Tips on Paper

To get the best results printing on uncoated paper, make sure you choose the right sheet for the job. Carefully consider all the functions your paper must perform. For example, do you need to reproduce critical color? The you might want a bright-white paper with good ink holdout. Do you expect people to archive your design? Then you should find an acid-free paper that won’t yellow or get brittle with age. Is the job going to be varnished or laminated after printing? Then the paper chemistry and ink should be compatible with the off-line process selected so that varnish or laminate will adhere properly. Pay particular attention to such factors as finish, color, brightness, opacity, and ink holdout.

The most common source of difficulty reproducing full color images on uncoated paper is dot gain, or the expansion of a printed dot’s size when it is transferred from plate to blanket to paper. Depending on the nature of the paper you choose, dot size can increase by 5 to 30%. One way to compensate for dot gain is to screen back the film somewhat so that the dots on film are smaller overall. Then when the press gains, the dots come back to full size. Adjusting for dot gain during the color separation process can increase contrast, improve highlights, and reduce muddying of shadow areas. If you use this technique, however, be careful not to screen back the type or you may experience some break-up. Make sure your pre-press supplier works together closely with your printer to “fingerprint” the press, and that you provide the separator and printer with actual samples of the stock to be used.

Consider reducing the line screen values of halftones, duotones, and four-color process subjects. The more open the paper surface, the more a lower line screen value will improve the result. In general, run halftones with screen of 133 lines or less and stay away from tints of 5 percent or finer, although under controlled conditions you can reproduce screens of up to 200 lines per inch and achieve even finer tints on smooth uncoated stocks.

For four-color work, consider using undercolor removal and grayscale component replacement to minimize total ink density. By using black ink to replace deep shadows in the other three process colors, you can reduce the problems caused by excessive dot gain and excessive ink density.

If you want the warm feel and texture of uncoated stock but need improved ink holdout for certain areas or images, consider applying an undercoat on the printing surface before you print the final design. Undercoats can be applied on an offset press as aqueous coatings (using special plates), on a silkscreen press with opaque white ink or UV-cured ink, or off press using hot-stamped white foil. These techniques can be especially effective with metallic, fluorescent, or enhanced inks. Pay close attention to registration on the second pass.

Waterless plates can print extremely fine detail without plugging on uncoated papers because they don’t require a fountain solution. In addition, waterless inks hold out very well on the paper surface, which makes it easier to keep colors saturated. Temperature-controlled inking, however, is usually required when running waterless or dry offset.

Work with the ink supplier on the ink’s tack. Because uncoated papers tend to lint more, ink tack should be as low as possible while still maintaining good trapping. If you must use higher-tack inks, consider slowing the press to minimize linting. Keep in mind, however, that low ink tacks are more likely to result in increased dot gain.

Consider increasing the color strength of the ink. By mixing your spot color inks with a higher relative amount of pigment or using enhanced process inks, you can use less ink and get better results. A thinner ink film, however, usually results in a higher tack.

You can add color to your job or create unique image effects by using colored or textured stock. Since most offset inks are transparent or translucent they absorb some colors of light and let other colors pass through. We see color depending on which parts of the color spectrum are absorbed by the ink and paper and which parts are reflected. Different color papers absorb different parts of the spectrum, affecting the tint of the ink. You may have to experiment a bit with slightly different ink hues to achieve the desired effect. Be sure to do an ink drawdown test before you print. Make sure the resulting color is what your client expects. Integrating textured sheets into your design can create dynamic effects at little added cost.

Consider alternative printing methods such as flexography or screen printing. These processes use opaque inks which can create much heavier color saturation than offset presses can achieve. There are tradeoffs, however. Many flexo presses cannot print high-resolution designs, although there are some on the market which can maintain 150-line screen resolutions. Screen printing can be an excellent choice for short runs, but be sure to work closely with your supplier to get the best results.