Frequently Asked Questions about Inkjet Printing

Transcription

1 Frequently Asked Questions about Inkjet Printing Q: What is inkjet printing? A: Inkjet printing is a non-impact printing process in which text and images are formed by the precise placement of really small (picoliter-sized) droplets of ink fired at high speeds from the nozzle of computercontrolled printheads. Droplets of cyan, magenta, yellow and black inks are combined to form precisely placed dots of various colors, which when viewed from a distance, composes an image. Q: What is a picoliter? A: A picoliter is a millionth of a millionth of a liter. Depending on the resolution of the printer, inkjet drop sizes range from 3 or 4 picoliters to more than 25 picoliters. Q: Why are wide-format inkjet printers more finicky to operate than desktop inkjet printers? A: Although both types of printers operate on the same basic principles, it's the width and variety of the media used in wide-format inkjet printers that create headaches. Media expansion, surface tension, and ink absorption are more complex on a larger piece of media. If the media stretches slightly or if the paper doesn't advance properly, causing noticeable blemishes in the output. Users of desktop printers also aren't likely to change media as frequently; they don't expect to be able to achieve equivalent levels of image quality and color reproductions on bond paper, backlit film, canvas, silk, and vinyl. Desktop inkjet printers also have an advantage in setting up files to print. The typical smallformat job adheres to time-honor size and orientation formats, whereas users of large-format inkjet printers struggle with tiling problems on oversized jobs and nesting issues on smaller-format jobs. In addition, most desktop-publishing-application software is designed for traditional 8.5x11 inch jobs and offset-printing ink standards. Users of wide-format inkjet printers typically have to work around these biases in application software. Q: What's the difference between a plotter and printer? A: When digital printing was in its infancy (i.e., 5 to 10 years ago), the term plotter was widely used to describe either a wide-format inkjet printer or a computer-controlled contour cutter. These days, the term plotter is more commonly used to refer only to cutting devices. (But beware! In some circles, the terms plotter and printer are still often used interchangeably.) Q: What is a piezo inkjet printer? A: A piezo inkjet printer uses one of two major types of printheads: drop-on demand or continuous flow. Drop-on-demand systems are those in which an electrically stimulated crystal changes shape, creating pressure on the ink chamber and forcing ink through the nozzles. Drop-on-demand printheads are made in Japan by Epson and in England by XXAR/MIT and are incorporated in wide-format inkjet printers from Raster Graphics, Xerox ColorgrafX, Mimaki, Roland DGA, Signtech, Vutek, and Nur (the Fresco). Continuous-flow inkjets use an electrical charge to deflect a continuous flow of ink. Different types of continuous-flow piezo inkjet systems are used in IRIS printers and the superwide NUR Blueboards. One of the benefits of piezo inkjet printheads in that they can be engineered for use with either waterbased inks or inks in which the colorants are suspended in a solvent such as oil, naphtha-alcohol, acetone, or a chemical called MEK (methyl ethyl ketone). Q: What is a thermal inkjet printer? A: As the name implies, thermal inkjet printers use heat as part of the process. In a thermal inkjet, the inks are repeatedly heated to create bubbles, which force the drops from the nozzles. Unlike piezo inkjets, which can be engineered for use with a wider variety of inks, all thermal-inkjet printers drop-ondemand systems that use water-based inks.

