Print Engine Technologies

• Thermal

  Thermal transfer technologies include Thermal Wax Transfer, Dye Sublimation or Dye Diffusion, and Multi-bit Thermal Wax Transfer. These technologies are similar, differing in ink formulation, thermal head design, and required controller complexity. The individual technologies, however, vary considerably in cost-per-copy and available print quality.

  Thermal transfer technologies implement 3-, 4- or more- color imaging through multiple passes of correspondingly colored thermal transfer ink coated sheets over the receptor media. Each ink sheet must be as large as the maximum image area on the media (e.g., 14 inches for legal size). One entire ink sheet is consumed for each color printed: one each for cyan, magenta, and yellow in 3-color printing, etc. The media and ink sheets are moved sequentially their entire length in front of a thermal head, which spans the width of the media. The thermal head can be controlled to provide pinpoint heating at any point via an array of heating elements spaced to the printer’s horizontal addressability. Horizontal addressability is established by the thermal head element size and spacing, and vertical addressability by motion past the head. Wherever the head applies heat, the ink from the ink sheet changes state (solid & liquid for thermal wax or solid & gas for dye sublimation) and adheres to the media.

  Implementation difficulties stem from the registration of multiple ink sheets, media quality, heating/cooling time, spot shape and consistency, and head element density (addressability). The ink sheets are roll-fed through the printers and aligned with the media for imaging. Because each color is printed separately, the paper feed system must grip the edge of the media to prevent it from shifting significantly, thereby reducing the printable area.

  Thermal Wax Transfer, a basic thermal technology, generally allows only single-bit (binary) color. The ink is designed to melt when heated by the head, and transfers full intensity dots on the media. As with all print technologies, data must be fed to the print head in a manner synchronized with the feeding of the ink and media.

  This technology affords a reasonable tolerance in print head manufacture due to the limited sensitivity to heating level. As long as each element along the head is able to reach the melting temperature of the ink, the ink will fully transfer to the media.

  Thermal wax transfer technology yields vivid transparencies and glossy prints, paired with the simplicity of the overall process. Since ink sheets cannot be reused, the cost per page is fixed regardless of coverage. Most printers based upon this technology require the use of specially coated paper to bond better with the resolidifying ink. Drawbacks include the different feel and specular reflectivity of the paper, its cost, and the inability to use stationary, preprinted forms, and other nonstandard media. Plain paper may be used in some implementations, but at noticeably reduced image quality. It has been noted that colors adhere better to previously printed areas than to uncoated plain paper. This led to the idea of preprinting with a transparent color. Xerox (Tektronix) offered an ink roll where the first “color” is a transparent pre-coat for plain paper, followed by yellow, magenta, and cyan. The transparent color is printed (pre-coating the plain paper) wherever any of the other colors will be printed, and for a spread of one dot in all directions. This leaves the plain paper uncoated in white areas. Users did not embraced this implementation since a high quality “laser” bond is still required and there is little cost incentive. Further, since the technology uses ink that melts at a relatively low temperature, transparencies sometimes do not survive long periods of overhead projection, and abrasion resistance can be problematic.

  Dye Sublimation uses a very similar mechanism, but it is capable of producing multi-bit color depth. Rather than providing an ink that has only a threshold melting point, the inks used with this technology sublime—they change directly from solid state to a gas, allowing a controlled fraction to be transfered. This sublimation requires a larger amount of heat, and a smaller spot size is required to compensate at least partially for spot growth and edge softness inherent in the process. However, when the dye sublimation ink reaches the subliming temperature, the large heat of sublimation (amount of heat that must be added to make an exact unit mass change phase) allows finer control over how much ink is transferred. Thus, accurate, multi-bit color depth can be more easily achieved. When ink sublimes, the gas that carries the pigment coats the area (dot) being imaged. If half the amount of ink is transferred to two adjacent spots, the entire spot area becomes half saturated (as opposed to one spot becoming completely colored and the other remaining white, relying upon the user to blur the combination).

  Dye sublimation prints are prone to slight blurring, which can cause a decrease in text and fine line legibility. Also, because of the relatively large amount of heat required to transfer the ink, thin lines in the feed direction, where only a single heat element of the print head attempts to provide enough heat to produce a dark line, can often be significantly lightened or lost completely. To reduce this error, printing speed is noticeably reduced from that of thermal wax transfer printing, and electronic preprocessing is frequently added to the controller. Finally, because less pigment can be transferred in the gaseous form, colors tend to be less saturated than those produced by other output technologies. To alleviate this problem, special print substrates have been designed which more readily accept the ink and retain the pigment on the surface so that more color remains visible. The drawback is the high cost of this required media.

