Commercial Printing & Anti-Offset Powder
What is Anti-Offset Spray?
 When ink is wet, offset powder prevents contact with other sheets |
1. Spray on blanket. Press sheet with voids caused from excess spray.
2. Tape pull from blanket. 3. Scanning Electron photo of tape pull. 4. Press sheet with voids caused from excess spray. 5. Spray particles that caused the voids in Figure 9 are stuck to the opposite sheet. 6. Scanning Electron photos of Figure 8 (left) and Figure 9 (right). |
Specks in printing from Anti-Offset powder (Figure 1) occur in sheet fed printing. The technology of anti-offset sprays has
Excess powder can interfere with applied ink on paper
progressed through recent years, and problems have been minimized with respect to their composition, application methods and ink rejection properties.When specks do occur because of powder, it is usually due to the use of too much powder.
This excess powder will interfere with subsequently applied ink. The function of powder powder is to create a physical gap between the printed sheets in a lift, allowing air to circulate to set and/or dry the ink film(s). (Figure 2)
The amount of powder run is typically governed by the amount of coverage on the job and is left to the judgment of the pressperson. In addition, the ink film thickness required to obtain the desired printing density may vary among inks, and could have a significant effect on the amount of powder which must be run. Figure 3 shows the amount of powder run on a job using inks which run at normal film thicknesses. Figure 4 shows the increased level of powder that was required to run a similar job but with ‘weaker’ inks which were run at high ink film thicknesses in order to achieve similar printing density.
Here are two examples of how running too much powder can cause specks in printing:
- Too much spray when running the first printing pass: This can result in the spray transferring onto the unprinted side of the sheet above. This spray can then contaminate the print during the second pass, and create voids where the ink being printed cannot contact the sheet. This spray will build up on the blanket (Figure 5) and can be removed using tape pull material for verification (Figures 6 and 7). The lift of sheets may have to be blanked through a bare blanket unit to remove the spray before the second side can be printed.
- Too much spray when printing the second side: This can result in the spray particles from the second side being imbedded in or sticking to the ink which was printed on the first side. Because these sheets are now blocked together, when they are separated in the bindery, the spray particles can pull ink and paper material away from the surface, leaving white voids in the print (Figure 8). Close examination will show that the materials which caused the void on one sheet will be attached to the sheet above it. (Figure 9) The photos in Figure 10 are S.E.M. photomicrographs of Figures 8 and 9. The photo on the left shows the voids in the sheet from Figure 8 and the photo on the right shows the particle which left the void stuck to the sheet above as in Figure 9.
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