Any flexographer probably knows that size matters. No, this is not about substrates or printing plates. We’re talking doctor blades here, and the three important dimensions are length, width and thickness.
Length is the dimension of a blade across the anilox roll. On a narrow web machine this may be as little as 13 or 14 inches, while on a big CI press it can be as much as 110 inches. It’s easy to think that it is okay if the blade pretty much spans the roller. However, getting blade length exactly right is important to the quality of the work produced by your press, the health of the anilox roll, and even how much time press operators spend cleaning the press and the floor between jobs. For example, a blade that is even 1/16th of an inch too short will leak. Sure, the blade may butt up against the end seals, but any gap that will let ink by ensures it will wind up in places you probably don’t want it to go.
If you took wood or metal shop back in middle or high school the teacher probably said something along the lines of, “You can always cut off but you can’t add on.” That applies to doctor blades just as it applies to that new trim you put on a window in your house last month. Except that a bit of trim molding that’s a couple hairs too short doesn’t leave a puddle of cyan ink on the floor. At Provident we measure and cut blades so we and our customers can be sure the blades are the correct length.
Once we have measured and delivered precut blades for a given press it’s a simple matter to order additional blades as needed, but it’s very important to account for any changes made to a press. This is one of the details we find when tracking down trouble spots. For instance, this can be something as simple as using a different type or make of end seal. We often use a paper gauge to make sure the fit between the blade and the end seal is accurate, but this can be thrown off if a different seal is used. Things like this may seem like minor details, but if you are running a couple of big CI presses and not all the blades are the correct length there can be problems that burn valuable time when the press operator has to clean up after a couple of doctor blades that were a little too short let ink puddle up on the floor.
This is the lateral measurement from the tip of the blade that does the actual metering to the back edge where it mounts into its holder. Typically measured in fractions from one to two inches it’s easy to think of width as not being all that important, yet it can cause a blade to wipe the anilox roll at the wrong angle, so it interferes with print quality, results in faster blade wear, and can even contribute to anilox roll scoring. For example, most press makers specify a certain blade width for their flexo presses, but there is nothing to stop a customer from running a blade of a different width. But what happens a 1.25-inch a blade is used when a 1-inch blade is specified? Nothing good. The blade wears faster and can bend, which affects how well it meters ink. And no, the cure is not more pressure.
One of the things we see is the correct blade width being used, but the wrong type of tip for the application, usually because the job had to be run and the blade was the one it that happened to be available. This can work, but it can still compromise print quality. We recommend making sure you stock the blade widths recommended by the press maker and also the right tips for each job. This means being proactive in anticipating the jobs that will be run in a given week, but it shows in the quality of the work your presses produce.
Measured in hundredths and even thousandths of an inch, doctor blade thickness—the measurement from the top to bottom surfaces of the blade—can be easily ignored or forgotten. Yet it has a major impact on how well a doctor blade works because a blade’s stiffness is directly related to its thickness. One would think that a stiffer blade is always going to be better than one with greater flexibility, but this is not necessarily the case. Too stiff a blade can trap particles of metal or dried ink, which can affect print quality and even lead to anilox scoring. One that is too flexible may not meter ink correctly until too much pressure is added to the process, leading to excess wear.
Although blade thickness is always clearly marked it can be easily overlooked in the rush to get a big job on press. Be sure your team takes an extra few seconds to make sure the blades being used are the right thickness. But even here, there are important nuances to understand. For instance, a 60- or 80- or 110-inch blade can be subject to flutter over its length, so it’s not unusual to use a stiffer blade to minimize flutter. The detail here is to use a blade with a different tip to ensure accurate metering.
Beware the Frankenpress
Questions about length, width and thickness are usually easily resolved if your presses are the standard models from any of the mainstream press vendors. But our time in many shops has shown us how presses change over time and involve parts from one make being used on an entirely different press, and some parts even being fabricated by local machinists. This gets the job done and keeps a press up and running, but can make finding the right length-width-thickness combination a little challenging. However, this is where onsite measurement and evaluation can make all the difference in how well your press handles the next big job—or the one that shows up ten months from now.
Whether your presses are well-used versions of the ones you bought new and use all standard parts or you have a fleet of Frankenpresses, length, width and thickness of your doctor blades are all intimately related to the quality of the work shipping out of your shop and to various operating efficiencies. My colleagues and I here at Provident Group are often asked to look at problems arising on customers’ presses and we often track a problem to incorrect doctor blade selection. Reach out to us so we can share our expertise on how to make sure these three simple measures of your doctor blades are in line with your goals for success and productivity.