The winning prescription for doctor blades
Numerous factors need to be considered when selecting doctor blades for flexographic printing. Different ink processes such as ultraviolet (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 uses 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’s 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. Here are two examples:
- If a high-line screen anilox is being used with ranges from an 800-line anilox up to a 2,000-line anilox, a thinner tipped blade will minimize dot gain resulting in the cleanest dot and the sharpest print.
The same is true for the anilox roll. A doctor blade tip that is too thick for the high-line screen anilox it is metering provides a footprint too large to shear ink properly. This 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 (1,000-line at a 1.5-volume), this will not be the case. The thin 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.
High-line screen concernsHave 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 "Doctor Blade Selection Tips" (above) explains how to match up the most affective blade tip thickness with the line screen of the anilox roller.
Flexo printing perspectiveNow, 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, newsprint, envelope and corrugated are the primary substrates used today, all of which require a different type of doctor blade, depending upon the type of printing. There are three types of printing within flexography: (1) Process, (2) Combination and (3) Line and Solid.
In Process printing a plate screen has an abundance of dots, 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.
Combination printing 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.
Heavy Line and Solid printing generally requires a 330-line anilox roll and below. These volumes typically range from 8.5 billion cubic microns 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 on the coating weight required. In this case, a thicker, radius edge blade is normally used.
Know your optionsMost doctor blades are available in three basic shapes or tip configurations: beveled, radius and lamella. The radius tipped doctor blades were introduced in the mid-nineties and were being touted as the universal blade of the future. With graphics becoming more advanced and anilox roll line screens increasing up to 2,000, 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 customized technologies by working with various tips, steel thicknesses and coatings. 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.