UV Energy-Curable Inks
There are two types of UV light curing technologies: mercury bulb UV light and now UV LED light.
UV energy-curable inks contain virtually no VOCs, so they are 100 percent stable and can remain in the press for very long periods of time. Maintenance of these inks on press is not required. Plates stay clean because the inks can never dry on them. Inks can be left on press for very long periods of time, including overnight.
These inks are also very durable. They are naturally water, chemical and solvent resistant. They are also very scratch and scuff resistant on most substrates. All colors are strong and vibrant because the inks are virtually 100 percent solids with no volatiles to evaporate. Opaque white inks are higher opacity than water- or solvent-based varieties. Low-migration formulations are now available for food packaging applications. Because of zero VOCs, energy-curable inks, too, can be a marketing advantage for converters selling to environmentally conscientious brand owners and customers.
In addition, UV LED curing provides complete “through the ink film” curing, allowing for much faster press speeds than mercury bulb UV curing. Press speeds are now pushing faster than 700 fpm. UV LED provides consistent light energy output for thousands of hours of run time, giving flexo printers a higher confidence level that the inks will cure properly. UV LED curing uses much less energy than mercury bulb curing and puts out far less heat, so it is a much better choice for printing on heat-sensitive substrates.
Unless your business is focused on long-run jobs where maximum press speeds are required, flexo printers should consider utilizing water-based and UV light energy-cured inks as formidable options to solvent ink technology.
In the “cons” column, UV light curing inks can swell photopolymer plates on long pressruns, causing dot gain. There is a potential for ink components to migrate via offsetting in rewind rolls or through packaging over time if not fully cured. There is also a potential for migration contamination of food products if the inks are not fully cured. It is hard for a printer to determine if the inks are fully cured when running.
These inks are not recommended for highly absorbent substrates, as UV inks can dive in and not fully cure, which could lead to migration problems. Additional press cooling controls are usually needed for heat-sensitive substrates. Heat is much less of an issue when using LED light curing.
Mercury UV bulbs must be replaced every 1,200 hours or so to ensure adequate energy on press to provide full ink cure. Substrate-cooling rollers or cylinders are required for heat-sensitive substrates. It is also a high-power consumption process. And the initial cost of UV LED light curing equipment is not only high but also increases proportionally with the width of the press.
Typical products printed with UV light energy-curable inks include:
- Shrink sleeve and shrink film printing
- Most surface and reverse printed flexible packaging materials
- PS labels
- In-mold labels
- Foil and film lidding materials
- Folding cartons
- Surface print film and paper applications where very high ink durability is needed
- Higher holdout coated paper and paperboard substrates
UV light-curable inks can be a skin irritant to some individuals, so hygiene controls are needed to minimize skin contact when handling inks. Also, UV light curable inks can migrate and possibly contaminate a packaged product if they are not fully cured on press, so process controls need to be implemented to minimize the possibility of under-cured inks.
Non-toxic, environmentally friendly ink cleaners are available for cleaning up UV light-curable inks. Alcohol-type cleaners will also work well. UV light-curable inks typically have a three-to-five-year shelf life because the photo initiators will eventually start to polymerize the inks in storage.
Inks for Success
Unless your business is focused on long-run jobs where maximum press speeds are required, flexo printers should consider utilizing water-based and UV light energy-cured inks as formidable options to solvent ink technology. They should be considered part of your future success plan.
About the Author: Bill Phillips is business development manager of liquid inks at hubergroup USA Inc, based out of Arlington Heights, IL.