It's just a piece of rubber. How important can it be? Many screen printers harbor that attitude toward the squeegee—that is, until squeegee-blade imperfections begin to affect the results they achieve on press. The sight of flawed prints is usually the catalyst that sparks the most interest in the squeegee, but spotting trouble so late in the workflow means downtime.
Dull edges, nicks, and uneven profiles are some of the problems that can prevent the squeegee from properly playing its vital role in transferring ink to the substrate. Normal wear and tear from printing can lead to defects in the squeegee blade, as can exposure to inks and chemicals, mishandling, and improper storage. The squeegee sharpener is the tool that screen printers use to restore the squeegee blade's edge so the squeegee can be brought back into action.
Squeegee sharpeners are available in different sizes and configurations (Figure 1) to accommodate a variety of squeegee sizes, performance requirements, and budgets. This article will discuss the technologies in use today.
Squeegee grinders use abrasive components to remove material from the squeegee blade. The two most common types are belt and wheel grinders. Long-belt grinders are the oldest kind. They use a continuous belt that spans the length of the squeegee blade so that the entire length of the blade may be held against the belt during grinding. Wheels or pulleys drive the belt.Advertisement
The belt grinder is the simplest and least expensive type of squeegee sharpener, according to Sylve Ericsson, executive vice president of Interchange Equipment, Inc. "You put your squeegee on the grinding belt and, depending on the operator, you will have decent quality," he says.
Operator skill is key when using long-belt grinders. These machines are typically manual devices, which means the operator must maintain the blade at the correct angle, ensure that end-to-end free-blade height is uniform, and carefully control the amount of blade material removed during the process.
"When you grind, you should not be taking more than 0.002 in. on each pass," Ericsson says. "That is very difficult to do manually."
George Rogers, president of Printer's Edge Ltd., identifies heat generation, tearing, and material buildup as some of the challenges associated with the use of belt grinders. "You develop a lot of friction and heat because you're trying to sand a pliable material," he explains. "Urethane rubber, being an elastomer, is going to give, and when you push it up against the wheel or belt, it deflects at various levels depending on how much pressure is put on. A tearing effect causes little peaks and valleys on the edge of the squeegee blade. Those peaks will break off, and the valleys tend to cause streaks in the prints."
Ground urethane can become lodged in the grit of the grinding belt. When enough squeegee material builds up on the belt, the machine may sharpen blades erratically or ineffectively. Rogers says lack of a uniform grind across the blade and a reduction in material removal are tell-tale signs of urethane buildup on the belt. But he also points out a noteworthy advantage to using belt grinders: Operators can build fixtures to hold squeegee blades at different angles. The use of such jigs enable the grinder to produce custom blade profiles and edge characteristics.
The short-belt grinder (Figure 2) also uses a grinding belt and pulleys or wheels as drivers, but the squeegee is held still while the belt-driving system travels from one end of the squeegee blade to the other. The single point of contact made between the moving belt and the squeegee blade's surface puts less pressure on the squeegee, which means a reduction in blade deformation during grinding. Edge straightness and uniformity of height also improve when the holding device and wheel movement are held parallel. Rogers says short-belt grinders are a better alternative to long-belt machines, but heat and buildup on the belt can still lead to problems.
"The wheel type of sharpeners are faster, more consistent, and easier for most screen printers to use," says David Landesman, co-president of Lawson Screen & Digital Products, Inc. This sort of grinder may feature a stone wheel, diamond wheel, or an abrasive belt wrapped around a wheel.
Diamond wheels are the most common types found in wheel-based sharpeners. They can be manufactured to produce a variety of blade profiles (Figure 3). Diamond wheels are designed to wear at a slower rate than stones, but both should be routinely checked for proper alignment. As is the case with long- and short-belt machines, the belt-wrapped wheel can generate excessive heat and lose efficiency as blade materials build up on the belt.
Manufacturers have added features and functions to their squeegee grinders in order to deal with temperature management, debris removal, and squeegee-blade handling. Cooled pulleys and tooling or machine components designed to naturally dissipate heat are some of the tactics used to prevent temperatures from climbing too high. Vacuum systems are integrated in some units to help remove urethane dust and particulates and prevent them from clogging the grinding media. In some cases, the operator need only attach a vacuum cleaner to a port (Figure 4). Finally, mechanical and pneumatic clamps are used to hold the squeegee blade steady during grinding, thereby further ensuring the production of a straight edge and uniform height.
Squeegee cutters use knives, either stationary or rotating, to remove material. Some machines heat the knives to aid the cutting process. Rogers says these knives are heated to 120-130°F, a bit more than half as hot as the melting point of the squeegee-blade material.
