Capillary films offer many benefits to industrial screen printers. The following discussion compares stencil films and direct emulsions, weighing the perceived cost advantages of emulsions agains the processing and performance attributes of capillary films.
Versatility is a hallmark of screen printing and a characteristic that has allowed the process to thrive, despite competition from other decorating methods. Compared to other printing technologies, screen printing provides a relatively uncomplicated method of image reproduction and supports a much wider range of inks and substrate types, sizes, and shapes. These capabilities have made screen printing a staple decoration method for a wide range of products, particularly in the field of industrial decorating. For screen printers to maintain their share of the industrial market against competing technologies, they must adopt a manufacturing mentality in material and equipment selection, as well as in the procedures they employ for printing. Such a manufacturing mentality is based on three tenets: * Control: Processing must be brought under control, meaning that results must be reproducible, tolerances met, and standards of quality maintained or exceeded. * Simplification of processing: Wherever possible, processing steps should be simplified and their number reduced to decrease the possibility of mechanical and human error. * Efficiency enhancement: Manufactured goods must be produced efficiently, which is accomplished by reducing production time and increasing throughput. The goal is to add value at the lowest possible cost. An effective production manager will evaluate every element of the manufacturing process against these requirements. If the manufactured item is decorated by screen printing, that scrutiny should include assessing the impact of ink, mesh, and stencil-material selection. This article will focus on the latter of these three elements and examine how the use of stencil films can benefit industrial screen printers. Control The function of the stencil is to block the flow of ink to non-image areas and to direct the flow of ink to image areas. Over broad image areas, the fabric "meters" the ink and is the principal determinant of the ink deposit. With fine line and halftone work, however, stencil material is adjacent to image edges, so the printed ink deposit becomes the sum of ink held in the mesh openings and ink held by the additional thickness of the stencil at those openings. Thus, control of stencil thickness is essential for controlling the ink depo-sit in any industrial jobs that involve fine lines or halftones. Control of the ink deposit, in turn, permits control of color, drying/curing parameters, and costs. The configuration of the stencil must be controlled in three ways: 1. It must be uniform. 2. It must be independent of the mesh weave and provide a flat surface on the bottom (substrate side) of the screen. 3. It must present a relatively thin profile to avoid both printing skips and excessive deposits that lead to collapsed ink columns, dot gain, undercuring, or unpredictable tonal changes where colors overprint. Controlling stencil configuration is more challenging with direct emulsions than with capillary films. Films are manufactured to tolerances of ±1 micron and maintain these tolerances throughout stencil processing. Achieving the same tolerances when coating screens with direct emulsion requires control of multiple variables, including coating-trough fill level, configuration of the trough's leading edge, and coating speed, angle, and pressure. Varying any of these elements can affect stencil thickness and uniformity. The bottom of a film stencil is inherently flat because of the polyester carrier base onto which the film is initially coated by the manufacturer. Achieving stencil-bottom flatness (a low Rz value) with direct emulsion also is possible by following initial coating and drying with successive face coats and intermediate dryings. Such procedures certainly can be made part of a manufacturing routine, but each coating pass has the potential of introducing errors and complications. Even direct emulsions with high-solids content are approximately half water, so they lose half their volume when they dry, and the emulsion contracts into the mesh structure. If additional coatings (with intermediate dryings) aren't made, the stencil bottom won't be flush against the substrate during printing. Squeegee pressure will force ink into the concavities at the bottom of the stencil, underneath what were supposed to have been the image edges. The result will be a serrated or "sawtoothed" edge in the print. Additional emulsion coatings may be added to build up the stencil and smooth over the concavities, but this can cause the emulsion-over-mesh (EOM) level to become inappropriately thick for many applications. Achieving a low Rz value with direct emulsion can necessitate a build-up as thick as 20 microns. Capillary film, on the other hand, can be purchased to match specific mesh counts and provide a low Rz value at an EOM of 5-7 microns or less (indirect films are flat at thicknesses of only 3-4 microns). One of the main drawbacks of using a thick stencil for high-resolution printing is that the stencil may become incompatible with the rheology of the ink being printed, which could prevent proper ink transfer. As a result, the printer may be forced to use ink additives and make adjustments to the floodbar and squeegee pressure, adding additional variables to the job. Another drawback of the thicker stencil is that if the ink releases from the screen, more of it will be used per impression than with a thinner stencil. The additional ink costs can easily nullify any savings from using direct emulsion rather than more expensive capillary films.
