Machine Design

Color Printing Uses Many Palettes

As engineering models increasingly are described by solid models, there has been a change in the character of hardcopy output devices found in engineering labs. Pen plotters are adequate for creating line drawings, but tend to be slow and run out of ink quickly if images on the page are characterized by large areas of color or solid fill.

Printing technology that is more appropriate for depicting shaded solid models include electrostatic printers and ink jets. Ink-jet printers today are increasingly popular for creating page-sized color output. Ink-jet printers capable of printing in full color can be found for between $500 and $1,000.

Ink-jet printers that provide resolutions of 720 dpi (dot/in.) are considered high-resolution devices. Typical prices start at $600. Printers designed to be used with PCs may quote two different sets of printing speeds, one for Windows, a second set when running under DOS. Under Windows, quoted printing speeds are on the order of 100 cps (characters/sec) at best. Under DOS, that figure rises to about 120 cps. Another measure of throughput is the number of color graphic pages/min (cgppm). Speeds of 0.7 cgppm are considered good for Windows performance. A good metric under DOS is 1.6 gppm.

Ink-jet printers from different manufacturers often use identical software drivers for operation under Windows. This means that control of factors such as brightness, contrast, and color saturation may be similar. Selection modes for dither patterns and media may be similar as well.

Ink-jet printers providing 360-dpi resolution cost in the $500 to $600 range. Printing speeds of 115 cps under Windows, 121 cps under DOS are available. Speeds of 0.7 cgppm under Windows are also considered good for this class of printer.

Entry-point printers start at $365. Resolution is in the 600 300-dpi range. Printing speeds available for these devices are 95 cps under Windows, 69 cps under DOS. Graphic performance of 0.4 gppm under Windows, 1.6 gppm under DOS is considered good.

Most ink-jet printers handle A-size media only, although a few B-size models are available. Maximum text and graphics resolution for these devices tends to be on the low end.

One difference found among ink-Jet printers is in the generation of the color black. The more-expensive devices usually print in four colors: cyan, magenta, yellow, and black. Low-end models may provide only the first three, then produce black by combining them. Knowledgeable graphic arts professionals say that four-color models generally produce richer blacks.

Another difference is in the type of media different printers can accept. Some can print images on card stock, and at least one manufacturer is planning a low-end printer capable of printing on cloth. Typical A-size printers can accept envelopes and transparencies.

Dye-sublimation is a relatively new technology that produces photographic quality results. It uses a transfer ribbon containing a dye that actually sublimates, or changes from a solid to a gas when heated. The gas is then absorbed into the surface of photographlike paper.

Another new color printing technology called phase-change ink jet provides image quality that approximates that of color laser printers. It uses dye-based inks diffused into wax color sticks that are solid at room temperature. The printer heats up the sticks in a reservoir. A printhead that operates analogously to that of an ink-jet head ejects droplets that solidify as they hit the paper. Because the ink is solid as it hits the paper, there is no wicking. A pressure-fusing process smoothes the solidified color dots and insures bonding to the paper or film.

Color printing with the thermal-transfer process requires four sheets of waxed ribbon for every color page. In the process, heat fixes an area of colored wax to the paper, and the ribbon is peeled away. The process is repeated four times on a sheet of paper to produce a full-color image. Mixed colors are produced by overlaying one semitransparent wax over another.

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