Tips & Tricks: Hue Variability from Sheet to Sheet

Error Definition and Effect
In the case of a paper delivery, one can as a rule assume that the same print result will be achieved across an entire print run. And yet, again and again we see variability in hue emerge in a production run of a single delivery. A printer has restricted opportunities to react to the differences in quality by means of the color control system. When the print quality shows constantly varying hues from sheet to sheet, the printer has no chance to provide uniform printing quality.

This error can reveal itself as a doubling of single colors and buildup of a line on the printing blanket. This error is distinctive in that the variations in hue manifest themselves according to a clear rhythm, i.e., on each fourth sheet or also on each sixth sheet.

Causes and Remedies
During paper manufacture, individual reels are cut lengthwise for web and sheet fed printing from a mother reel (drum). For sheetfed offset printing, the cutting occurs in the cross cutter. In this process, a mass per unit area of 480 gram per sqaure meter is cut out of various locations on the drum and other drum reels. The sheets cut in the cross cutter may display variable quality. These can include differences in smoothness, the strength of elasticity, or in the tendency to build up the line.

It is very helpful when the printer recognizes the cross cutter rhythm early on, and then provides the paper supplier with relevant information in order to receive subsequent delivery as quickly as possible. During paper manufacture, the reels in the cross cutter are cut "chaotically." This means, that reels with differing suitability for printing are cut from different drums and even from different locations on the same drum at the same time. For logistical reasons, paper mills are unable to resort to a "system cut," wherein neighboring drum reels and reels laid down successively one behind the other that demonstrate similar characteristics, are combined.

Case History
A brochure was printed in four-color on shiny coated paper employing a sheetfed offset printing process. During the print run, constantly varying reproduction manifested itself in a single series: one faultlessly reproduced sheet was followed by three sheets with visibly diminished ink density (illustration 1). The printer further reported that there was a build up of the line on the printing blanket.

Investigation
In order to establish the cause, pre-dampened test prints were pro- cessed from a series of the unprint- ed printing stock. The test probed the sensitivity of the line to the dampen- ing solution. At the same time, the paper strips to be tested were moist- ened with commercially available dampening solution in the dampening unit of the press. In the current exam- ple, a test print with faultless ink transfer was achieved, followed by three tests with poor ink transfer (illustration 2).

Conclusion
The investigation demonstrated that the sheets were cut in a fourth cross cutting rhythm, wherein in each instance, one reel demonstrated faultless printability and three reels indicated a pronounced sensitivity to the dampening solution. In the case of sheets, which were sensitive to the dampening solution, owing to the pull of the printer's ink the particles of the line were torn out, leaving sediment on the printing blankets and resulting in an interrupted ink transfer, with the consequence that tonal value was lost.