  Photo Sublimation is an interesting variation of dye sublimation. A cross with photography, called Pictrography, has been developed by Fuji Photo Film. Instead of heating individual resolution elements to transfer subtractive primaries, an optical exposure using red, green, and blue (additive colors) laser diodes is first created. A wateractivated thermal transfer process transfers the exposed (subtractive color) dyes to the substrate. The use of a sublimation transfer process maintains 24-bit color depth within a single spot. The focused optical exposure results in sharpness exceeding that of conventional dye sublimation, with resolutions exceeding 400 dpi.

  Inkjet

  Inkjets produce color by transferring ink onto print media. Since all colors are generated during a single pass of the paper, a print head can scan across the page, the page moves perpendicularly to implement the raster scan, and registration problems are reduced. Although registration is more controlled, issues such as spot definition, color saturation, and horizontal band misalignment can be quite significant.

  The reaction between the ink and the paper is of major importance, and several technologies have been developed which offer altogether different methods of retaining spot shape and color fidelity. Of these, Liquid and Solid Inkjet provide drop-on-demand, where ink is only used when it must be placed on the printed page. Thus, these technologies offer variable costs per page depending on the amount of ink coverage and are substantially less costly to use in some applications than in others. The Continuous Ink (Hertz) technology offers other advantages.

  Liquid Inkjet, the most common of inkjet technologies, refers to the room temperature state of the ink. Drops of ink must be propelled onto the print media. Various formulations of ink from different vendors display different abilities to resist problems such as wicking (where ink travels along paper fibers by capillary action) and absorption (where color saturation is lost by ink being pulled into and away from the surface of the paper). Two distinct technologies have been developed to deliver the ink to the paper: pulsed inkjet and thermal inkjet. In pulsed inkjet designs, ink fed to the print head is controlled by hydraulic pressure. When a drop is required, pressure is placed on the ink (generally by a piezoelectric device) and the ink responds by flowing from the inkjet nozzle. Often the nozzle itself is the source of the pressure, with the piezoelectric literally popping the ink off the head and onto the media.

  Delivery of thermal inkjet is slightly more complex. An element located in the jet nozzle heats the ink, causing internal bubbles to be formed. Once the bubbles become large enough, ink is forced out of the nozzle. This ejected ink creates a slight vacuum in the nozzle which draws additional ink into the nozzle. Liquid inkjet is the fastest growing technology in the color printer market due to it’s combination of low initial product cost, modest consumable cost, and quality approaching laser on plain paper and full photo quality on special paper. Speed is moving up, and the technoogy is even being applied for networked environments.

  In Solid Inkjet technology, the ink is a crayon-like solid at room temperature; it must be melted before it is sprayed onto a page. Special ink formulations have been developed which allow the ink to melt at a very precise temperature and solidify very quickly when dropped below that temperature. This eliminates two problems apparent in liquid inkjet: namely, those of non-uniform spot shape and color absorption.

  The ink is placed onto a page using a moving print head containing nozzles for each color. The ink hardens as soon as it contacts the paper, creating a uniform spot shape. Once the page has been covered with hardened drops of ink, a fuser applies high pressure and some heat to flatten the ink, smoothing the page surface and strengthening the bond between the ink and the paper. However, the residual roundness of the dots diminishes the transmissive saturation; additional pressure enhancers, heater rollers, and laminators are used to improve transparencies. Since almost all of the ink remains on the surface of the paper, very intense, saturated reflective colors can be produced.

  Continuous Inkjet (Hertz) is the antithesis of drop-on-demand; that is, the ink is sprayed in a continuous series of drops. When spots are not to be imaged, they are electrostatically deflected away from the media and diverted into a waste bin. The claimed advantage is that imaged spots can be finer due to the properties of a stream of ink. Compared to other inkjet technologies, continuous inkjet provides excellent spot size control using very fine jets of ink, making the overall quality of the output high. Hertz inkjet technology is represented by both Scitex’ Iris Graphics and Stork in an Edition of the CHQF Study, with Iris imaging up to 32 drops on a single 300 dpi dot and Stork imaging in binary at 762 dpi (30 dots per mm).

  Electrostatic

  Electrostatic printers create an image by harnessing the ability of static electrical charge to attract or repel toner. The image is first mapped onto a nonconducting surface in the form of electrically charged regions. Charged toner particles, the colorant used in electrostatic printing, are applied to this surface and are either attracted or repelled by the charged regions. A fusing process bonds the toner to the output media. Often the nonconducting surface is separate from the output media, requiring the additional step of transferring the toner to the media before fusing.