2 Q: What are some other differences between drop-on-demand piezo and thermal inkjet printer? A: As explained above, drop-on-demand piezo and thermal inkjets differ primarily in the manner in which droplets of ink are forced from the nozzles. This difference - the use of heat vs. the use of mechanical pressure - causes major differences in the operational life span of thermal and piezo printheads. Thermal printheads must be changed far more frequently than piezo printheads, simply because the repetitive heating cycles needed for thermal inkjet printing quickly degrades the performance of the printhead;s onboard electronics. It's interesting to note that wide-format thermal printers were adapted from desktop color printers designed for the consumer market. To make it easy for consumers to replace the printheads as often as necessary, the printhead was attached to the ink cartridge, so the printhead would be replaced each time the ink supply ran out. In contrast, wide-format piezo inkjet printers use more permanent (but more costly to replace) printheads, adapted from those originally developed for high volume, industrial inkjet printing applications such as personalization, package coding, and printing variable-data cards and tags. Although wide-format piezo inkjet printers have been hyped for their adaptability for a wider range of applications, manufacturers of thermal inkjet printers point out that piezo inkjet printheads require more space between nozzles than thermal inkjet nozzles. This affects the number of passes required to achieve a desired resolution. Piezo inkjet printers may also require vacuum pumps and ink-absorbent material to keep nozzles printing reliably. Repeated flushing of the system may be necessary to release trapped air. This head service can waste ink each time the printer is turned on. A service call is usually required to replace a piezo printhead. Q: Is a printhead the same as a print engine? A: No. A printhead is the component from which the ink is fired and is just one element of the print engine, which also includes the mechanics and firmware to control the movement and operation of the printhead across the media. When a printer manufacturer buys a printhead from Epson, Lexmark, HP, or other manufacturer, the printer manufacturer is given a certain amount of leeway to design firmware to control operating parameters such as firing rate and print modes. The printer manufacturer also engineers the paper-advance mechanisms that are so critical to the accurate placement of the dots. What all this means is that even though Roland Hi-Fi JET, Mimaki JV2, and Epson Stylus Pro 9000 all use the same Epson printhead, each company has custom-designed a different print engine to control it. For example, the print engines of the Hi-Fi JET and Mimaki were customized to fire water-based pigmented inks with a higher viscosity than the water-based dye inks used in the Epson Stylus Pro Q: Why can't I use just any ink in my printer? A: Ink is an integral part of an inkjet-printing "system" because the ink chemistry must have the viscosity and surface tension necessary to flow easily and reliably through the printerhead nozzles, but dry quickly enough for the paper to be advanced onto a take-up reel without smudging. Thermal inkjet manufacturers Encad, HP, and ColorSpan offer users a choice of at least two different types of inks engineered to work with their particular systems. Theoretically, piezo inkjet printers can handle a wider variety of inks. But most piezo inkjet printers on the market have been designed with the expectation that users will buy a printer for a specific range of applications, then stick with a single inkset for those applications. For example, Raster Graphics Arizona Digital Screen Press was designed to use 3M Scotchcal Piezo Inkjet Series 3700 solvent-based inks.

3 Q: What are the different types of inks? A: Ink is comprised of a base carrier (water or solvent), a colorant (a dye or a pigment), and small amounts of chemical additives to provide desired characteristics. Most entry-level wide-format inkjet printers use water-based inks, which are comprised primarily of distilled water, a benign solvent known as glycerin, dyes, or pigments and small amounts of UV inhibitors, drying agents, or other chemicals. Water-based dye inks are known for their exceptional color gamut and quick fading. A new breed of enduring-dye inks is extending the life span of prints created with dye-based inks, but these inks produce a smaller range of colors. Water-based pigmented inks are known for their high resistance to fading and typically produce less vivid colors than dye inks. Like pulp in orange juice, pigment particle can be anywhere from 50 to 500 times larger than the molecules in dyes, which are more like granules in Kool-Aid. Because the pigment particles remain suspended in the water or solvent, they can clog the nozzles of some printheads. Solvent-based pigmented inks combine fade-resistance with the ability to print directly on standard materials used for screen printing. But the use of solvents raises some environmental and in-shop health issues that many digital-only shops may prefer to avoid. The bottom line: There is no perfect ink. It all depends on your application. Q: Why do I need to buy a coated inkjet media? A: In theory, you could run any flexible substrate through your inkjet printer. But you'd have trouble optimizing the appearance of the final text and image because uncoated substrate does not react very well with inks. (e.g., compare a dot of ink on a napkin to a dot of ink on a film). The receptor coating on inkjet media is carefully designed to control the size of the dot, adhere the water-based ink to the substrate, and speed drying time. The back coating on the inkjet media helps keep the substrate flat and dimensionally stable as it moves through the printer. Uncoated vinyls and some other substrates can be used by wide-format inkjet printers that use certain types of solvent-based inks. Because the solvent bites directly into the surface of the substrate, a coating isn't necessary in order to