The Science of Doctor Blades
A review of the top issues concerning doctor blades.
THE ARTISTRY OF doctoring the doctor blade is giving way to science. Tom Allison, president of Allison Systems (Riverside, N.J.), remembers when he used to ask his dad what the press operators were doing as he watched them prepare the doctor blade for printing. After hushing his son, Allison's father would say, "Pressmen are frustrated artists; each one has his own 'pallet' of special things he feels that he alone can do to make 'great art' come off the press."
Where once true—when the performance of the doctor blade depended solely upon the operator's set-up—now stronger blade holders, higher quality blades, and blade monitoring software offer printers the opportunity to turn doctoring into a near exact science. Yet like any scientific process, there is room for error, and error means downtime, waste, and an inferior print job, said Johnny Stamey, technical product manager at Max Daetwyler (Huntersville, N.C.).
pP examines the most prevalent doctor blade mistakes and ways to prevent these costly errors.
Many doctoring issues in flexo have something to do with excessive pressure on the doctor blade. Scoring, premature blade wear, and blade wave—the top three flexo doctor blade problems according to industry suppliers—can all be corrected, to a degree, by decreasing the pressure on the blade.
Cause: Scoring not only mars a print job, but also permanently damages the surface of the anilox roll. The problem usually occurs as a result of excessive pressure on the doctor blade, causing the blade to trap insoluble particles and damage the anilox. Over time, explained Bobby Furr, technical product manager for HarperScientific.com®, a division of Harper Corp. of America (Charlotte, N.C.), "Material from the doctor blade goes back through the ink train, lodges up behind the doctor blade, and starts grinding on the anilox roll." The anilox cell walls are ripped apart by the trapped material—such as wear debris from the blade or ceramic pieces from the damaged anilox—and a groove is formed in the roll. "Most of the time it goes undetected until the line starts to appear in the printed substrate, and then it is too late," Furr said.