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Screen Printing Squeegees: How To Pull Prints
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.
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
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
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.
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?
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
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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