adhere the ik on to the media. But different solvent-system inkjet printers use different types of solvents, and they don't work equally well on all uncoated substrates. Pre-testing media is important. Q: Should I buy a printer that is faster or has higher resolution? A: It depends. If you are doing museum-quality fine art, just a few prints a day, or ganged-up jobs that will be cut into smaller, handheld prints, resolution will probably be more important than speed. If you're doing posters that will only hang for a day or two, a high volume of prints each day, or larger banners that will be viewed from a distance, speed may be a more important than resolution. Throughput speed can also be important if you find yourself having to reprint jobs that were botched in finishing department or if color isn't managed properly. Most printers today offer a choice of print modes in which you can sacrifice higher resolution for faster speed. Creating higher-resolution images (e.g., 1440 x 1440 dpi) can be a painstakingly slow process because the printhead must traverse back and forth across the same swath of media several times, with the media advancing at ultra-itsy-bitsy steps. On all printers, various print modes may involve one-pass, two-pass, or four-pass operation, with different nozzles firing during each pass to prevent the appearance of streaks due to misfiring jets. When printer manufacturers claim their printers can print 600-dpi images and have speeds of up to 90 sq ft/hr, be aware that the up to speed is typically for a lower resolution image. If you expect to always print in the best-quality mode, be sure to ask what the throughput speeds is in that print mode. Also note that RIP software can boost throughput speeds and offer additional print modes. Conversely, choosing a printer on throughput speeds alone is relatively futile if your RIP and workstation take hours to process jobs for output.

4 Q: What's the difference between true resolution, addressable resolution, and apparent resolution? A: The true resolution of inkjet printers is measured in dots per inch (dpi). A 600-dpi printer means that each dot is 1/600th of an inch in size, placed in a 600-space/in. x 600-space/in. grid. A 300 dpi printer, means that each dot is 1/300th of an inch in size, placed in a grid with 300 x 300 spaces/sq in. An addressable resolution of 600 dpi means that dots larger than 1/600th of an inch (e.g., 1/300th of an inch) are placed in a 600 dots per inch grid. Apparent resolution isn't a mathematical measurement, but rather describes how images are perceived by the human eye. But there's much more to image quality than resolution. In theory, smaller dot sizes will produce finer details, sharper text, and smoother curves. But if the dot's aren't shaped properly or placed precisely where they need to be, resolution really doesn't matter. In actuality, some 300 dpi printers are capable of producing output that looks better than images output at 1440 dpi. You will have to look and judge for yourself. Welcome to the graphic arts. Q: What's the difference between dpi and ppi? A: Even though many industry professionals use the terms dpi (dots per inch) and ppi (pixels per inch) interchangeably, they shouldn't. Dots and pixels are not the same thing: dpi is a measurement of printed dots per inch on a paper; ppi refers to the picture elements (pixels) gathered by a scanner or viewable on a screen. There is no one-to-one correlation between the resolution of digital data (600 ppi) and the resolution of a printed image (600 dpi). Q: What are the advantages and drawbacks of printers with more than four colors of ink? A: Printers with more than four colors of ink give you either: 1) the ability to achieve smoother blends and gradations between colors, or 2) the ability to hit a wider color gamut. CMYKcLm configurations add lighter densities of cyan and magenta ink to the traditional CMYK inks. Because the lighter cyans and magentas can be used instead of printing the more conspicuous dark inks in hightlights and midtones. The end result are fill areas that are less grainy and improved pastels and fleshtones. CMYKOG inksets add orange and green to the CMYK mix, expanding the range of colors that can be reproduced on the printer. While it's true that six-color printers have more printheads to maintain and more ink chambers to keep filled, six-color inkjet printers typically don't use significantly more ink than four-color models because smaller quantities of each ink can be used to achieve equivalent image quality. However, controlling ink amounts on a six-, eight-, or 12-color printer can be difficult and requires RIP software suited for the task. One potential drawback of using a six-color printer is that it can sometimes be difficult to try yo match colors created on an earlier-generation four-color printer. Q: What's the difference between expanded-gamut, hi-fi and Hexachrome printing? A: Expanded-gamut printing refers to any more-than-four-color process that expands the range of colors that can be reproduced compared to CMYK inks. In a hi-fi scenario, lighter densities of cyans and magenta are added to the CMYK inkset. As explained above, the lighter cyans and magentas help provide smoother blends and gradations between colors and between fleshtones. Hexachrome is a sixcolor process invented by Pantone (an ink manufacturer) that adds approved orange and greens inks to the CMYK primaries. Adding orange and green expands the range of Pantone Matching System (PMS) colors that can be reproduced on a printer. Q: Why is black represented by K in CMYK abbreviation? Why isn't it CMYB? A: K stands for key. It was traditionally the reference color used to register the other process colors in printing. In inkjet printing, the proper use of black is still fundamentally important to getting good color reproduction. Black is used to reproduce text and line art, neutralize the contamination of CMY inks, add density, and reduce total ink consumption in wide-format inkjet printing. Many of these black-generation functions are controlled by RIP software for wide-format inkjet printers.

