In this Part 2 of On Press Troubleshooting we will present some of the problems that can occur in printing press-room explained by some of the experts in this field.
According to Gordon Pritchard owner of The Print Guide (http://the-print-guide.blogspot.mk/) and one of the legends in this field doubling is often confused with slur as both exhibit an elongation of halftone dots. However, slur is usually an elongation in the direction of sheet travel through the press while doubling can be in any direction. Doubling (and slur) often manifest as a problem with the range of tones available in the presswork being compressed and loss of detail, particularly in the shadow areas (a.k.a. muddy halftones). Doubling can be caused by many of the same factors as slur. When the cylinders rotate the halftone dots are not placed in exactly the same position with every revolution. As a result the dots print up as double or multiple images. Doubling between units occurs when a blanket picks up a previously printed ink film. This is known as backtrapping. Examine the dots, or line art graphics, under a loupe to confirm whether the problem is doubling or slur.
Phantom effect, reappearance of the previous print run or the print run underway but out of shift.
According to guys from Brancher (http://www.brancher.com) 2 possibilities can cause this problem.
Worn blanket. Change the blanket. Even if the blanket seems to be clean, its compressible layer may be altered and the phantom image may appear later.
Form rollers incorrectly adjusted on inking rollers. Adjust the pressure settings of the form rollers on the inking rollers.
Crystallization Of Ink On Drying
When a job requires two or more printings with the ink drying in between, the dried ink sometimes fails to trap the succeeding ink properly, this is believed to be due to the non-drying oil or wax in the ink coming out or floating to its surface during drying, causing it to repel the following ink.
Crystallization can result from running too much ink too much grease or wax compound in the ink or for allowing the ink to dry too long and causing the ink film to harden appreciably before over-printing it. Too much cobalt or manganese driers can cause it because these driers cause rapid surface drying. When a job requires two or more printings, the first ink to be printed should be made without grease or wax in their formulas and no excess drier. At this stage a paste drier should be used or a bare minimum of cobalt drier used. Succeeding prints should be made as soon as the ink has dried sufficiently to permit handling of the sheets.
If an ink has crystallized, then an over-print compound added to the succeeding I ink to make it trap. Here the over-printed color may not adhere well even though the print may look all right; adhesion can be improved by heating.
According to Press Proof Printing ( http://www.pressproof.com) it is an condition that occurs when the ink is lacking proper varnish for the paper on which it is printed. * The dried ink film has no binding to the paper and will scuff off as a dry, chalky powder.
1. Excessive vehicle or varnish penetration
2. Stock is too acidic
3. Stock is too absorbent
4. Fountain solution pH too low
5. Ink too soft, providing no hold-out
1. Add body gum or water-repellent varnish
2. Check pH of paper, avoid highly acidic coatings
3. Avoid highly absorbent paper-drains ink solvent-use inks with higher solids
4. Adjust fountain pH to higher level (4.5-5.5
5. Increase driers to speed drying before vehicle sets into stock
*Use gloss overprint varnish to save a job that’s printed