- November 01, 2006, David Argent, Contributing Editor
Last month we discussed ink composition in relation to various printing methods. Now we’ll look at basic differences in ink delivery.
On the input side of the system, ink going to the press today is controlled carefully by correlated proofing methods, spectrophotometric color measurement, and automated dispensing systems. It then begins a controlled journey to the substrate. Following is a primer on the different metering devices.
Offset lithography works on a simple principle: Ink and water don’t mix. Images are put on plates that are essentially flat. The image areas are ink-receptive, and the non-image areas are water-receptive.
The water, known as dampening solution, contains chemicals to improve the demarcation between image and non-image areas. The plates are wet first by dampening solution and then by ink. The ink adheres to the image area, the dampening solution to the non-image area. The image then is transferred to a rubber blanket, and from the rubber blanket to paper. That’s why the process is called “offset.”
Since the ink film is split several times, offset inks are highly pigmented. Extremely thin-film, high-viscosity ink, which is printed from an essentially flat surface, leads to high-fidelity print. From run to run, and during a run, the amount of ink reaching the substrate is subject to variation, and operator adjustment is needed due to the changing chemistry of the dampening fluid, water take-up by the ink, and many ink film splits in rollers on the press. After the run, ink left in the press is not usable due to contamination by dampening solution and paper fibers.
Gravure printing also is simple, comprising a chromed roller with the image engraved into the surface in small cells. It is partially immersed in the ink fountain. The ink on the cylinder is wiped evenly with a doctor blade to leave ink only in the image cells. The cylinder is impressed on the substrate, and the ink empties from image cells to substrate. A simple process, with just one ink film split from an image that is flat to the substrate. The result is high-fidelity print. One source of color variation is from doctor blade wear on the cylinders causing less cell volume and the need for stronger ink. On a long run, less clear ink extender is used as the run proceeds. Re-use of press ink then becomes a complex ink management challenge due to strength variation.
Flexo is another simple process, with low-viscosity ink transferred from an engraved cylinder (anilox roller) to a resilient image plate and then to the substrate. There are two ink film splits: anilox to plate and plate to substrate. The image is in relief on the plate—a plus when printing rough substrates but a drawback in terms of image growth from impression of the printing plate to substrate. Press return ink usually can be put back in the press with little adjustment for repeats.
During the press run, each printing method transfers ink to the substrate in different ways. The amount of ink transferred is very small—from 1–5 microns. In each process it is essential to restrict variation in ink transfer to maintain quality and control waste. Often we hear qualitative statements such as, “When I change this, the ink transfers better.” Better to have quantitative data in a fixed process. The goal should be to eliminate variables in the process, so that ink transfer is fixed and print quality is predictable. The challenge is to find variables in the printing process that cause ink transfer variations—and eliminate them.
In stable processes, costs become stable. It is surprising to many in the industry that the rate of ink consumption is not measured more accurately and on a more consistent basis. Doing this not only allocates ink cost for specific jobs based on history; it also allows predictive costing for future print jobs.
David Argent has 30+ years of experience in the converting industry. He specializes in process analysis and improvement with particular emphasis on ink and coating design and performance. Contact him at 636/391-8180; firstname.lastname@example.org.