Color Solid Ink Printing

C. Wayne Jaeger, Ph.D., Xerox Corporation

How does a solid ink printer work?

Normally ink is thought of as a liquid. However, there is a printing technology that utilizes solid ink, also called phase change ink or hot melt ink. The names are often used interchangeably, but the term solid ink will be used in this description of the technology.

The concept of solid ink is that it is solid at normal ambient temperatures but in the ink-jet printing device, the ink is melted, converting it into a liquid that can be jetted much as any other liquid ink is handled in a piezoelectrically driven ink-jet printer (but not, of course, in a thermally driven bubble-jet printer). The real advantage of solid ink over aqueous ink is that the molten ink does not have to dry. Instead, it freezes (solidifies) almost instantaneously on the cool printing surface. This also means that solid ink does not dry out in the nozzles of the ink-jet, as aqueous inks are prone to do. In addition, solid ink does not wick into the paper as liquid inks do. It remains bound to the surface of the paper, resulting in more vivid colors and producing an enhanced color gamut.

After several attempts by various companies (Howtek, Exxon, Dataproducts, Hitachi, Spectra, Brother), Tektronix successfully developed and introduced a color ink jet printer in 1991 using solid inks. The first-generation solid ink printer had 16 ink-jets per color (cyan, magenta, yellow) and 48 jets for black. It printed an A-size page (8.5 x 11 inches) in just under two minutes. Since then, the technology has progressed to the point where the same size page can be printed at 24 pages per minute. The latest solid ink printer's resolution is more than four times the resolution (sixteen times the amount of data) of the first solid ink printer. The cost of the latest printer is less than one-tenth the cost of the original printer and further improvements are expected in costs and performance.

The first generation of solid ink-jet printers worked by printing ink directly onto the paper or transparency printing media. The printhead was rapidly shuttled back and forth across the page, as the paper was incrementally advanced upwards after each printhead pass. On each pass, a stripe 16 pixels wide was printed. (A similar strategy is still employed in most desktop aqueous-ink printers.)

The disadvantages of this approach made it quite clear that if solid ink were to succeed, a completely different printer configuration had to be developed for it. The printhead with ink weighed over 1.8 kg, or almost four pounds. The printer had to be placed on a very sturdy table to prevent them both from walking across the room as the heavy printhead shuttled back and forth. Most of the time to make a print was spent in decelerating the printhead, stopping, and then accelerating in the reverse direction. The ink drop placements going in one direction would be slightly offset from those of the ink droplets going in the opposite direction. Although the drop placement error was very small, the spatial frequency of the 16-pixels pass was in the resolution range for which the human visual system is most sensitive. To print secondary colors, two primary color droplets were overlapped, but the order in which the primary colors were printed changed when the printhead was printing in the reverse direction. For instance, printing a magenta droplet over a yellow droplet created a slightly different red than printing a yellow droplet over a magenta droplet, and this caused unacceptable hue shifts.

In addition, the gap between the printhead and the substrate to be printed must be consistent to give predictable drop placement. Printing on paper of different thickness changed the printhead/paper gap enough to produce visibly different prints. The complexity of precisely controlling the motion of the printhead and paper made it clear that in order to have better reliability, increase the speed (number of prints per minute), improve the image quality, and also decrease the cost of the printer, both the paper handling and the paper path had to be greatly simplified.

The key innovation of the solid ink printers developed by Tektronix1 starting in 1995 was the development of indirect printing. The concept was to replace oscillating motion with an ink-jet printhead that would rapidly and precisely spray-paint a complete image on a spinning drum, the print head moving axially like the cutting tool on a lathe as it deposited a spiral track. After the image is applied to the drum surface, it is offset (transferred) from the drum onto paper. This approach enabled a very simple paper path to be used, with the paper going straight through the printer in what is essentially an offset printing process. While this greatly simplifies the paper path, the indirect printing process places fairly severe constraints on the ink. The ink must be tough and hard at ambient temperature. The ink must be extremely clean and have a low melt viscosity so that it can be easily jetted through the tiny apertures of the printhead. (The printhead is intended to last the lifetime of the printer.) The ink must quickly freeze on the drum surface and stay in place on a rapidly spinning drum. Finally, the ink must easily and completely transfer from the drum to paper in the offset printing step.

The heart of the printer is an anodized aluminum cylindrical drum. A multi-aperture printhead as wide as the drum is used to precisely apply the ink droplets to the drum surface. The ink droplets are generated by a piezoelectrically driven printhead made of stainless steel. The printhead is not fully populated with apertures, but contains many spaced sets of aperture columns. Each aperture column is made up of four jets: cyan, magenta, yellow and black. The aperture columns are equally spaced across the width of the array. Each time the drum makes a revolution, each four-jet column prints any desired combination of cyan, magenta, yellow and black ink droplets on every pixel in the line over which the four-jet column passes. Each four-jet column prints simultaneously, printing parallel paths of ink droplets around the drum. In the next drum revolution, the printhead is incremented over so that the next set of drops is printed parallel to the first set. After each drum revolution, the printhead is moved over one step, until the entire image is painted on the drum. The total lateral movement of the printhead during the printing process is actually quite small and depends on the gap between the columns of each four-jet set of the printhead. Depending on the selected image quality, the drum makes approximately eight revolutions in the process of generating the image plus one additional revolution to offset the image to paper and simultaneously clean the drum and treat its surface for the next image. The keys to producing high image quality are the consistency of the ink jets and the interlace method for generating each set of parallel lines on the drum surface.

