UV Ink Sees the Light in Pad Printing
New options for production efficiency and image quality
Looking for a way to increase pad-printing productivity and efficiency while improving the durability and resolution of your prints? The introduction of UV-curable inks for pad printing may provide the answer. Learn what characteristics UV pad inks offer, where they can be used, and how their processing requirements differ from those of conventional inks.
One reason why pad printing is so versatile is because of the unique inks used in the process and the way they make image transfer possible. Unlike screen- and offset-printing inks, pad inks have to transition through several rheological states (changes in flow properties) in order for the image to transfer to the substrate. With properly formulated inks, you can achieve not only excellent ink transfer, but high print resolution and impressive production rates as well.
Historically, pad-printing inks have comprised a range of solvent-based formulations, including conventional solvent-based inks that dry by evaporation and epoxies that crosslink when exposed to heat. Recently, however, these formulation have been joined by ultraviolet (UV) curable pad-printing inks that offer the same characteristics of rapid curing and high durability as the UV inks screen and offset printers have been using for years. Today, three-dimensional metal, glass, and plastic substrates with recessed or textured print areas are ready candidates for UV pad printing.
UV technology and pad printing
UV curing was investigated in the early 1950's in the automotive painting industry, moved into offset-printing markets in the '60s, and began taking over large sections of the screen-printing market in the '70s. Today, UV-curable photopolymers not only include inks, but have expanded to encompass adhesives, silicone release coatings, pressure-sensitive adhesives, and numerous other coatings.
In pad printing, UV-curable inks allow users to achieve precise printed images that cure very fast and exhibit high abrasion resistance, great impact resistance, and incredible solvent resistance after curing. The introduction of these inks coincides with advances in printing equipment that allow users to take full advantage of the benefits of UV technology. Notable equipment enhancements include greater parts-handling automation, advanced timing control during printing, improved closed ink-cup designs, and automatic pad cleaning, to name a few. All these improvements have helped to make UV pad printing a reality.
Pad-printing ink rheology
Just like their solvent-based cousins, UV pad inks have unique rheology requirements. During the printing cycle, the ink is flooded on the plate and excess ink removed with a doctor blade (or ring on closed-cup systems) leaving ink only in the recessed etched areas of the plate. This ink must retain a specific viscosity in the printing plate until the silicone pad picks it up.
As solvent evaporates from the ink after pickup by the pad (UV pad inks also contain some solvent to provide the necessary rheological characteristics), the ink becomes tackier. With the new rheology, the ink's cohesiveness (ability to stick together) increases and causes the ink to adhere to the substrate and release from the pad during the printing stroke.
Curing UV pad-printing inks
As mentioned previously, solvent-based pad-printing inks are dried by solvent evaporation. Epoxies, on the other hand, crosslink when exposed to heat in curing, achieving greater molecular weight than standard inks. UV pad inks are similar to epoxy inks, except that they crosslink when exposed to UV energy, rather than heat. Exposing printed UV inks to very high intensity UV energy allows them to fully crosslink in seconds.
As UV energy penetrates the ink film, photoiniators in the ink react with other nearby molecules called mono-mers and oligomers, causing them to link and form a solid matrix of molecules. The whole reaction takes only a few seconds, and if the ink is properly formulated, can even occur in the presence of a fairly dense pigment load. The final cured product typically exhibits greater abrasion resistant than most heat-cured epoxies and has equivalent or superior solvent resistance.
The most efficient source of UV energy with sufficient intensity to fully cure UV inks is a medium-pressure mercury-vapor lamp. Some of these lamps are doped with various metals to shift their output toward particular wavelengths.
Heat-sensitive materials, including ABS, polystyrene, and other plastics, can also be printed and cured successfully with UV pad inks. In circumstances where these materials will be encountered, curing equipment should incorporate dichroic reflectors. The reflectors are positioned above the lamps and absorb much of the infrared (thermal) output while reflecting the desired UV wavelengths and minimal heat to the printed surface.
Comparing conventional and UV inks
Use of UV-curable pad-printing inks is ideal for most high-volume printing applications where solvent and abrasion resistance are required. In general, cured UV inks provide significantly greater abrasion resistance than conventional solvent-based inks and epoxies. One application in which this advantage has been validated is in computer-keyboard manufacturing, where characters are pad printed onto keys.
