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Why is our ink flaking off?
July 17, 2007
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Question: When we dry-trap in a second pass through some of our older sheetfed presses, ink sometimes flakes off the sheet. Is this crystallation or chalking?
Crystallization is legacy sheetfed ink problem, now that most printers wet trap on multi-color presses and aqueous (water-base) overcoat inline on press. The symptom: ink can easily be scratched off with your fingernail—the residue appears like dry flakes. This problem is very similar to chalking.
Different from Chalking
Crystallization occurs with overprinted, usually second pass, dry-trap colors, whereas chalking usually happens with a single color on paper. When dry trapping, make sure inks are wax-free because the wax in the bottom ink can prevent good adhesion for the top ink. If the ink still feels wet or smears when rubbed, it’s not crystallization or chalking problem but some other ink drying or adhesion problem.
Why It Happens
Chalking occurs because there is insufficient vehicle (fluid paste) binding the pigment (dry solids) to the paper. The cause is an incompatibility between ink and paper. Ink should always be formulated for the type of paper. Some cast-coated papers are very absorbent, for example. If the ink vehicles (oils and solvents) quickly drain into the paper, it leaves little behind at the surface to bind the pigment to the paper, thus can be scratched off.
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 17, 2007 | Comments (0)
