Screen Printing Articles Industry Alert Hot Shots Photo Gallery Message Boards Visit Our Advertisers: Clarke Systems Estimate Software International Sign Assoc. Matrix Payment Systems PRINTING United

When you consider that a skilled printer knows how to adjust his print stroke to compensate for variables like various durometers in squeegee blades, different viscosity inks, mesh counts in screens, and a wide range of substrates, you can see that there is a lot to learn. But since most of this is knowledge that is acquired through practice, many printers find it difficult to teach the knack of pulling a squeegee to newcomers. This is one reason why most of the people who regularly pull a squeegee for a living probably never received more than a half-hour's instruction in their entire career. So, for those of you who may have been slighted in the squeegee education department, here are the basics summed up in one place.

Mesh Tension: The force that pushes back
So many factors in screenprinting are interdependent that it becomes difficult to discuss one in isolation. This is the problem we face when we begin to take a closer look at how a squeegee works. It's really only one half of the equation. The squeegee pushes down against the screen, but the tension in the mesh of the screen also pushes back against the squeegee. . Screen tension is the other half of the action-reaction process that makes screenprinting possible, so we really need to begin our discussion of squeegees by understanding how the mesh reacts to the downward force of the squeegee.

We know the first part of the printing process begins with the squeegee forcing the mesh down against the substrate. The squeegee then moves forward across the screen pushing before it a wave of ink known as the working head of ink.

Only one small portion of the mesh, a narrow line of fabric directly underneath the leading edge of the squeegee blade, comes into contact with the substrate at any one time. Before the edge of the squeegee arrives at a given point, the tension in the mesh keeps the mesh suspended above the surface of the substrate. Immediately after the blade passes, the tension raises the mesh off the surface again. This rapid snap-off of the mesh is essential to creating a clean and unsmeared imprint. Without this effect most types of screenprinting simply won't work.

Off contact
To allow for this up and down movement, space has to exist between the print side of the mesh and the surface of the substrate. It need not be a great deal of space, 1/16" to 1/4" or less. This is called the off-contact distance.

There is no standard off-contact distance. It really depends on the size of the screen, the tension in the mesh, and preferences of the individual printer. The best off-contact distance is almost always the smallest that you can get away with.

Over the years handprinters have devised ingenious systems for setting off-contact distances, the least complicated being to attach shims to the underside of the frame with tape. Another popular method is to drive screws into the underside of the frame, allowing off-contact distance to be adjusted with a few turns of a screwdriver.

Whatever the distance, it should be equal at every point. Ideally, the screen will be level. When you gain some experience setting off-contact distances, an eyeball measurement should be sufficient, but you might find it easier to actually measure the distance for your first few set-ups.

The more highly tensioned the screen, the closer it can be positioned to the substrate. This is one reason why new screens print better than old ones and professionally made screens tend to work better than homemade ones. It's all a matter of tension. It takes far less downward pressure on the squeegee to close minimal off-contact distances than it does to close the large gaps necessary to compensate for under-tensioned screens.

The only way to print with a screen that has lost most of its tension is to exaggerate the off-contact distance. This allows the downward pressure on the squeegee to create enough tension in the mesh to make it snap off of the substrate. Of course, this means the printer has to work much harder, and the extra pressure generates extra wear and tear on the squeegee, the stencil, and even on the mesh. Admittedly, this is not much of a problem where the mesh is concerned; it's probably ready for the garbage can anyway.

Unfortunately, the tension created by large off-contact distances tends to be uneven. In any screen the mesh near the frame tends to be less flexible than the mesh in the middle. In a low-tension screen set at a large off-contact distance this effect is magnified, so that the ends of the squeegee have to work harder than its midsection.

Get a grip (How to hold a squeegee)
We know how mesh tension responds to the squeegee, now we're going to take a look at how the squeegee applies pressure to the mesh. The force behind the squeegee is supplied by the printer in a movement repeated many thousands of times in the course of a screenprinting career. The first thing a printer needs to learn about using a squeegee is how to hold it.

In the standard grip, sometimes called the pinch grip, the printer places both hands palm down on top of the squeegee handle and grasps it between his fingers and thumbs. The hands are placed far enough apart to apply equal pressure along the entire length of the squeegee. A few printers prefer alternate grips. What matters most is that the grip you use is comfortable for you.

In fact, the standard squeegee grip may not be ideal. Many printers complain of pain in their hands and forearms following long print runs. NIOSH (National Institute for Occupational Safety and Health) studies have found problems with both the pinch grip and the width of the standard squeegee handle. NIOSH recommends using a handle that's 18% wider than the standard handle. An alternative to replacing all of your standard squeegee handles would be to pad them with a compressible material to allow for a more open grip. There are now on the market a number of ergonomically designed alternatives to the standard squeegee handle, and it may be worth your while to investigate some of these. After all, you're the one that's going to be pulling all those thousands of prints.

