DOCTOR BLADE REMEDIES FOR ROLL SCORING AND UV SPITTING: MDC Soft Blade and MDC One-Step Doctor Blades In the Flexo world of printing, we encounter many questions about common problems regarding doctor blades, but two frequent topics of conversation are roll scoring and UV ink spitting. The type of doctor blade used can make an impact on final print results plus, several steps can be taken before start up to prevent score lines and UV spitting from occurring. The MDC Soft Blade is recommended to combat score lines and the MDC One-Step Doctor Blade has been proven to successfully reduce or eliminate UV spitting. Roll Scoring Description Roll scoring shows as thin, circumferential lines that appear on the anilox roll during a press run. As illustrated in the picture, these lines show across the length of the roller. Score lines can range from light streaking to deep, dark streaks, which are clearly visible to the naked eye. Once the roller is scored and streaking is prominent, the roller has to be taken out of the press and replaced, which can be expensive. The cost of press downtime, ink waste, substrate waste and labor all factor into the replacement roller costs. Visible Score Lines on the anilox roll Causes of Scoring There are three things that make contact with the anilox roller: the plate, the ink and the doctor blade. Because the doctor blade is the only stationary item that makes contact with the moving anilox roller, scoring issues tend to focus on the blade. Chamber or
holder alignment, chamber loading pressure, and the condition of the anilox surface are all factors that can affect roll scoring. Photograph shows two different pictures of scoring at different magnifications. The engraved cells are flattened down circumferentially around the roll. The distortion of the cells wall transfers this image to the substrate seen as a line or streak. Chamber or holder alignment is key to a successful run and quality print. As the doctor blade chambers age, wear and tear by bending and twisting causes some form of distortion to the holder itself. Because of this alteration, the holders may not line up parallel, the pressman will put extra pressure on the assembly to be sure the entire blades seats. Typically, the blade that touches the anilox first must deflect in order for the doctor blade to make contact with the anilox roller. It is important that the doctor blade extension is completely seated or resting against the stops in the holder and that all blades touch the roller simultaneously, both from the top and bottom and from gear to operator. Other factors, such as the holder not being cleaned properly, can also interfere with the misalignment of the blade set up. Attention to set-up detail is very important to help to eliminate scoring problems. Improper chamber loading pressure contributes to score problems. Excessive loading pressure will cause the doctor blade to bend or curl deflecting the blade, which results in wiping with the backside of the blade instead of by the tip. As mentioned above, the doctor blade should touch the anilox roller simultaneously. If too much pressure is applied, the doctor blade can develop a burr, which may break free and potentially cause scoring (see picture below.) As a rule, remember the required pressure is little as possible and as much as needed.
Excessive recast on the engraved cells of the anilox is another factor in roll scoring. When an anilox roll is engraved, the laser beam penetrates into the ceramic and a volcanic-type reaction takes place as the molten ceramic is displaced and forms a cell. In most cases, the cells have a hexagonal configuration. It is impossible to have every one of these billions of cells be completely flush on the anilox roll because of the casting method. The goal is to keep the cell walls as thin as possible to allow for the transfer of a thin ink film, which allows for easier control of the substrate and a sharper clearer image. If the anilox surface has a large amount of high spots, it may be necessary to polish the engraving. But, by post polishing, the cell walls become distorted and compromise the engraved cells from end to end, therefore minimizing the consistency of the ink film. The higher the line count anilox roll, the higher chance of scoring the anilox roll. High Spots on walls Notable high recast on the cell walls. These high spots first come into contact with the doctor blade, which can result in score lines. Doctor Blades and Roll Scoring As an example of how the doctor blade is affected by an anilox roll with uneven walls and excessive recast, the photo below was taken of a doctor blade that was removed from a deck with multiple score lines across the length of the roll. This blade has an extreme amount of roughness along the edges of the blade tip. This was a result of a very rough anilox roll with very high recast and uneven walls.
