While impact and thermal technologies require a print head to actually contact the page surface in order to print, ink jet dot-matrix printing is a "noncontact" technology; that is, ink jet heads never come into contact with the printing surface. Instead, liquid ink is literally spray-painted onto a page. Conunercial ink jet printers use one of two approaches to spray ink: drop-on-demand, and intermittent jet. These techniques are both very similar. The dropon-demand technique requires an individual command for every dot that is ejected from an ink jet print head, much like DMI print wires that are fired by discrete pulses. The intermittent jet approach fires streams of ink (a continuous series of dots) rather than individual dots. By timing the start and duration of each ink stream very precisely, it is possible to achieve extremely fme detail at resolutions that are now exceeding 360 x 360 dpi. For the purposes of this book, ink jet print heads are assumed to use intermittent jet technology.
Inside the Ink Jet Print Head
When compared to other types of print heads, ink jet heads are perhaps the simplest and most straightforward design. The ink reservoir is provided in a prepackaged container. The ink container might come in a replaceable cassette, or it might be integrated into a throw-away print head assembly. When the ink is installed, gravity and capillary action feed the ink along narrow channels, and the ink comes to rest at each nozzle, the exit aperture, where the ink is ejected. There might be 12, 24, 50, or more depending on the vintage and complexity of the particular ink jet head. The nozzles themselves are really nothing more than fine holes (each about ~ the diameter of a human hair) drilled into a metal plate with a laser. You might wonder how liquid will stay at the nozzles without spilling out all over the place. The answer is that the ink's viscosity and surface tension hold the ink in place until it is pumped out. Figure 3-18 shows you the internal components of a throw-away ink jet print head. Finally, every ink channel is fitted with an ink pump. In actual practice, an ink pump is almost microscopic, but it is the key element in any ink jet head. Because each pump requires electrical pulses in order to run, a series of contacts will run to a connector on the print head. Like other dot-matrix technologies, each ink pump can be fired independently for maximum flexibility in image formation. The circuitry needed to time and fire the ink pumps is contained in the printer's ECU, or on a supplemental driver board located close to the carriage transport system. There are two basic types of ink pumps: piezoelectric and bubble.
Piezoelectric Pump
In a piezoelectric pump, a ring of piezoelectric ceramic material is built into an ink channel as in 3-19. When a high-energy electrical pulse is applied across the ceramic ring, its piezoelectric quality causes it to constrict the channel. This causes a sudden displacement of volume that pushes out a single droplet of ink. After the electrical driver pulse passes, ceramic returns to its original shape, and more ink i s drawn into the channel to make up the expelled droplet. Piezoelectric ceramic requires short pulses (in the 5- to 10-!ls range) at high energy levels. Pulse amplitudes can be anywhere from 70 to 200 V depending on the particular design of the channel and the type of ceramic used. One pump is required per channel, and can fire at rates approaching 5 kHz (5,000 dots per second). This is one droplet every 200 J.LS.
Bubble Pumps
Bubble pumps (used in "thermal ink jet" or "bubble jet" printers) are also widely used to generate ink droplets; they are now perhaps the single most popular technology for disposable ink jet print heads. As you see in 3-20, nozzles and channel construction are very similar to piezoelectric heads, but ceramic rings are replaced by ring heaters. An electrical driver pulse fires a ring heater. In turn, this heats ink in the immediate vicinity. As ink heats, a bubble forms and expands in the channel. When the bubble finally bursts, its force ejects an ink droplet, and more ink is drawn in to fill the void. Heated ink droplets also dry faster on paper. Although bubble pumps are fast-working devices, they are limited to firing rates of 1,000 dots per second. Ring heaters (like the dot heaters found in thermal print heads) require a finite amount of time to cool after firing. If there is not enough cooling time, ink might actually dry out and clog inside the channel. However, bubble pumps do not require nearly as much energy to operate. Early units typically used 24- to 50-V pulses, but the new generation of mobile lap-top bubble jet printers uses even lower signals. You can see the proliferation of nozzles and electrical contacts in the HP ink jet print head.
Monochrome vs Color
Because the color of an ink jet's output depends entirely on the color of its ink, the ink jet printer has become a preeminent platform for low-cost, high-quality color printing. The principles involved in a color ink jet design are virtually identical to those of monochrome printers, but where a monochrome printer merely prints black and white (the presence or absence of dots), a color printer must interpret the colors of each dot, and provide a more sophisticated signal stream that will not only drive the print head, but will drive the proper color nozzles. This marginal increase in complexity also demands a slightly more complex print head. Essentially, a color ink jet head can have three or four separate ink systems integrated into the same head; three to handle the three key colors (yellow, magenta, and light blue), and a fourth color (black) might be included. As you might imagine, cramming three or four ink systems into a single print head is no small feat, and each ink reservoir is considerably smaller than the single-reservoir (monochrome) models; thus, color print heads have a much shorter life than monochrome print heads.
