- December 01, 2002, Dr. Richard M. Podhajny, Ph.D., Contributing Editor
Most flexo presses producing packaging run in the range of 300-1,000 fpm. Occasionally, a few customers want to increase their press speeds still further. Often, attempts to run at higher speeds lead to unforeseen problems.
Although mechanically most flexo presses can run at relatively high speeds approaching 2,000 fpm, doing so problem-free is still a challenge. Factors that prevent most printers from reaching these high press speeds often have little to do with the press itself but more with the ink formulations and substrate surface treatment limitations.
Ink feed: Today's high-performance presses utilize enclosed doctor blade ink feed systems that can have lower ink surface area in contact with the anilox cylinder. Maintaining good ink feed at high press speeds can be demanding on the ink feed. The pump ink delivery and position of the infeed may have to be adjusted as a significant increase in the press dynamics can create local ink starvation areas.
Anilox wiping and friction issues: Use of dual-blade inking systems brings higher friction between the doctor blades and the anilox roll. As the press speeds are increased, anilox roll surface temperature can exceed 140 deg F. At such a surface temperature, the ink can dry in the anilox cells.
As the inks can flash off their fast solvents, the ink can increase in surface tension and decrease ink wettability. If running water-based inks, cell shape and dimensions may affect the rate of ink flowout. In general, cells that have an edge (diamond, hexagon, etc.) will provide better flow than spherical cells just based on surface-tension dynamics.
Another important area for high press speed is the surface tension of the substrate you will be printing. In general, the higher the surface tension, the better the ink wettability and flowout, as well as the drying. If you are using in-line corona treatment, the faster you go, the less effective your surface treatment. As press speeds increase, the power of the corona should be increased, or additional corona treaters should be installed. Measure the surface tension of the web as a function of press speed with the corona treaters in normal operating mode. If you place an additional corona treater or increase the power, you should be able to bring the surface tension to the desired level.
Ink wettability: High press speed lowers ink dwell time before the inks hit the dryer. As a result, ink may need to be rheologically adjusted so it can perform at these higher press speeds.
Drying: Although most dryers have high capacity, as you increase drying speed you should determine if the retained solvent levels still are within your specification range. Dryer temperatures may have to be increased to accommodate the drying speed of some inks, such as water-based ink formulations.
The inks themselves should be reformulated for press speeds of 1,500 fpm. The reformulation may involve the addition of a fast co-solvent, a pH adjustment, or the addition of a more suitable surface-wetting agent.
One approach to running at higher speeds is to increase the color strength of the inks, so a thinner ink can be run. This will greatly improve the drying rate necessary for high-speed printing.
Foaming: Another phenomenon that can occur with high press speed is foaming in water-based inks. Although at 700 fpm you may not see any appreciable foaming, air entrapped in the ink may not be able to get out quickly at high press speeds; the result can be an undesirable microfoam. As such, this often requires a different defoamer to be incorporated into the ink system.
Dr. Richard M. Podhajny has been in the packaging and printing industry for more than 30 years. Contact him at 215/616-6314; email@example.com.