Doctor Blade Problems

——How to Eliminate them before they start

Editor’s Note: This article is the basis of the author’s Forum ’03 presentation, to be given during the Fundamentals session Monday, May 5, 2:20 p.m.-4:30 p.m. At that time, Stamey will present in further detail recommendations on how to eliminate doctor blade problems.

Sometimes the best plans for a “perfect press start-up” quickly come to a halt when you look at your first press sample. You wonder, “Where are these streaks coming from? Does my anilox have score lines? Do I have contaminants in my inks? Am I running the correct doctor blade? Why are my chambers leaking?” We have all had these problems and could probably think of many more. In today’s market, pressrooms have no time in the production schedule for troubleshooting, and no money in the budget for press operators to run waste. The bottom line is to produce—and do it fast, with little or no waste!

When looking for that perfect start-up, first ask yourself, “What type of work will I be doing? Will I be doing line work or process?” If you are using a low-line-count anilox, the roughness contributes to faster blade wear. Therefore, a heavier tip is needed, such as a radius tip blade (Figure 1 shows wear). If you are using a higher line count, the smoothness requires a lamella tip for the clean wipe (Figure 2 shows wear comparison).

Remember that the higher the line count of the anilox, the cleaner the wipe needs to be. When matching blade type to anilox, printers find that using lamella for high line screens and radius for low line screens results in better wiping and less blade wear.

For some applications, a plastic or plastic composite blade is used. These materials usually wear slowly, are easy on anilox rollers and are somewhat safer to use. Some plastic blades need to be thicker to provide the same rigidity as a steel blade. The additional thickness increases the contact area, however, which does not allow for as clean a wipe as a steel blade does. The majority of printers using plastic or composite blades are in the corrugated industry. Because the doctoring system on many of these presses is very hard to access and it can take hours to change a single blade, using a durable, plastic blade is a good choice.

Setup Procedures
Once you’ve decided on the type of blade to use, some simple but very important setup procedures will help ensure a problem-free press run. First, make sure you have a straight blade. The standard tolerance is 2.0mm to the backside of the blade in a 3-meter piece at maximum (or .08-in./10 ft.). If these guidelines are met, it will simplify the setup process.

Next, make sure the holder is clean of all inks, especially the inside of the holder where the blade is to be placed. Check the holder to ensure that it is straight, that no bolts are missing and that the holder is free of any damage that may have occurred during cleaning or handling. Be sure the part of the holder that comes into contact with the blade is free of nicks and dents; otherwise, once the holder is tightened, a wave in the blade will result. If you find any flaws, use sandpaper or a hand file to make the holder as smooth as possible. Finally, place your blade carefully into the holder.

With the blade in the holder, you are ready to tighten the bolts. Secure each bolt from the center of the holder toward the ends; alternate tightening bolts from each side. Once all of the bolts are snug, use a torque wrench to guarantee that even tension is applied. (You may want to contact the blade holder manufacturer for the exact recommended settings.) Remember that increasing blade pressure during the setup (before you start running the job) will affect your entire run.

There are two distinctly different types of doctor blade systems for the flexo industry: the reverse-angle blade system (Figure 3a) and the more commonly used enclosed doctor blade system (Figure 3b). Both of these systems have preset recommendations for the height of the blade, usually determined by the holder manufacturer. Check that you have a consistent height across the entire length of the blade. Any deviation can cause waves in the blade and leaking in the chamber systems. If you have waves in your blade, you will have to increase your blade pressure to get even ink metering.

The enclosed chamber holder will also be preset for the correct contact angle, which should be between 30 degrees and 35 degrees for the best ink metering results (Figures 3a and 3b). If you are not sure you’re using the correct angle, send the blades back to the supplier after they have been used on the press. Your supplier should be able to evaluate the blades and tell you exactly the degree the angles are set and if you need to make changes.

When the doctor blade is secure, you are ready to place the holder on the press. It is a good idea to periodically check all the holder components to ensure that the holder will be properly aligned when placed in the press. This is a critical step because the mounting brackets for your holder could be loose, you may have some ink build-up or a number of other factors could cause the holder to be improperly aligned.

Success on the Press
When the doctor blade comes into contact with the anilox, it should not run against a dry anilox (no ink). This will cause premature wear on the anilox, as well as possibly damage a new doctor blade. Always start up using the lowest possible doctor blade pressure. Try not to pay a lot of attention to the pressure gauge, if you have one. The best way to set the blade is to watch the anilox and stop applying pressure once you achieve a clean wipe.

The thinner the tip, the less pressure is required to achieve a clean and clear printing result. For the best results, use the same material on both sides of the chamber to eliminate uneven pressure. Increased pressure leads to a deflection of the doctor blade. This results in a reduced doctor blade angle and an increased contact area, so the actual wiping is done by the back of the blade and not by the tip (see Figure 4). As a result, excessive anilox and blade wear may occur.

Excessive blade pressure creates free-floating metal slivers and particles, which contaminate ink systems (see Figure 5). When a hard particle becomes trapped between a deflected blade tip and the anilox (Figure 4), the particle rides there, effectively destroying rows of cells. These rows of cells appear as thin bands running around the circumference of an anilox and are called “score lines.”

When the blade is set and running, make sure the cavity is aligned so the top and bottom of the chamber have equal amounts of pressure. Incorrect alignment can create uneven ink metering, ink leakage and uneven blade wear. Check the horizontal alignment in relationship to the anilox, as well.

If you have completed all these steps but still notice some streaks during the run, it’s possible that a foreign particle is trapped under the blade or there is a nick in the blade. Possible solutions are to change the blade, reduce the blade pressure, install ink filters or install magnets in the ink system. Using abrasive inks is another possible cause of streaks. A coated or stainless steel doctor blade will result in better wear when using this type of ink. If you are running water based-inks, which sometimes can be very corrosive, you may need to check and adjust your pH levels.

Common Problems
If you are having a lot of anilox wear as a result of excessive pressure on the doctor blades, look for the reasons and make adjustments. Try to install a limit adjustment to ensure that the chamber does not come into contact with the anilox. Wear may also be caused by having too large a contact area by the blade tip. Try the following suggestions to reduce wear:

• Reduce the contact area by using the correct doctor blade width (check with the holder manufacturer).
• Use a blade with reduced tip thickness.
• Check blade angles.
• Reduce blade thickness if you are using straight steel.
• Make sure you are using the correct material. Check with your doctor blade supplier to correctly match blade materials with your application.

Another common problem in the flexo pressroom is leaking in the enclosed chambers. Be sure the alignment is correct by checking the chamber. (Remember that it must be parallel to the anilox roller; use a machinist’s level to check this.) Also, the top and bottom blades must contact the anilox roller at the same time. A good way to check the contact is to engage an empty chamber (with no end seals) to the anilox and check the resistance between the two with a feeler gauge.

One of the biggest problems comes from the end seals. Be sure to use the correct sizes for your chamber and keep them properly maintained. In some cases, you may think your chamber is leaking, when actually the containment blade is re-doctoring the ink. To solve the problem, either reduce excessive blade pressure, adjust ink viscosity, use the correct containment blade for the holder or anilox, use a thinner material or try a plastic blade.