Mastering the doctor blade selection process, must start with understanding the many choices of doctor blades and their various uses. However, it is also important to recognize that what is available in doctor blades was developed for rotogravure...not flexo. When discussing with production managers as well as pressmen why they buy the blades they do, the answer is usually... "We have been buying this type for as long as we can remember". Thus, there exist many misconceptions and a need for more information on blade needs for our industry. I want to review many of these choices available and attempt to focus on what is optimal for the best performance, specifically for flexographic printing.
Trail Doctoring vs. Reverse Angle
To make intelligent choices we need to first understand the difference between these two systems. The major difference is that a trail doctoring blade scrapes the ink, while a reverse angle blade shears it off the anilox surface. Lets take a closer look!
Trail doctoring (sometimes called forward doctoring) is still the predominate way of doctoring in the gravure industry. The doctor blade is at an acute angle from its tangential line of contact and it scrapes the ink off the roller’s surface. To make it simpler it looks more familiar in a press setting. As we look at the slide this will leave a lubricating ink film on the roller, extending blade life. Because of the nip or hydraulic pressure created by the excess ink the blade will float with increased press speed. To adapt for this blade pressure is turned up. A stepped blade is most often used in this application.
Reverse angle doctoring is the dominate system in the flexographic industry. In this method the ink is sheared off the anilox roll surface at an obtuse angle from the tangential line of contact. Simplified in the press schematic, This gives us a more consistent ink film thickness by shearing the ink from the top of our laser engraved ceramic cells. Press speed has less of an effect on the ink film thickness. With less ink film, blade life is much shorter and effects roller wear. There are many reasons for different metering tips and minimal blade pressure is our goal.
To Get the Most Out of Our Blades
The anilox roll must be in good condition to start with. It should be manufactured for use with a doctor blade. "Superfinishing" will help to give a more consistent ink film and reduce wear on the blade. Ask your supplier if your anilox rolls are superfinished. If not, microscopic recast of ceramic may cause uneven blade wear and streaking.
- The doctor blade must be designed and manufactured for the specific application, whether it be for doctoring or retaining use. Considerations must include, machine width, press speed, the function and the location of the doctor blade on the roll, and the type of work (process, line, solids, etc.) and the length of run.
- The doctor blade must be accurately aligned and adjusted to the anilox rolls in the designed location of your system. This will help insure a clean consistent wipe across the anilox roll.
The blade should be set at the minimum pressure to create a good wipe. This would leave only the ink film designed by the roll, not excessive ink on the cell land areas.
Our Metering Tips and Blade Choices
There are four basic metering tips to choose from:
- The unhoned blade, that is the thickness of the material with a squared edge. For example .006 or .008 material.
- The beveled blade, example 2 which has a specified angled edge: such as 30, 45, or 60 degree angles.
- The stepped blade or reduced edge, has a tip reduction to a uniform thickness. (Such as an .006 blades reduced to .003 about 1/8" from the edge).
A stepped beveled blade combines both of the previous elements.
We need to look at our criteria for choosing our blades first:
| Criteria | Issues |
| Ink film requirements and type of ink/coatings applied. | The more abrasive the inks coating, the shorter the blade life. Corrosion from waterbased inks on long runs |
| Type of roller being used | Laser engraved ceramic, chrome, superfinished? |
| Blade life requirements | Blade break-in (honing), length of run. |
| Ink film thickness for quality of work you are doing. | Width of wiping tip determines cleanliness of ink film. The thinner the wiping tip the cleaner the wipe. |
| Safety | Number of wash-up and changeovers which affect how many times blades are handled. Ease of blade changes. |
The best test of blade and ink film requirements is to run characterizations in press. It is important to run enough blades in such a test as to minimize variations caused by differences in setup and use.
One important factor is the blade extension. The blade extension, depending on material, will cause a soft or hard wipe. A "hard wipe " is a blade with little deflection and will shear the ink cleaner from the cell land areas. The tradeoff is shorter blade life and more anilox wear. A "soft wipe" has more blade deflection and leaves more of an ink film on land areas. This gives you an inconsistent ink film thickness but causes less wear on the blade and anilox.
Spring strength of a doctor blade material determines the thickness and wipe you are looking for. A thin plastic blade of .0075 will have more deflection than a .060 plastic blades. Blue carbon steel at .006 will have more deflection than a .008 thickness. By adding a backup blade or shortening your blade extension, you can run thinner blade material with harder wiping results. Steel hardness has been a buzz word lately. After talking with several manufacturers hardness is consistent with a Blue steel ranging from 48-52 Rockwell C. It is said and believed , that a harder steel will cause excessive heat and wear at the Anilox surface.
If using blades for the first time. The best place to start may be to look at a blue steel .006 Unhoned blade. This should give you a consistent ink film, because your contact edge once broken in will not change. This is providing we have not changed blade pressure or angle by on press adjustments.
Reduced edge or beveled blades through wear will give you a thicker contact edge as the run goes on leaving a varied ink film during use. The beveled blade, of the same thickness and material of an unhoned blade, will give you an initial harder wipe. However , this will only be short term and or a limited size run.
Stepped blades, give a good clean wipe because of the thinner contact edge. The reduced edge will give a softer wipe with more deflection at the tip. They also have a reduced blade life for the same reason. If a consistent soft wipe is what you are looking for from your blade a stepped blade would be your answer.
The stepped beveled blade offers a combination of the previous but the initial beveled edge comes off during set-up and you are back to a stepped blade in a relatively short amount of time. The blade pressure needs to be extremely light for the bevel to have any effect on production even short term.
When it Comes to Blade Material Choices
There are several different blade materials with each manufacturer having their variations of raw materials. Blue carbon steel and white carbon steel are the most popular. Each is priced the same with basically the same characteristics. The choices between these two are cosmetic. Advantages of carbon steel are that you have one of the strongest spring strengths available and you can run a thinner blade to achieve the wipe you want. Always be sure you are receiving quality steel.
If corrosion of your blades is a problem stainless steel is available. These blades are of a softer material and usually wear faster than the carbon steel on ceramic. These are used most often on chrome anilox rolls to help reduce wear to the roll. To check and see if your current blades are stainless steel put a magnet to them to see if it sticks. Stainless steel does not have an attraction to magnets.
Blade life has been improved by the introduction of plastic blades. To get the spring strength and wipe comparable to steel, these materials are made of a heavier gauge and would need a reduced edge. These materials are most used on the containment side of enclosed doctor blade systems. Another reason for these blades is their resistance to corrosive materials such as high titanium whites and some coating materials being used. Safety is another advantage to these blades as you are less likely to cut yourself during installation and cleanup.
Composite blades offer us a combination of both worlds. These blades are being developed with improved spring strength. This offers us the chance to make them out of thinner raw material (currently some as thin as .025 with a reduced edge of .010 with a stepped metering tip). It also allows the safety, longer life of plastic, and the corrosive protection needed. There have been much advancement in this area as technology of combined materials moves forward. The path forward... composite or synthetic blades have made more changes in this area of flexo than any of the others.
For more information on this product, please contact us.