Screen Printing Squeegees: How To Pull Prints
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Sign China 2015 - September 16-19, 2015 in Shanghai, China


Screen Printing Squeegees: How To Pull Prints

Screenprinting is one of the most universal segments of the sign industry and one of the most profitable, but only if you know how to use and care for the tools of the trade.

By Bill Stephens

If you've been following along in this series you're now equipped with the perfect squeegee for the type of work you'll be printing. You understand squeegee angle and pressure, your off-contact distance is set, and the ink is in the screen. In short, you're ready to start pulling prints. So, let's take a closer look at exactly what you're going to be doing.

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  • One complete movement of a squeegee across a screen is called a stroke. There are two different strokes used in screenprinting: the flood stroke and the print stroke. Both are important and both have very distinct purposes.

    The Flood Stroke
    The flood stroke comes first. Its job is to spread ink across the screen in preparation for the print stroke. While it's doing this it also pre-loads the mesh openings with ink. With the mesh openings already filled, it takes very little effort on the part of the print stroke to transfer that ink onto the surface of the substrate.

    Hardly any squeegee pressure is used when making a flood stroke -- enough to move the ink across the screen but not enough to force it completely through the mesh openings. It's not always easy to judge the exact amount of pressure required, so the screen is usually raised off the substrate to prevent accidental imprints.

    Many printers regard the flood stroke simply as preparation for the print stroke. However, when printing with air-dry inks, it becomes vital. In fact, a flood stroke becomes the best way to make sure the ink keeps on flowing throughout your print run.

    This is why at the conclusion of your print stroke some ink will be left clinging to the threads of the mesh openings. This tiny bit of ink will be surrounded by quite a lot of air, creating ideal conditions for rapid evaporation of the solvent. Inks with highly volatile solvents, like lacquer-based inks, tend to dry very quickly. Parts of the screen, especially near the more highly detailed areas in the stencil, can be blocked in little more than seconds. This can be an even bigger problem if you are using a screen stretched with very fine mesh, or if the shop temperature happens to rise. If you are working in an unair-conditioned shop during the summer months, screens blocked by rapidly drying ink can mean missed deadlines and a bin full of ruined substrate.

    The flood stroke offers some very effective protection against this by depositing a thick protective barrier of extra ink between the air and the mesh openings. Smart printers make the flood stroke an essential part of their printing routine. It only takes a few seconds, and it can keep you from having to put your print run on hold for many minutes while you clear the blocked mesh.

    The Print Stroke
    With the screen loaded with ink, we're ready for the print stroke. While the print stroke is only a single movement of the squeegee across the screen, it actually performs three separate functions. First, it brings the mesh down onto the surface of the substrate, closing the off-contact distance. Secondly, it sweeps forward across the screen pushing a wave of ink before it. And, finally, it cuts away the excess ink that remains on the top of the mesh and carries it away to the far end of the screen.

    Let's take a look at the print stroke's first job, the downward movement that closes the off-contact distance. As it does so, it puts the mesh under added tension. The mesh will, of course, already be under some tension as a result of being stretched when the screen was assembled. This is called static tension, the innate tension of the mesh at rest. The additional tension created by the downward thrust of the squeegee is called dynamic tension. It is the mesh's resistance to this dynamic tension and its desire to return to its rest state that makes off-contact screen printing work.

    Only a fine line of mesh directly underneath the edge of the squeegee blade comes into contact with the substrate at any one time. The very instant the mesh is released from the pressure exerted by the edge of the squeegee blade, its own internal tension snaps it up off the substrate. With the mesh out of the way, there's nothing to interfere with the newly deposited wet ink on the surface of the substrate. The pressure of the squeegee tends to drag the mesh and stretch it slightly. These movements alone are more than enough to smudge the imprint. But in off-contact printing the interplay between dynamic and static tension tends to produce imprints that are clear and sharp.

    Now that we've dealt with the downward part of the print stroke, let's look at its second function. When the squeegee moves forward it begins to apply force to the ink rather than to the screen. The force applied to ink by the squeegee blade is called shear, a term that can be applied to any force that causes ink to flow. As the squeegee moves across the screen it sweeps a wave of ink before it. The force generated within this wave pushes ink through the mesh and down onto the substrate.

    So, now we've come to the third and final function of the print stroke. As the squeegee sweeps forward pushing the wave of ink before it, it simultaneously cuts off any ink that protrudes above the threads of the mesh. This leftover ink, not needed for the imprint, is gathered into the wave of ink being swept along by the squeegee. Ultimately, the squeegee will deposit it, along with the rest of the wave, in the inkwell at the end of the screen ready to be redistributed by the next flood stroke.

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    Double Stroking
    Sometimes we may not be entirely happy with the print we've created. Maybe the squeegee drifted slightly off track and missed part of the image. How do we fix it? The natural reaction is to make a second print stroke. Unfortunately, this is unlikely to improve your print, and may actually make matters worse. T-shirt printers sometimes use a technique called double-stroking, two successive print strokes made with the intention of building up a thicker ink deposit, a fairly normal practice when printing white ink on a black shirt, for example.

