Comets (gravure print)

Relatively short stripes that appear and disappear randomly on the printed web, making them difficult to detect. They are usually small particles that either pass through the contact zone between the doctor blade bevel and the impression cylinder and lift the doctor blade bevel in the process. This allows ink to pass under the doctor blade bevel for a very short time until it sinks again and scrapes off normally. Particles can also become trapped under the doctor bevel and be rubbed off by the cells until they can no longer lift the bevel. In both cases, the doctor blade will then be doctored off again correctly.

Solutions to comets:

• Check doctor bars for cleanliness. Inspect support doctor blades for waviness and replace if necessary.
• Check the impression cylinder for damage and replace if necessary.
• Check the roughness values (Rz) of the impression cylinder and repolish or replace if necessary.
• Check the colour for impurities, the filter for blockages and replace the colour, if it is very dirty.
• Use cleaned ink trays and equipment to avoid old residues in the fresh ink.
• Check the doctor blade setting, set doctor blade angle steeper and/or increase the doctor blade order.
  
  TKM recommends the TKM Magnet Series 8000 and TKM Ceraprint ink filters.