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Controlling Ink Flow with Combination Separations

(September 2005) posted on Wed Sep 14, 2005

How to combine the smooth gradiations of traditional halftones and the texture and details-carrying capabilities of index dots in the same design.


By Tom Trimingham

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One way to look at separations in screen printing is to view them in terms of ink flow. The ink flows across the screen, falls into the stencil opening created by the emulsion, and then releases onto the surface of the garment. Controlling how the ink flows through the stencil is the job of the specific color separation used to create the stencil. If you have ever printed a design that has long, horizontal lines that run parallel to the squeegee blade, you have seen how the squeegee will bump and stall on these areas as the "wave" of ink and blade of the squeegee hit the emulsion's edges. When you are aware of this issue you can easily avoid ink-flow problems by adjusting graphics, stencil thickness, or mesh count. A more complex issue is the control of the flow of ink through a halftone screen. The variables increase in this case as you now have to take into account the way that the halftone works with both the ink and the graphic to reproduce the tonal ranges in the original artwork. An interesting effect of ink movement through the mesh is that the shape and style of the halftone dots also direct ink flow in their own ways. In a traditional halftone pattern (Figure 1) the dots change in size depending on the percentage of coverage, while the center of the dots remains a consistent distance apart. This creates an environment with progressively smaller and smaller barriers that prevent excessive ink flow, especially when the dots become large enough that they overlap each other and leave tiny pieces of emulsion in between these shapes (one of the reasons that 70-90% tonal areas can fill in so quickly). The alternative to traditional halftones is what I call density halftones. These halftones are composed of equal-sized dots that create the illusion of a tonal range based on the distance at which they are spaced. The most popular of the density halftones is the index dot (Figure 2). Here, the dot is actually a single pixel in the resolution value of the image. For a size comparison, a 180-dpi index dot would be approximately the size of a 55-lines/in. halftone at 15%. There are many advantages to using index dots when it comes to ink flow. Dots that are all equal in size provide more control over the ink deposit (think of a saltshaker with equally sized holes) and will show significantly less dot gain over the course of a print run because there is far less overlapping of dots in the higher percentages. Another big advantage of an index dot in screen printing is that the dots stack next to one another and not on top of each other as they do in traditional halftones. Less stacking of dots means less ink spread caused by the ink being pushed around from the pressure of another dot on top of it. The downside to density halftones is that they don't reproduce delicate shading or tonal gradations very well. Everything tends to look grainy in an indexed image, and colors usually have a definitive edge. Some printers still swear by this method and use a very fine index dot (above 200 dpi) that is printed wet on wet to combat these problems. The theory is that the wet inks blend together and form a smooth surface without the grainy appearance, but it's just as difficult to manage as a traditional halftone with the advanced level of screenmaking and ink controls needed to achieve brightness without smearing or dulling. The textural look of a density halftone also requires several more colors to achieve the same tonal gradations as a traditional halftone. It is likely that six colors or more will be necessary to produce a multidimensional appearance in an index print. A simple solution to this puzzle is to learn how to combine both conventional and index halftones in the same set of separations so that each style of dot can be used where it works best. I find that combination separations work well with images that have both textures and smooth gradients or shadows. If the design has similar imagery throughout, then a combination separation is probably unnecessary. The design that I created for a local promotional company called for a combination separation because the figures were textural and the logo and background searchlights were very smooth and rendered with gradients (Figure 3). Producing a combination of halftone styles in the plates for this piece helped control the ink flow in the different textures and yielded the best finished results. Separation of a combination halftone If you have created your own art in Photoshop, then it is likely that you have all major elements confined to their respective layers. But if you were provided artwork, you will have to create some selection channels using paths or other methods to effectively isolate separate elements in your design. All of the major elements in my XFO design were isolated in their own layers in Photoshop. This made it an easier task to prepare the file for separation. The steps for creating a combination halftone require that you first prep the file, selectively separate out the elements that require traditional separations, isolate and create the index separations, and then combine them together to form the completed halftone images. Prepping the file The most important thing to remember when creating separations using the index method is that the final size of the dots must be equal to the dot size (also called pixel size) you defined for the index halftones in your design. This means that your image must be separated at final print size and output to film at a resolution that effectively holds this dot size on screen. A good way to test what size dot you can hold is to create a test file that has several different resolutions in it. Print and then examine the result to see what dots create the best tonal range in the finished print and still hold on screen. It is important to remember that you don't want to just print the smallest dots possible; you want to define the dots that keep the best tonal clarity. This means that you can see all of the transitions between values in a grayscale test. A smaller dot may make your midtones (25-65%) blend together and lose differentiation so that the image becomes visually flatter. I composed an index test file in CorelDRAW so that I could keep all of the resolutions printing with sharp edges. The bitmaps were imported and then printed together to see what could be held on screen with the best tonal range (Figure 4). For the test I used 230-, 266-, and 305-threads/in. mesh counts. I also exposed the test on some lower mesh counts to see what would hold up on a 180- or 190-threads/in. mesh for printing an underbase. The final results showed the best tonal range was reproduced at 190 dpi on 305-threads/in. mesh. Your equipment will influence the results you get, so test your own press and films to define your best range of dots. Once I determined the optimum resolution for the print, I resized the XFO image in Photoshop to 190 dpi at the actual print size without flattening the layers. I then duplicated my original file twice and deleted everything except the figures in the background on one file, which would be my index portion of the print because of its grainy appearance. I deleted this portion of the image from my second copy of the original file and kept what remained for my traditional halftones in order to capture the smooth metal gradients and the background searchlights. The files are now considered prepped because they can be separated in different modes and then recombined into a final set of positives. Separate the traditional halftones The file that needed traditional separations was composed of grayscale images. I used Photoshop's Curves menu to separate out two grays


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