The need for fast color hardcopy is often the primary justification for electrostatic plotters. Mechanical engineers can use area-fill color (up to 1,024 colors) and shading to make more realistic 3D images or to improve the display of finite-element analysis results, such as stress-strain distributions.
The electrostatic plotting process itself consists of applying a programmed voltage to a densely spaced array of nibs in a stationary writing head. As paper passes over the printhead, the nibs create electrostatic dots on the paper, which is exposed to liquid toner to give a permanent image.
Monochrome plots are produced in a single pass, color plots usually in multiple passes. Both single-pass and multiple-pass color techniques use the three or four-color process, with a layer each of cyan, magenta, yellow, and (sometimes) black. Each layer or color separation requires as much data as one monochrome image.
Multiple-pass plotters write the color separations sequentially, but single-pass plotters need all color separation information simultaneously. This requires greater processing power and more data storage. Single-pass plotters are quicker than multiple-pass units and need only one registration rather than one for each color.
Raster plotters have resolutions expressed in points per linear inch because they plot dots an entire line at a time. Electrostatic plotter resolution is generally 200 to 400 ppi. Naturally, higher resolution costs more. But because dots are printed by the line, space between lines is also a major factor in print quality. A 400-ppi print might not look as good as a 200-ppi print with denser line spacing.