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Printing Halftones Part One: Frames, Mesh and Moiré
By Bill Stephens
The halftone positive has just come back from the service bureau, and you can’t wait to get it on the light table so you can check it out with a pocket microscope. Some of the things you hope to find are halftone dots that are solid black and open areas that are completely transparent. You’ll also check to make sure that there aren’t any pinholes in hard-to-correct places, and you may even make a pin scratch in a black spot well out of the print area just to be sure that the emulsion really is on the right-reading side of the film.
In this case, the positive is perfect, and it looks like you're ready to get on with printing your first halftone. Before you go any further let's take a minute to be sure you've got everything you need.
We'll begin with the screen. A screen has only two parts, mesh and frame, but for printing halftones both must meet exacting standards. The frame has to be solid and well made; the corners square and stable. Mesh tensioned at 20 newtons or more places tremendous stress on a frame, enough to warp a flimsy frame. A frame has to be perfectly flat or it is useless for halftone printing. In fact, it’s not much use for any kind of printing, and you are probably better off discarding any suspect frames.
Originally, virtually all screen frames were made of wood, but wood eventually deteriorates under regular contact with water, and frames get wet every time you wash out a stencil or reclaim a screen. Steel was also used as a frame material, but steel is heavy and not even a good coat of paint can keep it from rusting under these conditions. Plastic screen frames do stand up to water very well, but they have never enjoyed great popularity because of problems with strength and durability. Light, strong and impervious to water, aluminum is the most popular frame material in current use. Mesh is usually glued to the frame after being brought up to tension in an external pneumatic or mechanical stretching system. Ideally, the mesh will retain virtually all of that initial tension through most of its working life. However, eventually wear and tear takes its toll and mesh tension drops off to a point where the screens will no longer print properly. At this point, standard frames can only be sent off to be re-stretched with new mesh.
Self-tensioning frames have become an increasingly popular alternative. These frames have their own built-in mesh-stretching mechanisms. This not only eliminates the cost of an expensive external stretching system, it allows a printer to make periodic adjustments to mesh tension. Screens that are always at optimum tension can be a terrific advantage in finely detailed printing.
Of course, self-tensioning frames cost more than standard frames, and offer virtually no benefit unless you also have a tension meter. A mesh tension meter costs about $600, but it can be a wise investment even if you never stretch a screen. Mesh tension can make such a difference to your results that you should probably check the tension in any screen you intend to use for printing halftones. Buying a mesh tension gauge can be an important step in upgrading the standards of your shop.
If you are considering self-tensioning frames as a way of saving money, don’t forget to factor in the ongoing cost of labor and materials involved in any screen stretching operation. And time spent stretching frames is that much less time available for printing.
Sizing things up
Free space also has to be allowed on both sides of the frame, not just enough to allow the squeegee to pass freely, but enough extra room to make sure the ends of the squeegee stay well away from the frame. Within an inch of the point where it’s glued to the frame, mesh lacks the flexibility to stretch the usual 1/16" to 1/8" off-contact distance. It the squeegee blade protrudes into this less flexible mesh, it meets with increased resistance, which the printer has to overcome by applying more downward pressure on the squeegee. The result is increased wear and tear on the mesh, the squeegee blade, and the printer himself, not to mention a fair number of misprints.
The length of the squeegee can actually serve as a guide to ideal frame width. Ideally, the squeegee will be long enough to overlap the print area by an inch or two on both sides. In hand printing, if the squeegee is too short it could drift off to one side during the print stroke and miss part of the artwork. On the other hand, you don’t want a squeegee that’s too wide either. The wider the squeegee, the more downward pressure required to make a print stroke.
So by choosing a squeegee that’s a couple of inches wider than the artwork and adding 8” to 12” to that, you have your minimum frame width. Then, allow at least this much space at both the top and bottom of the frame.
Monofilament mesh is marked with the thread count, a period, and then the thread diameter. For halftone printing you will be using monofilament mesh with a thread count of at least 255 threads per inch and above. As a rule, the more threads per inch, the better job a mesh will do of holding fine details like halftone dots.
