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.
We will start with 3 typical offset printing problems presented and explained by Bill Kozak in his Offset Pressman (http://offsetpressman.blogspot.com) blogspot.
High Dot Gain
Ink is not the only factor to consider in this case. There are many other factors to consider relating to your plates, pressure settings and chemistry. Let’s just consider a couple possibilities that may involve the ink itself that is causing high dot gain.
Cause: Ink viscosity too low.
Solution: Consult your ink manufacturer and request change. Increasing your viscosity may make an improvement, but cause problems in other areas. Ask your supplier for help to understand all the underlying issues.
Cause: Improper ink and water balance.
Solution: Run minimum water and the minimum ink to achieve proper density. It may be that you are not able to be within a proper ink and water balance window due to the inks properties. Ask your ink manufacturer for a certificate of analysis and pay attention to values that indicate such things as water pickup.
When troubleshooting setoff problems, understanding the problem requires a thorough understanding of the drying process. With heatset printing, the flash point of the ink will have a large bearing on whether it dries properly. In coldset printing where most drying takes place through absorption and evaporation, setoff is much more prevalent and in some cases may be unavoidable. Here are some causes of setoff that relate to the ink itself.
Cause: Poor ink strength.
Solution: Get a higher pigment strength in your ink to run a lower ink film thickness. This is especially important in coldset printing. Setoff cannot be completely eliminated in this case, but sharper points on the grater printing rollers and keeping nips away from image area may help alleviate the problem. With heatset inks, ensure that oven temp and cooling is sufficient. Quite often more heat does not make the problem better, but worse. Cooling is more often a key factor in ensuring that inks set properly.
Definition: Lateral streaks are not to be confused with gear streaks. These streaks do not coincide with the teeth of the gear that drives the plate or blanket cylinder. They do however occur laterally from the gear to operator side of the cylinder.
Problem: Ink forms are set too heavy.
Solution: When ink forms are set too heavy, they will often hit the gap too hard. This will cause a bounce and hence a slightly noticeable line across the printed image. Lighten the forms to the manufacturer’s specifications.
Problem: Bad bearings in form rollers.
Solution: Check each form roller bearing thoroughly. A bearing that is starting to wear will cause skips across the plate.
Problem: Main cylinder bearings.
Solution: Much like the roller bearings, the worn cylinder bearings will cause the same problem. Inspect and replace as necessary.
Problem: Rollers are too hard.
Solution: Change rollers to something softer according to the manufacturer’s specs. Generally, form rollers, and particularly the water form, are softer than the others. When they lose their hardness, plate wear and bounce will occur. Use a durometer to check.
Problem: Blanket is loose or smashed.
Solution: Lines across the blanket can actually be smashes. Additionally, the blanket should be torqued tight enough to not roll up on the cylinder. It must be tight enough to not move on the cylinder, but not so tight as to lose blanket height.
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.