Halftones Continued: Pushing the Limits
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Halftones Continued: Pushing the Limits

The history of screenprinting can be quickly summed up as a series of inventive solutions to printing problems.

By Bill Stephens

In fact, screenprinters occasionally forget that there are limits to what we can accomplish with screen, ink, and squeegee. We're going to begin this second article in our series with a look at some of those limits, unique problems screenprinters face when printing halftones.

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  • DOT Gain
    The first concern of any halftone printer is reproducing dots accurately. If you take care of the dots, the image pretty much takes care of itself. Accuracy doesn't just mean reproducing every dot; it also means keeping the dots on the printed page the same size as the dots on the computer screen.

    If you read the earlier article in this series, you already know about a problem called dot gain. This means that reproduced dots are larger than the dots in the original artwork. Dot gain can play havoc with a halftone, because all of the shades or tints that appear in the image are created solely by differences in the sizes of the dots. When dots become larger, the tints in the image grow darker. Worse, some tints disappear out entirely. Dot sizes in the 85% range and above are especially vulnerable. Deep grays are often transformed into solid black thanks to dot gain.

    Unfortunately, dot gain doesn't just affect the darker areas. It can make a noticeable difference in tints in the midtone range (40%-60%), where it can swallow up transitional tints, chopping up subtle blends into distinct bands of different shades. This is known as tonal jump.

    Dot gain isn't just a screenprinting problem. Every printing method capable of printing halftones has to contend with it. Unfortunately, it does tend to be more of a problem for screenprinters because the inks we print with have a greater tendency to spread out when they come into contact with the substrate. Not only that but there's more ink involved, since screenprinting produces the heaviest ink deposit of any print technology.

    But dot gain doesn't only occur during the printing process; part of the problem can be traced to the prepress area. Dot gain can occur whenever the original artwork is copied in another medium. It can be introduced when positives are made. It can take place during the stencilmaking process.

    The prepress area is the best place to combat dot gain. When you make computer-generated halftone positives, most graphic arts programs make it fairly easy to adjust dot size. The trick is in anticipating how much dot gain to expect from certain combinations of ink and substrate. It's slightly more complicated than compensating for 20% dot gain by making all the dots in your positive 20% smaller. Remember dot gain affects some tonal ranges more than others, and you have to concentrate your efforts on those areas, particularly vulnerable shades.

    Of course, you can't know how much dot gain to allow for without doing at least one test print. Get used to making test prints when a halftone job comes up. You may need to make several to help you weed out problems before you launch into your production run. Making several versions of a positive and shooting extra screens can seem like a waste of time, but in the end it not only saves time but a great deal of money. No matter how much it costs, it's almost certain to be much less expensive than replacing a stack of ruined substrate.

    Line counts determine mesh counts
    Dot size also helps us determine which mesh to use to print a halftone. When we make a stencil, those halftone dots become tiny bits of emulsion, many of them isolated from any other part of the stencil. This means their only support comes from the threads of the mesh. The quality of your print job depends not only on how closely those dots resemble the dots in your original artwork but on how securely they are fastened to the mesh threads. The appearance of your printed image depends to a great extent on how many of those tiny dots manage to cling onto the mesh despite being hit by streams of water and being scraped a squeegee blade. Over the years, screenprinters have worked out how much support a dot needs to be able to hang on.

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    The relative size of the dot and the mesh threads turns out to be critical. One common rule of thumb states that a dot must be at least the diameter of three thread diameters. Another, and probably more accurate method, computes the minimum width based on two thread diameters plus a mesh opening. Any dot that doesn't come into in contact with at least two threads is in danger of disappearing.

    This means that when selecting a mesh for a halftone job, we need to think of thread diameter before we consider mesh count (the number of threads per inch). Of course, the two are related. As mesh counts go up, thread diameters tend to shrink. Coarser meshes inevitably have thicker threads. But many meshes, although they have the same mesh count, are available with different thread diameters. Thinner thread diameters allow larger mesh openings and a greater percentage of open area, offering less resistance to the flow of ink. There is no firm connection between mesh openings and thread diameter. Openings can be smaller, larger, or equal to the diameter of the mesh threads.

