Digital Paper Properties | Manufacturing Trends | Guidelines | Digital Paper Sources

The variety of substrates available for digital printing has significantly increased from just a few years ago due to the growing popularity of digital printing and the increased level of technology now available. Digital presses are now able to accept a greater number of paper stocks such as card stock and textured papers as well as other substrates such as polyester materials, vinyl, transparencies, self adhesive label stock, textiles, and foil.

Digital Paper Properties

Compared to papers used for offset and other print processes, digital papers have some unique characteristics. Digital papers build up less static charge as the paper travels through the press. This is an important property because many digital papers are used on equipment that prints with electrophotographic imaging. During the electrophotographic process, the paper is given an electrical charge which attracts dry toners that are then heated and fused to the substrate. Electrophotography would not function correctly if the paper built up an electrical (static) charge as it traveled through the press, which conventional papers have a tendency to do. The digital paper must be charged at the appropriate time to attract the toner properly. Digital papers are also smoother than offset papers to allow for better adhesion of the toner.

Another important property to consider is the pH of the paper (acid or alkaline base). Many papers manufactured today for digital and conventional printing are alkaline based. Alkaline paper is much better than acid papers at preserving a printed piece for long periods. Anyone who has seen an old newspaper which has become yellow and brittle has seen the results of aging because of the acid base of the paper.

Several other important properties to consider when selecting a paper stock for digital print applications are:

• Finish: is the paper glossy or dull, rough or smooth.
• Uniformity: sheets that vary in their uniformity may produce inconsistent results in the print quality.
• Thickness: Most digital presses have limitations on the thickness of the substrates that can be used for printing. While limitations on thickness are becoming less of an issue as technology improves, it is best to consult with your print supplier for their capabilities before deciding on a specific paper stock for your application.

Manufacturing Trends

A few years ago, many digital web-fed presses used paper rolls that were punched along the edges so that the paper web could be transported through the press with a pinfeed system. Paper mills would send their paper rolls to converters to punch the pinfeed holes. The converters then sold the rolls to the printers.  Most presses today use a pinless transport system which has eliminated the need for the pinfed paper rolls which saves another step in the process.

Another trend that is gaining in popularity is the development of lightweight but strong paper stocks. Many organizations use lighter weight stocks in order to save on mailing costs. Financial and legal institutions can send more sheets of a thinner paper for the same price as older, thicker types of paper and yet the thinner paper must be as strong and permanent as the thicker traditional paper stocks. Several types and brands of paper have been developed that provide excellent strength, opacity, and brightness but also provide for more sheets per inch.

Digital papers with heavier weights are also being developed to satisfy customer demand. Paper sizes, such as 12 x 18 inches and 13 x 19 inches, are also in demand to accommodate the maximum size capabilities of digital printing equipment. Xerox, for example, has added a 14.33 x 20.5 inch size to its paper line specifically for the iGen3 digital press.

Guidelines

There are three potential problems that may occur with paper printed on a digital unit that do not occur with paper printed by conventional processes:

1. The path of the paper through the press: Paper traveling through a digital press may be turned in different directions and started and stopped at various intervals. These unusual movements through the press can cause paper jams and registration problems if the incorrect paper is used. The direction of the paper grain is critical to help ensure smooth operation. The grain of the paper also affects some finishing processes, so it must be a consideration for the entire production process.
2. Electrophotography process: The temperature and moisture content of the paper are critical for the paper to receive the correct electrostatic charge during the digital printing process, which will ensure proper image transfer and toner adhesion. Many digital printing companies have special areas that have the optimum temperature and humidity for paper storage and for production. Any paper used in digital presses, printers, and copiers must be properly acclimated to provide the best results.
3. Heat level: Digital papers must be able to withstand the high heat created during the process of image fusion.

Don't assume that any type of substrate that you have used with conventional printing will automatically be suitable for digital printing. Make sure you choose a substrate that has been tested and approved for digital printing, otherwise you may encounter poor results. Your print provider will be able to recommended paper and other substrates that will help make you selection easier.

Many digital printing systems use electrophotographic processes and dry toners, which use heat to fuse toner to the substrate. Some papers may not be able to withstand the high temperature required. The faster a digital press operates, the more heat is generated. This can cause paper jams, curling, and the paper may lose some of its quality, so choosing the correct paper designed specifically for digital printing equipment is very important.

Besides toner based printing, ink-jet technology is used with some digital equipment. The resulting print has more gloss than dry toners so using a glossier paper would not be as distracting. Uncoated paper also works well with ink-jet presses.

Direct imaging presses have different requirements than fully digital presses. Most of the direct imaging presses utilize waterless offset printing technology. Although the presses run conventional offset stocks, problems can arise with paper fibers adhering to the image carrier (picking) because of the waterless technology. Fibers can then be transferred to the printed areas which produces poor quality. Paper manufacturers are constantly developing paper stocks that are stronger and cleaner to help eliminate the picking problem that can occur with waterless technology. The illustration below shows the results of a digital application printed with substandard paper stock using waterless plates.

You may decide that a test run or proof may be the best way to determine if the substrate that is to be used is the best choice for the application. An advantage with digital equipment is that a proof or small test run is much less expensive than with traditional equipment. There is very little make-ready involved with digital equipment and the cost per sheet is the same regardless of the quantity, so testing various paper samples for your print project is easy. If your application has never been printed with digital equipment, it is probably a good idea to ask for a proof or test run before going ahead with the job, and in many cases, your print provider may require it.

Choosing stocks is a subjective process so there are no absolute rules when selecting paper or any other type of substrate for your project. Your application may require high contrast between the glossiness of the print and the glossiness of the paper. You just need to make sure that the substrate you choose is compatible with the digital equipment on which it will be run. Your print provider can be very helpful in choosing the most appropriate stock for the job.

Digital Paper Sources

Digital paper sources provides a complete listing of vendors and their Web sites which will help you gain additional knowledge about a particular brand or type of paper.