September 2007

How Digital Printers Work

lance around your desk. Chances are you’ve got a desktop laser printer or inkjet printer nearby your computer Gfor making prints from the document files you or others create. These printers, so ubiquitous today, Graphics revolutionized the printing industry twenty years Printing ago by giving us other ways besides offset printing to reproduce documents.

Mailing In the early days it was relatively easy to Under One Roof determine whether a document had been This feature facilitates the use of personalization printed using an offset press or a copier. But as in marketing, on-demand printing for books and TechneGraphics, Inc. digital printing technology improved and quality manuals, and just-in-time inventory management Park 50 TechneCenter reached offset level, it became clear that digital 2002 Ford Circle based on short run lengths. printing augmented and expanded the demand Milford, OH 45150 for printing in general. (513) 248-2121 How does digital printing work? Digital printing begins with creation of the Fax (513) 248-5141 Today the major difference between offset and document file – the words and images that will digital printing lies in what each can be used for, Web site: print on the page. Regardless of what program is www.techgra.com and the specific situations when one technology used to create the file or any of its components, offers an advantage over the other. File Transfer site: the file is converted into a raster graphics image. A www.tgidirect.net What is digital printing? raster is a grid of x and y coordinates on a display FTP site: The term digital printing can be broadly defined space; a raster image file identifies which of those ftp.techgra.com to include any reproduction method that uses coordinates to illuminate. electronic files and dots to produce an image A raster image file is sometimes referred to as a Email: [email protected] using ink, toner, inkjet, or other dye- or pigment- bit map because it contains information that is based imaging system. Because a digital printer directly mapped to the display grid. BMP, TIFF, re-draws the page image for each print rather GIF and JPEG are all examples of raster image file than relying on a press plate to carry the image, types. The action of converting a file to a raster Would you like a digital print requires no setup sheets and each image file is known as raster image processing or more tips? sheet can contain a different image. RIPping. To prepare for digital printing, all files Try our weekly email must be RIPped to create the bit map that will The advantage of digital printing newsletter – Tuesday guide the imaging device (usually laser or inkjet) The characteristic of re-drawing the image for Tips. To be added to to print the dots in the right place. each page means that digital printers can do two the mailing list, send a things very efficiently: print sets of multi-page As we mentioned before, digital printing devices request to: documents one set at a time; and personalize the use a variety of technologies to create the image: [email protected] image on a page to a single person or company. wet or dry toner, inkjet, and dye- or pigment-based How Digital Printers Work (continued) systems. The most frequently used are dry toner- flow, thermal drop-on-demand, or piezoelectric based printers and inkjet printers. drop-on-demand) to force the liquid ink out of its cartridge, spraying it onto the paper. Inkjet Laser printers use the pulses of light from a laser droplets are usually between 50 and 60 microns – beam to create images on a light-sensitive surface. smaller than a human hair (70 microns) but larger The images are formed from dot matrix patterns, than a toner particle. typically 240 x 240 dots per inch, 300 x 300 dots “Toner particles per inch or 600 x 600 dots per inch. When printing photographs, an inkjet printer are attracted to the using specially formulated paper can achieve near- The laser printer uses technology similar to a photographic quality. Some inkjet printers print photoreceptor, then copier, based on the principle of the attraction on paper or other substrates dispensed from rolls; transferred to paper.” of opposite electrical charges. Using the bit map this enables printing high resolution images in information from the RIPped file, the laser beam large format for products such as posters, signs, exposes an electrically charged photoreceptor, and displays. changing the charge to the opposite of the rest of the photoreceptor. Toner particles are attracted to Digital printing and paper the photoreceptor, then transferred to paper. The Papers designed for digital printing have different toner is fused to the paper by passing the paper properties than paper used for offset printing. In through hot rollers (approximately 400 degrees). particular, the paper must be formulated to react appropriately to heat, pressure, and the chemistry The heat required to fuse the toner to the paper of toners or inkjet inks. introduces some limitations to the type of stock that can be used in a laser printer. You may have had the experience of using an inkjet printer to print on a sheet of paper that Toner has always worked well in your laser printer, but Toner is a fine, negatively charged, plastic-based having the image not reproduce clearly or having powder. The plastic particles ensure that the toner the ink seep through the sheet. Or paper may eject “... the toner must will “melt” when heated by the fuser. Toner is with a bad curl from your desktop laser printer, adhere to the dots of manufactured by mixing pigment (either black the result of moisture on the surface of the paper a bit map...” or colored) with molten polymer, then cooling rapidly evaporating during the toner fusing the mixture and crushing it in a milling process. process. Yet other papers are not able to withstand This creates toner particles of between 7 and 10 the high heat in a laser printer and react by microns. cracking, curling, or improper fusing of the toner.

The size of a toner particle is relevant to the Why explain how things work? resolution of a printed image. Because the You may be wondering why we would devote toner must adhere to the dots of a bit map, it is an entire issue of Printips to a discussion of important to maintaining the resolution of the equipment. Part of the reason is that we want you image that the toner particles be no bigger than to know something about the way our imaging the dots. High speed digital production printers technologies work, since we believe the knowledge such as we have use microfine toner; this is why we will help you evaluate our recommendations when can hold a finer resolution than is possible with a we advise offset vs. digital printing. desktop laser printer. It will also help you understand why we may How does an inkjet printer work? change our recommendation based on the actual An inkjet printer uses extremely small droplets image you need to reproduce. We want you to be of ink to create the image on the paper. The fully informed so that you will always understand ink droplets are controlled by digital signals your options and make the best, most informed that use one of three methods (continuous decision. Digital Printing Tips hen designing a document that CMYK appears muddy and grayish and will be printed on a toner-based may appear fuzzy due to mis-registration digital printer, keep these tips in of the CMY. Wmind: TRICKS • Keep print coverage low on fold lines. tips • For screens, blends and gradients, use tint Cracking at the fold line is a common values with a wide range. Narrow tint problem with toner-based digital & ranges mean wider bands of tint value printing. Scoring on the fold line helps “Scoring on the fold will be used at output, which leads to prevent cracking; so does designing so line helps prevent that there is lower toner coverage at the banding. We recommend a range of cracking...” 15% to 85%. fold.

• Don’t create black type from CMYK. If Also remember that the range of papers that can printing in full color, be sure that black be used in digital printing is not as wide as offset type is set to print as K rather than as a printing. The greatest limitations are the thickness blend of CMYK. Black type created as of the paper and the kind of coating on the paper.

heat and pressure. The second is to produce a paper with the right surface and brightness to Why are the keep the image sharp, and the right opacity to Q. prevent image show-through. Finally, the digital & paper choices for digital color printing puts different demands on the sheet than digital black and white printing. Qquestions andA answers printing so limited? Coated stock in digital printing has been an “...produce a paper issue. The coating on papers produced for with the right surface offset printing can’t withstand the high heat and brightness to environment of digital printing. This once keep the image There are two primary represented a true limitation to digital printing; challenges to developing however, as customers have requested coated sharp...” paper for digital printing. stocks, paper manufacturers have risen to the AThe first is formulating. a paper that will run well demand and begun to develop coated stock for regardless of the paper path (straight or curved), digital printing, and equipment manufacturers toner chemistry and developing system, and fuser have found ways to lessen the effect of the heat. 513-248-2121 PRSRT STD Fax 513-248-5141 US Postage PAID 2002 Ford Circle www.techgra.com Permit #271 Milford, OH 45150 [email protected] Milford, OH

How Digital Printers Work

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