    It's one thing to try this when you're printing on an absorbent surface like a T-shirt, quite a different matter when printing on less absorbent surfaces. Extra ink on the T-shirt simply ends being driven deeper into the shirt. On a nonabsorbent surface it has nowhere to go except under the edges of your stencil. It will remain there creating ghost images or a halo effect (faint outlines around the imprint) on all your subsequent prints until it either wears off or you stop and clean off the print side of the screen. Since most sign materials tend to be nonabsorbent, you only get one chance to make a good impression.

    Push or pull?
    It is common to speak of "pulling a print", and this describes exactly what most printers do. But it is also possible to push the squeegee. In fact, many printers find that pushing a squeegee is quite a bit easier than pulling one. Even those who stick to a pull stroke for most of their printing sometimes switch to a push stroke during long print. Pushing uses a different set of muscles, and it can provide a welcome break. It may, however, produce a slightly heavier ink deposit, so make a few test prints when you first change directions. If you find the change in directions produces a noticeable difference in your ink deposit, chances are you can compensate by making a slight change in your squeegee angle.

    Squeegee speed
    There is no standard print stroke speed. It varies, depending on factors like the viscosity of the ink, the thread count of the mesh, and the substrate. The only rule is a print stroke has to allow the ink time to flow onto the substrate. Obviously, the longer it takes the ink to move, the slower your print stroke has to be. So, a thicker (higher viscosity) ink will require a slower print stroke than a thinner (low viscosity) ink. Many of other factors that determine stroke speed are also those that affect the flow of ink. Finer meshes and stencils that contain more highly detailed images will impede the flow of ink, and so they, too, will require a slower print stroke.

    The slower the print stroke the more time it allows for the ink to seep onto the substrate, so when all other factors are equal, a slower print stroke tends to produce a heavier ink deposit. This makes a strong argument for keeping a consistent printing speed throughout a print run when a consistent ink deposit is important.

    Screen tension, too, can be an influence on printing speed. Generally, higher-tensioned screens that allow for closer off-contact distances make for faster printing.

    Squeegee maintenance and storage
    With proper care a squeegee should last many years, but careless handling has retired many a squeegee blade well before its time. The critical area is the printing edge. Any nicks and gouges will certainly show up as streaks in your prints. Normal wear and tear will, however, eventually do in any squeegee blade. Pulling hundreds of prints will gradually round off the edge. As the edge grows duller, the squeegee will deposit more and more ink. At some point the blade will become so rounded-over on the edges it will no longer produce acceptable prints. The solution can be as easy as reversing the squeegee in its holder, which some types of holders allows you to do. Alternatively, you can either replace the blade, or sharpen it.

    Squeegee sharpeners cost many hundreds of dollars. They restore the printing edge by cutting away a thin layer of the old blade. There are two basic types. One type cuts away the worn material with a knife. A second type grinds it away. Sharpeners that use abrasives can be further subdivided depending on whether the abrasive they use is mounted on a wheel or on a belt. Many economically minded printers have built their own sharpeners employing nothing more complicated than a sheet of sandpaper fixed to a board with wooden guide rail mounted next to it to keep the squeegee straight.

    When caring for your squeegee, next to looking after the edge, the most important thing is to keep the blade clean. If you allow ink to dry on the blade and become hardened, it will be much more difficult to remove. Dried ink on the printing edge will leave streaks in your prints and can even damage the mesh in your screen.

    It's a very good idea to make a habit of wiping off the blade as soon as you've finished printing. The only cleaning tools you need are some paper towels, or rags, soaked in mild solvent. Never immerse the blade, however. Solvent will soak into the blade and cause it to swell and soften. Prolonged contact may even cause warping. Even when wiping down the blade, try to use as little solvent as you can.

    What about the solvent in the ink? Doesn't that affect the blade? Of course it does. The solvent in the ink will cause some softening, which is why it's a good idea to have more than one squeegee on hand. Ideally, they need to be able to rest for a day or so between print runs to give the solvent they've absorbed a chance to evaporate. It is absolutely essential that a blade is allowed to rest before you try to sharpen it.

    Over time, repeated contact with solvents will gradually change the durometer of the blade. Paradoxically, although the short term effect of solvent is to cause the blade to become softer, repeated exposures will gradually make it harder and more brittle, a phenomenon known as drift.

    You can minimize the effects of solvents by using higher durometer squeegee blades. These tend to be more resistant to penetration. Not only that, but they also stand up better to the general wear and tear of printing. But contact with highly aggressive solvents will eventually affect even the highest durometer blades.

    Careful storage is another key to prolonging the life of your squeegees. Assembled squeegees should be laid flat. Never stand them upright with the weight of the handle bearing down on the blade, which could cause the blade to warp.

    Unassembled lengths of squeegee blade should be stored flat. This is especially important to remember when you first begin to buy full lengths of blade. The manufacturer may ship it coiled, but you need to uncoil it and lay it flat before you store it. A blade that remains coiled up may retain a slight curve throughout its lifetime. The ideal place to store your squeegee blade is a cool, dry room, away from dust, direct heat, and out of UV sources like sunlight.

    This brings us to the end of our series on squeegees. Next time we'll take a look at another important process, reclaiming screens. We'll show you how taking good care of your screens in the reclaiming tank can mean additional dollars in your pocket.

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