Success doesn’t come easy these days; we have to work at it. Sometimes solutions to our problems are right at our fingertips and we forget the solution is so close. Tools of various kinds help us keep our businesses running smoothly. They are used to measure, adjust, fix, and/or control the things we do in the shop. Yet we fail miserably when it comes to using probably the most useful tool in our arsenal to solve a problem. I am able to confirm this often by asking simple questions to member inquiries.

　　Most of us have digital cameras. What would happen if the digital camera exposure meter quit working? How many photos would be lost because you guessed the wrong exposure? With today’s camera we don’t even have to think about exposure. It’s all automatic. However, I’m sure some of you can remember the problems involved with manually setting the camera exposure. This same situation occurs every day the screen making department of your shop.

　　Let’s take a look at just some of the variables in the screen room.

　　* Humidity changes
　　* Old emulsion
　　* Hand coated screens are different
　　* Exposure lamp intensity variations
　　* Different mesh thicknesses
　　* Variations in light source distance
　　* Colored and white meshes
　　* Integrator set wrong
　　* Screens partially pre-exposed

　　All of these variations negatively affect the quality of the screens produced in the screen room. Controls need to be in place to monitor these variations. Without controls the screen department is hampered in producing a quality screen for their internal customer C the pressroom.

　　Now, let’s get back to that control tool for the screen room that seems to hide in the drawer. It is simply called an exposure calculator. There are many types and styles offered by manufacturers. In the event that you don’t have one, go get one at your screen printing supply company. It’s a tool that should be used frequently to keep screen making variations in control. Without it you are only guessing at exposure time. A senior quality control guru once said “without data you only have an opinion.”

　　The tool is not complicated to use. It is a special film positive photo tool that allows you to compare several exposures, while only making one exposure to a test screen. The typical exposure calculator has several rectangular blocks of varying density, which act as filters to block a percentage of the light striking the emulsion. Each block contains a definition target to allow for evaluation of the image resolution at that exposure value.

　　To use it you simply expose the calculator on a screen in place of a normal film positive. Test exposure should be twice the length of your normal exposure for the calculator to work properly. This will obviously overexpose some areas of the screen, but that is to be expected. If you are using the calculator with new equipment or a new emulsion, exposure history is not available. In these cases just double the manufacturers recommended exposure data found in the literature you received from them.

　　Each density block has an exposure factor that corresponds to the amount of light the area received. The clear area with no filtration has a factor of 1, while the block in the center has a factor of 0.5; this means that the 0.5 factor received only half the amount of light used in the test exposure. The exposure is doubled to get the exposure test in the center of the target range. If your test exposure is accurate then the emulsion area under the 0.5 factor will be the same color as the rest of the emulsion. The next target up the range will be lighter in color.

　　If you are using SBQ photopolymer emulsion or a one pot emulsion you will not be able to see a color change on the exposed emulsion. The scale in these emulsions will appear as a range of shades in a single color, rather than revealing different colors. Underexposed areas will appear lighter and during washout a milky substance, especially on the squeegee side, will appear.

　　After determining the correct exposure based on the exposure calculator scale, I would recommend that you again double that exposure and run the test again. This will reaffirm or not the results of the first test. At this time you can also look at resolution. It is a measurement of the stencils ability to reproduce fine detail. Checking resolution is done by examination of the lines of text in different point sizes and test images called resolution targets. Don’t be fooled by resolution, many times an underexposed resolution target appears sharper than a correctly exposed one. It is important to keep in mind that durability not resolution is the important issue. Underexposed screens are not hardened sufficiently to stand up to the wear and tear of printing operation. Exposure test should be run for each emulsion and screen mesh that is used in the shop. Detailed record keeping is a must for each combination.

　　Now that you have determined the correct exposure time, your control procedures are not done. There is another factor that needs to be monitored, and that is the strength of the exposure lamp. Unfortunately as time goes by the intensity of the UV lamp diminishes. Its ability to produce UV radiation declines much faster than its ability to produce visible light. You won’t be able to notice the UV difference and eventually you will be producing underexposed stencils. It is easy to monitor the light. All you need is an exposure control or grayscale.

　　The exposure control scale is basically a small film positive strip containing a range of graduated neutral film densities. Some have 21 steps and others 14 steps. They are small enough to be placed away from the image area and still be exposed with the film positive. They can later be blocked out. Expose it with the screen and record which level washes out at the best test result. As long as this level remains constant on subsequent exposures, the screen should be properly exposed. Once you have determined your ideal exposure time using the exposure calculator, the exposure control scale will keep your exposures on target and keep tabs on your UV lamp in the exposure unit.

　　In summary get the unused photo tool out of the drawer and put it to use. Even though making test exposures takes time, it pays off in the long term. Repeatable quality screens that are correctly exposed save time and money, eliminate employee aggravation, and prevent premature screen breakdown on the press. All add up to better employee moral and greater profits for your company.