  Xerography, sometimes called Transfer ElectroPhotography (or TEP), creates prints on plain paper. A pre-charged photoconductor (a drum or belt that conducts only when exposed to light) is discharged by a scanning laser or LED array to create a partially discharged latent image. Magnetically controlled toner is placed in contact with the photoconductor and repelled from the charged areas. This toner image is electrostatically transferred to plain paper (or transparency), then fused by heat and/or pressure. Magnetic control usually involves small iron particles that can be attracted by a magnet. In the case of dual-component toning, iron particles are mixed in a controlled ratio with electrostatic toner particles in a developer; they separate through the opposite pulls of the magnetic and electrostatic screen at the photoconductor allowing reuse of the developer. In the case of mono-component toning, small iron particles are encased within the electrostatic particles; they are consumed.

  Color xerography works in much the same way, except that multiple colored toners must be fused to a single output page. This requires that the toners allow the transmission of light in order to provide overprint capability. One way to lay toner on top of previously applied toner is to complete the imaging, transfer and fusing of one color before beginning the next. In order to improve print speed, however, multiple color toners are combined during the stages prior to fusing. If combined at the imaging stage, toners must not interfere with one another, either optically or electrostatically. Combination during transfer is a more common approach which provides a surface, perhaps the output media itself, as a repository for singlecolor images which are placed directly on top of one another. If an intermediate surface is used, the images are transferred to the output media before fusing takes place.

  The details of implementing dry-toned color create some difficulty in retaining color purity in each developer station. Since the process is based on static retention of toner particles, it becomes difficult to completely remove excess toner from one stage to totally eliminate contamination of the next. Maintaining edge sharpness as colors build one upon another is also difficult. Color electrophotography has the potential for increased quality by emulating multi-bit color depth. Like many technologies, it is difficult to obtain many intermediate shades with electrophotography, due to the threshold nature of the process (a high process gamma) and a significant process sensitivity to temperature, humidity, and barometric pressure. However, the threshold nature is adaptable to high incremental resolution. Therefore, if the timing of the scanning laser beam is controlled to a small fraction of a normal resolution dot, the size of the toned spot may be correspondingly controlled. Control to fractional steps of a resolution dot (but never less than one dot) is the basis for Hewlett-Packard’s Resolution Enhancement technology (REt). Control within a single resolution dot to one part in 256 would be equivalent to screening at the full resolution with 8-bit color.

  In practice, a small matrix of spots may be treated as a single halftone cell so that the requirements on a single spot are relieved. Since the laser scans in one direction, it is most efficient to construct the halftone cell matrix as a 1 by N matrix; that is, one spot perpendicular to the scan, by 2 or 3 spots in the direction of scan. For example, at 400 dpi a 1 by 3 cell generates a magazine-quality 133-line screen, and a 1 by 2 cell generates a 200-line screen that is difficult to notice without a magnification aid. Canon and Xerox use such techniques. It is, of course, possible to combine such “contone” techniques with conventional screening or dithering. Xeikon’s DCP-1 achieves 16- shade contones at 600 dpi, which are subsequently screened at 150-lines to achieve full 24-bit color depth. This yields near-photographic image reproduction. Note whether or not screening is explicitly referenced, the same results, that of multiple high resolution spots providing a full range of color, is obtained.

  Liquid TEP. While xerography uses dry toners, it is possible to implement a variation of transfer electophotography using liquid toners. Instead of delivering toner particles using magnetic control with iron particles, very small toner particles may be suspended in a liquid dispersant (similar to direct electrophotographic dispersants) and are brought in contact with the photoconductor. This method allows the average particle size to decrease from 5-10 microns for dry-toned systems to the submicron range, allowing multi-thousand dot-per-inch resolutions. However, dissipating the carrier fumes can be problematic, and achieving adequate density requires a delay during developing (slower throughput.)

  ElectroInk is a novel high-speed solution to this problem developed by HP/Indigo after working on the problem for more than a decade. A rough tentacular particle shape allows fast developing of liquid toner. To accomplish this, however, a two micron, tentacular particle size is employed. With this particle shape, there is a strong tendency for ElectroInk to adhere to itself, allowing rapid, complete purging of the toning station and subsequent use by another color. Imaged onto an offset blanket, this property along with some heat allows the image to be peeled off and transferred to the final substrate. The liquid toner has many similarities to offset ink, with potentially lower noise levels than dry toning systems.

  Direct Electrostatic printing uses a stylus to apply charge directly to the print media. A dielectric coated media is used to store the charge pattern and provide a suitable surface for toning. Charged toner particles are suspended in an electrically inert dispersant and swept across the page; the toner adheres to the appropriately charged regions and is repelled from those of opposite charge. Excess toner is removed before the paper passes by the stylus for the next color. Implementations may use multiple styli and/or toning stations for different colors. Liquid toner has the advantage over dry toner for this application because it uses finer toner particles and is simpler to design for large format printing. Drawbacks in using liquid toner include possible ventilation and chemical disposal problems caused by the complex solvent used, a low maximum density, and a need for special dielectric coated paper.

