Anything that slows down or interrupts his quest to combine flawless print and output efficiency is reviled by the press operator. This includes back doctoring which is wasteful, disruptive and a general pain in the backside. The good news is it can be easily overcome with the right containment blade.
Back doctoring explained
No one would debate that back doctoring (also referred to as “trail doctoring”) is a nuisance. What operator wants to stop and clean the press more often than he already has to? It is also an expensive problem. Lost press time and wasted ink and substrate cost money – and delivering output with print defects costs customers.
To explain back doctoring, let’s compare what’s supposed to happen to what’s actually going on.
Both the metering blade and the containment blade are thin strips of material that play a key role in transferring ink from the anilox roll to the substrate. But the two blades have very different purposes. In order for the doctor blade chamber to function optimally, each blade has to be chosen carefully.
Ideally, the reverse angle blade acts as the true doctor blade responsible for delivering a precise amount of ink to the plate and ultimately, the substrate. It is designed to wipe the excess ink from the anilox roll before the transfer takes place to ensure that the correct volume of ink is transferred. Metering blades need to be stiff and rigid to prevent hydraulic pressure from lifting the blade as the roll turns, letting ink pass underneath and over-inking the anilox roll.
The trailing blade acts as a capture or containment blade and holds the ink within the confines of the chamber. Essentially, you want the containment blade to function in the opposite way in terms of how it interacts with the ink. Located in the opposing position in the chamber, its function is to collect and contain the excess ink from the roll after the transfer takes place. In this case, you want the hydraulic force from the rotation of the roll to lift the tip of the containment blade, letting the ink flow underneath – in one direction – and back into the chamber where it is reclaimed for future use. Whereas stiffness and rigidity are required of the metering blade for effective wiping, the containment blade should be thin and flexible in order to create a one-way “valve.”
Back doctoring occurs when the containment blade meters – or ”doctors” – the ink instead of allowing it to re-enter the chamber/funneling it back into the chamber. A containment blade that’s too stiff or rigid (such as steel) will prevent the ink from passing under the blade and being reclaimed. The “back-doctored” ink builds up on the outside of the blade, pools at the ends of the chamber and eventually slings onto the press and web. (This occurs more frequently when the containment blade is installed in the bottom position of the chamber where gravity pulls the ink downward.) The resulting mess requires the press to be shut down, cleaned and reset by the operator before he can continue with the print job. At higher press speeds or smaller anilox diameters, the centrifugal force of the roll as it turns is greater and the problem is even worse.
How to stop back doctoring
The key to eliminating back doctoring is using the right containment blade.
The ideal containment blade should be thin and flexible enough (such as plastic) to lift and create a one-way path for the ink to return to the chamber. Unlike the metering blade, where a precise area of contact enhances metering performance, a fine contact area with the roll is not required (why pay for a bevel when it’s not needed?) for the containment blade to perform its function. Choosing a straight or radius edge will optimize blade life and cost.
Polyester containment blades
Polyester has been the preferred material for containment for a long time due to Its flexibility and stability at thin gauges. The material has a low coefficient of friction and is highly solvent-resistant. Polyester blades cost substantially less than steel blades (roughly one-third to one-half) and are safer for press operators to handle. The material is soft and contains no sharp fragments which can break off, become lodged against the anilox roll and destroy the surface engraving as the roll turns. The material is also non-abrasive and won’t cause excessive wear on the roll.
Back doctoring is a common pressroom problem that, fortunately (but less commonly), has an easy fix: converting to a plastic containment blade. By choosing plastic, a printer can ensure that the containment blade will succeed in doing its job – keeping unused in ink the chamber and preventing it from escaping and making a mess – even at high press speeds.
Read our blog post, “Top 5 Reasons to Switch to Polyester Containment Blades” to learn more about the benefits of using plastic containment blades.