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Technical Bulletin 15-12/86; 2-3/87

Problems: Premature plate wear and picking in the solid and shadow areas.

Causes: Improperly adjusted presses or overpacked press condition.

Corrections: For an improperly adjusted press, adjust the press according to the recommendations below to allow for the optimum length of plate run; for an over-packed press, check and correct press packing using the method below.

Improperly Adjusted Press

Adjusting a printing press to obtain good print quality as well as optimum length of run can be a complex procedure. Several factors acting independently or in combination can result in plate wear. Since each press requires different pressure settings, this bulletin is not intended to provide specific procedures but to advise of the possible problems that may occur if the press is improperly adjusted.

Two mechanical factors which have been observed to affect the length a plate runs are discussed below.

1. Impression Cylinder Pressure

Insufficient back-cylinder pressure causes ink to pile on the blanket in the shadow and solid image areas. The piled ink becomes tacky and provides an opportunity for paper lint and offset spray powder to accumulate, resulting in image pick in those areas.

Insufficient back-cylinder pressure can also cause blanket toning in non-image areas. All printing plates, regardless of the manufacturer, will accept a minute amount of ink in the background areas which is transferred to the paper. When impression cylinder pressure is too light the ink is not transferred as well and builds up in the non-image areas on the blanket. This situation is often mistaken for a plate problem rather than a improperly adjusted impression cylinder. Repeated use of strong plate cleaners in an effort to eliminate the blanket toning can dramatically shorten the life of a plate.

2. Dampener Setting (Conventional Dampening)

A dampening form roller that is set too tight to the oscillator, will not run at the same peripheral speed as the plate. The dampening form roller cover will abrade the plate image mainly in the solid areas and the shadow areas of halftones causing drastically reduced length of runs.

Most literature that pertains to dampener adjustments is rather vague (adjust the dampening roller until it is snug, firm, etc.) in describing the setting to the vibrator, while specifying a (light) setting to the plate. Refer to the press manufacturer's service manual for correct procedures and pressure settings.

Over-Packed Press Condition

Minimum plate-to-blanket pressure improves printing quality and extends plate life. Excessive pressure will cause blanket skid on the plate creating copy register and quality problems as well as premature image wear.

Pack to Print

The following procedure will establish the actual running conditions of press packing and the trueness of the cylinders. It will identify optimum printing pressures.

* Select a plate with large solid and halftone screen areas (not line copy). Use a dark-colored ink.

* Pack the plate and blanket cylinders to the press manufacturer's specifications.

* Wash the gum off, roll up the plate and pull several impressions (until the copy is no longer washed out).

* Stop the press. Remove .004, of packing from under the plate.

* Remount plate, wash old image off blanket and run about 10 more sheets.

* Inspect copy. If the image is broken, the plate and blanket packing were correct. Replace the packing that was removed and run. If the print is still good after the .004, was removed from the plate packing, it's an indication that the blanket or plate cylinder is over packed. Continue to pull packing from the plate until the print becomes broken.

Check Ink Roller Settings

Since the removal of a substantial amount of packing may affect the roller settings, it is recommended that you conduct an ink band test at this point to determine whether they will require any adjustment.

Replace .001, to .002, under plate to obtain a good print. Measure the thickness of excess packing.

Now, replace all the packing that was taken from under the plate, and remove the excess amount from the over-packed blankets. Then reset the blanket-to-impression cylinder pressure and run the job.

Note

The primary advantage to this method of determining correct printing pressures lies in the fact thatonly one printing point is affected by removal of packing from the plate cylinder during your check, whereas two printing points would be involved if you removed it from the blanket cylinder.

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