- March 01, 2008, By David Argent Contributing Editor
Drying of liquid inks is a simple mechanism requiring complex engineering. About 20 years ago I worked on computer-simulated drying to develop ink formulation inputs. There was interest in running OPP snack food webs on flexo CI presses with water-based ink to limit solvent emissions. The questions posed concerned what changes were needed in a water-based formulation to maintain existing line speeds. The simulator tracked evaporation of volatiles as the web passed through the press from unwind to rewind.
Between-color and overhead dryer parameters were based on designs already running on presses. A proven solvent ink system was used on the input side with colors at 28% solids and white running at 45%. Solvent composition was 80:20 Ethanol:N-propyl Acetate. Dry coat weight of color was 0.8#/ream; the white was 1.2#/ream. The graphic was a three color trap: color in deck 1, trapped color in deck 2, and backed with white in deck 6. The web was 45-ga OPP. Web temperature was held to 130 deg F to avoid distortion.
The virtual press ran at 750 fpm. Output was calculated to track volatiles in #/ream retained in the ink film as the web traveled through the press and to compute the length of overhead dryer needed to dry the print completely. As expected, the between-station dryers could dry the ink only somewhat due to the minimal dwell time. In practice, this can lead to the need to space traps on the press.
A standard water ink based on a blend of 85:15 water:N-propyl alcohol with the same solids and application weight was tested in the same press setup. The graph shows trapped colors would be prone to picking and bleeding, and the overhead oven would need to be double the length of that indicated in the base line solvent ink. The virtual press showed that a stronger water-based formulation could be dried as well as solvent ink. It was then a question of formulating a stronger ink system with higher application solids and finding a metering system to apply lower weights.
For years some printers ran these high-strength, water-based flexo inks on snack food films, but the need for higher line speeds and some technical limitations pointed the market back to solvent formulations. The lessons learned are still valid, and improved software is used by equipment manufacturers today to optimize the converting requirements for which the equipment is being purchased.
Process improvement expert David Argent has 30+ years of experience in process analysis with particular emphasis on ink and coating design and performance. Contact him at 636-391-8180; firstname.lastname@example.org.