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Tips for Optimum Screen Exposure

(December 2012) posted on Tue Feb 12, 2013

This guide examines stencil-material characteristics, issues related to light sources, and methods for testing you can use each time you create a stencil.

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By Screen Printing's Solution Sourcebook

Understanding key variables in the screenmaking process is the only way to guarantee optimum stencil exposure in screenmaking and performance on press.

Light sources
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.


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