For halftone work, we usually prefer to use the mesh with the smaller thread diameter. Smaller threads not only do better at holding fine details, they also result in smaller ink deposits.
Less ink means less dot gain. As you recall from earlier articles in this series, dot gain can wipe out many of the details in a halftone image, and anything we can do to eliminate dot gain is likely to result in a better print. On the other hand, we also need to make sure enough ink gets onto the substrate to create an opaque imprint.
When printing with UV inks, we are invariably trying to leave the smallest possible ink deposit. This almost inevitably means choosing the mesh with the smallest thread diameter. In UV printing, thin ink deposits facilitate curing because they allow UV light to penetrate the entire link layer.
The other factor that determines the size of mesh openings is tension. Even if thread counts and thread diameters are exactly the same, the mesh at a higher tension will tend to have larger mesh openings. Tension, therefore, becomes another vital factor in halftone printing.
It is important to understand that there is no one ideal tension. It varies according to the mesh. Manufacturers usually specify the optimum tension for each of their meshes. This information, usually given in table form, should be available either on their website or in their catalog. But in the absence of better information, keep your halftone screens at an absolute minimum of 20 newtons tension. In general, higher tensioned mesh not only does a better job of holding fine detail, it makes for easier printing all around.
Different weaves and different colors
Another rule for halftone printing is to use only dyed mesh, at least when using direct stencil materials. White mesh causes light scatter during exposure, which tends to undercut the positive, and creates halftones that are irregular in shape. This, too, can contribute significantly to moiré problems.
Mesh count and halftone line count
We also learned that many printers choose mesh counts based on the halftone line count. There are various versions of this rule of thumb and depending on which version you use, the factor can be anywhere from four to five times the halftone line count. This method can work well if you use actual mesh counts and accurate halftone line counts. If you intend to employ this method, it helps to have the information available. Mark the mesh count on all your screens and make sure line count appears somewhere on the halftone positive. This can make a big difference if the job has to be rerun at a later date, usually long after the original screen has been reclaimed.
Moiré can be caused by artwork; someone in the photograph wearing a tweed jacket is the classic example. It can be caused by poor contact between the positive and the screen during exposure. It can even arise from the interaction between the halftone imprint and the textured surface of certain substrates.
Moiré is most likely to appear when one dot pattern is superimposed on another, so it most often occurs in print jobs that involve multiple screens. Still, it can’t be entirely ruled out even in a one-color job, because two patterns are automatically present in the halftone grid and the weave of the mesh.
Of course, it often arises when halftone dots line up in a certain way with the threads in the mesh. Instead of switching mesh, you could try changing the angle of the positive on the screen. Even a shift of as little as 5 to 7 degrees might clear up the conflict. Now, of course, you have a stencil that doesn't quite line up with the frame of the screen, which can cause alignment problems with press and substrate. It can be easier just to change the angle of the mesh in the screen. Stretching services will often stretch the mesh at whatever angle you specify. With your mesh at 5 to 7 degrees you can eliminate moiré and still position your positive squarely within the frame. The screen stretching service may charge more to angle the mesh, but anybody who has battled moiré problems would consider it money well spent.
Moiré is unpredictable. Unfortunately, it often doesn’t show up until you’re actually looking at a final print. Most solutions involve either re-shooting or re-stretching screens. In some cases, even positives have to be redone. All of these solutions take time and sometimes the problem doesn’t get solved on the first try. Keep this in mind when agreeing to a delivery date for any halftone job. Make sure you factor in enough time to work out any problems
As you can see, halftone printing isn’t a great deal different from regular screenprinting. It just takes greater care and precision. And that care and precision has to be applied to every step of the process. If you get used to hitting those standards, and applying them to all of your screenprinting work, you’ll not only become a better printer but a more efficient one. Most screenprinting problems arise from simple sloppiness, but by meeting higher standards of care and accuracy you’ll automatically be eliminating a lot of the problems that waste time and resources.
Next time, our series on printing halftones continues with a close-up look at making halftone stencils.
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