Printing/Coloring on Aluminum Foil
The same presses and the same types of plates used for printing paper and plastic films are used for printing on foil and foil paper laminations by rotogravure, flexography, letterpress, lithography or silk screen processes. Inks for all the processes are readily available in formulations expressly made for printing on aluminum.
Rotogravure printing on aluminum foil is recognized for its quality of reproduction, color, uniformity and the wide range in tonal effects which can be achieved. This process can utilize very fine screen half tones, and also the widest possible range of colored transparent inks, having unusual brilliance. Printing by rotogravure is an economical process for large volume runs for the production of paper-backed foil labels and over wraps. The process is not generally economical for short runs because of the high cost of the printing cylinders.
Ink viscosity and depth of etch of the cylinders are somewhat more critical for foil than for paper printing. Where practical, it can be helpful to have halftone illustrations and type on separate cylinders for more even inking. Ink drying is somewhat slower on foil than on paper and about the same as on cellophane, but over-all faster than for any other process delivering similar printing quality.
The quality of printing by this method on foil is also excellent with modern inks developed for printing foil at production speeds. Quality of all types of inks, including transparent, is generally excellent. Sheet fed presses are frequently used for foil, but, with proper drying units, web fed presses also may be used successfully.
Sheet fed lithography on foil is economical for short and medium size runs because of the relatively low cost of plates and short setup time. Moreover, unrelated jobs can be placed on the same plate, where press size and color runs permit. Any weight of stock from lightweight paper backed foil to heavy carton stock can be printed. Lighter stock is, however, more critical in feeding, registering, and stacking.
Dry spray powder is needed to prevent offsetting in delivery piles. Liquid drier added in proper proportion to the fountain solution also aids in ink drying. Ink water balance requires careful adjustment. The number of sheets per lift (stack-up of printed sheets at delivery end of press) can be critical to prevent pressure offsetting, caused by excessive weight of stock. Offsetting also is affected by type of backing, stock and ink, ink coverage, and drying.
Flexography is a rotary method of printing from raised characters on rubber plates. Printing is usually roll-to-roll web, with inks that dry rapidly by solvent evaporation. The process is economical because of simplicity of press design and the ability to set up and pre-register off the press with a saving of press time. For these reasons, short runs are also economical.
Flexographic printing is primarily useful for printing line designs, but halftone reproductions in lower line/inch range is utilized for some applications. When satisfactory foil inks are used, details of operation do not markedly differ from those for paper. The same remarks concerning drying of rotogravure inks also apply to Flexography.
Letterpress can be used for printing foil-paper laminations. The quality of printing on foil with letterpress is good for line designs, but is not thought by some to equal the quality of gravure or lithography for halftones on foil.
Dry spray powder is generally needed to prevent offsetting in sheet-fed delivery piles. A kiss impression minimizes embossing of the foil stock. Cylinder packing of oiled manila tympan sheets or pressboard has been found helpful. Full-strength inks allow application of thinner ink films and thereby minimize offsetting problems. Size of lifts depends on the same factors as with offset printing.
While sheet-fed is most common in letterpress, high-quality, process-color printing on foil also is done on rotary web letter presses.
The silk screen process produces an unusually high-coverage, heavy ink lay, and is used to give unique effects and for many specialty applications. Though by no means confined to these applications, it is particularly good for small runs for posters, displays, and test purposes or sampling, and for some problem jobs. For suitable applications, it offers impressive economies, over the multiple strikes which would be required of conventional printing processes to obtain similar effects. Presses range from hand operated printing frames, to automatic sheet-fed presses with drying ovens.
Printing Inks for Foil
Since foil for many printing applications is wash-primed, or coated before printing, a basic requirement is that the inks adhere to the specific surface involved. The inks must also be able to withstand the further converting processes, and also conditions of end use. As in coating materials generally, printing inks are today available in all forms, e.g. opaque, gloss, transparent, etc.
A large number of different resins, varnishes, solvents, plasticizers and pigments are now available to the ink makers. Inks may be formulated for foil to give desired performance in the following respects.
- Matte of Satin Finish
- Color Stability
- Freedom from residual odor
- Heat block resistance
- Abrasion resistance
- Alkali and acid resistance
- Resistance to Alcohol
- Resistance to fats and oils
- Resistance to water
Ink companies make special inks for given end uses, and often for specific presses. Cooperation between the printer and the ink maker is, therefore, very important for printing on any material.
Coatings for Printing
Prime or wash coatings are nearly always used on foil that is subsequently to be printed. The major function of the wash coating is to provide anchorage for ink. Another function is to provide a barrier to prevent offsetting of undesirable materials from the paper to the foil surface, prior to printing or coating. Wash coating is commonly done in conjunction with the laminating operation as previously described. A common wash coating for rotogravure and flexographic printing is shellac applied from a dilute alcohol solution at about 0.05 dry pound per 3,000 sq. ft. Vinyl and other types are also used.
Heavier coatings, about 0.5 to 1.0 dry pound per 3,000 sq. ft. are applied for lithographic and letter press printing. These coatings are generally vinyl copolymer or nitrocellulose. The inks used must, of course, be compatible with the type of coating.
A second coating of an appropriate film-former is sometimes applied over the printing to give scuff resistance or impact lubricity.
Designing for Printing on Foil
Foil offers the artist an entirely unique surface on which can be produced a multiplicity of attractive effects, not obtainable on most other graphic arts materials. One may choose intriguing contrasts between bright metallic surfaces and muted, glowing hues. Iridescence, in the whole scale of the spectrum may also be called forth, and dramatically played against flat, opaque areas of black and white and color, in line, halftone, and massive solids.
Beyond the constant elements of good taste and design, foil demands only an understanding and a discriminating use of the dynamics of its surface. Unlike paper, foil is fluid in the play of light and shadow. It will tend to pick up the tones of its surroundings. Thus, it can shine brightly white or receded into black or gray or reflect the color imparted by any adjacent object. While this mirror-like quality is the essential excitement of foil design, it is a characteristic which must be kept uppermost in the mind of a designer, and exploited or muted with intelligence and discretion, depending upon both interaction of all elements and the purpose of the design.
The few restrictions attaching to design in foil become readily apparent as the work progresses and are easily accommodated. Overuse of open foil surfaces can fail to achieve a desired result.
Type on foil, unless fairly large, is often overpowered by the surrounding brilliance and may prove difficult to read. Reverse type, except in display sizes, is generally to be avoided directly on bare foil.
Opaque white is put down first if it is desired to avoid reflectivity in a particular element of the design; white also provides a superior surface for fine screen process work, but the foil still can be used to unique advantage by being allowed to shine through in appropriate areas of the process halftone sections of the design. Needless to say, the designer must always know the process by which the work is to be printed. As was indicated in the discussion of the various printing methods, each establishes certain limits within which the artist must work. In this connection, of course, foil does not differ from any other stock.
In the preparation of artwork and mechanicals for printing on foil, the methods for printing on any other stock are used. No special preparatory techniques are required.