Ultraviolet curing (UV) inks are not new to the market, but they have continued to build momentum as the ink of choice for more and more screen printing applications. UV inks are not the answer to everything. However, from environmental advantages to faster production speeds, more and more evidence is being gathered that suggests that UV is the best choice for many, if not most, situations in today’s screen printing environment.
Before UV inks and UV curing systems (dryers) became a viable option for multi-colored screen printing, heated conveyor dryers were the only option to dry each color before the next one was applied. In order to run an automatic screen printing line with several colors, multiples of machine/dryer combinations (sometimes as large as thirty feet long for each color) had to be connected. Systems running conventional inks have a difficult time running faster than 60 prints per minute due to the limits on how fast the ink can be cured. The faster the machine runs, the larger the drying system needs to be. As one can imagine, this became an extremely cumbersome and expensive venture for multi-color screen printing units.
Multi-color screen printing units today use ultraviolet lamp systems that provide output in the 300 to 400 watts per inch (WPI) range and spectral output in the specified range needed for optimum curing. Most UV lamps use mercury and additives to modify and concentrate the output. A typical 10 inch lamp system running at 300 WPI will produce 3000 watts. Special cooling and exhaust systems are required as well as proper shielding to restrict UV exposure around the machine.
With this type of UV curing system, multi-color machine lines can be assembled with automatic print/cure color modules that are as small as five feet long for each color! Automatic lines to apply up to six colors are possible, saving the screen printer both floor space and electrical energy. In addition, speeds of over 100 prints per minute are possible because of the instantaneous curing time from one color to the next.
UV curing equipment generally falls into two main categories: conveyor systems for use with semi-automatic screen printing machines, and curing systems designed and built in conjunction with automatic screen printing equipment. Semi-automatic machines can be converted with the use of new screens and the addition of a UV curing unit. Units are available in many configurations from flat sheet systems to 3D container printing units that rotate the bottle or container in front of the lamp. Older, automatic screen printing systems can be retrofitted as well. It is recommended to work with a company that has experience with these types of conversions if one is considering the addition of a UV curing system.
From a machine standpoint, many factors are involved with assuring the desired results using UV inks. Probably the main concern with UV inks, which must be monitored carefully, is the amount of ink that is applied to the plastic substrate. UV inks are somewhat translucent by nature because the light energy needs to reach the bottom of the coating at the substrate surface for proper curing. If too much pigment is added, it can block the energy from curing the entire coating correctly. To compensate for this, a finer screen mesh is required. A 355 to 420 mesh screen is recommended versus a 230 mesh screen that is common with conventional inks. In addition to the mesh, the trend is to utilize aluminum frames for the screens. Aluminum frames are recommended for close tolerance printing due to screen tension and warp issues. Non-flat screens that have warped are difficult to set over the surface to be printed. It is very important to deposit the correct thickness of the UV curable coating by adjusting the mesh count and other variables for a specific application. Another area of concern is the durometer or hardness of the squeegee, as well as its angle and position. In printing cylindrical objects, the screen normally moves horizontally to stroke and rotate the bottle while the squeegee is positioned on its center. As the squeegee V point wears and various pressure settings are used, the squeegee will move off center. An off center squeegee in cylindrical printing will deposit more ink and possibly blur the image. Without creating a blur problem, moving the squeegee off center can help control the ink deposit or opacity in a UV ink application.
Ink Advantages and Disadvantages
UV inks certainly bring many advantages to the table. First, UV inks can provide economic benefits for the screen printer. Although UV inks are generally more expensive, the operator can lay down a smaller amount of ink because of the finer mesh used versus conventional solvent inks. In addition, after the volatiles are dried by heat, the layer thickness of solvent inks can be reduced by as much as 30 percent. With UV inks, 100 percent of the ink deposited stays on the plastic part – nothing is flashed off. This can help offset the total cost of the inks by using less.
As mentioned earlier, the greatest economic advantage that UV inks provide is the increased production speeds that can be realized. In many cases, UV inks can run at much higher speeds than conventional inks because of the curing time involved. Again, the floor space savings and potential energy savings realized through the use of UV inks are significant.
UV inks also are operator-friendly. With solvent-based inks, the operator usually must incorporate the appropriate amount of hardener and/or thinner to adjust the viscosity and drying characteristics. On the other hand, UV inks generally do not need any type of additive to boost adhesion or change the characteristics of the ink. Also, because UV inks must be exposed to UV light to cure, the ink does not dry on the screen. Conversely, keeping the ink in the screen during breaks or even overnight is not a problem. Solvent-based inks will dry and clog the screen if not cleaned properly right after a run or at the end of the day.
In addition, environmental benefits exist with UV inks. When solvent-based inks are cured, emissions must be vented properly. And because solvents can be combustible or flammable, there are many regulations on shipping, storage, and disposal that do not accompany UV ink products. The only emission that is produced by UV curing is a small amount of ozone gas of which is easily disposed.
Finally, UV inks provide a glossier cosmetic finish on the plastic substrate. And because a finer mesh is used, more detailed, finer copy can be screen printed.
Although utilizing UV inks poses many advantages, it is not the answer for all applications. First, it does not provide the opacity of solvent-based inks, which can cause problems on dark plastic substrates. For instance, a yellow will not keep its color over a dark blue or black plastic. In addition, there are some plastic substrates that do not accept UV inks at all. Certain polyesters must be printed with special solvent-based inks. Also, there have been challenges with using UV inks with special processes such as elongation for in-mold decorating and vacuum forming applications. Solvent-based products have generally worked better for these processes; however, new UV products are being produced practically on a daily basis with properties that were previously unavailable.
Plastics Decorating would like to thank Henry Newman of H.L. Newman Equipment Co., Inc. (847-831-0220), Bob Chadwick of Comdec, Inc. (800-445-9176), and David Cordell of Nor-cote International (765-362-9180) for their assistance with this article.