From oil to acrylic to rubber to soy, the modern letterpress printer has many ink options. Despite their different bases or carriers, drying times, and pigment quotient, nearly all inks available to letterpress printers today are made for offset lithography, so some special allowances must be made when using these inks in a letterpress environment.
Many letterpress printers struggle with getting good results with metallic inks. In general, metallic inks can be considered an ink which has a finely ground metal as the pigment used. Traditionally gold has been simulated with powdered bronze pigments. Today many inks are made with aluminum powder and a tinted vehicle (varnish) in order to obtain the correct color of gold intended. The aluminum gives the metallic sheen, and the color in the mix gives the special color to the inks.
Metallic inks give a truly shiny surface effect on very smooth paper. On coated stock, the effect can be quite similar to hot stamped images. As the paper stock becomes more fibrous, the effect is dulled down considerably. Many printers will use a base layer of a printed color of ink just to coat the surface of the paper prior to printing the metallic images. This does two things; it seals the pores in the paper surface by filling them with ink, and it flattens the fibers so that the surface will be smooth and will allow the metallic pigment to lie flat on the surface of the paper to provide the greatest sheen.
Metallic inks come from the ink manufacturers in two different forms. In one type, the ink is premixed with metal powder and vehicle, and it may be used directly from the original container. In the other type, the metallic pigment is provided in a separate container from the vehicle of the ink, and the two must be mixed just prior to printing. There are some driers and modifiers in inks which can discolor aluminum and bronze, and if mixed and stored, the metallic particles may tarnish or oxidize, and will not have the proper color when printed.
As with any other ink type, it is good to obtain some metallic ink to test before offering to use it on a job at hand. Deadlines do not always allow the leisure to experiment in order to obtain the best result, and failure can be costly in terms of both materials and reputation. Likewise, one should never assume that a metallic ink will perform correctly on an untested substrate.
Light Ink on Dark Paper
When light ink is used by itself, it is often the desire of the printer to create a light image on a dark background, but this is seldom very effective if the background is very dark. It may be necessary to print the ink twice in perfect register to obtain enough ink density to cover a dark color paper. Opaque white can also be added to other colors to give them a bit more opacity, but one must be careful as the color printed will also be lightened if too much white is added.
If available to the printer, hot stamping an image with colored hot stamp “foil” (may be metallic or pigmented) is the best way to obtain sharp, opaque color images on dark backgrounds. If carefully registered, however, printed white or light colored inks with multiple “hits” can give a pleasing result. It is best not to assume that the final color will appear as desired. Test the effect prior to agreeing to use it for a client or customer.
It actually takes very little ink to initially ink up a press, and most beginners tend to apply too much ink, causing problems with image quality and drying characteristics. Most printing manuals suggest starting with a “glob” of ink about the size of a pea. That would be enough to fully ink a small platen press, but not enough for a larger one, or a Vandercook or other cylinder press. It is good, however to start with too little ink and add until you have enough for the task at hand. Most printers will use minimal ink until they get the press makeready done, then add ink until the image is properly printing. This is a good general procedure as too much ink can mask some problems which should be resolved by proper makeready.
There are techniques which can be used to increase the amount of ink deposited on the paper without adding more ink to the inking system. One common method is double rolling. In this method, the press is tripped off impression for one cycle, building up more ink on the plate or typeform. This is particularly helpful when printing large solid areas of ink on the page. In the second cycle, the press is returned to impression and a sheet is fed in.
Another method is to make a double hit impression. That is, leaving the sheet on the platen or cylinder in register, and giving it another impression after ink has been distributed again to the plate or typeform. The key to quality with this method is maintaining precise register between one impression and the next. This is used quite often for large blocks of solid color, but extraordinary care must be taken when printing small copy, as a small difference in sheet placement could greatly affect the quality of the image.
Ink Drying Times
Most inks dry within a few hours. If printing on very porous paper stock, the ink may dry almost immediately, and can be handled very shortly after printing. If an ink does not cure fast enough, extra drier can be added to the ink to get it to cure more quickly, and when printing on certain substrates, like plastics and plastic-coated paper stock, special inks must be used which are designed for that particular material.
When too much ink is applied, the drying time can be greatly extended. If the ink depends on absorption for part of its curing (as do most rubber-based products), too great an ink thickness can really inhibit the drying of the ink film. When driers are added, a skin can form on top of the ink film, so the sheets may be handled, but full cure may take several days in some cases.
Inks which dry primarily by oxidation (as most oil-based inks) generally dry faster and can be handled sooner after printing. Special inks have been designed for printing on plastic and other non-absorbent surfaces. These inks dry solely by oxidation, and care must be taken that they are not left on the press during breaks in the day’s work. The ink will setup on rollers and ink disks or cylinders, and will require extensive cleaning if fully cured.
Generally, material should not be cut or folded on the day it is printed. Allowing the ink to cure overnight is generally sufficient to allow further handling of the printed sheets and further processing in the bindery.