Drying Time in Today’s Print Market
Today’s print markets have changed in that there is a movement toward shorter runs and faster customer delivery times. Fast-drying aqueous coatings allow these jobs to be processed quickly and efficiently to that end. Before aqueous coating availability, lengthy ink and overprint drying delays had to be endured before attempting the work & turn printing of the second side of a substrate.
While there are numerous factors that influence the ability to effectively work & turn (back up a job), perhaps the aqueous coating itself remains the most important. The aqueous coating formula must be one formulated specifically with work & turn performance (formulated for blocking resistance) as criteria. The coating must be capable of drying fast and hard in order to pass a face to face block test. Blocking resistance is a necessity since a dry first sidecoating is exposed to the pile heat build-up of second side coating (IR) drying.
Typically, aqueous coating drying systems include hot air flow, Infra-red, and exhaust/evacuation to accelerate aqueous coating drying at high press speeds. Aqueous coatings dry by means of evaporation of the coatings’ water, ammonia and co-solvents components (may be up to 40%), and absorption into the substrate.
The substrate type and the ink coverage influence when a job can be backed up. Substrate type affects absorption, and ink coverage affects setting (heavy four color process work sets slower than a light screen job). In any case, faster setting inks yield a faster turn time. Further, the thinner the aqueous coating weight or film thickness the faster it can be effectively dried.
Another factor involves the size of the load. A small load will have more air in it effectively airing the sheets, preventing blocking. Periodic physical airing of the load can also help prevent blocking. Load temperature is critical in preventing load blocking. It’s amazing how much heat can build up in a pile even after the pile is removed from press end.