As more UV curing inkjet printers come to market, the safety aspects of these new inks and printing systems need to be understood by those considering a purchase. To protect both production workers and print customers, safeguards must be put in place. The major safety issues with UV-curable inks, unlike solvent printers’ flammability and high levels of vapors, are skin irritation, inhalation of reactive materials from the printing and curing process, and eye damage.
To help define the issues, I consulted scientists and engineers at DuPont Imaging Systems. DuPont manufactures UV-curable inks and UV-cure printing systems, and prides itself for being the safest chemical company in the world.
UV-curable inks are very reactive materials and are classified as hazardous. They must be treated carefully to prevent injury. Skin irritation is the most significant risk from liquid or incompletely cured ink and can cause a rash. Long term exposure may cause sensitization in some individuals, causing them to react violently to subsequent low-level exposure to the ink or its vapors. To further confuse the issue, some individuals may react to one brand of ink, but not to another.
Once completely cured, UV ink is safe. To assure complete curing, individual equipment manufacturers’ UV exposure recommendations need to be followed. If the ink smears or the image can be damaged by a thumb twist, it is not fully cured. It is not always easy to tell if curing is complete. So even if the image appears to be cured it may not be.
Be aware: incomplete curing is sometimes used to demonstrate ink flexibility and good adhesion on difficult substrates. This is especially true when demonstrating flexibility on fabrics. This practice is misleading and hazardous, and over time the substrate will cure and the flexibility and adhesion may change, usually resulting in a stiffer image on fabric and poor adhesion to difficult substrates. Post print curing can be accomplished by long term exposure to a UV source such as the sun.
Disposal of waste UV ink must be done carefully, preferably by a professional hazardous waste company, as UV ink dumped into water will kill fish.
Some UV inks are not well designed. Obvious separation of the ink in the bottle or the need for continuous agitation to keep inks from settling are signs of a poor ink formulation. Use of such inks can result in many printing, color and curing issues, because the formulation will vary over time as the ink is used. Settling may also occur in the ink supply system to the printheads as well, a problem for inks of all types.
In choosing and operating a UV-curing printing system there are several significant safety considerations. First are the effluents (waste elements) from the process. These include fine ink droplets from the printheads, the ozone produced by UV lamps, and materials evolving from the substrates being heated by the lamps. These materials concentrate in the air around the printing area of the printer. If not vented properly, these contaminants may cause health problems for the operator who spends a lot of time near the printer. Some printers have venting systems built in. DuPont’s 22 UV printer, for example, allows the hazardous vapors to exit through its venting system when the door is opened. Some printers offer no protection from these vapors.
THE UV LAMP
Ultraviolet lamps on UV-curing printers need to be very intense to enable curing at fast printing speeds. This intense light can cause eye damage in much the same way as high intensity sun exposure. Cataracts and retinal damage result when there is prolonged exposure to UV radiation. The damage is cumulative and occurs without pain. Therefore, it is imperative that operator exposure to working UV-cure lamps be minimized. Both direct and reflected UV light are hazardous.
A general rule of thumb to share with operators is, “if you can see the light, you are being exposed to UV radiation.” Some printer manufacturers have chosen to take this hazard seriously and others have not. Look for a printer with a UV-blocking glass window or solid door enclosure, and a brush on the bottom of the lid that prevents UV light from escaping. If the printer’s built-in protection is inadequate, eye protection should be required of everyone near the printer during operation. Remember, since there is no pain associated with UV eye exposure, eye damage won’t be detected until it is too late.
With the development of this technology we have traded solvent fumes for reactive inks and UV light. This generally proves to be a good tradeoff, and with proper safety procedures in place, many substrates can be printed safely while maintaining good quality and durability.