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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.


Underexposure Unfortunately, most screen printers don’t understand what complete exposure means. Many believe it is the shortest exposure in which the emulsion doesn’t fall off the screen when developing with a garden-hose sprayer. If this is how you pick your exposure times, you are grossly underexposing the emulsion and contributing to your haze problems.

Use of hot solvents Use of hot solvents causes emulsions and polyester mesh to swell, trapping pigments in the weave of the mesh. Haze removers have difficulty reaching trapped pigments, which reduces their effectiveness.

Delayed ink removal The longer you wait to remove inks from screens after production, the more difficult it becomes to remove the resulting ink stains. This is especially true for solvent-based and many UV inks. Applying a slowly evaporating ink wash to screens right after ink is removed is an effective way to reduce ghost images when immediate reclaiming is not feasible.

Incorrect use of reclaiming chemicals Misuse of screen-reclaiming products contributes to ghosting problems. All screen-cleaning products should be applied to both sides of the screen and brushed into both sides of the screen. Remember to remove these products from both sides of the screen with a high-pressure wash . What some perceive as emulsion-haze problem can actually be a procedural issue—for example, pressure washing from only one side of the screen.

Latent or phantom haze This type of haze is virtually invisible on the screen when viewed head on.
There is no apparent stain, and the screen mesh may appear like new. Only when the screen is viewed at approximately 45° does the latent image become apparent. Latent haze may show up in the most unlikely places, such as embedded in the design of a subsequent job. The printed image shows areas of lighter color anywhere the image overlaps a previous job’s latent haze. If screens are not processed correctly and carefully during each step, a thin film of chemical residue may contribute to latent haze. For example, underexposure and improper developing cause difficulties in reclaiming and leave the stencil susceptible to other chemical reactions with inks and ink removers. These reactions can cause physical changes to the mesh that may contribute to latent haze.

Possible remedies for latent ghost haze include keeping a close control over all screenmaking and cleaning procedures, using mesh-preparation products that rely on surface treatments and wetting agents to balance the surface tension of the mesh, and using a slow-acting, two-part haze remover that contains sodium hypochlorite and can be dried on the screen.


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Comments

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