Drying Aqueous Coated Printing
When it comes to drying aqueous coated printing, hot air movement is the most effective element. IR also aids in drying inks and coating and can compensate for a short drier as well as control pile temperature. The exhaust air evacuation system should always be run at 100% with the damper wide open, maximizing efficiency in order to remove moisture-laden air from the press area and optimize drying conditions. This will also help control the press room humidity.
Extended delivery is always a benefit giving the printed/coated sheet more time to dry before sheets are piled in delivery. Modern presses are also often equipped with hot air knives. It is advised that they be always run at 100% volume with the temperature setting at ambient. While by design the volume of airflow can be controlled, as well as the temperature being blown at the sheet, it is advised not to try to control the pile temperature using them.
It is well to remember that aqueous coatings are thermoplastic, dictating that they will re-soften when heat and pressure are applied. It is essential that the pile temperature be maintained within the recommendations of the coating supplier. Pile temperature is the result of the IR input and stock thickness.
A delivered sheet should be dry to the touch. If the first side coating is not effectively dried it may be re-wet by second side coating contact in the pile. Re-wetting can also occur if the first side coating is dried effectively, but the second side coating is not, resulting in the second side coating re-wetting the first side coating. This is not re-softening of the first side coating. Picking or sticking of the coatings will likely be observed. Successful work & turn printing/coating requires that both coated sides need to be effectively dried to prevent re-wetting.
The pitfall of work & turn two-side printing and aqueous coating is overheating the sheets during the second print/coat pass. Overheating is a sure way to reach a condition where sheet blocking will occur in the load ruining the job.
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