|Figure 1 Steps in Stencil|
|Processing step||Direct emulsion||Capillary film|
|Drying after screen prep.||*|
|Coating for flatness||*|
|Washing out coating trough||*|
|Totals||14 steps||8 steps|
Reduction of processing steps Film systems require significantly fewer processing steps than direct emulsions (Figure 1). As a result, they introduce far fewer variables that affect stencil thickness (Figure 2), greatly reducing the possibilities for error. Even if processing steps for direct emulsions are automated with coating machines, the machines still require human tending. And, even with automatic fill-level maintenance, these coaters have difficulty matching the close tolerances of the industrial-grade coating equipment used by stencil-film manufacturers. The use of emulsion burdens the printer with finishing a "raw" material, when material in a more finished state--film-- is readily available. Film is sometimes dismissed as "fragile," yet water-adhered capillary film is capable of printing 10-30,000 impressions. If the film is wet-adhered, dried, and reinforced with emulsion, it can yield as many as 50-70,000 impressions.
|Figure 2 Variables Affecting Stencil Thickness|
|Variable||Direct emulsion||Indirect capillary film||Capillary film|
|Dilution of sensitizer||*|
|Wetting dwell time||*||*|
|Coater fill level||*|
|Coating trough edge||*|
Elevated efficiency If we look at material costs only, we see that film costs more per square meter of stencil than direct emulsion, even if we allow for spills, emulsion wasted in the coating trough, and throw-away emulsion in the bottom of the bucket. However, if an automated stencil coater is used, its cost and amortization schedule must be included in the equation. More importantly, the costing must include labor expenses for tending to the coating machine, touching up pinholes, and completing the additional processing steps required for emulsions. Consider the example of a European company specializing in screens for CD printers. They employed two workers to apply 21 x 21-cm sheets of capillary film to prestretched, degreased screens and produce more than 250 exposure-ready screens/hr. In an attempt to reduce labor costs, the company purchased an emulsion-coating machine that could simultaneously coat five CD screens. They found that they could achieve satisfactory quality only with multiple coatings and infrared intermediate dryings. Using this regimen, it took five minutes to produce each screen. Because there were five stations, production averaged one screen/min or 60 screens/hr, assuming no rest periods for the workers. Film also adds to the value of the decoration of manufactured goods because of its superior resolution and imaging properties. The combination of flatness and thin profile inherent in film, more difficult to achieve with emulsion, simply yields superior images. Capillary film: Worth a second look When we compare the impact of using capillary film versus emulsion, the variation in cost per printed item often amounts to just a fraction of a cent, given the far more significant costs of labor, fixed overhead, substrates, mesh, and ink. In short, any shop that demands the highest level of image quality along with the greatest control, fewest processing steps, and most efficiency can benefit from the use of capillary stencil films. About the author Donald Marsden is director of industrial commercial services for Ulano Corp., Brooklyn, NY, an emulsion and stencil-film manufacturer. A graduate of Princeton and Johns Hopkins Universities, he joined the company in 1972. Marsden is a member of the Academy of Screen Printing Technology and the Society of Glass and Ceramic Decorators and has written articles for trade publications worldwide. His industry contributions earned him the Sundeen Award from the International Graphic Arts Education Association in 1992. Editor's note: This article was updated from an earlier version that appeared in the Sept. 1999 edition of Screen Process magazine in the United Kingdom.
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