Doctor Blades - Flexo's Competitive Edge!, 1992 Corrugated Containers Conference Proceedings

Robert Thompson--Doctor blades, just small parts in the Flexo Printing process, are responsible for increases in print quality, consistency, and speed that has allowed Flexo to be competitive with other printing methods.

Your Anilox roll is designed to deliver the correct amount of ink, from its cells, for desired density and resulting print quality. Let’s imagine that we have here a plain glass here. Then we pour in some purple Cool-Aid. Just a little bit will cover the bottom of the glass. Then we put it onto a white cloth or paper, look down through the liquid and you will still see the cloth through the liquid. Pour in a little more and the color gets darker but you can still see the white underneath. A little more and the white disappears, the color is full density now and no matter how much more purple Liquid you pour in, the color will remain the same.

This applies to ink coverage too. Once the proper amount of ink is applied to the substrate so that the fibers or surface cannot be seen, then any more is too much and will only serve to make the dots grow and the edges waffle. To accomplish this there should be little or no ink on the surface of the roll when the transfer is made from the cells to the plate.

Many printers, both past and present, use rubber rollers to apply and doctor ink at the same time from their anilox rolls. This method works well but, there is a film of ink left on the anilox roller and it is necessary to change to rubber rolls of different hardness to compensate for various press speeds and anilox roll cell counts; a time consuming and costly process.

This is where the doctor blades comes in. Doctor blades shear ink from the surface of the anilox roll consistently delivering the prescribed amount of ink regardless of press speed.

In tests done by an anilox roll manufacturer using a film thickness gauge it was found that on the average, a two roll inking system left a 4 micron ink film on the anilox roll. When doctoring with a blade an average ink film thickness of 2 microns was recorded. Flooding begins at 6 microns.