There is much more to coating, drying and exposing a screen than what most people realize. And, at least 80-percent of people are doing at least one part of the process wrong. I have visited over 7,500 shops from California east to India, and from Canada south to Latin America. Very few shops are doing things right.
There are far too many variables present in the screenprinting process to have even one error. The error will snowball and create havoc. There has always been a delicate balance between chaos and control.
Screens should be coated on the print side first, with enough coats to fill the mesh openings with emulsion. The only way to know for sure if the emulsion has completely filled the mesh opening is to look at the squeegee side to assure that it is shiny with emulsion. The guy who tells you to coat one over one or two over one or whatever has no idea what he is talking about. He doesn't know how thick your emulsion is, the solids content, the temperature or the speed and angle of your application. All these, and more, affect the amount of emulsion that is applied in each pass. The only way to be sure that you have filled the mesh openings is to look for the shine on the squeegee side.
After you have made sure the squeegee side is shiny, you make a final pass on the squeegee side to push the emulsion through to the print side. Additional coats on the squeegee side will build up the stencil on the bottom of the screen. The most important thing you will ever learn in screenprinting is:
"The wet ink deposit is always equal to the stencil thickness!"
You want thicker ink, you use a thicker mesh and more emulsion. Simple to a point, but of course too much emulsion and you have a mess on your hands.
After the screen is coated it must be dried with the print side down. This is to assure that the stencil will be on the bottom of the screen. If you dry the screen with the print side up, your emulsion will seep back to the squeegee side and you will have a problem with image degradation and more.
Never use heat to dry emulsion. Thank about it! The only way to "dry" emulsion is to dissipate the moisture. Heat will not dissipate moisture. Consider the hot tub or spa - heat holds moisture, folks! The best way to "dry" emulsion is with the use of a dehumidifier that moves the moisture out of the room.
A great number of screenprinters are not allowing their screens to throughly dry before exposing the emulsion. This causes the emulsion to be underexposed, due to the moisture still in the emulsion and makes reclaiming difficult and time consuming. A fully exposed screen will reclaim quickly and easily with no "color" left in the mesh. You'll save time, chemicals and water in your reclaiming process. Plus, you will have a screen with great detail and no pinholes. Allowing the screen to "dry" for at least 24 hours will allow it to acclimate well enough if you are using a dehumidifier.
For exposing emulsion you need a single point light source. Fluorescent and Quartz Halogen bulbs are inadequate for exposing quality screens. They do not contain enough UltraViolet Energy to properly harden the emulsion, causing polymerization or crosslinking. It is not the intensity, brightness or wattage that exposes the screens. It is the UltraViolet Energy that does the trick. You need a bulb in the 380 to 420 Nanometer range - like a metal halide lamp. These are available from most plant growers.
A 1,000 watt Metal Halide bulb will get the job done in a very short time. You can buy these at most lighting stores - like the Light Bulb Store franchise in the USA or at a good nursery where they sell African Violet growing equipment. Stores like Texas Hydroponics will be a good bet for an
inexpensive unit. I especially like their Hydrofarm Xtrasun/Powerhouse 1,000 Watt XtraSun Metal Halide, 1,000 Watt Ballast Combo Unit, but they have other light sources that may prove to work equally well. They are very knowledgable about lighting systems. Just be sure that you let them now that you want a unit that projects in the 380 to 420 Nanometer range and that you need at least 1,000 watts for speed. You will need a ballast, lamp, socket and reflector which you can get as a complete system. They have a wide range of HID Systems online at:
Texas Hydroponics :: Grow Lighting :: HID Complete Systems
Always use an exposure calculator or step wedge to find your correct exposure. You will need to test each mesh count as the thickness of the mesh and the size of the mesh opening will allow differing amounts of emulsion to be placed on the mesh and thus the exposure will be different. If you change any part of the process you will need to revisit your exposure test to assure that you are exposing fully.
Never use high pressure to develop a stencil. It is not necessary to use high pressure. Soak the exposed emulsion for a few minutes and then rinse with light pressure. The unexposed emulsion will wash away easily and quickly. There is no need to stand there spraying water on the emulsion. It is not going anywhere until it has become soaked with water and allowed to soften.
When developing your stencil, feel the squeegee side of the screen. If you feel a bit of slime or if the color of the emulsion comes off on your finger tip, you are underexposing your screens.
Obviously, there is more to this, but hopefully this will help dispel some of the rumors that are in current circulation. We have trained thousands of screenprinters at seminars, open houses, at the School of Screenprinting and in our consulting around the world. Once they understand the process they can make a perfect screen the first time and each time afterwards.
questions about screens 