Tests conducted for a leading manufacturer compared keys printed with conventional materials (an epoxy ink overprinted with a UV-curable clearcoat) to keys printed with a UV-curable ink and no clearcoat. The test used a mechanical "finger" that repeatedly actuated printed keys to determine how many strokes it would take for the print to rub off.
When exposed to this abrasion test, the clearcoated epoxy print began failing after approximately 1-2 million actuations. The UV-ink prints, however, showed abrasion resistance up to 13 million actuations. Not only did the UV ink provide abrasion resistance 1300% greater than the epoxy/clearcoat prints, it also shorted the printing/drying cycle from 7 min/ part to 15 sec/part--a 96% time reduction.
UV inks also offer excellent solvent resistance after curing. Part of this resistance stems from the high crosslinking density (high molecular weight) of the cured ink film. The high degree of crosslinking between molecules will not allow them to unbind from each other and dissolve in the solvent. Additionally, UV inks can be tailored with chemistry that is resistant to specific solvents.
For comparative purposes, let's consider the changes you would experience if switching from a two-part epoxy to a UV ink. First, you avoid the need to mix inks prior to use (unless you are using a color-matching ink system). Naturally, you also save time in curing since the long, high-temperature curing cycle required for epoxies is replaced with almost instantaneous UV curing. A beneficial side-effect is that you decrease the possibility of heat-induced substrate damage. And UV-curing systems require a fraction of the space needed for drying conventional solvent-based formulations.
Additionally, two-part epoxies have a short pot life after mixing and can begin to gel in the ink cup on your press after only four hours. This leads to production delays as you stop to clean the cups. But, with UV inks, pot life is virtually limitless--other than pausing occasionally to adjust the ink's solvent level, you need not stop production. See Table 1 for more feature and cost comparisons between UV and two-part epoxy inks.
|Comparing UV and Solvent-Based Pad-Printing Inks|
|UV ink||2-part epoxy|
|Average labor costs per part||$0.001||$0.12|
|Curing time per part||2 - 4 sec||10 min - 8 hr|
|Average kilowatt hours per part||0.001||0.2|
|Curing/drying floorspace required||1 - 3 sq ft||2 - 20 sq ft|
|Cost of ink||$50/lb||$28/lb|
|Cost of curing system||$5-10K||$3-10K|
There are, of course, special considerations to address before converting to UV. For example, you'll face new equipment expenses in the form of curing systems and UV measurement devices, which we discuss in following sections. If you print on substrates with low surface energies (polyethylene, polypropylene, etc.), the parts will require flame or corona treatment to ensure good adhesion by the UV ink. And you'll need to add new quality-control measures to ensure that UV prints are processed correctly.
Pad-printed images are typically applied to a smaller surface area than screen-printed graphics, which means pad printing requires a smaller curing system than typically used in UV screen printing. Most frequently, the curing system takes the form of a small conveyorized unit with a hood that shields the lamp/reflector assembly.
If your facility already has UV-curing equipment to support screen printing, the equipment may be suitable for curing pad-printed items as well. Just verify that it provides the correct lamp and UV output characteristics. Most pad-printed UV inks can be effectively cured with a medium-pressure mercury-vapor or halide-doped lamp. The lamp should have an input power rating of at least 300 watt/in. and deliver the majority of its energy in the 365- and 254-nanometer (nm) wavelengths. Reflectors can be used to focus or diffuse the lamp energy. IR-absorbing dichroic reflectors are recommended for use with heat-sensitive substrates.
Depending on the lamp power, you may need to adjust the curing unit's belt speed to ensure that prints receive the proper UV exposure. If measured with a radiometer, the curing unit should provide a UV dose of approximately 2 Joules/sq cm with wavelengths in the 320- to 390-nm range. Where the substrate allows, ink crosslinking speed can be further enhanced by preheating the prints at 130-150°F (55-65°C) for approximately 5 sec prior to UV curing.
UV print production
To achieve successful results when pad printing UV inks, you must control two key elements of the process: the thickness of the printed ink deposit and the curing environment.
Ink-deposit thickness Pad printing provides excellent ink-thickness control, which is especially good news if you are printing UV inks. Pad-printing plates are manufactured to precise etch depths, and the resulting ink-deposit thickness they provide can be accurately controlled. Most printers use 15-25 micron etch depths on steel plates. Photopolymer plates may also be suitable for UV applications, but they require greater care in exposure and developing to control etch depth.
