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Shedding Light on the Coating Process

(February 2008) posted on Mon Feb 04, 2008

Discover where mistakes are most likely to happen in the screen-coating process and what your screenroom staff needs to do to avoid them.


By Gordon Roberts

Many screen printers have made the switch to capillary film, which eliminates most of the problems involved in the scoop-coating process. The down side, of course, is that it’s more expensive. However, it might be worthwhile having someone cost out the savings. Capillary film, when applied properly, will always outperform direct emulsion. The ease of application and the amount of time you save in screenmaking might just make it worthwhile.

 

Third basic truth: The screen is not ready for exposure until the emulsion is completely dry

I often hear grumbling among screenprep staff about the number of pinholes they have to touch up, and I hear them blame the phenomenon on dust, miniscule scratches on the glass surface of the exposure apparatus, film quality, etc. Here is a fact to ponder: Most pinholes that occur in emulsion-based stencils are due to inadequate drying. If there is any moisture left in the screen when the emulsion is exposed, it will impede the photochemical reaction and wash out when the screen is rinsed out after exposure, resulting in minute pinholes.

Here is another fact to consider: If you use heat in your drying process, it is possible for you to dry the emulsion’s outer layer too quickly, sealing in moisture and causing pinholes. I strongly believe that the best way to dry a screen is through dehumidifying the air surrounding it and by drying it over an extended period of time, preferably overnight. If you think residual moisture might be the source of your pinhole problem, leave a screen overnight in a room with a decent dehumidifier that is capable of bringing the humidity level in the room below 40%. Expose the stencil the next day, and I suspect you will see a great improvement

 

The whole truth: You cannot cut corners when making stencils—period

Once again, we return to the one truth that seems to be most self-evident and almost always is disregarded in our work environments. If we constantly cut corners and make compromises to get the job done on time, then we are most likely causing the problems that force us to constantly play catch up.

If a screen is not perfect when it leaves for the press, then it will, at best, slow down the print run—and at worst, not perform at all. When this happens, we throw everything into turmoil, quickly re-shoot a screen and, as always, cross our fingers and hope everything will work. It might take a little longer to achieve the kind of results necessary in the stencilmaking area, but quality in this area will allow for a much smoother workflow. The time is always well spent.

In my next column, we’ll look at the problems wrapped up in the highly misunderstood piece of equipment that you use to expose stencils. Before we go there, order a new bulb for your exposure system right now, along with an exposure testing kit. I have a feeling you will need both by then.

 

Gordon Roberts has a history in screen-printing production management that spans more than 25 years. He has held supervisory positions in shops that represent a broad spectrum of application areas and markets, including printed electronics, apparel, signage, and retail graphics. Roberts has presented training courses on the basics of screen-printing production and on shop management for the Screentech Institute and is presently a consultant for the screen industry. He can be reached at screenconsult@aol.com.

 


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