This blade was taken from a print deck with no scoring on the anilox roll and was metering very smoothly. This is typically how a worn doctor blade should look. When scoring problems are encountered due to older presses, vibration, unstable pumping systems, or an unknown variable, the best way to solve this is by using the MDC Soft Blade, a nickel-coated specialty blade. Using too much pressure on the doctor blade because of a mechanical issue on the press is one reason to use coated blades. Using a coated blade is less likely to cause a burr and can withstand more tension than an uncoated blade. If the ability to control the ink film is a result of using too much pressure on the blades, then a MDC Soft Blade should also be used. Uneven cell walls or recast causing score lines on the roll and on the print can be addressed by using a nickel-coated Soft Blade, which is recommended specifically for this problem. On some presses, no matter how minimal the recast, the slightest unevenness of the cell wall can cause a score line. A MDC Soft Blade acts as a selfrepairing blade and is more forgiving as it compensates for the surface roughness and seats itself. The advantages of using a MDC Soft Blade for scoring problems are: Longer blade life because of the coated steel Clean doctoring because the blade is stronger Corrosion resistance because the blade has a barrier coating UV Spitting and the Right Doctor Blade: MDC One-Step Today, the quality of the UV flexo market includes extremely enhanced graphics. As UV flexo continues to make tremendous strides within the printing industry, UV spitting still exists. While the saga of what component to blame for spitting is an ongoing process, we hear about the inks, anilox rolls, doctor blades, angles, plates and other factors as the cause. Since all of these variables play an important role as to how the press runs, the fact of the matter is, UV ink transfers differently on the press versus water and solvent inks. Water-based inks and solvent-based inks normally run a viscosity of 25 to 27 seconds on a #2 Zahn cup, but UV inks transfer very differently. These inks, in many cases, can have a 5 to 7 times difference within the transfer zone of the press. Because of UV ink being thicker and transferring differently, there some physics that take place at the transfer point between the engraved cylinder, doctor blade and plate cylinder. Due to its thixotropic properties, when the UV ink is transferred from the anilox roll, metered by the doctor blade and transferred to the plate, the ink frequently does not release without spraying. The result is the appearance of ink droplets on the print that are referred to as UV spitting.
What exactly is happening? Visualize your windshield wipers on high speed while it is raining and the water is being wiped thoroughly off of the glass allowing you to see clearly. Now, visualize a thin coat of slushy ice from freezing rain on your window. While the windshield wipers are on and because the slushy ice is thicker than water, ice spits behind and under the wiper creating a uniform sheet of ice residue on your window. Your window is not completely clear and your vision becomes impaired. If you reduce the rubber wiper to 1/4 its size in width, the ability of wiping the slushy ice would be much more enhanced because the wiper would not flex as much and, therefore, result in a cleaner wipe and sharper view. This same analogy can be used when wiping UV inks. When you increase your press speed as the anilox roller is turning, the UV ink resists itself to transfer. If you increase your blade thickness, the doctor blade will not deflect as much while it meters the ink, but you compromise your ink film thickness resulting in a less than desirable finished printed product. As the ink hydroplanes and builds up behind the blade, the blade starts to flex. This allows ink to drool under this portion of the blade and transfer to the plate and to the substrate. While many companies and experts have found ways to minimize the problem, from the doctor blade perspective, doctor blade experts have researched and attacked this spitting phenomenon by focusing on the specific characteristics of the doctor blade. Pressroom testing has proven that using a thicker blade minimizes UV spitting. When applying coatings and adhesives, thicker blades are normally used because of dealing with a very viscous fluid. This thicker blade effect theory is well accepted in coating and adhesives applications because we need a blade that will resist the force or heavy volume of this type of fluid while allowing it to meter and pass onto the next process. The compromise, however, is that the printed substrate can suffer because the ink film is not as thin as it should be to meter evenly, therefore affecting our process dots, as well as our reverse and positive type, barcodes, etc. Addressing the UV spitting problem head on. Research, development and testing lead to a highly technological blade that incorporates the alloys and the manufacturing design needed to eliminate or substantially decrease UV spitting without compromising print results. The MDC One-Step blade has made an enormous impact in the pressroom.
MDC One-Step profile This specialty doctor blade has been precision engineered to be thicker at the base (where the blade is installed in the holder) and extend close to the contact metering point. The tip of this doctor blade is designed to maintain the same quality and characteristics of standard edge. The unique design of the blade allows the thicker ink to meter itself just like a normal doctor blade, but because of the strengthened base, the amount of flex is decreased therefore creating consistent ink film metering without creating the ink hydroplaning effect behind the blade. This, in effect, eliminates or substantially lessens UV spitting. By stiffening the base of the blade, the blade holds its position, allowing for a correct shearing (metering) of the ink. This results in minimal or no UV spitting. To summarize, UV ink printers are making tremendous strides in printing high quality graphics and using the MDC Soft Blade and the MDC One-Step doctor blades can be the determining factor to achieving a good, quality print.