    As halftone line counts go up, the dots get smaller. To hold those smaller dots we have to use progressively finer threads and higher mesh counts. Eventually we come to the finest mesh screenprinting mesh available (27 microns, or about a third of the width of a human hair). Unfortunately, halftone screen frequencies keep going up and dots become so small we can no longer hold them, even with our finest mesh.

    The highest line counts most screenprinters will ever print are in the 85 to 100 LPI range, or about the same frequency used in newspaper halftones. Depending on the type of ink we use and the substrate we're printing on, the limit may come at a much lower line count. The mesh we use has to have openings large enough to allow the particles of ink pigment to pass through. Mesh is also the major factor in controlling ink deposit, so when we select a mesh, its ability to hold halftone dots may be secondary to how much ink it leaves behind on the substrate. For an example of the limits imposed by ink and substrate, you have only to look to T-shirt printing, where few printers print even 65 LPI halftones, with most sticking to far lower line counts.

    To print the best possible halftone image, you have to be able to print as many dots as possible. However, rarely will we be able to print all of the dots in a halftone. As we get near to the limits of what given mesh can handle, significant portions of the dots in both the higher and lower percentages fail to print. Dots in the highlight areas, 15% and smaller, may be so small in comparison to the mesh threads, that the threads themselves block the tiny stencil openings. At the same time, spreading ink can overwhelm the small non-printing areas in dots of 80% and above, causing them to fill in. Whether or not such losses are acceptable really depends on the job.

    The best results come, of course, when we stay away from the limits. Always use the lowest halftone line count acceptable for the job at hand and mesh with the highest mesh count you can comfortably work with. A quick and rather informal way of checking whether or not the mesh in your screen can handle a halftone is to lay the screen on a light table and lay the positive on top of it. Use a magnifying glass to see how the dots match up to the mesh threads. Remember, they should span two or more. The alternate and more mathematically precise method is to measure the diameter of the smallest halftone dot and use the two-threads-plus-a-mesh-opening formula.

    Many printers base their decision on which mesh to use by a formula that links mesh count to halftone line count. The problem is that there is considerable disagreement about how the two relate. Depending on the screenprinter you talk to, the mesh count should be 3.5 times, 4 times, 4.5 times or even 5 times the halftone line count. The difference of opinion should tell you something about the reliability of the formula. The problem is that this method ignores a number of key variables like mesh tension, which can make a considerable difference to thread count. (See my article "Mesh Part II: The Tension Rises" available on this website.) Still, if you want to use a factor of 4 or even 5 times halftone line count as a guideline to selecting an appropriate mesh, you should end up somewhere in the ballpark.

    Fortunately, as sign printers, the halftone jobs we get will rarely be seen close up. If you read the first article in this series you know that distance is a factor in choosing halftone line counts, and the further away the viewer stands from the halftone, the lower the line count you can safely use. Most signs using halftones can safely be printed with mesh counts well under 380 threads per inch.

    One variable at a time
    There is no doubt that screenprinters face a few special problems when printing halftones. Don't let them discourage you from taking on your own halftone jobs. Every year screenprinters successfully print millions of halftone impressions.

    If the sheer number of rules seem overwhelming and the standards that have to be met seem high, remember that you are entering an area that pushes the limits of screenprint technology and rules are simply the best way of eliminating the numerous variables that seem to affect every aspect of screenprinting.

    Beginning to exert some control over those variables is the first step in being able to consistently produce good work. Once you succeed in establishing a set of guidelines that produce good results every time, you are free to push the limits or even cut the occasional corner. But push one limit or cut one corner at a time. At least then, you'll have no problem spotting the source of the problem when things go wrong. And the solution is as easy as going back to your original standard.

    In the remaining articles in this series, we're going to give you a set of guidelines to work with. We're going take you step-by-step through the process of printing a halftone. In the next article, we're going to begin at the beginning by taking a look at artwork -- everything from shape of halftone dots to how to create the perfect positive.

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