Back trap mottling

 Backtrap Mottling is mostly frequently seen in the ink printed in the first or second printing unit of a multicolor offset printing press. The cause lies in the partially absorption of ink into the paper.  The partially variable immobilization of the ink leads to a variable splitting of the ink on the blankets.
An uneven (thicker/thinner in various areas) ink film remains on the paper surface, which the human eye sees as mottled. The partially variable ink absorption is configured differently on each of the following sheets, as a result of which the mottling effect increases rather than diminishes as experience shows.

                              Backtrap mottling can be triggered by an uneven formation which could be in the base paper stage and/or by a binding agent migration during the applied coating.

                              Another cause for backtrap mottling can be the surface energy on the paper. If this energy is too high it can influence the ink setting in a negative way.

                              There is a similar situation as far as ink is concerned. A higher surface energy value changes the rheology and the tack value. Faster ink setting with different tack values also has an influence on mottling. Faster ink seting and faster setting behavior of the paper influences mottling.


Solution to the problem.
                                The uniform base sheet formation is important. For uniform coating application and even binder/functional additive migration, which control open and close coating, one must provide an even coating thickness on base paper surface. Also, it minimizes the base paper influence on final coating.

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