Comets

Problem case

Causes for comets
Relatively short lines, which appear at random on the printed web and disappear by themselves, which makes them hard to spot. The lines do resemble a comet trail, hence the name. Often caused by a small particle which either passes through the contact area between gravure cylinder and doctor blade, causing the doctor blade to lift up for a short distance, releasing ink in the process. Sometimes it can be particles stuck underneath the blade and wear off on the engraved area until they are worn off. in both cases the doctor blade continues to doctor correctly.

Solution

Solutions for back doctoring
1) Check doctor blade alignment and doctor blade holder, replace wavy doctor and back-up blades. Clean doctor blade holder.
2) Check printing cylinder for damaged areas, polish or replace if necessary.
3) Check Rz roughness values for chrome surface.
4) Check ink for dirt and plugged filters, change ink inf necessary.
5) Use clean ink pan before printing as often residues wash into the fresh ink.
6) Check doctor blade angle and setup. Adjust for higher and/or harder setup if necessary.

TKM recommends to filter ink with TKM Magnet Serie 8000 filter, use TKM Ceraprint.