by Curt Baskin, Sun Chemical
UV- and LED-curable screen printing ink systems are relatively new and growing technologies for many high-tech graphics printers, and with good reason.
UV inks offer an environmentally safe solution that can reduce a printer’s carbon footprint. They also eliminate the space required for long jet dryers or wicket dryers. No residual solvent is left in the printed sheet, and the inks offer better screen stability and color consistency.
One key benefit of curable screen inks is that printing can be done at faster speeds of 1,200 to 2,000 sheets per hour when using certain cylinder presses. Additionally, new designs in traditional UV curing equipment, such as certain curing units, now reduce the excessive heat that previously distorted the substrates.
When comparing UV-curable ink systems vs. approved solvent ink systems (Table 1), the benefits of UV inks are apparent. However, the preexisting specifications needed to convert to UV-curable printing typically require approximately three to six months of testing, and some printers are leery to spend the money needed to adequately perform that testing.
Those printers that do make the investment, however, will find the long-term benefits of switching to UV will save them money in the form of time savings, reduced energy costs, increased manufacturing space, better print quality, faster throughput with cylinder presses and environmental benefits.
Types of UV and LED curing ink systems for the high-tech graphics market
Whether used for automotive, appliance, medical, computer and business equipment, cellular phones or other high-tech parts, UV curing ink systems can be used in a variety of applications.
Special blending and color matching systems are available for each of these ink systems to offer the colors that original equipment manufacturers (OEMs) require. Since OEMs want their high-tech colors to go outside the scope of standard Pantone color targets, a special blending system was designed to achieve the more difficult colors needed to meet specifications.
High-tech applications where UV ink curing systems are available include the following:
Thermoforming and vacuum forming
UV- and LED-curable ink systems can be used for decorating parts that are used for thermoformed and vacuum-formed applications, as well as POP displays. Thermoformed or vacuum-formable plastics, such as those used for bicycle helmets, use deep draw UV ink systems that require flexibility and high elongation properties.
Printers in the in-mold decorating (IMD) and in-mold labeling (IML) industry require robust technologies. There are UV-curable formulations available with very high performance. With extraordinary elongation properties, this UV ink can stretch up to 1.5 inches on 3D plastic parts, such as automotive interiors, appliance parts and radio dials in cars. These inks also have excellent washout resistance and work well with a variety of resins. Formable UV hardcoats are available for use on IMD parts.
UV panel display inks used for automotive and appliance applications can be used with adhesives and pass OEM specifications. Developed for polycarbonate and treated or coated polyester used in the manufacture of panel displays, membrane switch overlays and nameplates, the UV ink curing system offers excellent adhesive delamination resistance and flexibility. A separate UV ink curing system also can be formed up to an inch and passes the Ford Motor World specifications – an extremely difficult metric.
Another UV ink curing system has been developed for printing graphic overlay windows. The transparent inks offer outstanding clarity and excellent scuff resistance. They do not require protective masking after printing is completed, eliminating the use of protective masking of the windows during transport. This takes a major post-processing cost out of the loop – saving manufacturers money and time.
One UV ink system on the market can be used to print directly on decorated coated metal appliance parts and metal signs that pass the Tabor Test. Unlike most metal signs using UV ink coatings, this flexible ink system allows for embossing, debossing and bending without fracturing.
Printers must use extended-life pigments and a durable, clear overprint to achieve approximately five to seven years of outdoor durability. These inks also offer gasoline resistance and can be used to print on metal gas pumps, rather than using pressure-sensitive decals. This provides faster installation and better durability.
A UV-curable, screen-printable overprint clear ink that delivers high gloss, excellent abrasion resistance and enhanced lightfastness has been designed for use with digital inks. Special nonyellowing screen-printable opaque whites also can be used to back up digital inks. Screen printing ink systems used with digital inks enhance performance.
Processing UV-curable inks
The key to ensuring that UV-curable screen inks perform with optimum results on all of these various applications is making sure the correct equipment is used in the printing process. For example, the use of correct lamp systems will reduce the degradation and yellowing of substrates and prevent adhesion problems, while achieving optimum results.
Curing units must offer high enough energy levels without excessive heat that distorts the substrates. The right level of milliwatts (in addition to millijoules) and energy is vital for the proper cure of heavy ink deposits when printing. These units should have iron and gallium capability for this market segment. For LED curing lines, the use of rubber conveyor belts will reject dirt and contamination, thus reducing rejects. Anti-static equipment and cleaning rollers should be set inline to reduce possible rejects from contamination, too.
It is highly recommended that printers use a radiometer and establish a standard setting to maintain a control for their curing units. Standard data can be used for job setup and provide the ability to track lamp usage. A consistent setup of lamps, power/wattage percentage and belt speed is key in consistent, proper cure. For LED curing units, printers should contact the manufacturer for advice on a measuring device.
Lamp configuration is critical to processing UV inks. Four key areas of importance include UV irradiance (mW/cm2 or milliwatts), spectral output (nm or nanometers), UV energy (mj/cm2 or millijoules) and IR energy (excessive heat that can distort substrates as measured by Fahrenheit heat at the focal point of mj/cm2).
Lamps and reflectors should be maintained according to manufacturers recommendations; using gloves when cleaning the bulbs and keeping reflectors clean is vitally important. The longer UV lamps are used, the less effective they become due to the bulbs losing their output. For medium-pressure mercury vapor lamps, output typically drops off after 1,200 hours of use.
Measure the UV output from the lamps with radiometers regularly to ensure consistent curing, and use UV-blocking sleeves on overhead lights to prevent curing in the screen. Additionally, never use solvent-based inks under UV lamps. If solvent-based inks are used, then maintenance often will be needed.
Use the appropriate equipment to ensure the proper mixing of UV inks. The proper formulation of metallic and special-effect colors is necessary to be successful. When using UV inks, keep work areas and equipment clean. Screens, squeegees, mixers and other equipment should be cleaned after every job.
UV inks should be stored in black polyethylene containers to avoid curing from ambient light and premature gelling. Most UV inks have a shelf life of 24 months from the date of manufacture and should be stored between 40 and 90 degrees Fahrenheit. This does not include inks with special-effect pigments or inks that have been modified. Review material safety data sheets to ensure the proper disposal and usage of UV inks.
UV-curable screen inks are readily available for screen printers in the high-tech marketplace. Printers who choose to make an investment in the equipment needed to switch to UV-curable printing will discover considerable savings over the longer term in the form of time, reduced energy costs, increased manufacturing space, better print quality and environmental benefits.
Curt Baskin is a field marketing manager for Sun Chemical and is responsible for a wide range of UV-curable ink systems for the high-tech market, including the PD series of inks for panel displays, OEM series of inks for OEM performance, VAC series of inks for thermoforming and vacuum-forming applications, DMU series of inks for IMD and IML applications, MTG series of inks for formable and nonformable hard coats, UVT series of inks for graphic overlay windows, COMETAL inks for printed coated metal and the DSC series of inks for UV screen-printable overprint clear inks with digital inks. He can be reached at [email protected] or at 708.236.3798.