"It'll never melt the blade whatsoever," he says. "If you're trying to cut a stick of butter and you run a knife under hot water, then it's easier to cut the butter than if the knife were cold. It lowers the cutting resistance."Advertisement
Cold-knife machines may come standard with cooling/lubricating systems to aid the cutting process and prevent heat buildup from occurring (Figure 5). Some squeegee cutters use knives that can be bought at hardware and grocery stores, such as box-knife blades and injector-style razor blades. Other units require the use of proprietary blade systems. Either way, these machines can remove squeegee-blade material while generating minimal amounts of debris. That attribute makes squeegee cutters popular in industrial and specialty screen-printing operations in which production must be confined to a clean room.
Squeegee cutters are generally more expensive than belt and wheel systems, but the performance they're designed to deliver can offset the price. Peter Herman, manager of sales and marketing for Thieme Corp., says cutting—whether with a hot or cold knife—produces a more precise edge than what can be realized by grinding.
"What type of edge finish are you looking for, in terms of the finish of the print edge, the shape of the edge, and the quality of the print you're trying to achieve for your application? If you're not looking for detail, or if you're in an application where you're laying down a lot of ink or adhesives, of course the belt or wheel will meet your needs," he says. "Sooner or later, they all get the job done, based on the application."
Operator experience weighs heavily on the edge quality produced by a grinding machine, but Ericsson says the level of quality a cutting system can generate depends largely on maintenance. He points out that knives, whether fixed or rotating, must be in perfect shape in order to work as expected.
Squeegee sharpeners are available with analog and digital controls, as well as manually and automatically actuated components. Some units combine manual and automatic processes and functions. A basic machine may simply have an on/off switch and mechanical clamps, while a more advanced unit may offer digital controls, programmable cycles, user-defined material removal, pneumatic clamps, and more.
Digital input allows the user to program the sharpener's operating functions. Number of passes, cutting depth per pass, and cutting speed are some examples of parameters that may be accessible. Rogers says digital controls and readouts are for those who want to cut away a precise amount of material on each pass and not have to rely on a dial indicator. "And some guys want every little gadget they can get," he adds.
Clamping systems eliminate some of the operator intervention in the squeegee-sharpening process. On the mechanical side, you might find levers or knobs as access points to the clamping mechanisms. Pneumatic systems are pretty much hands-off, and Landesman says he finds them to be quicker and more consistent. Regardless of which clamp type you deem most compatible with your needs, Ericsson cautions that the clamps must never squeeze the squeegee blades, which creates the risk of blade distortion and, consequently, an uneven edge.
Angle adjustment is another consideration. If all you'll ever need is a square edge, then you can probably live without it. Even though a square edge is suitable—if not preferable—for many screen-printing jobs, the need may arise for a different profile. As noted earlier, diamond wheels can shape various blade profiles as they grind. But some sharpeners come standard with angle adjustment. This function allows the operator to adjust the squeegee's position in such a way that the sharpener can produce single and double bevels at virtually any angle.
How little material can the machine remove, and how accurately can the unit remove it? These questions are addressed by a system's minimum cutting depth and blade-height tolerance. Wheel and belt sharpeners offer minimum cutting depths measured in thousandths and hundredths of an inch, respectively. Knife units support minimum cutting depths measured in hundredths or thousandths of an inch. Blade-height tolerance refers to variations that may occur from end to end on the squeegee blade. Squeegee sharpeners listed in the product-comparison chart range from ±0.0004 to ±0.006 in.
Other specs to keep in mind are maximum squeegee length and cutting speed, both of which should be selected to match the applications you normally handle. If possible, leave some room for growth. Machines on the market can accommodate squeegee sizes for use in printing garments, graphics, industrial jobs, and more. You can even tackle 181 in. of squeegee blade at once with one of the models listed in the product-comparison chart. And manufacturers will generally build the machines to custom lengths as well.
Screen printing isn't a one-squeegee-fits-all world, so it's important to find a sharpener that can properly recondition the types of blade materials you use, whether they're soft and pliable or hard and reinforced, and do so at a speed that prevents press downtime. As always, you should consult with manufacturers about these and other performance specifications so that you can make an informed decision.
"You can have the best press, best dryer, and best exposure unit in the world, but if you don't have a sharp, clean squeegee blade, your print will look bad no matter what," Landesman says. "Screen printing is an integrated approach to decorating. Too often we make the mistake of focusing on just one or two elements of the process. The squeegee blade is one of the critical elements that has been most ignored in the industry."
Squeegees and squeegee sharpeners probably aren't the hottest topics of discussion around your shop's water cooler, but you can bet word will quickly get around when a squeegee-related problem creates a bottleneck on your shop floor. Giving the squeegee the care and attention it deserves will ensure that high-quality prints continue to flow from your presses.
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