How Flexographic Printers Can Control Doctor Blade Performance on Press

On press? Oh no! Something’s wrong! Correcting the issue can compound the problem. Try diligently to avoid error as you implement the crisis management plan.

Ready? Here we go!

• Step one: Diagnose the problem
• Step two: Apply corrective measures
• Step three: Develop and then implement best practices (standard operating procedure or SOPs)
• Step four: Train, educate and reinforce those SOPs to make sure operators catch the trouble at setup
• Step five: Reap the rewards of what you sow

Welcome to production floor management! The ever-present mission: make packaging lines profitable and keep the presses humming—every hour of every shift. Realize every component, no matter how insignificant it appears—a doctor blade, at first glance a single, flexible piece of metal or plastic, for example—can do considerable damage and destruction.

Marty Cansler, managing director, sales and technical support at Daetwyler, encourages every package printer to adhere to these steps. His reason: Quality print results go out the window with a bad blade—either a poor choice, improper set, worn surface, bad end seal; even the lingering impact of hard pigments and high opacity. Everything comes down to one simple fact: viscous liquids can be vicious!

Instead of leaving it at that, Cansler urges printer/converters to examine the most common pitfalls associated with less-than-stellar selection of the right doctor blade for the target application—beginning with back doctoring and progressing through white ink wear, UV spitting, color shifts and inconsistencies.

Back Doctoring

“Back doctoring seems to be the most common of all the issues we are asked to address. It’s primarily a problem on the back side of a central impression (CI) press—decks six through 10 on a 10-color machine,” Cansler observes.

“At first glance, it can appear to be a chamber leaking issue, often associated with a worn or improperly set end seal,” he continues. “Ink often fills the drip pan as it falls from the bottom of the chamber. If the blade chamber is not parallel to the anilox roller, the ink often moves to the lowest point of the chamber as it drops. This leaves press operators to misdiagnose back doctoring as an end seal problem.”

Back doctoring is much more than a simple waste of ink, according to Cansler. Inevitably, spilled ink covers press parts that eventually must be cleaned, and the dirty residue disposed of. That means downtime!

“The most common cause is overpressure of the chamber. Extra chamber pressure re-doctors any excess ink, as it makes its way back to the bottom of the chamber. If the press operator believes the chamber is leaking, he will often use more pressure in an attempt to correct the issue—which compounds the problem,” he says. “Another false correction is to slow the press speed to help control the centrifugal force.

Cansler lists the most common corrective measures to address this issue as:

• Ensure the end seals match the radius of the anilox roll
• Be certain the chamber is parallel to the anilox
  • The alignment should be correct on top and on the bottom of the chamber
• Use a thinner containment blade (as compared to the wiping blade), so the hydrodynamic force of the ink can force the ink back into the chamber
  • The thinner blade acts as a check value to help the ink re-enter into the chamber and also contain any excess from leaking out
• Lastly, use a containment blade that has a release coating, so the ink can freely flow back into the chamber

“A good example of a blade with such characteristics is Daetwyler’s Pearlstar,” Cansler notes.

Most importantly, work with a blade supplier that can help correctly diagnose the problem, apply corrective measures and train your operators on proper setup and application of best options for doctor blade technology.

White Ink Wear

White ink is often a challenge for any doctor blade. Why? Cansler offers details. “Pigments in white ink are made of titanium dioxide. This very hard pigment is required to bring opacity levels to a higher value. White ink often results in additional blade wear and multiple press stops per week to change severely worn blades. It’s associated with a prevalence of dirty print, where additional downtime is needed to clean plates.

“The most common corrective measure is using a longer running, more durable doctor blade. Multiple options are available from most blade suppliers, but the key is to match the white ink blade life with the blade life obtained from the standard line and process colors, so the press only needs to stop once to change all blades.”

Cansler maintains, “Printers often try to save money on blade costs and will not consider the costs of press downtime in the equation. If a standard blade costs $10 and a premium coated blade costs $50, the additional cost of $40 is quickly recovered by saving one press stop.”

Elaborating on that thought, he says, “If downtime rates are in excess of $500 per hour and four, 15-minute press stops per week are prevented, the savings is in excess of $460 per week. For a 50-week production year, that’s a savings of more than $20,000 for one station, on one press.”

