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UV Screen Printing Inks for Plastics
by Jeff Morris

Technology Feature
July-August2003



UV screen printing inks have been available for decades, but like any maturing process, the technology has grown leaps and bounds in recent years. Improvements in specific substrate formulations have made printing on difficult substrates possible, and ink manufacturers have developed inks with viscosity and rheological properties that have been optimized for every variable in the process including machine type, machine speed, screen mesh, squeegie type and more. Today there are as many ink types and additives available as there are substrates.

Many decorators have the ability to print both UV and solvent-based inks, and deciding when UV inks offer an advantage is something your ink supplier should be able to assist you with. For instance, high speed/high volume jobs such as printing HDPE shampoo bottles is a perfect application for UV inks. Due to their lack of solvent, UV inks offer much improved process control in that they do not clog or dry in the screen, the viscosity remains much more constant, and color variance due to the continuous adding of thinner is eliminated.

Also, typically UV inks are more chemical and scratch resistant than solvent-based inks due to the improved hardness of the cured ink film. Cosmetically, UV inks generally exhibit greater gloss and smoothness as well. These are a few examples of ways that UV inks can be more desirable than conventional inks, and once you’ve chosen to use UV, your options will be greater than ever. Your ink supplier should be able to offer you information that will help you maximize your potential with UV inks.

Your ink supplier will fall into one of two categories: Either they are knowledgeable and armed with the latest technology to help you find inks that will not only adhere to your substrate but improve your process and your bottom line, or they are not knowledgeable and thereby are holding you back from reaching your full production and profit potential. To find out which type of supplier you currently have, let’s discuss the process and hopefully you’ll be able to ask the right questions of your supplier in order to get the most from your UV ink.

Unlike conventional inks, UV inks contain no solvent and require UV radiation to cure. Substances called photo initiators are added to the ink’s formulation, and when exposed to UV energy, they start the reaction that results in a cured ink film.

In recent years, many new types of photo initiators have become available, some better than others. Some UV ink manufacturers have started using lesser quality initiators to save cost. If you’re printing on ONLY white substrates, your bulbs are fresh, reflectors clean and focal point optimized you need not worry about the quality of the photo initiator in your UV ink. On the other hand, if you print colored substrates and your UV curing system is less than perfect you’ll need to insist on only high quality photo initiators in your ink. These higher quality products may be more expensive, but you’ll rest easy knowing the ink you’re using day in and day out will cure properly every time.

Some manufacturers offer curing promoters that you can add to your ink when you see a curing problem. For instance, let’s say you’re printing solid black ink with a wide mesh count of 255. Even with the best photo initiators, you may see curing problems. If so, call your supplier. They should be able to supply you with a curing promoter (usually a pure, high quality photo initiator) that you can add to your black ink to solve the problem.

Not all processes are the same, so why should you settle for ink designed for someone else’s machine speeds and mesh counts? Do you find that your ink is too thin, and runs through your screen? Is your ink too thick, resulting in clogged mesh or poor screen flooding? Even if your supplier only has one viscosity for ink there are still ways to adjust your ink’s viscosity and rheology to make it work better for you and your individual needs. First, a layman’s explanation of viscosity and rheology: rheology is the term used to describe the study of an ink’s flow characteristics and is not to be confused with viscosity. Viscosity is a term we’re all familiar with, and any machine operator understands that some inks are “thick” while others are “thin”.

Your ink supplier should be able to clearly explain what effect their ink’s viscosity and rheology (or flow characteristics) will have on your process. If you’re having problems with ink running through the screen during set-up, there are additives that can reduce the ink’s flow while maintaining the thin viscosity that you require at high speed when the machine is running. On the other hand, if your ink appears to be clogging in the screen when using certain high mesh counts or doesn’t completely flood the image area after each stroke, there are additives that will break down the ink’s viscosity or increase flow to help cure this. An ink’s flow properties also have a significant effect on the visual appearance of a cured ink film, and your supplier should be able to explain why one ink looks smooth and glossy while another looks mottled or matte.

For years, the “knock” against UV screen printing has been that the colors are too translucent and don’t cover dark substrates well. Experienced printers understand how to manipulate the process to achieve the best coverage, but what about the inks themselves? How can they be made to better cover dark substrates?

Some UV ink manufacturers will tell you that there is nothing they can do with the inks to improve the opacity, other than make certain process recommendations, i.e. mesh count, squeegie durometer/angle, etc. Fortunately, there are also some manufacturers who have done their homework and have developed color concentrates coupled with high quality photo initiators that can make it possible to print more vibrant, opaque colors than ever before. This is a fine balance because as you know, UV ink relies on the penetration of UV energy through the entire ink film to achieve a full cure. This is why it is important to ask your supplier about the interaction between the concentrates and photo initiators to be sure you’ll get a full cure, even with much improved opacity.

These are just a few topics that your ink supplier should be able to address, and with the correct information you should be able to ask the right questions to improve your printing process with UV inks.

Jeff Morris is the vice president of sales with Ruco USA, a Division of Comdec Incorporated. Ruco USA is the Exclusive North American Importer of Ruco Printing Inks of Germany. For additional information contact Jeff Morris at (800) 445-9176, email jmorris@comdecinc.com or visit: www.rucousa.com.