5 Q: What is a RIP and what does it do? A: From the description above, you'd think that inkjet printers are incredibly compex machines. But in reality, they are relatively dumb - just waiting for instructions on what to do. Most of the functionality of an inkjet printer is determined by the RIP (raster-image processor) software or print controller. The RIP takes an ordinary data file, sets it for the appropriate output size, and then tells the printer exactly where to place the droplets of ink on the paper. There are dozens of RIPs on the market because RIPs also provide some of the functionality needed to customize inkjet printers for specific applications. For example, Wasatch PosterMaker, Onyx PosterShop, Scanvec-Amiable's Poster PRINT and AccuPrint, and 3M Cactus RIP's are designed primarily for printfor-pay production environments. Certain Agfa RIPs or the new BEST Color RIP provide functions that enable wide-format inkjet printers to be used for imposition proofing or contract color proofs. Many RIPs geared specifically for the sign market have sophisticated design and layout programs that avoid the need to use Photoshop or QuarkXpress. Other RIPs enable wide-format inkjet printers to be networked with color copiers or film recorders. Q: Why are ink-and-media profiles so important? A: In commercial offset printing, printers use cyan, magenta, yellow, and black inks manufactured with Standard Web Offset Printing (SWOP) standards. In wide format inkjet printing, the magenta ink produced by one manufacturer can vary significantly from the magenta ink produced by other manufacturers. And how the ink ultimately appears on the print depends on the certain brightness, absorption, and reflectance characteristics of the substrate and it's inkjet-receptive coating. In order to provide proper instructions to the print engine in terms of how much ink to lay down per pass, your RIP software needs data about the color properties of your inks and your media. Color profiles (ICC and others) provide this information. Q: Why is calibration so important? A: Calibrating your printer is a way to make sure that the colors you get from your printer today will closely resemble the colors you got from your printer two weeks ago. It's a way of tweaking all of those variables that contribute to the way colors are reproduced. For instance, the performance of your inkjet printer can be affected by in-shop humidity or aging printheads. Calibration routines help accommodate for these variables, so your printer continues to produce colors that are consistent from one week to the next. Calibration is often part of the printer's firmware or included in the RIP software. Q: Why is lamination so important? A: Many inkjet prints (up to 75% by some estimates) are laminated. In addition to adding rigidity and protection, lamination adds to the perceived value of the print. Different films change the look of the print - - intensifying colors or adding satin, textured, or high-gloss finishes. Lamination also extends the range of products that can be created from a single wide-format inkjet printer. With the right combination of laminating film and application techniques, it's possible to produce an extensive menu of products for your customers without altering any of the synergistic elements in your finely tuned inkjet-printing system. For instance, without changing inks and media, you can use different finishing materials to produce floor graphics, presentation graphics, indoor way-finding signage, P-O-P displays, and tradeshow graphics. Although lamination adds to production time and requires skilled operatord, many print-for-pay inkjet shops regard finishing as an important profit center. Q: Why is laminating inkjet prints so tricky? A: Many newcomers to wide-format inkjet printing find themselves reprinting jobs ruined when the laminating film peels away from the print (delamination). This is typically caused by the wrong choice or materials, or attempting to laminate inkjet prints that are not fully dry. Sometimes prints that feel dry to the touch, may not in fact be adequately dry, due to the presence of glycol, an oily-type solvent used in nearly all water-based inks.