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Photonics Ensures Quality of Flexographic Printers' Rolls

Photonics Spectra
Jul 1997
Ruth A. Mendonsa

It is certainly an understatement to say that maintaining consistently high-quality graphics is a priority in the printing industry, and manufacturers of printing equipment face a formidable task in providing high-performance products.
In flexographic printing, ceramic-coated anilox rolls with laser-engraved cells that serve as ink reservoirs are used. The laser engraving produces millions of cells that are smaller than a human hair and minutely different from one another. These cell irregularities can cause inconsistencies in ink volume along a roll, which can impair print quality. The traditional cure is to measure the cells' liquid by conventional optical microscopy or by spreading a set fluid amount over a portion of a roll, blotting with paper and then measuring the saturated portion of the paper to determine cell volume. Even a slight deviation from prescribed fluid amount can skew test results, and human error comes into play.
Harper Corporation of America in Charlotte, N.C., a manufacturer of flexographic anilox rolls, has found a better way to ensure that its products produce consistently high-quality print. The company uses the RollScope from Tucson, Ariz.-based WYKO Corp. to check roll surfaces and cell volumes. The instrument employs optical phase-shifting and vertical scanning interferometry to quickly and accurately assess the anilox roll surfaces in 3-D.
Harper uses the RollScope as a manufacturing setup tool. Once the laser-engraving process begins, it must run until it is done, and engraving times average eight to 12 hours per roll. If anything goes awry, the process has to be redone. To avoid this problem, the technicians do quick test burns at one end, covering a few thousandths of an inch. They use the RollScope to check each burn for cell volume and make adjustments until they reach the proper volume before a roll is engraved full width. When they finish engraving a roll, they inspect the entire surface using the RollScope.

Speed and accuracyare key
Steve Tucker, a technical specialist at Harper, said the company chose the RollScope because it is accurate, offers high repeatability and is fast. It takes only 15 to 20 second to obtain a measurement; human error is virtually eliminated. The device incorporates a low-noise, 8-bit digital output charge-coupled device comprising 355,200 elements in a 480 3 740-pixel array. Vertical scan range is up to 500 µm with vertical resolution <10 nm. Harper also takes the portable RollScope to its customers to assess their anilox rolls.
Tucker said it's as if the company has "stopped using a yardstick and begun using a micrometer." Since using the RollScope, the company has cut manufacturing tolerances for cell volumes by 50 percent.

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