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Preventing and Removing Haze

(December 2013) posted on Wed Dec 18, 2013

The products and processes that can streamline your screen-reclaiming efforts.


By Screen Printing's Solution Sourcebook

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Haze removers traditionally contain some form of caustic ingredient, such as sodium or potassium hydroxide and/or sodium hypochlorite, and may include a solvent or solvent blend, as in the case of most single-component haze removers. They remove ink and/or emulsion ghost-haze images (stains) that remain in the mesh after removing the ink and emulsion from the screen during the reclaiming process.

Haze
The first thing we think about when discussing ghost images is a stain of the printed image that remains in the mesh after reclaiming. The severity of ghosting often depends on the type of ink used. For example, catalyzed inks contain solvents that react with low-surface-energy substrates to improve ink adhesion. These types of inks can create ghost haze that is difficult to remove. Inks also contain pigments that get trapped in between the knuckles of the mesh, bond to the mesh, or become partially dissolved by hot solvents and slightly penetrate the mesh (Figure 1).

Another area we think about is the negative image created by the stenciled parts of the screen. Here, the emulsion or film used to define the customer’s artwork most often causes the ghost haze; however, the ghosting can be attributed to a combination of the emulsion and the ink staining the mesh in these areas (Figure 2).

Other considerations regarding haze
Underexposure prevents the emulsion that encapsulates the mesh from the squeegee side from absorbing enough UV light to become firmly anchored to the screen. When developed, most of this emulsion rinses away along with the unexposed emulsion from the image area. The only firmly anchored emulsion resides on the side closest to the light source—the substrate side. This leaves mesh threads on the squeegee side exposed and susceptible to staining from ink and emulsion. Additionally, a reaction may occur between partially hardened emulsion, which may remain on the squeegee side of the screen, and the ink. This reaction makes reclaiming difficult and often results in excessive ghosting.


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