- October 01, 2000, Richard M. Podhajny, Ph.D., Contributing Editor
One reason the drying rate of flexo inks has become an important subject is the general trend toward the use of thinner ink films. In the narrow web sector, demand for higher quality process printing has changed the printing press and the flexo process inks. Inks today need to be stronger since they are applied by a finer anilox roll.
Whereas 400-line screen anilox was adequate in the past, today standards require process printing with anilox rolls of 800-line screen or higher. Inks applied from these finer anilox rolls have to dry more slowly if they are to stay open on the anilox rolls and the printing plates. Yet, at the same time, inks must dry quickly to maintain productivity. As a result, the process inks are stronger and are applied in a thinner ink film.
Over the many years I have been involved in formulating inks, I have yet to find a good quantitative method of predicting ink-drying rates. Unless you have a press with appropriate drying, it is difficult to predict the ink-drying rate on a specific press with a given ink.
The basic problem of any laboratory attempt to mimic a high-speed press is that lab printing units produce prints at a much slower press speed, and the lab oven does not have the same drying efficiency.
Many laboratories use a conveyer unit equipped with an electrical hot air dryer (similar to a hair dryer) to provide the heat and air flow. The problem with these units is that the samples are printed separately before they can pass through this conveyer dryer. The result is that the ink has lost much of its solvent, and the drying test provides little assurance that the ink will dry effectively on a high-speed, commercial press.
The drying speed is determined at a specified oven temperature at the point where the ink will not set off. This is the "tack-free drying point." Once the tack-free drying point is established for a given ink formulation, you can compare other ink formulations to it. Even though this method uses a hot air dryer, its correlation to drying on a commercial dryer is rarely attempted, and its use is restricted to laboratory comparisons.
An experienced ink formulator once explained that it is possible to assess the relative drying rate of an ink by conducting a drying test using a NPIRI grind gauge that was developed at the National Printing Ink Research Institute. This gauge is a finely, mechanically engraved tool that has two identical parallel channels, which range from 0-25 microns. The basis of this test is that you can place an ink in question and compare to a standard.
Both inks will "dry" and create a "tack-free" length along the channel. Using a stopwatch and determining the tack-free point length in the channel on the NPIRI gauge, you can make a relative comparison of ink drying.
You can make a permanent image of this NPIRI drying comparison. To do this, you compare your ink to a standard using the NPIRI gauge by imaging the two ink channels (after an interval of time) in the gauge by placing an absorbent paper onto the gauge and rolling over it with a rubber roller.
The image created then can be measured, and the length of the tacky portion of the image is the relative rate of drying. (The dried or nontacky portion will not transfer to the paper.)
Although this method's usefulness depends on using an ink standard, it has proven to be an invaluable tool for many laboratories. Application of this test with the quantitative aspect is particularly well suited for routine quality control.
More quantitative methods are available, but they require the use of specialized equipment and are more cumbersome.
Dr. Richard Podhajny has been in the packaging and printing industry for more than 30 years. Contact him at 215/616-6314; firstname.lastname@example.org.