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Preventing and Removing Haze

(December 2011) posted on Wed Jan 11, 2012

Ghost haze is a problem you can conquer when you arm yourself with the proper chemicals and use sound techniques.

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In most cases, however, printers experience haze caused by both the ink and the emulsion. Two-component haze removers usually work best in such situations, followed by haze removers that contain a caustic ingredient and some form of solvent. Typical two-component haze removers work most effectively by applying the first component to a dry or nearly dry screen. These systems are less harmful to the mesh, but they work slower than caustic systems. For maximum effectiveness, they should be allowed to dry naturally on the screen. Once dry, they are activated by applying a solvent ink wash onto the dried haze remover. This ensures that ink and emulsion stains are treated. If you find that time is a factor, or you do not want to use two products for removing haze and are not as concerned about potentially harming the mesh, you should select a caustic haze remover that contains solvent.

Ink and emulsion are not always to blame for haze. Other contributing factors, such as process variables, are often overlooked. Process-related variables contribute to the severity of ghost images as much, if not more, than your inks and stencil systems. Since process-related variables are often more controllable than the type of ink or emulsion you use, let’s take some time discussing how these variables contribute to ghosting.

Low and unstable screen tension Low and unstable screen tension can contribute to high degrees of ghost haze. Low-tension screens require excessive off-contact on press in order to achieve proper peel/snap-off during printing. This causes inordinate mesh elongation as the squeegee stretches the screen during the print stroke. At this point, the ink transfers through the mesh, and pigment particles get trapped in the knuckles of the mesh when the screen relaxes to its resting position. High, stable tension reduces ghost haze.

Unnecessary use of roughening agents Roughening agents, or abraders, improve stencil bonding and durability, but they should not be used unless absolutely necessary. An alternative is to use a degreaser/wetting agent that treats and conditions the mesh for improved stencil durability.
Improper drying Improperly dried and/or exposed emulsions are the leading cause of emulsion ghosting. Emulsions with residual moisture do not cross-link thoroughly. If the drying area has a relative humidity greater than 50%, residual moisture will be present in your coated screens.

