readings are taken at regular intervals and then compared to the baseline reading to determine if the lamp is within tolerance."

"Solatell takes it a step further with the Sola-Scope system, which is capable of storing up to 90 readings. These readings can be downloaded into a PC spreadsheet and compared numerically and graphically. Once downloaded, the readings become a valuable source of historic information that can prove, or disprove, the ongoing performance of a UV curing system."

TAPP Technologies has been using a Solatell spectroradiometer over the past year, and Swartz says it has allowed him to accurately monitor the more than 60 UV heads that they run in their operation. "It allows us to predict when a bulb is ready to fail, and also gives us an good idea why it is failing," says Swartz.

Swartz has used tapes and on-web radiometers. Tapes, he says, are not accurate and too subjective; on-web radiometers are not good for higher speeds. With the Solatell's portable spectroradiometer, Swartz downloads the data into a PC and analyzes doses and spectral intensities over all of the bandwidths.

Running UV inks

TAPP Technologies has been running UV inks for about nine years. Overall, Swartz thinks they are easier to run than conventional inks, since you don't have to mix fountain solutions. On the flip side, UV inks are extremely temperature sensitive, and transfer and lay down is more sensitive. Swartz keeps a close eye on process and ambient conditions, with temperature-controlled rollers and air conditioning on the press.

According to Midlik, one of the largest markets for UV flexo printing is flexible packaging—especially printing on film substrates. "We tell our flexo film converters to constantly measure dyne levels. UV flexo inks require that the dyne level of the film measure 44 at a minimum. We also recommend that converters install a corona, or a flame treater in-line, to ensure that dyne levels match the required levels."