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Dealing with ink dispersion.

May 1, 1995 12:00 AM

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Having dealt with several ink properties relating primarily to an ink's performance on press, this month we examine an aspect that may be considered manufacturing related. However, it subsequently has considerable bearing on how well ink ultimately will behave during printing and afterward.

The subject is dispersion. Inks are a complex mixture of pigments, varnishes, waxes, solvents and other additives. During manufacturing, these ingredients are blended carefully and milled to ensure a smooth, homogenous product free of lumps or large particles.

Pigment is the principle ingredient in an ink that requires dispersion. Pigments are produced by a chemical reaction, which takes place in large vats, between petroleum-derived ingredients in an aqueous solution. Following the reaction, the resulting pigment formed falls to the bottom of the vat as a precipitate. Filtration removes most of the water, and the pigment and remaining water are collected from the filter as a press cake, a paste-like product. The press cake is placed in hot-air circulating ovens to remove remaining water, and pigment clumps are pulverized to a fine powder.

Inkmakers receive pigments in this form and, as part of the manufacturing process, disperse the powder into a suitable varnish to produce ink. However, during drying, pigment particles join in clumps called agglomerates. While the pulverizing process produces what seems to be a fine powder, many agglomerates remain and are too coarse to produce suitable ink.

This is where milling comes into play. The pigment and varnish mixture is put through various equipment depending on the type of ink and its consistency. Paste inks are put over a three-roll mill. The mill consists of three hardened steel rollers rotating at different speeds, creating a shearing action while pressing together by hydraulics. This combination of energy application breaks down pigment agglomerates to a very fine particle size--as small as two to three microns.

Other dispersing techniques include kinetic dispersion (KD), which resembles a giant high-speed blender for liquid inks such as flexo and gravure. Shot mills contain steel shot rotating at high speed. Ink is forced through the shot, dispersed and exits through a mesh screen that allows ink to pass but retains the shot in the mill.

At the end of the milling process ink takes on a smooth, silky texture, and to the casual observer may appear to be fully dispersed. However, looks can be deceiving as some large clumps can remain, creating the need for a test to determine the degree of dispersion.

Enter the grind gauge. Designed by the National Printing Ink Research Institute (NPIRI) and based on a similar model for the paint industry, the gauge consists of a steel block with two precisely engraved channels, carved deep at the top and shallower toward the bottom. A scale on the side indicates depth in microns. Ink is placed in the channels and drawn down with a steel doctor blade. As the channel gets shallower, large particles show up as scratches, and the point where they start can be read on the micron scale.

Different printing processes require varying dispersion levels. Corrugated inks and news inks can be coarse, while gravure and lithographic inks require the finest degree of dispersion. If grind gauges indicate many large particles remain, further milling is done. The grind test is one of the last quality control procedures conducted during manufacturing.

Hickeys--little voids or particles surrounded by a void in the print commonly referred to as a doughnut--are one common problem occurring during printing. When they occur, dispersion frequently is called into question. However, close inspection of the particles usually reveals they are too large to be detected using the grind gauge, since they are drawn down by the main ink mass and not shown as scratches.

Particles of this size must be detected by a wash-out process consisting of a 500-mesh screen at the end of a canister into which the ink is placed. Ink-dissolving solvent is mixed into the canister, and the ink's vehicle portion gradually passes through the screen. Ultimately, only large lumps remain, which can be isolated and examined under a microscope.

While ink-skin particles, undissolved resin and large foreign objects can be detected by the wash-out process, don't automatically attribute the problem to insufficiently ground ink. By far, paper fibers and poor housekeeping are most likely to cause hickeys.

Unfavorable dispersion can cause plate or cylinder wear, lack of gloss, plugged halftones and poor rub resistance. In lithographic printing, tinting and scumming can occur if the pigment is not well dispersed and protected by the vehicle.

In conclusion, favorable ink dispersion is essential to obtaining optimum performance on press, and careful application of the grind gauge test is important to produce top-quality ink.