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UV or Not UV in ScreenPrinting Inks
Despite UVís somewhat troubled childhood, maturity has brought it wider acceptance, and in some areas of screenprinting it is even beginning to dominate. Today, virtually all CDs and DVDs are screenprinted using UV inks, and UV is seen as screenprintingís best hope in the ongoing battle with digital print technologies.
The principal reasons for this rapid growth are two-fold. For one thing, UV inks dry very rapidly. (Weíre talking three seconds or less!) For another, UV inks are about as environmentally friendly as you can get, because they produce almost no emissions. Unlike conventional inks, UV inks contain no solvents. The solvents in air-dry screenprinting inks evaporate as the inks dry releasing Volatile Organic Compounds (VOCs) into the atmosphere. VOCs have been a prime target in the battle against air pollution because they are a key contributor to harmful pollutants like ground-level ozone. Some VOCs are classed as Hazardous Air Pollutants (HAPs) and are subject to special regulatory controls under the Clean Air Act.
In many solvent-based inks, solvents make up more than 50% of the ink; in some, they make up more than 70%. Thatís a lot of solvent to evaporate away into the air. So, you can see that a large-scale print operation switching to a zero-emission print medium is going to make a big difference. For this reason alone UV inks are often seen as the wave of the future.
What is UV printing exactly?
No waiting around for prints to dry, no storage racks to hold drying substrate, and no bulky heat-drying equipment, UV can make a big difference to the speed and efficiency of conventional print operations. When printing multicolor jobs, the second color can be printed almost immediately. Some new high-production presses can print several colors at once by passing prints through intermediate UV curing stages. Prints can be handled and packed for shipping the minute they emerge from the curing unit.
Another interesting feature of UV inks is that when they cure almost the entire ink deposit is transformed into a solid. In fact, UV inks are 100% solids. As the solvents in a solvent-based ink evaporate, the ink deposit will continue to shrink until only the pigments and resins remain. But with UV ink, everything you print onto substrate stays there.
A gallon of UV ink is more expensive than its solvent-based counterparts, but that cost doesnít reflect the fact that you are not paying for a lot of solvents that are going to evaporate anyway. Because of their high solids content, UV inks tend to go a long way, typically about double the coverage of solvent-based inks.
How UV inks work
Ultraviolet light is present in sunlight, although it is beyond the visible light spectrum. Itís what your sunglasses and sun blockers are supposed to protect you from. Indeed UV-curable inks will react to sunlight, which is why you will never find a UV printing operation with an unfiltered skylight over the press. Like visible light, UV is a form of radiation and UV curing is lumped together with other forms of radiant curing like EB or electron beam curing.
Like conventional inks, UV inks contain resins and pigments. But in place of solvents, they contain monomers and oligomers, small carbon-based molecules that have the capacity to form into long chains called polymers. If polymers are like a chain, then monomers (which provide the inkís adhesive properties) are like the links in that chain. To get them to start forming chains, UV inks contain highly light-sensitive components called photoinitiators. They are sometimes called photo-catalysts.
The curing process is called polymerization. Sometimes the term crosslinking is used, which means that the monomers in one polymer chain begin linking to similar monomers in an adjacent chain to form a network. In the most common form of UV curing, (free-radical curing) this all happens very rapidly ≠ within seconds.
UV on the Production Line
There are two types of systems: focused, which is used for curing flat printing, and unfocused which is most-often used for curing three-dimensional objects. More complex curing units will mount several lamps and have controls that can adjust the intensity of the UV output. A combination of adjusting UV output and belt speed allows the operator a fair degree of control over the curing rate. This is necessary because not all UV inks, or even different colors of the same ink, cure at the same rate.
What kind of printing operation could benefit from a switch to UV inks?
UV printing can be expensive to get into because you will have to purchase a curing unit, UV inks, and most likely have to replace the mesh in your screens or buy new ones. Most UV printing is done with mesh counts of 380 threads per inch and up, because the idea is to lay down the thinnest possible layer of ink. Calendared mesh is often used in UV printing. This process flattens the print side of the mesh to further reduce thread thickness, which of course, determines ink deposit. Since mesh prices increase as the mesh count goes up, expect to pay top dollar for your screens.
The reason for this skimpy ink deposit is that UV rays must penetrate completely through the ink right to the bottom to affect a cure. This is also why UV inks are almost never opaque. A lot of effort in product development has gone into making them more opaque, but that is one area where they still canít compete with solvent-based inks.
What are the advantages of using UV?
A second reason is that because of their rapid curing ability, UV inks can increase production speed. Furthermore, a UV curing unit occupies less space than the drying racks or heating ovens used to dry solvent-based inks. Facilities that formerly operated heat dryers can also find their utility bills dropping. While UV curing units produce heat, itís nowhere near what a heat-drying unit creates.
Another big advantage of UV inks is that they donít dry in the screen. Air-dry inks continue to increase in viscosity as you use them because the solvents in them continue to evaporate. UV inks do not increase in viscosity. The result is you donít have to shut down in the middle of a production run to clean your screen. In some facilities using UV inks, inks are left in the presses overnight.
Because of the fine meshes that can be used, UV inks excel in printing fine four-color work and produce sharper prints. Many printers also combine UV inks with solvent-based inks to take advantage of the benefits provided by both types. Needless to say, such combination jobs have to be approached cautiously and with careful testing to insure compatibility.
What are the disadvantages?
A UV printing operation depends on its curing unit. If it fails, your production is completely shut down, so maintenance becomes a big issue. UV lamps have to be replaced after about 1,000 to 3,000 hours of use. And their output really should be monitored throughout their life cycle by regularly checking their output with a radiometer.
If youíre thinking about converting to UV, be sure to budget for training employees to handle the new technology. Itís quite a step for someone used to dealing with conventional inks. You should also expect to encounter a few failures as you mount the learning curve.
Health and safety concerns have to be addressed. Some people find the odor of UV inks unpleasant, so you may have to add some ventilation. UV inks can cause skin irritation, and exposure to UV light can also cause a skin redness known as erythema. Needless to say, eye protection should be worn by anybody that might be exposed to UV light rays. UV shielding should always be in place on the curing unit.
Because of the heat generated in the curing unit, some heat-sensitive materials can only be printed with difficulty and great care.
Many of UVís problems date from the early days of the technology so be sure you have the most up-to-date information before making a decision about the process.
Getting set up for UV printing
How busy does your shop have to be to consider going into UV? Mr. Stine notes, ď We have sold units to shops that do only $500,000 a year.Ē
With its good environmental record and the continuing development in all aspects of the technology, UV may well represent the wave of the future for screenprinting. Most of the growth in UV printing has come within the last five years, and as time goes on the maturing of the process will no doubt produce cheaper entry-level equipment.
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