Fluid Movements
Suppliers trace popular and pending doctor blade usage trends.
By Susan Friedman
The smooth, even business of ink metering isn't without its grayer areas. Yes, flexo printers have long been heavily loyal to doctor blades, particularly chambered systems...but metering roll applications still remain. And yes, gravure printers still favor trailing blade configurations...but reverse angle's potential hasn't faded away. Here, suppliers explore the sometimes uneven ink metering terrain.
Flexo: chamber-made?
For many flexo printers, the decision to use doctor blades instead of a two-roll metering system has been pretty, well, cut and dry.
"There are not many scenarios where a two roll system will out-perform a doctor blade system," contends Matt Burie, account executive, at Printco Industries. Burie emphasizes doctor blades' strength lies in their basic function—metering excess fluid off the unengraved areas of the anilox roll or gravure cylinder. "By allowing the anilox or gravure roll to transfer as much fluid as it has been designed to carry instead of flooding the printing plates, sharper, cleaner print is achieved that is essential for process work," he explains.
Tom Allison, president, Allison Systems, seconds the "process work advantage" line of thinking. "Two-roll inking is mostly extinct in flexo due to the need to produce high-end graphics on packaging, which translates into the need for good control at high and especially low color densities," he comments. "To do this, the doctor blade and blade system must prevent dot gain due to surface ink left on the anilox roll. Regardless of how fine the anilox screen and how small and well controlled the cell volume, the surface ink left by a two-roll system is usually not acceptable."
Perhaps the two-roll metering system has become an endangered species of sorts. But don't put it on package printing's extinct list just yet.
"Flexo printers running chrome-surfaced anilox rolls will typically print with a two roll system, as the doctor blade wears the chrome anilox very rapidly," points out Anthony Foley, national marketing manager, Edward Graphics.
According to Foley, the two-roll system is still a presence in low-end flexo applications where printing requirements are not so critical. "One of the benefits of the two-roll system is the ability to cheat on your ink delivery to the printing plate," he explains. "By applying more pressure to the rubber metering roll, you reduce the amount of ink delivered to the printing plate. By reducing the pressure on the rubber metering roll you deliver an increased ink film to the surface of the printing plate. Simply put, the two-roll system has more latitude in its ink delivery."
Andrew Schipke, president of FIT, sees another usage window for two-roll systems, albeit a narrow one."The only case where one could rationalizea two-roll system," he asserts, "is where a vast variety of print types(such as solids, screens, and low-end process) is being done on a press where changing aniloxes is too time consuming."He cautions, however, that consistency and overall quality will not match what can be achieved with chambered ink metering.
Suppliers maintain doctor blades—particularly chambered systems—hold not only quality but also operational advantages over two-roll systems.
A doctor blade system is not speed-sensitive, Burie notes, while a two-roll system will vary its fluid laydown depending on operating speed, due to hydraulic forces between the rubber roll and the anilox or gravure roll. The force of the fluid being pushed through the nip will actually deflect the rubber roll and cause inconsistent laydown.
In addition, the constantly recirculating chambered doctor blade system requires only a few ounces of fluid for start-up, leaving it less expensive to operate than a two-roll system, Burie adds. More efficient use of inks or coatings also makes clean up and maintenance faster, he says.
Chambered systems have even helped solve peripheral press design challenges. They have been the standard for some time on central-impression flexo presses, says Allison, due to the need to doctor and contain ink on the up and down sides of the CI drum. What's more, he points out many newer automated or semi-automated wash-up systems depend on the chamber to act as a cleaning device as well as an inking device.
Taking on gravure's angle
If chambered systems offer such a bevy of benefits to ink metering, why aren't more of them being put to work on gravure presses? Foley says testing is being conducted in gravure with enclosed chambers, but does not know of a North American printer running with them on a production basis.
He explains that because gravure presses typically run with the blade in a trailing, or positive doctoring configuration, the conversion to enclosed systems—which doctor in a reverse, or negative doctoring configuration—is difficult.
Edward Graphics' testing of chambered systems for gravure has found that the process's chrome-coated cylinders can be damaged or worn fairly quickly when doctoring in enclosed systems' negative configuration. Foley says "drying in," or ink drying in the cells of the engraved cylinder, can also become an issue with chambered systems. The ink does not have much time to re-wet and therefore can lose color strength as the run progresses.
Schipke is a bit more optimistic about the potential of reverse-angledoctor blade metering for gravure package printing, but acknowledges only time in the lab and field will tell. "Reverse angle metering is what is done in flexo, and today many gravure converters are looking at it as a means to more efficiently deliver ink or coating to their presses," he comments, emphasizing the chamber's benefits for controlling small amounts of expensive coatings and delivering multiple colors across a cylinder.
Schipke says advancements in loading reverse-angle chamber systems to the gravure cylinder now allow a non-oscillating doctor blade to accurately wipe a chrome gravure roll without causing accelerated wear to the surface.
FIT has several reverse-angle doctor blade systems running in gravure, continuous-pattern printing or coating applications, achieving "commercially acceptable quality" at 2,000 fpm, Schipke reports. In packaging, where there may not be a continuous engraved printing pattern around the entire cylinder, he says the main concern is the quality of the reverse angle wipe at extremely high speeds, going from image to non-image areas. To address this and other reverse- angle doctoring issues, FIT has R&D agreements in place with several gravure converters as well as an in-house test unit.
The viability of reverse-angle doctoring for gravure harbors a lengthy history of healthy debate and rigorous testing—two practices likely to endure for a bit longer.
"In gravure, attempts to use a reverse angle in lieu of a trailing doctor blade have been made since the '50s," says Allison, who explains the first reverse angle efforts were thwarted by lines of engraving across gravure's smooth cylinder face, such as the first line of a column of type. These jammed the thin tip of the reverse angle doctor blade and destroyed the blade, the cylinder, or both.
"Useful blading and engraving technology to avoid this sort of thing is in the field and, with the chamber, promises faster clean-up and job changeover for gravure," Allison states. In addition, he notes newer alloy steel blades with laser-hardened tips can eliminate easily broken feather edges and last 10 to 100 times longer than carbon steel blades.
Allison is quick to point out, however, that concurrent improvements in trailing blades might make pulling them off a press about as easy as pulling teeth. "The gravure trailing blade's special role in color and image control, particularly when working directly on difficult-to-wipe gravure cylinders, is being coupled with various quick clean-up schemes and control software. It's alive and well."