As a general rule of thumb, the greater the pigment content of the UV ink film, the more opaque the ink layer will be to UV energy and the longer it will take to fully cure the ink. The thicker the printed ink film, the better its abrasion and solvent resistance. Note that if an ink is over-pigmented and printed in a thick layer, it will only achieve a surface cure. Therefore, UV inks for pad printing must be formulated with precise control of pigment concentration.
Controlling the curing environment To ensure that you can consistently deliver durable, high-quality prints, it's necessary to maintain proper curing conditions for every job you produce. This means you'll have to expand quality-control to include regular monitoring and maintenance of the curing system.
Previously we mentioned the radiometer. This device is essential for making sure that the curing energy delivered by the system remains consistent and matches the curing requirements of your ink. Two types of radiometers are available: one that measures real-time UV intensity (mw/sq cm), another that measures output intensity over time, providing readings in Joules/sq cm (mw/sq cm x seconds = mJoules/sq cm). Radiometers are offered in standalone varieties that are sent through the curing system to record UV output data. However, some curing systems feature built-in radiometers that continuously monitor UV energy from within the system.
If lamp-power, belt speed, and other curing-system settings remain un-changed, you will still record a decrease in UV energy output as the lamp ages. Over time, the UV output of the lamps will gradually diminish, reaching a point where it becomes impossible to achieve a proper cure. For this reason, it's important to make daily or weekly lamp-output measurements a regular part of your production process with UV inks. The output data should be recorded after each measurement, and a lower limit (we recommend a 30% reduction in output) es-tablished to help determine when lamps must be replaced. Eventually, your records will help you predict the life you can expect out of a new lamp. Typical lamp life is 1000-2000 hr.
Printing multiple colors
UV inks can be used for single and multicolor jobs. In multicolor work, particularly jobs that involve process color, the inks can often be overprinted wet-on-wet, then cured in a single step after printing. In such situations, it's important to print the inks in the correct color order. In general, yellows and reds allow more UV to transmit through them than blues, violets, and greens, so it is best to print the more opaque colors first.
However, some jobs incorporate multiple colors that require independent curing. With current pad-printing and curing equipment, this means you'll have to remove printed parts from the pad press after each color is applied, run them through the curing unit, and return them to the press for printing of the next color. Consequently, you'll face increased production time and, possibly, color-registration challenges. As the use of UV inks in pad printing expands, however, pad-press manufacturers may develop multicolor machines that incorporate curing stations between each printhead, similar to the UV-curing units found between each press on multicolor inline screen-printing systems.
Where to start
Rather than jumping into UV pad printing with both feet, we suggest a more cautious approach that lets you judge the suitability of UV for your typical applications.
1. Begin by sending samples of your substrates to UV pad-ink suppliers for ink compatibility testing. In writing, state your objectives for these tests, including the abrasion and solvent-resistance characteristics that you expect from the cured prints. When the printed samples are returned, analyze them carefully to make sure the results are satisfactory.
2. If you're happy with the test results, begin researching UV-curing equipment, measuring tools, and inks. Collect information about curing-system features and costs from equipment suppliers. Explain your expected ink-volume needs to ink suppliers so that they can provide accurate quotes.
3. Carefully weigh the performance attributes of UV inks against the equipment/supply costs and procedural changes they require. Determine if converting to UV is a cost-effective option for the types of products you print and customers you serve.
4. If you decide to move into UV, begin ordering the inks and equipment. Expect lead times of 6-12 weeks.
5. Install and adjust the equipment to match your production needs and curing requirements. Test the UV inks you ordered on typical substrates as you fine-tune the curing system.
6. Implement process controls to ensure quality printed products. These controls should cover ink thickness and UV intensity measurements, lamp-life monitoring, regular maintenance schedules, color accuracy, etc.
7. Once your system is in place and UV production begins, reassess your ink-inventory requirements and adjust as necessary.
Are UV inks the perfect solution?
Moving into any unfamiliar technology requires research, testing, and procedural changes. This holds true for pad printing with UV inks. Adding UV technology to your production mix will require you to understand UV ink chemistry, invest in new equipment, and implement new quality-control procedures. And to bring employees up to speed about the new technology, you can expect to invest in training as well. By following a careful approach, however, you can realize increased productivity, lower energy costs, and high-quality products that exceed your customers' expectations.