Too often, improperly exposed screens make their way to printing presses and inevitably cause downtime. There are many reasons this happens. Short cuts, inconsistent procedures, unsubstantiated adjustments in exposure, and hurried attempts to get to press reaffirm our infamous motto, “There’s never enough time to do it right, but always time to do it over.” Understanding the factors which affect stencil exposure will result in greater consistency in the final print, increased stencil longevity, and less downtime on press. As with other production areas in screen making, determining and choosing proper exposure parameters depends on several factors including the type of equipment, the exposure time and distance, the stencil material and its application and drying conditions, the stencil development method, the quality of the film, and the mesh count and color.

　　Emulsions used in screen printing are sensitive to wavelengths of light between 340-380 nanometers (nm). Therefore, using a bulb that emits its highest energy percentage within this range is important. Metal halide, mercury vapor, and fluorescent are acceptable light sources. Much of their emitted light energy falls within the desired range. Unacceptable light sources include quartz, halogen, tungsten, and sodium vapor which all emit low amounts of UV light in the acceptable wavelength range. Even though fluorescent is considered acceptable, it is a multi-point light source, a major deficiency when trying to accurately reproduce the artwork. Single point light sources (e.g. metal halide) emit straight shafts of light. Because the light rays are straight, there is less chance of light scatter causing inaccurate reproduction of the artwork on the film.  Multi-point light sources, like fluorescent, emit light in all directions and increase the risk of light scatter. Furthermore, improvements in exposure time and stencil resolution can be achieved by using higher intensity (wattage) lamps. Higher lamp wattages will reduce the amount of time necessary for full exposure. In short, the higher the lamp wattage, the shorter the exposure time. Shorter exposure times also reduce the amount of light scatter which can lead to undercutting the film and inaccurate reproduction of the artwork.

　　To this point, time has been presented as the metric for exposure. However, time is relative to the light intensity at different exposure distances as well as the age of the light source. When new, a bulb will emit a certain amount of light energy in the desired wavelength range. As the bulb’s age increases, the relative energy at the desired wavelength can shift to something outside the preferred range. To prevent this, a light integrator is used. They adjust the actual exposure time to ensure the amount of light energy at the ideal wavelength is received by the stencil. However, once a bulb reaches approximately 1000 operating hours, it should be replaced as the light energy emitted in the desired wavelength is extremely low and an integrator can do very little to improve this.

 　　While many exposure units are self contained to make calculating exposure distance effortless, large format printers may still rely on one, or several, moveable light units to tackle their enormous frame sizes. When using a single-point light source, the minimum exposure distance should equal the diagonal length of the emulsion area on the screen. Placing the light source at this distance will ensure that the entire coated area of the screen will expose evenly. Any closer and the stencil area at the outer edges will be underexposed while areas in the center risk being overexposed. Moving the light source beyond the minimum distance will increase exposure time.

　　While a formal treatment of proper stencil application and drying conditions is outside the scope of this piece, suffice it to say any variation here can directly impact exposure. Every mesh and stencil coating method used in a print shop requires its own exposure calculation. In other words, just because you coat all mesh counts using the same technique doesn’t mean the stencil thickness is the same on each one. Stencil thickness is a major factor when determining exposure time. Thin stencils will require less time for UV light to penetrate and cross-link the material. As stencil thickness increases, so too should exposure.

　　A major contributor to downtime caused by inadequate stencils is due to the lack of environmental controls in the screen making area, specifically the stencil drying area. Since screen making requires the use of water, the air in many drying areas is too humid for stencils to fully dry. If moisture is not being removed in your drying area stencils will not completely dry and cannot properly expose. Furthermore, the stencil can delaminate from the mesh during printing, can prematurely breakdown on press, can be besieged with pinholes, may expose unevenly and will certainly be difficult to reclaim. Employ the help of a dehumidifier to keep the relative humidity (RH) in your drying are between 30%-50%. Keep the temperature in the drying cabinet between 27°C-37°C (80°F - 100°F) and make sure there is plenty of air circulation.

　　Although many do not realize the impact development procedures have on the quality of the stencil, there are several variables which should be standardized in this step of the process. Water pressure, nozzle pattern, distance, duration, and water temperature all play a role in determining the final stencil quality. Establishing standard operating procedures can alleviate many headaches that arise from inconsistencies in developing the stencil.

　　Film quality can obviously impact stencil exposure so be sure the density in clear areas of the positive measure below 0.05D so that light energy can easily pass through to harden the emulsion. Density in the dark image areas should measure above 3.20D in order to properly block light energy from reaching areas of the stencil which should not be hardened by light. These measurements can be obtained with the use of a transmission densitometer.

　　Mesh color can influence the quality of your stencil. White mesh, while cheaper, allows light to refract within the threads and cause undercutting in the stencil. If fine detail is present in artwork, the better choice is to use a dyed mesh (yellow, amber, etc) that inhibits light refraction and will produce better detail. White mesh is fine for solid areas of color and bold type but should never be used for fine detail artwork. It’s also important to remember that lower mesh counts will require longer exposure times than finer mesh counts. Low mesh counts hold more emulsion and therefore produce thicker stencils than finer mesh counts. As presented earlier, thicker stencils require longer exposure.

　　Understanding the factors which determine stencil exposure is more involved than one may first realize. Make sure to fully address and establish standards in your shop for the discussion points included here. In the end, you’ll realize that properly and fully exposing your stencils will produce consistency in your printed images and provide better longevity and abrasion resistance on press.

[时间：2007-04-20  作者：佚名  来源：信息中心]