Ed #8 Digital Variables

Digital Defined

In essence, digital printing technology allows you to connect your computer directly to a professional print shop. You can send a project online directly to a digital press and produce thousands of copies in a single day. But that’s just one of many advantages. With most types of digital printing, there is no film making, color proofing, stripping or plate making; there is no press make-ready, no time consuming changeovers and less waste. And because the images that appear are generated digitally, they can most often be changed on the fly, just as your desktop printer reproduces the different pages of a presentation without missing a beat. Digital technology permits quick turnaround, low-quantity print runs. Books and brochures can be printed on-demand, as they are ordered, eliminating the need to print large quantities of materials and then hold them in inventory.

Most toner-based digital printing systems rely on some type of electrophotographic printing technology. Electrostatically charged particles of toner are attracted to areas of the paper that have received an electrical charge. Then the toner is fused to the paper to form the image. In color printing, toners of the three subtractive primaries — cyan, magenta and yellow — and black are laid on top of one another.

In xerographic systems, a light source scans the image and reflects it onto an electrostatically charged photoreceptive drum. The drum passes over a toner roller, and dry or liquid toners are attracted to the charged image areas of the drum. The drum deposits the toners on an electrostatically charged piece of paper or other substrate, and the toner is fixed to the substrate by heat and pressure. The image and any remaining toner particles are then erased from the drum, which is ready for its next image.

Laser printing systems also rely on electrophotographic technology. The artwork is scanned, converted into digital data and then transferred onto an electrically charged drum using either a laser or a light emitting diode. Toners are attracted to the image areas on the drum, which then transfers them to the substrate.

Rather than relying on electrophotographic technology, some digital presses use ink jets that “print” using drops of ink applied to the substrate to create the image. Drops leave the nozzle at a rate of up to one million per second, producing a glossy image with a look that comes close to that of a continuous tone photograph. Continuous jet printers use electrical charges to guide the placement of the drops on the substrate. Drop-on-demand inkjet printing applies drops of specially formulated liquid or solid inks in response to a digital signal.

Hybrid technologies combine digital and conventional offset technology. One type of hybrid technology press combines an inkjet or xerographic system with an offset press, combining the low cost of web printing with the ability to print personal information. Direct imaging (DI), also known as digital offset, presses work like a standard sheetfed offset press, but the plates are imaged and prepared right on the machine. DI presses can match the performance of high quality conventional offset presses, including the ability to handle fine screens — in fact, DI presses are sometimes used to provide a preview of how a project will look before it is printed on a large offset press. No system is perfect, however. Although digital offset presses can accept digital data directly, they do not allow for variable data, because once the plates have been prepared, they can only print identical copies of the same image.

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