Darkening

When ink darkening occurs, the darker colours are transferred from the blanket over the printing plate onto the succeeding inking system, where they contaminate the light colours.

During wet-on-wet printing, the tackiness of the inks must be correctly formulated. Nevertheless, it can happen that the darker colours from the first printing units contaminate the lighter colours (mainly yellows) in the printing units.

Remedy:

• Change to a different tack of ink
• Print lighter colours first
• Check that the screen rulings are correct for the press colour sequence
• Check that the fount chemistry is correct (PH, Alcohol and IPA)