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Coated and Uncoated Paper:
Uncoated papers have no added layer on the surface and are completely natural; however, the surface may be sized with starch. Uncoated papers receive their smoothness in the calendaring stack, are generally more absorbent than coated papers and are available in many different textures, colors, weights and finishes. Coated papers, in contrast, have a smoother finish, aren’t very porous and may be calendered or super-calendered off the papermaking machine. Ink will, more or less, sit on a coated paper’s surface and take longer to dry—producing sharper, brighter imagines because the ink doesn’t bleed into the paper and blur the halftone screen.
Coated paper: Coating produces papers with excellent ink holdout, which is ideal for color reproduction—detail is not lost and fine text holds up well—making them a popular choice for products ranging from books and magazines to annual reports and advertising supplements. Coated papers are often called art papers and are commonly found in glossy art books and textbooks containing photographs or illustrations. They have exceptional runnability and printability, as well as a consistently high-quality surface, stiffness, bulk and opacity.
Coated Paper
Today, coated papers account for approximately 20 percent of all paper sold in the United States. Modern coatings are composed not only of pigments and binders, but also of additives and fluid (water), and modern coated papers are usually supercalendered, which subjects the coating to pressure-polishing—improving smoothness and gloss with the least reduction in bulk and light-scattering coefficient. Coated papers may be graded in different ways, including by use or by function, such as coated litho grade (water-resistant sized, one-sided coated paper) or coated book grade (suitable for reproducing halftone photomechanical illustrations).
Although coated papers can come in dull or glossy finishes, these shouldn’t be confused with varnishes or lamination. These are separate processes that are added at the end of a job. They can add an extra bit of gloss or durability to a printed piece that regular inks can’t provide. While lamination can work well with either a coated or uncoated stock, varnishes are most effective on coated stocks.
Coating can be applied by a number of different methods. At least ten types of coaters have been developed, including dip, brush, knife, roll, air-brush, spray, extruded, print, cast and strip coaters. Coating can be applied by rollers on the papermaking machine (film coating or machine coating), or by rollers or blades off the machine (conversion coating). Conversion coated papers generally have a thicker coating and are of a higher quality. Blade coating produces a matte (dull) surface. Gloss is produced by calendering and super calendering papers after they have been coated. The highest possible gloss is obtained through cast coating, which can be done on or off the papermaking machine.
Uncoated paper:
This category of papers—encompassing groups like writing, text, cover, bristol and many papers enhanced to run on large digital presses—represents the majority of papers specified by printers and designers. Uncoated papers are increasingly specified because of the understated, subtle message they’re able to convey. Uncoated grades are softer and warmer and give an approachable appearance. The challenge is to ensure that uncoated print results are able to match the performance of coated products. However, good quality uncoated papers are formulated to strike the delicate balance between ink holdout and ink.
Printing on uncoated papers:
Printing on uncoated papers can be as straightforward as printing on coated papers if separations are properly prepared. Because ink soaks into the paper, precise halftone dots spread and deform. This is called dot gain. By opening the separation, actual dot size is decreased and the space surrounding the dot is increased. These adjustments allow the press to carry more ink, increasing color saturation, clarity and contrast.
After a wet press sheet has been approved, all ink colors should be increased by approximately five points based on densitometer readings. This increase will compensate for any dryback.
It is essential to keep in mind the physical characteristics of uncoated papers when planning a printing job. Good quality uncoated papers are so smooth that it’s sometimes hard to tell that they’re not coated. Yet uncoated papers are porous, so inks, varnishes and coatings perform differently than on coated papers. New technology in the prepress area and pressroom has enabled designers to use premium uncoated papers in exciting new ways. With current prepress technology, the natural surface of these papers becomes perfect for four-color process printing.
Inks on uncoated papers:
Because uncoated papers tend to be more porous, quick-setting, positive hard-drying inks are recommended. Press stable (stay fresh) inks should not be used as they encourage retarded ink drying and abrasion on uncoated stock – especially important when the work has large solid areas of print.
-Depending on the image, better results may be achieved with the addition of fluorescent ink to magenta and yellow process colors. Fluorescents will increase the perceived reflective light of the paper surface, thus increasing brightness and clarity.
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