MULTI-COLOR PERFECTING

  Process inks for 8 and 10 color printing presses.

Eight and ten color sheet-fed perfecting offset presses have given new impetus to the offset process, as demonstrated by recent sales figures for German printing press manufacturers. The trend is being fueled by sophisticated technology and user-friendliness, together with increased productivity. The impressively high quality standard of products created with the perfecting process could not have been achieved without improving printing ink quality, and adapting those inks to printing requirements. During production, for example on an eight-color perfecting press, the challenge comes not in printing the first four inks on the first print side, but after turning, when the inks on the first side come into contact four times with the impression cylinders. There must be almost no alteration in the first side image despite this high-pressure contact. The differences between the first and perfecting sides must be minimal.

Another challenge arises in terms of the inks’ stacking characteristics, since the stack always contains superimposed sheets, freshly printed on each side.

An optimum compromise in terms of ink properties must be found in order to meet all the requirements of the perfecting process, some of them quite contradictory.

The REFLECTA^® 8000 ink series and the mineral oil-free REFLECTA^®-ECO 8500 series represent an almost ideal response to the demands of the perfecting process.

Ink consistency

Extensive experimentation has demonstrated that inks must have very good free flowing characteristics and not too much tack. Free-flow capability is important not only in terms of proper movement in the ink ducts, but also for good ink splitting in the printing units and the formation of smooth surfaces on the print. Tack influences how the sheet releases from the blanket and from the impression cylinders of the perfecting printing units after turning.

The surfaces of the impression cylinders are lightly structured in order to minimize sheet adhesion and ink setoff. An extremely thin ink film forms on the impression cylinder, and can largely prevent any marking by the coarse structure of the cylinder. The ink film must not, however, become so thick that the surfaces become sticky, since this can damage the printed image. On the other hand, the tack must be just high enough that the screen tonal value gain remains within tolerances. If these prerequisites are met, high production rates can be achieved with no impairment in quality, and with long intervals between impression cylinder washes.

Setting characteristics

Relatively slow-setting inks show the best printing performance. When fast-setting inks and absorbent papers are used, the tack of the printed ink film increases very rapidly following ink/paper contact, due to the penetration of low-viscosity binder constituents into the capillaries. After turning, this can result in ink adhesion (buildup) on the impression cylinders caused by the inks on the first-print side, causing marks on the printed image and reducing washing intervals. Excessively "sticky" ink surfaces also interfere with separation of the sheets from the blanket surface, which can cause doubling and recoiling of the sheets. A very fast-setting ink would be advantageous, however, with regard to stack ability of the printed items.

Despite these contradictory requirements in terms of setting characteristics, we have succeeded in finding an acceptable compromise that meets both demands. In any event, the subject with the highest ink coverage should be run on the perfecting side, not the first side.

Gloss

The gloss of a printed item is influenced strongly by the ink, but to an even greater extent by the printing stock itself. For perfecting on gloss-coated papers, high ink gloss is an essential quality feature. A slight decrease in gloss is unavoidable due to contact between the first-print side and the structured surfaces of the impression cylinder.

Color intensity

Practical experience has clearly shown that high color intensity is an essential prerequisite for inks used in the perfecting process. High color intensity is needed, among other reasons, in order to achieve high solid-tone densities with low ink-film thickness. That in turn has a positive effect on dot gain and stack ability.

Printing ink stay-fresh characteristics

The REFLECTA^® 8000 and REFLECTA^®-ECO 8500 ink series all have stay-fresh characteristics, meaning that the inks can remain overnight in the ink duct without forming a troublesome skin. Given the adjustment time for an eight-color press, which depends on the form being produced, the fact that the ink stays fresh on the press is an important prerequisite for this procedure.

Printing stocks

Modern perfecting presses are now capable of processing any standard printing stock, from 50 g/m² grades up to light (0,4 mm thickness) card stocks. Short-grain papers have the most stable running characteristics, since they require almost no transfer correction. Of course printing stocks also influence press speed during production runs. Matt coated papers present relatively few problems, since there are no obvious gloss differences in perfecting printing.

Good quality recycled papers also cause no difficulties. Stocks with very fast absorption and fast-setting inks both tend to have equally detrimental effects, and result in greater ink piling on the impression cylinder. A paper with good wet picking resistance is essential for perfecting, since damping is applied more frequently than on four color presses.

Damping solutions for perfecting presses

Our standard COMBIFIX^® -XL product, as well as our SUBSTIFIX^® HD designed to reduce isopropanol content, are two outstanding damping solution additives for perfecting. COMBIFIX^® -XL and SUBSTIFIX^® HD offer the following advantages:

• Rapidly establish a stable balance between ink and damping solution.

• Quick runoff of printing plates.

• Very good printing plate protection.

• Establish and maintain pH between 5.0 and 5.3.

With SUBSTIFIX^® HD it is also possible to reduce the isopropanol concentration in the damping solution to between zero and 5 volume percent. Hydrophilic rubber rollers or ceramic rollers in the damping unit reinforce this effect.

Anti-Setoff Powder

Since fast-setting inks can be used only to a limited extent in perfecting, and sheets freshly printed on both sides are always in contact in the stack, anti-setoff powder is an absolute necessity, especially with heavy printing forms. Starch powders are recommended, with particle sizes on the order of 20 mm depending on ink film thickness and stock quality. Since anti-setoff powders always have a negative effect on the rub resistance of printed items, highly rub-resistant ink series must be used, particularly in perfecting presses. Our REFLECTA^® and REFLECTA^® -ECO series meet most of these requirements.

Summary

The modern perfecting process as applied in sheet-fed offset printing places additional demands on printing inks. Our REFLECTA^® 8000 and REFLECTA^®-ECO 8500 ink series have demonstrated their qualifications in practical use. Both series offer an impressive range of application capabilities:

• Stay-fresh characteristics in the press.

• Minimal buildup on the perfecting side.

• Minimal difference between front and reserve sides when perfecting.

• Very good drying in the stack.

• High gloss.

• Very good rub resistance.

• Suitable for presses with or without turning.

Their essentially unchanging ink/damping solution characteristics guarantee stable results from the first sheet, and very little startup waste. This stability means high-quality printing. Technical Information sheets are available for each ink series. Please contact us for more information.

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