Tips for Optimum Screen Exposure
This guide examines stencil-material characteristics, issues related to light sources, and methods for testing you can use each time you create a stencil.
Understanding key variables in the screenmaking process is the only way to guarantee optimum stencil exposure in screenmaking and performance on press.
Exposure lamps used in screenmaking come in a wide range of spectral outputs, intensity levels, and light-delivery geometries. These particular features aren’t discernable to the naked eye; however, they are critical to stencil performance, including resolution, durability, and reclaimability. Whether direct emulsion or film, every stencil material uses a sensitizer that reacts to specific wavelengths of the light spectrum.
Using the right kind of light is the foundation of good exposure. Only a fraction of a lamp’s rated input power is converted into output at the correct wavelengths of light required to harden a stencil. The useful output portion is known as actinic light, with wavelengths corresponding to blue, violet, and ultraviolet. Figure 1 shows spectral outputs of common exposure lamps. Note that metal-halide, multispectrum, and certain specialty fluorescent lights produce output that is rich in actinic light. Other types of lamps are not suitable for producing high-quality stencils.
Figure 2 shows the output spectrum for a metal-halide lamp, overlaid with two curves—one representing diazo absorption; the other diazo sensitivity. The sensitivity curve identifies the light wavelengths at which the sensitizer will react. The absorption curve peaks at 373 nm in the UV range, demonstrating the way that light output from this type of lamp is blocked by a diazo-sensitized stencil. Note that the peak in intensity corresponds to the tail of the absorption curve. Light is absorbed here with less intensity, but with more penetration. Metal-halide lamps, with peak output of 390-420 nm in the blue-violet range, penetrate the whole emulsion layer throughout the exposure period. As such, they’re the best choice for diazo stencils.
Figure 3 shows a comparable situation when a multispectrum lamp—sometimes referred to as a tri-metal-halide or iron lamp—is used to expose a photopolymer emulsion. The absorption peak is at a shorter UV wavelength of 342 nm. This shifts the intensity peak into the range of 360-390 nm, where multispectrum lamps are strongest in output, making them the best choice for photopolymer emulsions and films.
Two types of light-sensitive chemicals are used in direct photostencils. Diazo and dual-cure types can be grouped together, as the diazo sensitizer in both that primarily determines exposure length and degree of latitude in exposure time. One-part photopolymer emulsions and films use SBQ sensitizer, which is designed to react much faster than diazo when exposed to the right kind of lamp. Diazo and photopolymer stencil materials differ in wavelength, which is why we see a somewhat complicated relationship in their relative photographic speeds.
Too many factors affect image resolution to use it as a guide for determining exposure time. For example, filled-in detail doesn’t necessarily mean the stencil is overexposed. A rip in the exposure blanket or poor vacuum caused by a leaking seal can hinder image resolution, even at a fraction of the correct exposure time. Incompatible combinations of screenmaking materials, such as coating white mesh with photopolymer emulsion and then exposing with a fluorescent tube, can also lead to low resolution.
Optimum exposure time is determined by assessments of a stencil’s depth of cure. A proper cure means the stencil is completely cured through its full thickness. Various techniques allow effective evaluation of depth of cure. One popular method is the exposure calculator. It uses a series of increasingly darkened, neutral-density filters, overlaid on a repeating design. It allows multiple exposures of 100%, 70%, 50%, 33%, and 25% to be simulated in one step.
After exposing and processing a test screen with an exposure calculator, the finished stencil must be evaluated by the color-change method, not for resolution. For example, yellow diazo sensitizer shows up as a strong, yellow undertone where residual, unused diazo remains. Correct exposure is determined as the time taken for the yellow diazo sensitizer to be bleached out completely. In testing, no yellow undertone should be seen on one of the middle sections of the calculator image. Complete exposure is only indicated when the color remains unchanged for two successive steps. This type of calculator works very well with diazo stencils.
Two separate color changes often happen simultaneously with dual-cure stencils, with the extra dual-cure component causing a fainter, but more persistent, color change. This trick is to determine when, exactly, the diazo part stopped changing color.
No color change occurs with photopolymer stencils. Although the type of exposure calculator just discussed might be useful for determining the degree of resolution available at different exposure levels, it does not indicate the extent of cure. An alternative method involves the use of a grayscale-sensitivity guide, which is a phototool that features a 21-step non-halftone grayscale, with density increases of 0.15 at each step. More grayscale steps harden as exposure time increases. The sensitivity guide, when used correctly, helps determine optimum exposure with only one test.
Yet another test method uses no film at all. Instead, it involves the use of a digital radiometer to determine the point at which all of the sensitizer in the coating is used up. Here, a 365-nm filter is fitted into the light integrator’s photocell, and the photocell in placed in the vacuum frame behind the coated mesh. Exposure is then started. No light is able to reach the photocell at the beginning, because the sensitizer is highly absorbent. During exposure, as sensitizer is used up, the emulsion coating allows more light to reach more areas of the emulsion coating. An increasing amount of light is measured by the radiometer, and then the light gradually levels off.
The light integrator
The light integrator steps in to compensate for lamp degradation and power fluctuations. Matching the photocell filter to the sensitivity curve of the emulsion is important, because a disproportionate amount of stencil hardening is caused by those wavelengths that are most penetrating and usefully absorbed. Depending on the type of stencil material, the required wavelengths might reach outside the UV spectrum and into the blue and violet range. The photocell filter must be able to tell the integrator when adequate amounts of those wavelengths have reached the stencil.
Post exposure can be useful for improving the resistance properties of a stencil, but the benefits depend on the type of emulsion used.
Diazo emulsion or film A yellow undercast signifies unused diazo sensitizer in an underexposed diazo stencil. The partially exposed sensitizer doesn’t wash out from the stencil during processing, because it has already reacted with and attached to the polymers and resins that make up the stencil. After drying, it is possible to re-expose the screen, bleaching out the remaining diazo and further crosslinking the stencil to improve its solvent or water resistance.
Dual-cure emulsion or film The situation is the same with dual-cure products when underexposed. But unlike purely diazo stencils, correctly exposed dual-cure stencils can benefit from post exposure. That’s because the secondary crosslinking system can be made to polymerize further, even after all of the diazo is used up. This typically improves solvent resistance and can simplify reclaiming.
Photopolymer emulsion and film These emulsions benefit most from post exposure. Unlike diazo, which can be used with 100% efficiency when exposure time is long enough, photopolymer molecules can be very stubborn. Only some molecules react very quickly to create the short exposure times for which photopolymers are known. The rest of the molecules aren’t aligned correctly and crosslink only with difficulty. Increasing exposure time causes a loss of resolution and detail with little payback in terms of improved durability.
Unused photopolymer During development, when the stencil is wet, some of the unreacted molecules will realign and be available for crosslinking the second time around, thereby resulting in improved solvent and water resistance.