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Technical Bulletin 1-4/96

There are three common print quality press problems that have become more prevalent since the introduction of alkaline base papers. They have a history of occurring on either web or sheetfed presses at various press speeds, and with a variety of paper, press, ink, and fountain conditions.

The print quality problems are often referred to as:
* Gradual highlight dot loss
* Vanishing dots
* Hollow dots

Possible Cause

There is general industry agreement that these common problems may occur because of a gradual chemical contamination caused by calcium carbonate byproducts in the press, and on the blanket. There are two conditions of blanket chemical contamination of which press chemical contamination may be a contributing factor:

Condition #1

Blanket chemical contamination forms a "non-ink-receptive barrier" to the ink in the image area of the blanket. The visible, or invisible, barrier on the blanket prevents proper ink transfer from the plate image to the blanket. As a result, printed images, highlight halftones, text, and solids appear broken, grainy, or lack sufficient solid ink density.

Classic Symptoms: Printed sheets exhibit gradual highlight dot loss, grainy highlight dots, grainy solids, broken solids, or short press life.

Condition #2

Blanket chemical contamination may gradually "build up" around the edge of the halftone dot in the blanket. This build-up forms a gap that prevents ink transfer from the plate to the blanket. The highlight halftone printed image appears as a hollow dot.

Classic Symptoms: Examination of the affected image area with a 20X loupe reveals hollow dots, broken images or grainy solids.

Since these problems occur when the plate image is ink-receptive and all press pressures are in specification, it is suspected that many times the problem is diagnosed as a plate problem rather than a chemical contamination problem in the press.

How to Test for Highlight Dot Loss Due to Blanket Contamination

Step 1 Stop the press and rotate the cylinder by 1 or 2 rows of dots.

Step 2 Restart the press.

Step 3 Run press sheets and examine the image with a 20X loupe.

If the print quality improves, contamination is a possible cause. The blanket may be non­ink receptive in the image area due to a chemical contamination from the paper, fountain, or ink (movement of the cylinder moves the image to an area on the blanket that is ink receptive if the suspect contamination exists).

Solutions

Temporary Corrections

When blanket contamination is the source of the problem, you can use this temporary remedy which may work for short press runs.

1. Stop the press.

2. Wash the blanket and plate surfaces with a mild press wash.

3. Restart the press.

Rewash the blanket whenever the problem recurs. A major disadvantage to this remedy is the loss of valuable production time. For a longer-term correction, use the following procedure. This will allow for a longer press run, but will not permanently fix the problem of blanket contamination.

1. Stop the press.

2. Clean the blanket with a mild solvent.

3. Wash the solvent off of the blanket.

4. Flush the fountain system.

5. Deglaze and decontaminate the ink train and rinse with a vinegar/water combination.

6. Restart the press.

7. Repeat the procedure if press contamination is not completely eliminated.

Eliminating Blanket Contamination

To eliminate blanket contamination, we suggest the following experimentation as an on-going method of improving print quality issues.

Experiment with different papers, inks, blankets and press conditions.

1. When the problem occurs, stop the press.

2. Change one condition (e.g., clean the press, change paper, etc.) and restart the press.

3. Record the response.

4. Repeat for each condition.

5. Select and use the conditions that reduce or eliminate the problem.

Consult with your paper, ink, and fountain solution manufacturers.

When contamination symptoms (highlight dot loss) occur, record the conditions; that is, materials, adjustments, and run numbers. Save samples of the plates, blankets, papers, and inks. Take this information back to the materials suppliers to work out a solution for the contamination problem.

Copyright 1996 Imation. All rights reserved.