In sheet-fed offset printing, the conventional ink is still not thoroughly dry when the sheet arrives at the delivery pile. Consequently one of the major requirements in sheetfed offset printing is the avoidance of smearing, offsetting (transfer of ink onto the following sheet), and blocking in the pile (sheets lying on top of each other sticking together).
The application of powder offers a remedy yet brings many negative consequences, such as soiling of the press, quality defects in the print area (reduced gloss), and finishing problems.
Due to the powder layer between the sheets, the wet ink surface does not come in contact with the following sheet. Only a small proportion of the delivered powder actually arrives on the sheet. The faster the offset press runs, the smaller the powder yield that actually reaches the sheet, the remainder contributing to soiling of the press in the delivery area. The longer the offset press, such as an eight-color press for straight printing and perfecting (4/4), the longer the ink has to remain fresh to avoid build-up on the impression cylinders, and the more crucial the pile formation in the delivery becomes. Consequently, with long, fast-running sheet-fed presses, powdering has to be intensified. Powder extraction systems in the delivery actually remove large quantities of powder, though a residue still remains, which necessitates cleaning work in the press, though considerably less often.