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Giving up the Ghost: Tips for Preventing and Removing Haze

(July 2007) posted on Wed Jul 11, 2007

Ghost haze is a problem you can conquer when you arm yourself with the proper chemicals and employ sound techniques. This article examines products that help remove ghost images as well as methods you can use to reduce or prevent their occurrence.

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By Dave Dennings

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 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 assures us 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 processrelated 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. High off-contact is how one achieves the proper tension necessary to effectively transfer the ink to the substrate when working with improperly tensioned screens. Unfortunately, excessive off-contact causes inordinate mesh elongation as the squeegee stretches the screen during the print stroke. At this point, the ink transfers through the mesh and much of its pigment particles get trapped in the knuckles of the mesh when the screen relaxes to its resting position.

Screens that have high tension initially but are not stabilized properly prior to printing may experience a lot of tension loss, which results in the same pigment-particle entrapment explained above. Some meshes also exhibit lower elongation and relaxation during stretching and printing than others. Thorough in-house testing will prove which ones perform better. High tension is good, but high, stable tension is better for reducing ghost haze, as well as print misregistration, distortion, and color shifts.


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