A good example of this coated blade technology is Daetwyler’s Optilife Plus, Cansler notes. “This blade has a multi-layered coating—soft on the surface to protect the anilox roll from wear combined with a durable, hardened coating underneath to protect the blade against the aggressive titanium dioxide particles.”

Key things Cansler finds necessary to mention to ensure this coated blade technology reaches its optimum life are:

• Proper settings achieve light blade pressure. This is important for any doctor blade, but the projected savings can also be erased if improper settings prevent the blade from reaching its potential
• End seal life needs to match blade life. Otherwise, the press needs to stop and the savings are not achieved
• The anilox engraving must be considered and designed correctly, so the best opacity levels are achieved and the anilox surface is polished correctly to ensure optimum blade life

UV Spitting

UV spitting is a common issue in narrow web printing. Cansler explains, “It is often a result of trying to doctor a more viscous ink from the surface of the anilox. It is especially present in the process colors, since the problem seems to be pronounced in the higher linescreen anilox roll, where more shallow cells are present.”

In making further observations, he says, “Thinner doctor blades are often used in higher line anilox rollers to protect the thin cell walls, however, the end result from using this more viscous ink is additional blade flex. As the hydrodynamic force of the ink pushes against the thinner blade, it flexes and the blade allows a small amount of ink to pass under the blade tips. This is seen as small spots referred to as ink spitting.”

Cansler contends, “To control ink spitting, a doctor blade with a thicker body is used. However, the blade must be greatly reduced at the tip to be certain the higher linescreens and shallower cells are metered correctly and consistently to avoid dot gain/dirty print. This is easily achieved using a special doctor blade. Daetwyler’s solution is called the One Step. This doctor blade is designed to eliminate the need for a support blade, often used to control blade flexing, requiring only one step in mounting.”

Another often overlooked area of UV spitting is the anilox engraving as it contacts the doctor blade. Cansler describes the process this way. “As the doctor blade pushes ink into the cell, the air in the cell must have some way to escape, so the ink doesn’t trap an air pocket in the cell. This force in the cell can create a positive pressure that causes the ink to pop as the doctor blade moves beyond the cell opening, resulting in ink spitting.”

When addressing this trouble, he tells printers, “By slightly lowering a cell wall in the front and the rear of the cell, it gives the air somewhere to escape. This special engraving can be found in Sandon Global’s Fluid Hex engraving. If this engraving is combined with the One Step doctor blade, the best possible result can be achieved. This combination results in an ultra-clean wipe to minimize dirty print and dot gain, thanks to easy control of more viscous ink and smooth flow of ink from cell to cell.”

Color Shifts

“The battle for consistent color throughout longer pressruns is at the top of many printers’ list of areas for improvement!” Cansler acknowledges, “Most printers have color under control for the short to intermediate runs, but from time to time, they end up in a fight to control consistency from roll to roll on those long runs.”

Admitting, “The industry has gone to great lengths to improve plate, anilox roller and doctor blades,” he says, “Today, the flexo process is in a great position when it comes to consistency and quality. The best move flexo printers have made recently is stepping away from the old standard radius, or round tip doctor blades, to a machined tipped doctor blade.”

Again, Cansler provides a list of considerations when contemplating a change for color consistency:

• Radius tip, or round edge doctor blades, increase in contact area as they wear. That means less ink is being removed from the anilox roll. This increase in ink film thickness equates to more ink being transferred to the plate and color shift begins
• Color shift is always a slow transition, so at first, it’s not easily seen and detected
• Changing to a new blade in the middle of a run will change the color, regardless of the type of blade tip. The new blade will have a smaller contact area and will wipe clean. Press downtime is avoided
• Increasing or decreasing doctor blade pressure will also affect color
• The best fix for maintaining color consistency is to start out with and set color while using a Lamella tip, or elongated bevel/multiblade tip configuration

Consistency Rules

“The overall consistency of machined tip blades is well documented, thanks to years of trials and testing,” Cansler states. “Printers spend huge amounts of time and money finetuning so many areas of the process and still have color issues. That’s a pity.” At times, he believes very simple measures are called for:

• Change to a consistent machined tip blade that wipes clean for the entire life of the blade and long runs
• Add a coating that lowers the co-efficient of friction and helps the wipe to be even cleaner for longer

Cansler emphasizes his starting position. “Remember, we don’t have to stop the press just to change blades. Avoiding the need to, by applying SOPs, ensures long runs are consistent and on color. Plus, with no additional press stops, those long runs can be much more profitable.”