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Stencil Washout Techniques, Part I
By Dawn M. Hohl, Screen Printing Consultant, Uncommon Enterprises LLC
In Part I of this two-part series, we will discuss some common industry ailments, washout variables, and automatic washout systems. In Part II, we will examine exposure control films and washout testing.
Stencil development or washout is given little weight in the bigger scheme of the screen printing process. It seems simple and easy, and not a variable worthy of concern or extra time. But when images carry halftones and fine detail, washout becomes an important sub-process that will impact print results. Just like any other variable, it must be controlled and repeated to attain consistent tonal-range reproduction and print quality.
It is important we understand how stencil development methods impact our image and explore the best methods for washing out today's emulsions. We'll examine some common industry screen ailments, washout variables, the Screen Printing Technical Foundation's washout testing results and, finally, recommendations for best practice. We are primarily discussing direct emulsions and, to some degree, capillary films.
Common Industry Ailments
The application of a tonal curve generally is necessary to screen print halftones and four-color process accurately. No amount of underexposure will remove the need for a curve in screen printing, except perhaps on extremely low-line counts. The simple truth is: You cannot open up the small highlight to mid-tone dots enough to eliminate the need for a tonal-curve application. So if a curve is needed, why not expose the stencil to a fuller resistance level and help eliminate stencil-related issues in production? As you will see later, it is possible to achieve similar tonal curves from longer exposure and harder development compared with underexposing the stencil.
A related practice is post-exposing the stencil in an effort to recover some of the resistance underexposure has caused. While this can be useful when working with SBQ emulsions, it is not effective at all on diazo stencils and provides little benefit with dual-cure emulsions. For diazo and dual-cure emulsions, the unused diazo literally washes away during development, so there is no longer any sensitizer for post-exposed light to activate.
KIWO states in its dual-cure emulsion literature: "To improve the resistance by 15 percent, the post-exposure time needs to be six to 10 times the original exposure time." For SBQ type emulsions, a post-exposure of one to two times the original exposure time on the squeegee side can be used. Therefore, post-exposure adds an extra processing step and costs valuable time in screen making. However, keep in mind that while post-exposure may improve the resistance of the remaining stencil layer (in the specific emulsions mentioned), the unexposed stencil material, particularly on the squeegee side, has been rinsed away during development. Obviously, this missing emulsion can't be post-exposed, and compromises to some degree the stencil adhesion. Overall, post exposure is not worth the effort in most cases. It is much wiser to expose the stencil properly in the first place.
The primary exposure is your main tool for creating a durable stencil. It is estimated that 80 percent or more of printers regularly underexpose their stencils, many unknowingly. So even if you are not intentionally underexposing screens, an exposure calculator should be used to determine and check proper exposure on a regular basis. Many articles are available on the Internet on how to use and read a stencil exposure calculator. Emulsion manufacturers also can take you through the process.
If you have the following problems during washout, your stencils are probably underexposed:
Water Pressure, Spray Pattern and Distance
I would agree with both of these statements if the stencil is underexposed, and most of them are. But with a properly exposed stencil, the increased pressure and water droplet dispersion do a better job clearing out the emulsion in unexposed areas. High-quality emulsions can easily withstand pressure if exposed properly. You'll see results later to prove this.
The tougher and more water-resistant stencils available today actually require higher pressure to clean out of all the unexposed emulsion. Here is a demonstration test to prove the idea: Coat a screen with a dual-cure emulsion and allow it to dry, but don't expose it. Now wash out half of the unexposed emulsion with a pressure washer and half with a garden-hose nozzle under regular tap pressure. A haze of emulsion left in the mesh is sometimes noticeable on the side washed out with the garden hose, but not on the pressure-washed side.
Other advantages can be realized from using a pressure washer for stencil development including better edge definition, halftone sharpness and cleaner mesh bridging. Moiré also can be reduced as the dots become more accurately formed and opened. Other benefits possible are improved screen-to-screen consistency, less operator variation and simplified training.
Actual water pressure is related to spray pattern and nozzle distance from the screen. High water pressure can be quite detrimental if the spray pattern and distance are not appropriate, so some conditions need to be observed:
When using garden hose pressure, select a nozzle that has a strong fan spray pattern and don't be afraid to get close to the screen. There also is a special spray nozzle designed for washout.
Duration on Each Screen Side
But testing indicates that equal time spent on both sides of the screen during washout can effectively clear out the image areas better on direct emulsion. The length of time spent on each side also is an important variable. That being said, for capillary film, it is wise to perform most of the washout on the substrate side as the squeegee side is more venerable.
Soaking a screen will help to rehydrate and soften the unexposed emulsion making washout faster and easier. Finer definition images can benefit with improved resolution, but soaking is not recommended for softer type emulsions such as some SBQs. Of course, soaking is simply not practical for large screens, and extra attention should be given to the washout duration in this circumstance. Any development procedure should begin by wetting both sides of the stencil as a type of soaking stage. If the screen is totally immersed, the time in the water should be controlled.
Another related issue is how uniformly the whole area of the stencil is developed. The entire stencil should be washed evenly, a difficult task when washing out by hand. There can be a tendency to focus on fine image areas (color bars, etc.), but it's best to train screen makers to treat the entire stencil the same. Uneven treatment of the non-image areas will raise the risk of leaving unexposed emulsion on the screen, which can cause scumming. A good washout procedure should specify even and consistent motion across the entire stencil surface.
Exposed emulsion is not waterproof and will absorb water, swell and become soft. So you can wash out or soak too long. Developing stencils with a pressure washer can reduce overall washout time, which is particularly important for SBQs that get soft after long washout times. Super-long washout efforts will not fix image areas that won't open up. Several other issues may be responsible for difficult washouts and should be identified and corrected. Refer to SPTF's troubleshooting guidelines at SGIA.org for more help.
Automatic Washout Systems
Automated washout systems control all of the washout variables to deliver consistent results. This is not to say manual washout cannot be fairly consistent with good practices and procedures, but taking the human variation out of the equation can certainly make this process more fail-safe and efficient.
Of course, like any automatic equipment, do your research and be clear how the machine deals with the washout variables we have already discussed. These variables also must be set consistently and with deliberation to get the best results.
Be aware, switching to an automatic washout system will most likely require an adjustment of the halftone curves. Determining tonal curves is highly dependent on screen repeatability, and the washout technique can make a significant difference in the detail present in the stencil. Automatic washout systems will generate their own unique tonal profile in the stencil, so a new tonal profile should be determined through testing.
Additional benefits of automated systems include water recycling, labor savings, sound reduction, elimination of repetitive-motion injuries and lower humidity. They can be a great investment and provide greater productivity and consistency. Companies offering automatic stencil washout systems include Image Technology, Chemical Consultants Inc., RhinoTech Inc., SaatiPrint, Interchange Corporation, Zentner Systems, DLP Imaging and ESC GmbH & Co.
Soaking stencils should be carefully controlled to avoid problems from overly swelled stencils. Soaking for too long can cause over-washing, blistering and emulsion delamination, especially if the stencil is underexposed. Chemical Consultants Inc. currently offers a soaking tank specifically for stencil development.
In part II, we will examine exposure control films and washout testing. Keep an eye peeled for it.
This article appeared in the SGIA Journal, 2nd Quarter 2008 Issue and is reprinted with permission. Copyright 2008 Specialty Graphic Imaging Association (www.sgia.org). All Rights Reserved.
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