Pressure
Some printers, mostly those at the beginning of their career, feel they have to use a lot of pressure on the squeegee. Pressing down harder on the squeegee is almost a natural reaction if the ink deposit appears to be too light. Applying additional force to the squeegee does increase ink deposit, but it does so in a roundabout way by bending the blade of the squeegee, which changes the effective angle it makes with the screen. The angle at which the squeegee makes contact with the screen can alter ink deposit, but you don't need pressure to do that. You could increase the ink deposit just as easily by increasing the squeegee angle.

A good rule is to use no more pressure than the minimum required to produce a successful print. A great many printers use only enough downward force to close the off-contact distance. This is usually sufficient. If you can feel the blade bend, you're overdoing it.

Too much force exerted on the squeegee can not only prove tiring, it can be a contributing factor to repetitive stress injuries. It's also a major contributing factor to wear and tear on stencils, screens, and squeegees.

Excessive pressure can also lead to messed-up prints, especially if screens are at less than optimum tension. A heavy hand on the squeegee can actually drag the mesh slightly in the direction of the print stroke. When the mesh moves, the stencil moves with it, enlarging the printing areas and shifting the printed image enough to cause registration problems.

The solid areas of the stencil also move, dropping part of the non-printing area into newly deposited wet ink. On subsequent prints, this wet ink is transferred to the substrate leaving a faint outline shadowing parts of the image, a phenomenon known as ghosting. This misplaced ink will appear on every subsequent print until it either wears off on substrate or the printer stops and wipes it off the bottom of the screen.

Squeegee angle
Almost without exception, squeegee blades used for sign printing are of the square-edge profile. To make a print stroke the squeegee is tilted toward the printer so that only one edge, the leading edge of the squeegee blade, comes into contact with the screen. The angle that the blade makes with the surface of the screen is known as the squeegee angle.

Squeegee angle is really a matter of preference, which varies considerably from printer to printer. Factors specific to the job at hand also help determine squeegee angle: the type of ink, mesh count, substrate, and squeegee durometer. The most important factor of all is ink deposit.

If you think of a squeegee blade with the blunt end of the blade being pressed straight down on the screen as being at 90 degrees, the closer you hold your squeegee to the 90 degree position the less ink it will lay down. Conversely, the sharper the angle of the blade to the screen, the greater the ink deposit. Hold the squeegee too nearly vertical or angled too far over and it will not print effectively. Most printers hold their squeegees at a squeegee angle somewhere between 65 and 75 degrees off the screen, that is tilted about 15 to 25 degrees from the vertical position.

With an automatic press you can set the squeegee angle very precisely. The holding mechanism will also maintain whatever angle you select from print to print. In hand printing, however, consistency can be a problem. The printer sets the angle all over again with every pull of the squeegee.

The pressure the printer applies to the squeegee can also have some effect on ink deposit. The term squeegee angle refers to the angle the blade makes with the screen before any pressure is applied. But blades are flexible and even a modest amount of pressure will cause even the higher durometer blades to flex slightly. This effectively changes the angle of the blade in relation to the screen. This angle formed by the blade under pressure is called the effective angle or the angle of attack. Strictly speaking, it's the angle of attack, not the squeegee angle that determines the ink deposit.

For example, if all other factors remain the same, a lower-durometer blade will tend to leave a heavier ink deposit than a blade of higher durometer. The reason is that the lower-durometer blade bends more under pressure creating a sharper angle of attack. This sharper angle generates additional downward force on the ink, which tends to leave a heavier ink deposit.

Printers usually think of squeegee angle as the factor that controls ink deposit, because the angle of attack is difficult to see and almost impossible to measure -- particularly by the person pulling the print. It's easy enough to adjust the squeegee angle and the pressure used during a print stroke. In fact it can be done at any time during a print run, allowing the printer to make adjustments to ink deposit based on the appearance of the previous print.

The only other methods of controlling ink deposit, like changing to a different mesh count or using additives to change the viscosity of the ink, have to take place before the print run begins.

Wetting up
One of the most common questions beginning printers ask is, "How much ink do I use?" Probably the safest answer is: "More than you think." You need enough in the screen to pull a dozen or more prints before you have to add more ink. At the same time you don't want to pour in so much that it begins to work its way up the sides of the frame and up the squeegee blade and onto the handle. The ink in the screen should fit comfortably in the screen's inkwell, a solid area of the stencil at either end of the image area where the ink can safely be deposited at the conclusion of the squeegee stroke.

The first few prints
Be prepared to sacrifice some substrate when you start to print. Sometimes it takes several prints to get things rolling. That's why it’s a good idea to have some clean scrap material nearby to use for test prints. Whatever this material is, it should have a surface similar to your substrate. It should also have similar dimensions. At the very least it should be the same thickness as your actual substrate so you don't have to readjust your off-contact distance.

Assuming everything is working properly, it's now time to begin the actual print run.