Ink Consideration
The kind of ink used in ink jet printers is typically an indelible, solvent-based chemical that is resistant to drying in air. As a result, most ink jet heads can be left unattended for prolonged periods of time (often several days to several weeks) without fear of nozzle clogs due to drying. Most ink jet printers also have a type of "ink cap" sponge in the printer's carriage horn position that wipes each nozzle whenever the head reaches its home position, and covers them whenever the printer is turned off. This can be an important factor in older ink jet printers because sponges become dried and hardened with ink (as well as age), so they are less effective at keeping the print head clean. Cleaning sponge replacement should be a routine procedure when servicing older ink jet printers. Sooner or later, solvent will evaporate into the air. Evaporation begins to increase the ink viscosity inside each channel; it becomes thicker. In early stages, this can cause ink to sputter or travel off course to the page. In advanced stages, solvent might evaporate entirely, or enough to allow ink to dry and harden in the channel. This is a clog. The afflicted nozzle(s) might still fire electrically, but no ink will flow until the clog is cleared. The ink jet head will have to be cleaned or purged. Clogs might also be dislodged through normal use. Once any viscous ink is forced out through normal use, proper operation will return automatically. Of course, disposable ink jet print heads (which do not support purging) can simply be replaced.
Recycling Consideration
One of the side effects of disposable ink cartridges or ink jet print heads is that a huge amount of waste material is generated. When coupled with the relatively high cost of original manufacturer's ink cartridges, the expended ink units open a wide market for recycling. If you stroll through your favorite computer store, you will probably notice "ink cartridge refill kits" sitting near the ink cartridges, so the drive to recycle has made it to the retail level. Unfortunately, the refill kits are not compatible with all print heads, are not much less expensive than new print cartridges, and generate just about as much waste material as throwing the old ink cartridge/print head away. Chapter 7 discusses ink jet recycling in more detail, but if you plan to handle recycling on a regular basis, be sure to obtain the proper tools and materials for the job, preferably not from retail vendors. If you do not have the inclination to recycle ink jet cartridges yourself, there are a number of companies that accept and rework cartridges as a specialty.
Paper Consideration
When ink droplets leave a nozzle, they are still in a liquid form. Once a droplet reaches paper, it must dry almost immediately so that the finished page can be handled. This is not always easy to accomplish if you are using the wrong paper type. Paper must accept ink into its fibers just the right way to dry it quickly, yet leave droplets on the surface for a crisp image. If paper absorbs ink too readily, the dried image might appear light or faint (lacking contrast). This is a typical problem with standard-weight xerox grade paper. If paper does not absorb ink quickly enough, ink might remain a liquid, which can smear and smudge when touched. Although this is less common, gloss or specially coated papers might act this way. To guarantee just the right drying characteristics, there is a specially made ink jet paper impregnated with clay or solvent-absorbing chemicals that cause ink to dry quickly while leaving a clear, dark image. The best way to determine the compatibility between paper and ink is to test the printer in actual operation. Either ink or paper (usually the paper) might have to be changed to optimize the printer's performance.
Advantages and Disadvantages of Ink Jet Printing
Ink jet dot-matrix technology offers a method of "noncontact'' printing that can mark a wide variety of surfaces and paper types. Printing speeds rival any DMI printer, yet operation is very quiet. Nozzles and ink channels are incredibly small, so dot resolution can be extremely high (the head in 3-21 is used in a 300 x 300 dpi ink jet printer). Ink jet heads have no mechanical parts, so they are exceptionally inexpensive to manufacture, enjoy high reliability, and a long working life-some piezoelectric heads are rated for more than 1 billion dots. The low power requirements have made ink jet technology the forerunner of mobile, battery powered printers. Unfortunately, ink jet heads are sealed devices. If one ink pump fails, the entire head must be replaced. Costs can also add up; even though ink cartridges can be made inexpensively, they can still be expensive items by the time they reach store shelves. Given the fairly limited number of pages handled by a single ink cartridge, the per-page cost of ink jet printing can still be high. The ink itself can be a frustrating problem. Ruptured print heads or leaking cartridges can spill thick, indelible ink everywhere. Fabrics and other porous materials are particularly susceptible to permanent stains; even your skin can be stained. While contemporary commercial "disposable head" printers have become very clean, there is still a risk of ink spillage.
