Mark Vruno

What are colored blotches on my sheets?
July 19, 2007

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Question: My press sheets have been marking with different colored spots or blotches, almost like they’re stained. Are there different types of mottling?

There are many different types of mottle, such as back trap, water interference, paper, ink or squeeze/impression pressure mottle. Mottle, in any form, is a non-uniform, uneven, blotchy appearance.

Separate Cyan from Magenta
Back trap mottle often occurs in blue colors where cyan and magenta overprint. If the ink tacks are correct, then separating the cyan and magenta farther apart in sequence often solves the problem.

Too Damp?
Water interference mottle occurs if the press operator is running too much dampening solution. Excess water on the plate transfers to the paper preventing later-down inks from lying smoothly.

Check Out the Paper
Lightweight papers may have a wild paper formation of fiber distribution, and the ink will print with varying density or gloss. This is especially noticeable in large areas of what are supposed to be uniform solids or tints. Examining the paper with transmitted back-lighting will illuminate its uniformity.

Stain Testing
To get some future indication of paper performance before going to press, a simple and fast field test is to perform a K&N stain to determine the uniformity of holdout or absorbency.

Train at RIT
The Rochester Institute of Technology (RIT) offers various training seminars at its Printing Applications Laboratory in Rochester, NY.

Posted by Mark Vruno on July 19, 2007 | Comments (1)