A print head is a discrete device responsible for actually applying permanent print to the page surface. Four major technologies have evolved to accomplish this process: impact, thermal, ink jet, and electrophotographic. Printers are typically categorized by the particular technology that they use (e.g., thermal dot-matrix printer, ink jet dot-matrix printer, and so on). As discussed later, however, EP printers form images through a process rather than a print head. Impact technology is just what the name implies; characters, symbols, and (sometimes) graphics are literally struck onto a page surface through an inked ribbon of fabric or plastic. The force of impact leaves an ink impression of whatever was to be printed.
You might encounter two types of impact print heads: the character print head and the dot-matrix print head. Character print heads (also called daisy wheels) are the simplest and most straightforward type of print head design, as shown in 1-10. Modeled after a conventional typewriter, a daisy wheel is little more than a print wheel containing a fixed selection of preformed letters, numbers, pllllctuation, or other symbols. Each is reversemolded onto a single plastic support structure. When an ASCII character is received, the print wheel rotates so that the desired character is positioned in front of the platen. The character is then rammed against the page by a solenoid. Impact takes place through an inked ribbon, so the character's image is transferred to the page. Although the daisy-wheel printer produced some excellent print, the drawbacks were easy enough to recognize. First, the continuous spinning and ramming of the daisy wheel created a serious clatter. Also, mechanical limitations of the daisy wheel limited the printer's maximum printing speed. Constant flexing of daisy wheel "petals" frequently resulted in broken print wheels; so regular print-wheel replacement was a must. Next, the print size and font was static, so it was impossible to select different fonts or type sizes without replacing the daisy wheel (graphics were out of the question). Character printers have long since been obsoleted by dot-matrix designs, and it is highly unlikely that you will ever fmd a character printer still in service. This book will not cover character printers further. While impact dot-matrix print heads offer the versatility and reliability that daisy wheels do not, the technology also suffers from some drawbacks. First, impact print heads need a substantial amooot of energy to operate. This demands a relatively large power supply, and the print head becomes extremely hot during operation. Dotmatrix printing is not terribly fast. Per-character printing rates are faster than those of daisy wheels, but to achieve a comparablequality print, the dot-matrix head might have to make several passes to complete a single line of text. Impact dot-matrix printers also make a great deal of noise. The high-frequency chatter of continuous wire impacts can become very annoying. Thermal dot-matrix print heads overcome some of the limitations of impact print heads. A serial thermal print head is shown in 1-13. Instead of physically moving print wires in and out, a thermal print head uses an array of microscopic heater elements. An electrical pulse from driver circuits will cause a dot heater to warm very quickly. This leaves a corresponding dot on the page surface. As with impact dot-matrix heads, an array of dot heaters can be fired in any desired sequence to produce virtually any letters, numbers, or graphics. Because there are no moving parts in the print head, its operation is totally silent. An alternative to a serial (or moving) thermal print head is the line-head technique shown in 1-14. Aline-head is essentially a thermal print bar. It contains a horizontal row of dot heatersone heater for every possible dot along a horizontal line. Instead of a vertical column of dots forming full characters as the head moves across a page, an entire line of text or graphics can be formed one row at a time. Line-head printers offer the advantage of simplicity; no carriage transport assembly is needed to carry a print head back and forth. Facsimile machines typically use thermal linehead printing. Thermal print head technologies have proven to be a handy, reliable, and quiet alternative to impact print heads, but there are some serious trade-offs. Because a finite amount of time is required to heat and cool a heater element, thermal printing is a relatively slow process. Thermal printing is also limited to a single pass (such as fax printing), so the print is generally quite legible, but its overall appearance is poor. Another strike against thermal print heads is their need for heat-sensitive paper, which has proven to be fragile, expensive, and hard to handle or store. Normal paper could not be used because the temperatures required to mark normal paper would burn it. This restriction can be overcome by using a "transfer" version of a thermal print head (serial or line). Instead of the head actually contacting a page, a transfer ribbon is inserted between the two. The head heats corresponding points of wax on the transfer ribbon that melt onto the page. While this made thermal printing a bit more practical for "plain paper" printers, the technology never really gained broad acceptance. Ink jet dot-matrix print head technology evolved to offer an alternative method of quick, quiet, inexpensive printing that would provide print quality superior to impact print heads and still work on almost any type of paper. This method of printing draws liquid ink from a central reservoir, then "spray paints" the desired characters or graphics onto a page surface through a series of independent nozzles. Ink jet printing speeds can easily match that of an impact dot-matrix printer. There are two primary methods of commercial ink jet printing: piezoelectric jet and bubble jet (also called thermal jet). A piezoelectric jet print head uses liquid ink from a small, local reservoir to fill a series of ink channels illustrated in 1-15. While older ink jet heads typically provided 9 or 24 ink channels, contemporary ink jet heads can provide 50 ink channels or more. Each channel is covered by a nozzle, which is nothing more than a microscopic hole drilled into a metal plate. A small piezoelectric crystal in each channel acts as an "ink pump." A short, high-energy pulse from the printer's driver circuits vibrates the crystal, which ejects a single droplet of ink from the corresponding nozzle. Bubble jet print heads are very similar to piezoelectric jet devices, but the resonating piezoelectric crystals are replaced by small heater elements. An electrical driver pulse heats ink in a channel. A bubble forms and expands until it bursts. The force of a bursting bubble ejects a droplet of ink from the nozzle. Modern ink jet print heads offer an astounding mix of advantages. They require relatively little power (an ideal attribute for mobile printers), yet they can generate high-quality text and graphics at resolutions that rival EP printers. Ink jet operation can be slow when printing high-resolution graphics, but the low cost and disposable print head design often makes the trade-off worthwhile. Ink jet technology has also been adapted to inexpensive color printing.