  Direct Imaging, first demonstrated by Océ at the 1996 CeBIT exposition, is another variety of electrostatic printing technology. Wires are embedded circumferentially along a cylinder at the print engine’s base resolution, such as 400 dpi, and brought in contact with a dry toner development station. Toner is attracted or repelled from each wire depending upon the voltage applied to it. The cylinder is rotated an amount corresponding to the orthogonal resolution and the voltage is changed as desired. Thus, a toner image is created on the cylinder. This process is implemented for each color separation component which are combined in tight register onto a transfer drum. The color image is then transferred to plain paper and fused as in electrophotography. In OcOcé’s implementation, only one color can be transferred to any individual resolution element on the transfer drum. Therefore, the CMYK primaries cannot be combined to form the RGB secondaries. OcOcé therefore adds RGB primaries to achieve the full eight-color binary requirement. This single color per resolution element approach allows a thin layer of toner which may be fused by pressure and only a little heat.

  Others

  In Photographic Film Recorders, an imaging device, such as a laser (LED or CRT), provides the light to expose color slide or print film—silver halide-based—to record the image. The specific technique may employ either a monochrome light source used once for each of three color filters, three colored scanning lasers, or a color source. Some film recorders incorporate full chemical processors, delivering finished films or photographs in minutes. Since images are formed by combining light, film recording devices use the additive primaries of red, green, and blue and can take advantage of the wide color gamut of photographic film. Depending on the type of film—and the optical system resolution—a very high resolution image can be created. Recent advances in speed and cost-per-print have been significant. This technology is used extensively to create consumer photographic prints, I has also been used to produce microfilm, 35 mm slides and large-format positives.

  Digital Offset Plates. Presses print by transferring ink to plain paper; color offset uses CMYK inks. The page image is put on the press via printing plates, one for each color separation. Plates may be made on various substrates, including metal, plastic, and paper. The ink is transferred from the plate to a rubberized offset blanket, and then to plain paper mounted on a rotating cylinder. There are both continuous web and cut sheet designs: each use one cylinders per color, with the paper being transferred from one to the next within the machine. Plates are created using various technologies. While the immiscibility of oil and water allows oil-based inks to stick to the plates only where there is image, waterless plates have recently gained significant interest. On the other hand, the trend from molten lead reliefs of mechanical type slugs to photographic “cold type” technologies is quite mature. Laser imagesetters—capable of setting pages with unrestricted image sizes integrated with type—have generally replaced CRT and flash/mask typesetter designs. PostScript now accounts for a significant majority of current imagesetters. Direct PostScript-to-Plate imagesetters are gaining momentum.

  Digital Offset Presses. If such a PostScript-to-Plate imagesetter is mechanically integrated with an offset press, digital PostScript is converted to an ink-on-plain-paper printed image in a single machine. However, for color printing, four plate-making stations—one per color—might be required. A cost-effective design would share a single platemaking function and automate plate mounting, cleaning and color control. While such a complex piece of equipment represents a high capital investment, it is capable of both very high multi-copy speed (uncollated copies of the same image) and very low cost-per-copy. Presstek has pioneered this technology.

  The Dry Silver process uses a silver-based photosensitive emulsion coated on either a paper or polyester base imaged with red, green, and blue light. Like conventional color photo emulsions, it is a multilayered product. However, heat rather than wet chemistry is used to develop the image. This affords advantages in product simplicity, cost and maintenance. However, implementations to date have yielded prints that are unstable and deteriorate relatively rapidly with normal environmental light or heat. Few applications have found the cost/quality benefit sufficient to make this technology more attractive than either film recording or dye sublimation.

  Cycolor is a unique imaging process based on micro-encapsulated toners—related to the technology used in “carbonless” paper. Cycolor uses three wavelengths of light to expose treated paper and a pressure-based “developer” to image the toners. The Cycolor paper contains eight micron capsules, equally distributed for each of the three colors. Contained within every capsule is a photosensitive material and correspondingly colored dye. Exposure to the specific wavelengths of light causes capsules to harden. These selected capsules are then crushed by rollers, releasing the dye onto the paper to form the image. The process is capable of multiple intensity levels (by requiring more or less intense light to cause more or less intensely-colored capsules to harden) but prototypes failed to create a smooth image. The high pressures necessary created noisy images and significantly limited throughput. Although machine cost is not very high, cost-per-copy is not very attractive. Drawbacks in the areas of imaging speed, limited color saturation, and the impact of process noise on achievable resolution and color depth limit the technology’s applications. Initially targeted towards copy machine applications, two printers in development were canceled.