Discover how UV inks differ from other formulations and what it takes to print and cure them successfully.
By Bea Purcell
Depending on the ink system, certain colors within the system will have the tendency to cure faster than others. By the same token, certain ink systems will cure more slowly than others. Ink-manufacturer recommendations and your own experimentation will help guide you to the proper curing parameters for each ink line and color you use.
The inks should cure properly because my "puck" (radiometer) is showing the correct millijoule reading as recommended by my ink supplier. Radiometers are useful tools in measuring energy output. However, there is no correlation between numbers measured by different radiometers for the same curing unit. Employing the same radiometer to measure various curing units in the same shop is useful only for comparative purposes.
Ink manufacturers may have a published range of ideal curing conditions that were determined under controlled laboratory conditions, and these can serve as a starting point for your own measurements. Just keep in mind that in screen printing, where ink-film thickness can vary, the number of millijoules and, more importantly, the milliwatts necessary to cure an ink will also vary.
The lamp is bright, so it is still good. A UV lamp emits three different energies: ultraviolet, visible light, and infrared. As the lamp ages, the brighter and hotter it becomes. Simultaneously, the ultraviolet energy emitted by the lamp decreases. Since ultraviolet energy is invisible, regular measurement with a radiometer and good record keeping with a radiometer is the only way to ensure that UV lamps are operating correctly.
The ink is bad because it is not sticking to the material. There is a fine line between adhesion and cure. Ink film that is not cured thoroughly will not adhere to the substrate. Assuming you're printing a colored ink rather than a clear coat, you can determine whether the problem is adhesion or cure by slowing the conveyor speed and increasing lamp wattage. In other words, you eliminate curing as a variable by curing the dickens out of the ink. If the ink still doesn't stick, you know you're dealing with an adhesion problem.
You can also print a clear coat and check for adhesion (using the cross-hatch and tape test or a similar method). The clear coat should cure faster than pigmented inks, so if the clear adheres to the substrate but the colored ink does not, then the problem is cure. If the clear ink film lifts from the substrate like a banana peel, then the problem is adhesion.
The ink has a different smell; therefore, it is not good. Every UV ink has its own distinct smell in the container, depending on the monomers that are used in the ink. Smell has very little to do with the ink's performance. If smell is an issue, odor-masking agents may be added to the ink during or after manufacture.
All curing units are the same. Performance differences always exist among curing units, even in models from the same manufacturer used in the same production area. Among other things, these differences can stem from lamp age, use of different reflector geometries, and differences in the way the curing equipment is maintained.
Make UV work for you
UV technology gives screen printers the capability to print faster, more efficiently, and more accurately on a wide range of materials. But to realize the full benefits of UV, users must understand the nature of UV inks and the equipment required to cure them. UV technology leaves little margin for error, but through testing, regular measurement of curing conditions, and accurate recordkeeping, you can enjoy all the advantages that this technology has to offer.
About the author
Bea Purcell is product manager for Nazdar, Shawnee, KS. She has more than 18 years experience in screen-printing and UV-curing technology and has held positions in technical services, ink research and development, training, sales, and marketing. Purcell holds a bachelor of science degree in chemical engineering from the University of San Carlos and a master of science degree in education from the University of Southern California.
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