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Print Process Descriptions: Printing Industry Overview: Plateless Processes

Electronic Printing

The most important electronic processes are xerographic and laser printing. With one major exception, xerographic and laser printers operate on similar principals. In both processes an image is recorded on a drum in the form of an electrostatic charge. The electrostatic charge is then transferred to a sheet of some material, generally paper. A conductive fine dry powder, the toner, is then spread on the paper. The toner is attracted to the electrostatically charged areas of the paper, thereby converting the electrostatic image into a visual one. The paper is then heat treated to melt and affix the toner to the paper (Adams 1988; Bruno 1990; Hawley 1981).

Laser printing and xerography differ in how the image is inputted and how the electrostatic image is formed on the drum. In xerography, light reflected off a hard copy of the text or pictorial image (e.g., a printed or illustrated page) is projected on to the drum though a camera lens. In laser printing the image is inputted in digital form from a computer. A laser is then used to project the image onto the drum (Adams 1988; Bruno 1990; Hawley 1981).

The input and output capabilities of electronic printing continue to improve. For example, raster image processing has made the integration of text and graphic images much easier. (Until recently, most computer output devices formed text and graphic images as a series of dots. With raster image processing, the image is formed as a series of lines.) The resolution of laser printers is good but still falls far short of the resolution achieved with phototypesetters. To produce high quality reproductions of fine type and halftone screen images, a resolution of at least 1,500 line per inch is required. However, in 1990, the highest resolution laser printers could achieve was a density of 1,200 X 600 dots per inch (dpi) while most achieved resolutions of only 300 X 300 dpi.

Currently, electronic printing is used primarily for short-run in-plant and quick printing. Another use is for the production of proof copies of printed materials which will be printed using one of the traditional printing technologies. These proof copies are much less expensive than phototypeset proofs. In desktop publishing, electronic printing is often used to produce a camera ready copy of a document that is then printed using one of the traditional printing technologies. According to Michael Bruno, the current markets for desktop publishing include demand publishing, book review copies, college texts, workbooks, technical manuals, and parts catalogs (Adams 1988; Bruno 1990).

Ink-jet Printing

Ink-jet printers operate by spraying a pattern of individual ink droplets onto a substrate. The application of the dot matrix image is controlled by computer input. The two types of ink-jet printers differ in whether the "jet" of ink droplets is continuous or occurs only when a drop of ink is needed to form part of the dot matrix image. In continuous spray systems, an electric charge is used to deflect ink drops not needed to form the image to an ink recycling unit. In a drop-on-demand system, drops of ink are produced only when they are needed to form part of the image. Drop-on-demand systems are less complicated than continuous systems and use less ink; however, they print much more slowly (Adams 1988).

The advantage of ink-jet printing is the speed with which it can do addressing and print variable information on repetitive forms. For these reasons ink-jet printers are credited with revolutionizing

the direct mailing business. Other applications include printing bar and batch codes and printing variable information on computer letters, sweepstakes forms, and other personalized direct mail advertising as well as on payroll checks and other business forms.

Furthermore, because it is a non-impact printing process, jet-printers can be used to print on almost any surface despite the material, texture, shape, or resistance to surface pressure. Because of this versatility, ink-jet printing is used to print on substrates as varied as plastics, sandpaper, and pills (i.e., pharmaceuticals) (Adams 1988; Bruno 1990).

The major disadvantage of ink-jet printers is the low resolution of the images produced. The poor resolution is the result of at least three factors: even on the best machines no more than 300 dots

per square inch are possible; a certain percent of the dots applied are misdirected; and the dots of inks used tend to spread as they dry (Adams 1988).

Magnetography

Magnetography is similar to electronic printing except that a magnetic, and not an electrostatic, photoconductor is used. The toner must, of course, be magnetic material. Magneto-graphic printing is competitive with traditional printing methods, such as lithography, for small runs of up to about 1,500 copies. Drawbacks include slow speed, high toner costs, and the inability of currently available printers to do color process printing (Bruno 1990).

Thermal Printing

In thermal printing, an image is formed by a chemical reaction that occurs when portions of a thermal-coated paper are subjected to heat. The printing element consists of one or more heated pins or nibs. Currently thermal printers find use in facsimile machines and other office applications. A shortcoming of thermal print is that it tends to fade over time. In certain applications such as fax machines, thermal printers are being replaced by electronic printers using plain paper (GATF 1992b).

Ion Deposition Printing The ion deposition process is similar to electronic printing and other electrostatic processes. The four basic steps of the process are: 1) an electrostatic image is generated on a rotating drum using a directed array of ions; 2) toner is attracted to the latent image on the drum; 3) the toned image is transferred to plain paper by cold pressure fusion; 4) toner residue is removed from the drum by a doctor blade and the drum is ready for re-imaging (Bruno 1990).

Ion deposition printers are used in various business applications such as printing invoices, reports, manuals, forms, letters and proposals as well as in specialty printing applications such as tags, tickets, and checks (Bruno 1990).

Direct Charge Deposition Printing

In direct charge deposition printing, the image is generated by a direct voltage carried by ionized air. The process differs from ion deposition printing in that the image is projected on to a dielectric belt and not a drum. A major advantage of the direct charge deposition printers is the durability of both the dielectric belt and the imaging head which can produce up to 200,000 pages and five million pages, respectively, before replacement. This technology is used primarily for printing business forms (Bruno 1990).

Mead Cycolor Photocapsule Process

The Mead Cycolor Photocapsule Process combines microencapsulation technology used in carbonless copy paper with photopolymerization technology found in UV curable inks. The process

uses two coated materials, the Cycolor film and the Cycolor receiver sheet. The coating on the Cycolor film is embedded with millions of microcapsules that contain a liquid acrylic monomer, a yellow, cyan,or magenta leuco dye base, and one of three photoinitiators. Each of the photoinitiators is sensitive to the spectrum of visible light corresponding to the final color of the leuco dye itself. Leuco dyes are dyes which have been rendered colorless by the addition of a chemical group referred to as a color block. The color block can be removed and the appropriate color developed by reacting the dye with an acid. When the Cycolor film is exposed to colored light, the photoinitiators sensitive to the particular color cause the monomer to polymerize and harden. The contents of the unexposed microcapsules remain in a liquid state (Bruno 1990).

The Cycolor receiver sheet is coated with an acid resin that, during processing, reacts with the leuco dyes in the film to remove the color blocks and form color dyes. The receiver sheet can be either paper or a transparency. To print the receiver sheet, it and the exposed Cycolor film are brought into contact under pressure by feeding them between two rollers. The pressure breaks the unexposed microcapsules on the film, releasing the colorless leuco dyes, monomer, and photoinitiator. Subsequently, the leuco dyes react with the coating on the receiver sheet to form colored dyes and the monomer hardens as well. The result is a continuous tone color image (Bruno 1990).

Currently, the Cycolor process is used for color copiers, 35mm slide printers, color computer printers for desktop printing, and color video output for electronic imaging (Bruno 1990).

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