In sheet-fed offset printing , the printing ink is not yet completely dry at the point in time at which the printed sheet is placed on the delivery pile . As a result it for smearing , set-off and blocking may occur the arc. Smear is to be understood as the smearing of the ink , setting off means the transfer of ink to the subsequent sheet in the stack and blocking is what the printer calls the sticking together of sheets on top of one another . To prevent this, the powder machine sprinkles a fine powder onto each sheet so that the sheets in the stack do not touch each other directly. A distinction is made between coarse powder for cardboard and fine powder for paper. The printer can choose the amount of powder as required, depending on the type of paper and paint application.
However, this powdering leads to new problems such as soiling of the printing press, loss of gloss in the printed image and possible difficulties in further processing. Especially with print products that are personalized in a further step by digital printing , the powder can lead to problems when the paper is fed in and to contamination of the digital printing system.
Mainly starch from corn or calcium carbonate with defined grain sizes are used as powder . However, only a small part of the powder dispensed reaches the printed sheet. The faster the machine runs, the lower the amount of powder arriving, while the rest of the machine in the delivery area is dirty.
With very long offset machines, such as As an 8-color machine for the face and reverse printing, has the color on the printed sheets will stay fresh for a building to avoid the paint on the impression cylinder. This leads to increased problems in the delivery pile and increased powdering. Powder suction systems remove large amounts of excess powder here. The fine dust particles below 10 µm in grain size are critical , as they can damage the lungs. Powder-air mixtures based on starch also harbor the risk of dust explosions . A distinction is made between two types of construction, the continuous pollinator and the stack pollinator.
- Helmut Kipphan (Ed.): Handbuch der Printmedien, p. 259 f. Verlag Springer, November 2000. ISBN 3-540-66941-8