What is a Doctor Blade ?
Doctor Blades are actually an essential part of commercial printing. While there are many methods of printing, there are two predominant methods that use Doctor Blades. They are called Rotogravure (gravure), and Flexographic (flexo) printing.
In both Gravure and Flexo printing, the doctor blade is technically a metering blade. The word doctor is thought to come from German roots, where it was first called a “ductor blade”, meaning to draw across a surface. The doctor blade meters (controls) the amount of liquid ink that is deposited on a printing cylinder. It does this by wiping the cylinder surface of excess ink while the cylinder rotates in an ink supply. While gravure and flexo are totally different processes, the blade meters the ink in both.
Rotogravure is a printing process that has roots that go back at least 100 years. The modern gravure printing press typically will have 8 to 10 printing units or stations. Each station can carry its own color ink and has its own printing cylinder and doctor blade. The doctor blade is normally made of special carbon steels and is heat treated to a hardness that will allow it to wear very slowly. The printing cylinder carries the image to be printed. It is made of a steel core that is plated with a special copper alloy. The copper is then engraved with very small cells that make up the image. These cells are shaped like inverted pyramids. They are very small. You can line up 100 to 300 of these cells within one inch. After engraving, the copper is then plated over with chromium. The chrome is hard and allows the cylinder to run many copies of a print without wearing out. The doctor blade steel is not as hard as the chrome on the cylinder. This design allows the relatively low cost blade to wear out before the expensive cylinder. Doctor Blades will last from one working shift up to several days. There are many factors that affect the life of doctor blades. The type of inks, the color of the ink, and the cylinder surface can all affect the blade life. In addition to all of these, the way that a press operator sets the doctor blade angles and pressures on the cylinder can have major effects on the blade and even the cylinder life.
Flexographic printing is not as old as rotogravure. Its history as a major printing process probably goes back 30 years. In flexo printing, the image cylinder (called a blanket or sleeve cylinder) is separate from the inking cylinder. The image cylinder is made of a steel or composite core that has a polymer covering (blanket). This cover carries the image by using raised areas. These raised areas on the blanket cylinder are supplied ink by a second cylinder that is called an anilox cylinder (roll). This anilox roll is normally made of a steel core that is plasma sprayed with a special hard ceramic coating. This coating is then polished smooth, and then it’s engraved with a honey-comb shaped cells that carry ink. The engraving is usually done with a laser beam. The anilox roll gets ink from a closed ink chamber. This ink chamber rides on the anilox and is filled with ink using a pumping system. It is rectangular in shape. There are 2 doctor blades (one on the top and one on the bottom) used in the closed chamber. One acts as the metering blade and controls the amount of ink that fills the tiny cells and the second blade acts as a sealing blade, to prevent ink from leaking out of the chamber. The chamber also has seals on each end, that are usually made of plastic or a felt material. The doctor blades can also be made of plastic in some types of flexo printing. The anilox roll cells can be much smaller than gravure cells. There can be as few as 100 cells per inch up to 2,000 cells per inch.
Doctor Blades are manufactured in a wide range of sizes and shapes. Steel blades can range from .004” (0.10mm) to .010” (0.25mm) in thickness. They can also have special shapes to fit the application. The most used shape in both gravure and flexo is called a lamella tip. This tip is a reduced step area that is typically .060” (1.4mm) wide and is usually ½ of the base metal thickness. A typical blade that is widely used is .006” (0.15mm) thick with a .003” (0.075mm) thick lamella (reduced step). The lamella (reduced step) allows the doctor blade to meter very fine ink cells without leaving any ink in the areas of the cylinder that are not designed to print. Doctor blades and printing cylinders can range in length from 7” up to 120”. The small sizes can print labels and other small printing jobs, while the largest sizes are for printing the very high volume publication and packaging items.
By Steven Voorhees
Advanced Edge Concepts Inc. President
VAmatco Technical Coordinator