Learning to print with right pressure

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Posted: Thursday, August 5, 2010 4:10 pm

The press operator had complained that the iron-to-iron settings were misadjusted on one of the Goss Community units that he was operating.

The issue: Print images had extremely grainy halftones, holes in the solid areas of the print and irregular edges on the solids being printed. To overcome the glitches, the press operator increased the ink. The tactic improved the solid areas, but did nothing to improve the halftones. More perplexing, the printing problems mainly affected only one side of the web, in this case the 10-side of the printing unit. The iron-to-iron setting was checked at all points on the unit, and the results indicated that everything was within normal tolerances.

Upon closer inspection, I found that the blankets were covered in paper lint, particularly on the 10-side blanket. The ink rollers on the 10-side of the unit were covered with paper lint as well, indicating poor or no pressure against ink drums or the plate cylinder.

After inspecting the roller settings, I found that the roller stripe was very weak and even non-existent in several areas. The ink roller system was then cleaned and readjusted to meet Goss' 3/16-inch clearance specs.

(The only exception is the 1/4-inch spec governing form rollers to the plate cylinder, due to irregularities that can exist across the surface of the cylinder.)

Dry run

A "dry print" test was then conducted to show the press operator how well the freshly set ink rollers would transfer ink to the plate. The plate was cleaned and then the ink form rollers were turned on and then off to create two stripes (inside and outside ink forms) for evaluation on the plate. The ink stripe on the plate was uniform with a smooth continuous film of ink in the stripe area. The edges of the ink strip were also smooth and uniform.

The next step is to test the transfer of the ink stripe from the plate to the blanket. Only the operator side of each the blanket was cleaned. This was to show the press operator the importance of cleaning blankets on a regular basis. After the blankets were cleaned, the impression was turned on and the press was moved just enough to transfer the ink stripe from the plate to the blanket (less than a quarter-turn).

The blanket was then inspected. The results showed a smooth continuous stripe on the blanket on the operator side. There was a stripe on the drive side as well, but it was difficult to determine how well the stripe had covered this surface area.

The next step was to transfer the stripe from the blanket to paper. The unit was moved so that it would take less than a quarter-turn to cause an impression between the blanket cylinders. The unit impression was turned on.

A sheet of newsprint as wide as the blanket cylinder and at least two feet long was positioned next to the blanket cylinders. As the unit was inched, it drew just enough newsprint between the cylinders to transfer the stripe from plate to paper. At that point the impression was turned off and the newsprint was removed for evaluation.

Clues pay off

The result? The stripe on the newsprint was smooth and uniform on the operator side, verifying that the iron-to-iron pressure setting was reasonably close. However, the ink stripe on the drive side of the press was not continuous and the stripe's edge was very irregular.

The real culprit wasn't the iron-to-iron setting. Instead, the operator had two issues: First, the blankets hadn't been cleaned in more than a month. Secondly, the newspaper used very poor quality newsprint from a paper salvager.

The lesson: It doesn't matter if you have correctly set ink rollers, iron-to-iron settings and proper blanket heights.

The press will not print good quality if the ink rollers and blanket cylinders are not cleaned regularly, especially if you are using poor-quality newsprint.



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