Screen Printing - December 2012/January 2013 - (Page 8)

SOLUTION SOURCE BOOK TIPS FOR OPTIMUM SCREEN EXPOSURE Understanding key variables in the screenmaking process is the only way to guarantee optimum stencil exposure in screenmaking and performance on press. This guide examines stencil-material characteristics, issues related to light sources, and methods for testing you can use each time you create a stencil. 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. Resolution 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. Testing 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 reMercury Vapor sidual, unused diazo remains. Correct exposure is determined as the time taken for the yellow diazo senMetal Halide sitizer to be bleached out completely. In FIGURE 1 The different lamps used to expose stencils emit very different parts of the light spectrum, which is why it’s important to match the exposure lamp to the type of stencil material being used. 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. Light-sensitive chemistry 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 8 SCREENPRINTING Multispectrum 420-nm flourescent White flourescent

Table of Contents for the Digital Edition of Screen Printing - December 2012/January 2013

Screen Printing - December 2012/January 2013
Prepress for Special-Effect Garment Printing
Tips for Optimum Screen Exposure
Maximizing Image Size and Position
Understanding Screen-Tension Loss
A Look at Screen Lift-Off
Staging Garment-Printing Jobs
How to Control Dye Migration
Fine Tuning for Fine Details
Application-Specific Considerations for Graphics Printers
Upping the Ante at SGIA 2012
Statement of Ownership
U.S. & Canadian Directory
Opportunity Exchange
Advertising Index

Screen Printing - December 2012/January 2013