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The Correlation Between Doctor Blades, Ink Film Thickness and Flexo Printing
Category: Flexographic Printing | 10/07/2008 - 16:49:27
With changes developing rapidly throughout the Flexo printing industry and pressroom personnel turnover happening so frequently, companies sometimes fail to recognise the technical progress that is taking place and continue operating using familiar processes, ultimately creating many problems on the press. As technical consultants to printing operations all over the world, we frequently see revenue that is being lost as a result of not recognising and adapting to changes that are occurring.
Doctor Blades for Flexographic Printing
There are numerous factors to be considered when selecting doctor blades for flexographic printing. Different ink processes such as UV, water or solvent require the use of different doctor blades. High volume anilox rollers demand one type of blade, whereas a low volume, high line screen anilox roll will achieve greater quality by using another type. Likewise, line and block solid printing utilises still another blade versus what is needed when printing high-end process work. Similarly, coatings and adhesives will demand different blade thicknesses and tips. Add to the mix that a specific blade will perform better for longer runs than shorter, and the decision becomes even more complicated.
It is no wonder failures on the press can easily be experienced due to the lack of knowledge about which precision doctor blade to apply when printing a certain job. The economical impact of not knowing which should be used can run into hundreds, if not thousands of dollars. Knowing the physics of what happens at the transfer point on the press is extremely important.
If a high line screen anilox is being used with ranges from an 800 line anilox up to a 2000 line anilox, a thinner tipped blade will minimise dot gain resulting in the cleanest dot and the sharpest print. Imagine shaving with a new razor blade and getting a good, clean, close shave. If the same blade is used for a couple of weeks, the blade tip wears and it gets wider. A close shave is not achieved because it simply is not shearing the whiskers off sharply.
The same is true for the anilox roll. A doctor blade tip which is too thick for the high line screen anilox it is metering provides a footprint too large to shear ink properly. This in turn will result in an ink film thickness that is too thick and ultimately leads to dot gain.
If a thinner tipped blade is being used, your dot gain will be minimal. Many printers consider this a “catch 22” meaning they are concerned that if they go thinner, their blade will wear faster. This is true with heavy volume rollers, however, if they are printing a thin ink film (1000 line at a 1.5 volume), this will not be the case. The doctor blade will last just as long as a thicker blade.
In the case of a low line screen with high volume anilox roll, there is a bigger ink film (cell volume) which provides the blade with better lubrication. There is also a wider cell wall which offers better support for the doctor blade.
Concerns When Using High Line Screens and Doctor Blades!!!
Have you ever wondered why a high line screen anilox (800 or above) is more susceptible to damage than a low line screen anilox roll (400 and below)? It’s because the anilox roll company is engraving double the cells per inch on the anilox, equating to double the amount of engraved cells for a 400-line anilox versus an 800-line anilox. The more cells that are engraved on an anilox roll, the smaller the opening of the cells and the thinner the cell walls become. The thinner cell walls are more susceptible for damage, which sometimes shows itself in the form of score lines.
As a general rule, the chart below can be applied when choosing the most affective blade tip thickness with the line screen of the anilox roller:
Putting Flexo Printing into Perspective
Let’s look at some of the most common types of printing performed in the industry and the different substrates they are applied to. Film printing, tag and label, liquid packaging, newsprint, envelope, and corrugated are the primary substrates being used today, all of which require a different type of doctor blade, depending upon the type of printing to be achieved. There are 3 types of printing within flexography:
Process, Combination, and Line and Solid
Process printing is a method by which a plate screen has an abundance of dots on the plate, sometimes as small as a 1% dot. A high line screen anilox roll would be used; therefore, requiring a precision thin doctor blade tip of 75 to 95 microns. (See chart above)
Combination printing is a process that typically deals with vignettes. This can take printing from highlights to some very solid printing work. Within the vignette is an array of dots on the plate that graduate from a larger to smaller percentage dot. This requires an anilox in the mid-to-high range; therefore requiring a doctor blade tip of 125 microns. (See chart above)
A Specific Doctor Blade may be used to achieve certain Vignettes
Heavy Line and Solid printing generally requires a 330 line anilox roll and below. These volumes typically range from 8.5 BCM down to 4.0 BCM and can vary, depending upon what the printer is trying to achieve. With varnishes, adhesives or coatings volumes can go up as high as 20.0 BCM, depending upon the desired coating weight that is required. In this case, a thicker, radius edge blade is normally used.
The Perception . . . . .
Most doctor blades are available in 3 basic shapes or tip configurations. For many years, the basic configurations of these blades were all that the printing industry knew. The radius tipped doctor blades were introduced in the mid-nineties and were being touted as the blade of the future. They were considered the universal blade. With graphics becoming more advanced and anilox roll line screens increasing up to 2000, the radius tip has reached its limits.
While all three blade types are still offered, there are many other variations available. Doctor blade manufacturing companies have developed customised technologies by working with various tips, steel thicknesses and coatings. New blade technologies are continuously being developed with the specific printing applications for which they are used in mind. Additional advancements are also taking place to address specific printing problems. As a result, print issues such as chatter, gear marking, spitting, scoring, streaking or excessive blade wear can all be decreased or in many cases eliminated.
In summary, talk to your blade supplier and find out what’s new in their doctor blade product line. It may surprise you, resulting in a tremendous cost savings that could positively affect your bottom line financials.