It starts seemingly simple enough—with visible ink leakage dripping in the catch pan under the doctor blade chamber. Yet, when seals and calibration settings are checked, there doesn’t seem to be anything wrong. But the proof is right there in the pan and on your equipment.
Your problem could be due to back doctoring.
When excess ink remains on an anilox roller, it can cause drips and icicle-like buildup that impacts the overall performance of the press. This is particularly common on central impression (CI) presses, where backside decks rotate against the containment blade and leave residue.
The Perfect Storm
On a CI press, the chambers and blades are positioned as mirror images from one side to another. On the front side, metering blades are on the bottom and containment blades are on the top. Ink is typically pumped in at the top of the chamber, filling up to the containment blade. The bottom (metering) blade transfers the ink to the plate and then to the substrate. The anilox rolls turn in a clockwise direction with metering blades removing excess ink before it passes to the plate.
In this scenario, there is rarely enough ink left on the anilox roll to cause back doctoring or icicles. It doesn’t mean the potential for the problem isn’t there—it’s just that the orientation of the chamber helps mitigate any back doctoring that would occur if aligned differently.
On the back side, however, metering blades are on the top and the containment blades are on the bottom. This is where the challenge typically begins, as it is the “perfect storm” environment for back doctoring. Gravity, orientation and quality of components all come together to make back doctoring a likelihood under normal pressrun conditions.
Like all businesses, print profitability comes down to a series of choices regarding quality and cost. The best equipment in the world will fail to perform to expectations when inferior components are used.
Press plate cylinders turn counter clockwise, while anilox rolls turn clockwise. The top mount position of metering blades contributes to excess ink being left on anilox rolls after transferring to plates. That excess ink rotates around to the containment blade, and if the blade is too thick, too tight or too rigid, it can cause the ink to back doctor and create “ink icicles.” These buildup formations dry and hang down from the chamber, causing a mess and impacting print performance.
Why Has Back Doctoring Become More Common?
High speed presses were designed to increase productivity and profitability by increasing output. But the faster rotating anilox roll and sheer centrifugal force of moving components comes with some drawbacks. Unfortunately, speed and precision have always worked against each other, and back doctoring is one such result of faster presses.
Photos courtesy of Daetwyler
For many years, the choice of the chamber manufacturers was steel on the metering side and 0.015 Mylar (plastic) on the containment side. With the advent of turbo wash or automated washup systems, many printers found they had to run steel blades on both sides to avoid damaging the lighter plastic blades. Automated wash systems were too vigorous for plastic blades that were either being damaged, broken, or weakened by the wash solutions or pressure. The easy answer, of course, was to use steel blades instead of plastic, because they are simply more durable.
But it’s the use of steel for containment blades that is one of the key causes for the modern day back doctoring phenomenon. One of the principle concepts of flexography is that containment blades should always be thinner than their metering brethren. This allows any type of debris that might pass through the inking system to circulate back to the thin containment blade and pass gently without restricting it. In addition, thinner blades would allow debris to pass without lodging behind them and potentially scoring anilox rollers.
While steel blades offer excellent durability in the face of wash cycles and cleaning, they are also prone to warping and bending the thinner they become. Ink and debris can pass through the uneven edges of the blade, and the resulting dried up icicles hang from the bottom of the chamber. Not only does this cause performance issues and downtime to clean and reset equipment, but crucially, the real problem is that it costs print companies time and money.
Because plastics can retain their shape and structure even when quite thin, this material is often preferred in many doctor blade applications. However, quality is a major consideration when it comes to plastics. Poor composition or low quality manufacturing can create plastic blades that snap, crack, or were never “true” and smooth to begin with. This is what makes “discount” or “value” blades so worrisome, and why many print companies are simply unwilling to take a risk on a Mylar blade. After all, if downtime for washing is expensive, switching out broken blades is even more costly.