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.
Unnecessary use of roughening agents Roughening agents, or abraders, improve stencil bonding and durability. These products cost more than concentrated degreasers and microscopically scratch the mesh, which can lead to premature ghosting. These products are beneficial at times, but they should not be used unless absolutely necessary. An alternative to abrading the mesh is a product used as a degreaser/wetting agent that treats and conditions the mesh for improved stencil durability. If used consistently, it may reduce the severity of ghost haze.
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.
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.
To illustrate this point, conduct this simple test. Expose a screen using an exposure calculator or perform stepped exposures ranging from under to over exposure using your own positives. Develop, and reclaim the screen, no printing is required. Pay attention to the areas that are more difficult to reclaim and you will see these areas represent the shortest exposure times and leave a noticeable haze in the screen (Figure 3).
Use of hot solvents Use of hot solvents causes emulsions and polyester mesh to swell, trapping pigments into 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 solventbased inks 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 and/or procedures Misuse of screen-reclaiming procedures and products contributes to ghosting problems. Some printers take procedural shortcuts that they believe contribute to their efficiency. They become so concerned with throughput and turnaround times that they don’t realize where the bottlenecks truly are.
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