The Science of Doctor Blades
Hazing
Cause: Hazing is "typically the result of low doctor blade angles," said Ritz. Low wiping angles of less than 55°, he explained, cause the blade to "hydroplane" over the gravure cylinder, leaving too much ink in the non-print areas. In some cases, the existing friction level between the blade and cylinder may result in hazing, and may call for a new blade. "When hazing appears, most printers increase the pressure on the doctor blades, and the hazing disappears, but only for a short time, and then it's back," said Ryan Platt of FLXON, Inc. (Charlotte, N.C.). "The reason it reappears is because as blade-to-cylinder pressure increases, so does the contact area, and the more contact area the more friction." Increasing pressure on the blade is also a short-lived fix because "more pressure drops the contact angle even further," Ritz said. "For a few minutes, the increased pressure stops the haze due to the blade creating a new sharp corner against the cylinder. After the blade wears into this new angle, the contact patch increases and the hazing returns often worse than before."
