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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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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 is greater than 50% relative humidity, residual moisture is present.

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 their pigments into the weave of the mesh. Haze removers have difficulty reaching these 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 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. 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 and under 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. The stain is not apparent, and the screen mesh may appear like new. Only when the screen is viewed at approximately 45° does one see this latent image from a previous job. Latent haze typically causes no problems for screen printers, but on occasion, it 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. This is how the phantom image shows itself. There’s a restriction of the normal flow and/or transfer of ink through the screen and onto the substrate in these overlapped areas.

Long printing runs Mesh eventually loses its ability to fully recover or regain its original physical structure in the areas of the printed image, which leads to a condition called mesh bruising. Thread compression differs in image and non-image areas, leaving a permanent impression of the printed image in the screen, even after the screen has been fully reclaimed. The image reappears in the subsequent job.


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