The process of printing an image on paper breaks down to three basic steps:

  1. A drum maintenance unit cleans the drum surface of any residual ink from a previous print and applies an extremely thin layer of silicone release oil to the clean anodized aluminum print drum surface.
  2. The uniformly heated (135 °C) printhead sprays microscopic drops of molten ink onto the rotating print drum very precisely. The print drum is maintained at an intermediate temperature (65 °C). The ink droplets striking the oiled print drum change almost instantly from a molten liquid to a malleable semisolid.
  3. The paper to be printed passes through a preheater into a pressure nip formed by a pressure roller and the print drum. Under heat and pressure the image transfers from the drum onto the paper in a single pass. By the time the paper exits the printer the ink has fully set and the print is immediately ready for use.
Figure 1
Figure 1. Solid ink printer with offset printing. Molten color ink is sprayed onto the drum by an ink-jet printhead.

There are no solvents and hence no drying time. The prints are completely water-fast. Because the inks are not liquid when they come in contact with the paper, the ink fuses to the paper rather than soaking into it, giving vivid colors on a wide range of papers. The order in which the secondary colors are printed is always the same, which gives consistent and predictable color. The process of printing on a drum and then transferring the image means that the drum-to-printhead distance is always the same. This consistent gap makes possible accurate and predictable drop placement, thus producing enhanced image quality.

Figure 2
Figure 2. Solid ink printer with offset printing path. Semi-solid color ink transfers from the drum onto a heated sheet of paper. The duplex path allows two-sided printing.

Solid ink technology has proven to be a good solution for office and workgroup users. One of the disadvantages of the technology is that it requires 12 to 15 minutes to be ready to make a print from a cold start. Once the printer is turned on, it is best to leave the printer on continuously. During any extended inactivity, the printer goes into a standby mode in which the temperature of the ink reservoir is allowed to drop to just above the freezing point of the ink. The printer can then be "awakened" and ready to print in just a couple of minutes. It also does not require any purging of the ink to prepare the printhead as is required from a cold startup. An "Intelligent Ready" feature of the printer learns the normal office routine and will have the printer up to temperature and ready to print when office activity begins. The "Intelligent Ready" will learn when the weekends occur and remain in the standby mode.

Because of solid ink's good image quality and low cost, photographers are now using it to generate proof sets for school pictures. Many schools are using the printer technology because of its ease of use in loading the ink sticks and supplies and because it can print on just about any paper. Solid ink technology is best, the more it is used. It is unaffected by humidity or temperature and is consistent week after week and month after month through many years of use.

Figure 3
Figure 3. Cut-away view of a solid ink printer
  1. Front Panel Display. Intuitive front panel interface eases installation, helps with the management and troubleshooting of the printer and gives local access to advanced features
  2. Paper Trays. It has large paper capacities with a 100-sheet multipurpose tray (Tray 1) and a 525-sheet main tray (Tray 2). Additional trays may be added. Supported media sizes range from 3.5" x 5" to 8.5"x14" legal size pages.
  3. Print Drum. Solid ink technology mimics the process of larger, commercial offset printing presses by spray painting the image onto the high precision drum before transferring the image to paper. This helps deliver fast print speeds while avoiding issues common to lasers, such as challenges with accurate registration.
  4. Ink-Jet Printhead. A full page width ink-jet array is a permanent printer component made of stainless steel. It has 1236 ink-jets capable of producing on-demand 24,000 drops per second per jet. In other words, nearly 30 million ink droplets per second are accurately and precisely applied to the print drum from the image.
  5. Paper Path. The simple paper path allows the use of a broad range of media weights. The built-in duplexer allows the automatic printing of both sides.
  6. Paper Preheater. Slot through which the paper passes to raise the temperature of the paper surface to make it receptive to transfer of the print image. (For simplicity, not shown in this diagram.)
  7. Ink Loader. Clean, environmentally friendly solid ink sticks are easily loaded into the printer through a top door. Modeled after the way staples are loaded into a stapler, the slots are keyed so that only the right color ink can be installed. Ink can be loaded into the printer mid-job, and unlike any other technology the printer can be "topped-off" prior to large print jobs - helping customers keep their printer up and running.
  8. Ink Melter. The ink melter is at the end of the ink load chute. (For simplicity, the ink melter is not shown in diagram.) Ink is melted on-demand. The melting of the ink is controlled by the level-sense in the ink-reservoir of the printhead.
  9. Electronic Access. Side panel access to all connections means customer does not need access to the back of the printer - allowing customers to save space.
  10. Melted Ink Reservoirs. Four reservoirs hold the melted primary colors, yellow, cyan, magenta, and black. Melted ink flows into the ink-jet printhead where the ink is precisely sprayed onto the print drum.
  11. Maintenance Kit. The only replaceable part is the maintenance kit, which comes in 10,000 or 30,000 page capacities and is easy and inexpensive for customers to replace.
  12. Waste Ink Tray. Emptying the waste ink tray is the only other regular maintenance required, and is accessed through the same side door as the maintenance kit. It is an easy and clean process.

1 Xerox purchased the Color Printing and Imaging Division of Tektronix, January 1 2000.