The Significance of the Doctor Blade

Its purpose: The job of a Doctor Blade is to prepare a Rotogravure Cylinder or Anilox Roll so it can do it’s job. The Cylinder or Roller has to deposit the proper amount of pigment, so that the required color density is achieved on the substrate without any rejectable defects. The blade must leave as little ink as possible on the top of the cell walls and at the same time fill each and every cell. It must be able to do this consistently across the entire face length of the cylinder and do it at any speed and with any with any color ink. In performing its task, the blade must not damage the cylinder or roller from the time of its initial loading, all the way to the point when its useful life is over. And lastly, the blade life needs to be as long as possible but not at the expense of the cylinder or roller and its relative cost to the printing job must be low.

The issues of importance: Material Specifications, Mechanical Specifications and Properties, Edge Configuration, Blade Selection, Installation, Cylinder Considerations, and Other Considerations.

Material Specifications: The raw material used to make a doctor blade must be homogeneous in microstructure. Cleanliness is the aspect of the microstructure that specifies a blade can only contain the elements needed to give it the mechanical properties to do its job and not contain any elements that will damage the cylinder or roller. This is important for any doctor blade material: carbon steel, stainless steel, plastic, fiberglass, composite and so on. back to top

Mechanical Specifications and Properties: A blade must posses a specific tensile strength, hardness and wear resistance. The thickness and width tolerances, flatness, surface finishes and camber (curvature) all effect the printing performance. Camber, flatness and width variations will effect the initial wiping between the blade and roller. All of these properties effect the amount of loading force needed to cleanly meter the Cylinder/Roller. back to top

Edge Configuration: Metering is actually performed by the tip of the doctor blade. For Rotogravure doctoring a reduced thickness tip, referred to as a lamella tip, is the tip of choice. This configuration allows the blade thickness to be twice that of the tip. The lamella blade produces a combination of stiffness with contact tip flexibility. In flexo, the tip configurations vary. Choosing the proper tip is dependent on combinations of variables. They include: the line count and finish of the Anilox roll, the ink or coating being applied, the rigidity and alignment of the blade holder/chamber, roller width, printing speed, complexity of the art work and so on. The criterion has to point to the tip that performs adequately for the majority of the press variables. Flexo tips include pre-beveled, lamella, rounded (bullet shaped) and square edged. Each tip has its application were it best performs. back to top

Blade Selection: As with choosing a tip, the criteria needed will include all the same variables but also includes the substrate, the amount of blade that sticks out of the holder, ink pigment size, ink pH, press cleanliness and more. When choosing a blade, it has always been the practice of Benton Graphics to provide samples of different combinations of blade materials and tip configurations, so the press team can see which perform the best. back to top

Blade Installation: The optimum blade may perform poorly if it is not installed properly. The lack of parallelism between the roller and the blade holder / chamber will result is reduced blade and roller life. The same is for a condition called skewing, where the parallelism is correct in one plane but not in the second plane. Uncleanliness of the clamping seat causes the blade to be wash-boarded or skewed when clamped. Improper installation is the biggest problem with blade printing performance and short blade life. back to top

Cylinder/Roller Considerations: The surface finish of the roller and the TIR (total indicator run-out) are the two most important factors that effect the operational relationship of the cylinder/roller to the doctor blade and or blades. back to top

Other Considerations: The mechanism that applies the force that pushes the doctor blade into the Cylinder/Roller can vary from simply being a mechanical screw or level action, to a combination of pneumatic and hydraulic cylinders. The operation and maintenance of this mechanism can greatly effect the performance and life of the doctor blade. Excessive loading force is a considerable problem causing premature cylinder and blade wear. In addition to proper selection, operator training is of equal importance. back to top

Summation: Since 1977 Benton Graphics has been helping the Printing and Coating Industry choose the doctor blade best suited for each application. To better serve our customers, we are now manufacturing plastic metering and containment blades. Benton Graphics is committed to research and development. This commitment will keep our customers supplied with the best materials, edge configuration and service available in the industry. back to top

For more information on this product, please contact: info@teamflexo.com

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