Electrophotographic Printing Technology
Electrophotography is a well established and versatile printing technology. Its first application was in 1960 when it was embodied in an office copier. The process itself bears a strong resemblance to offset lithography. The role of the printing plate is played by a cylindrical drum or belt coated with a photoconductor (PC) on which is formed a printing image consisting of charged and uncharged areas. Depending on the implementation of the technology either the charged or uncharged areas will be inked with a charged, pigmented powder known as toner. The image is offset to the paper either by direct contact or indirectly via a silicone-based transfer drum or belt (similar to the blanket cylinder in offset lithography). Early copiers imaged the material to be copied onto the photoconductor by means of geometrical optics. Replacing this optical system with a scanning laser beam, or a linear array of LEDs, which could be electronically modulated, formed the basis of today’s laser printers. As a technology it spans the range from desktop office printers (4–10 ppm) to high-speed commercial printers (exceeding 100 ppm). Although capable of E-size printing its broadest application has been in the range of 8 1 in to 17 in wide, in color and in black and white.
Printing Process Steps
Electrophotographic printing involves a sequence of interacting processes which must be optimized collectively if quality printing is to be achieved. With respect to Fig. 23.12 they are as follows.
1. Charging of the photoconductor to achieve a uniform electrostatic surface charge can be done by means of a corona in the form of a thin, partially shielded wire maintained at several kilovolts with respect to ground (corotron). For positive voltages, a positive surface charge results from ionization in the vicinity of the wire. For negative voltages, negative surface charge is produced but by a more complex process involving secondary emission, ion impact, etc., that makes for a less uniform discharge. The precise design of the grounded shield for the corona can have a significant effect on the charge uniformity produced. To limit ozone production, many office printers (<20 ppm) employ a charge roller in contact with the
FIGURE 23.12 Schematic of the electrophotographic process. Dual component evelopment is shown with hot roll fusing and coronas for charging and cleaning. (Source: Durbeck, R.C. and Sherr, S. 1988. Hardcopy Output Devices. Academic Press, San Diego, CA. With permission.)
photoconductor. A localized, smaller discharge occurs in the gap between the roller and photoconductor, reducing ozone production between two and three orders of magnitude.
2. The charged photoconductor is exposed as described previously to form an image that will be at a significant voltage difference with respect to the background. The particular properties of the photoconductor in this step relate to electron hole generation by means of the light and the transport of either electron or hole to the surface to form the image. This process is photographic in nature and has a transfer curve reminiscent of the H and D curves for silver halide. The discharge must be swift and as complete as possible to produce a significant difference in voltage between charged and uncharged areas if optimum print quality is to be achieved. Dark decay must be held to a minimum and the PC must be able to sustain repeated voltage cycling without fatigue. In addition to having adequate sensitivity to the dominant wavelength to the exposing light, the PC must also have a wear-resistant surface, be insensitive to fluctuations in temperature and humidity, and release the toner completely to the paper at transfer. It is possible for either the discharged or the charged region to serve as the image to be printed. Widespread practice today, particularly in laser printers, makes use of the discharged area.
Early PCs were sensitive to visible wavelengths and relied on sulfur, selenium, and tellurium alloys. With the use of diode laser scanners, the need for sensitivity in the near infrared has given rise to organic photoconductors (OPC), which in their implementation consist of multiple layers, including a submicron- thick charge generation layer and a charge transport layer in the range of 30 µm thick. This enables the optimization of both processes and is in wide-spread use today. A passivation or wear layer is used for OPCs, which are too soft to resist abrasion at the transfer stage. In many desktop devices the photoconductive drum is embodied in a replaceable cartridge containing enough toner for the life of the photoconductor. This provides a level of user servicing similar to that for thermal ink jet printers having replaceable printheads.
3. Image formation is achieved by bringing the exposed photoconductor surface in contact with toner particles, which are themselves charged. Electrostatic attraction attaches these particles to form the image. Once again, uniformity is vital, as well as a ready supply of toner particles to keep pace with the development process. Two methods are in widespread use today: dual component, popular for high-speed printing and monocomponent toners commonly found in desktop printers. Dual component methods employ magnetic toner particles in the 10-µm range and magnetizable carrier beads whose characteristic dimension is around 100 µm. Mechanical agitation of the mixture triboelectrically charges the toner particles, and the combination is made to form threadlike chains by means of imbedded magnets in the development roller. This dense array of threads extending from the development roller is called a magnet brush and is rotated in contact with the charged photoconductor (Fig. 23.10). The toner is then attracted to regions of opposite charge and a sensor-controlled replenishment system is used to maintain the appropriate ratio of toner to carrier beads.
Monocomponent development simplifies this process by not requiring carrier beads, replenishment system, and attendant sensors. A much more compact development system results, and there are two implementations: magnetic and nonmagnetic. Magnetic methods still form a magnetic brush but it consists of toner particles only. A technique of widespread application is to apply an oscillating voltage to a metal sleeve on the development roller. The toner brush is not held in contact with the photoconductor but, rather, a cloud of toner particles is induced by the oscillating voltage as particles detach and reattach depending on the direction of the electric field. Nonmagnetic monocomponent development is equally popular in currently available printers. There are challenges in supplying these toners in charged condition and at rates sufficient to provide uniform development at the required print speed. Their desirability derives from lower cost and inherent greater transparency (for color printing applications) due to the absence of magnetic additives.
One way of circumventing the limitations on particle size and the need for some form of brush technique is to use liquid development. The toner is dispersed in a hydrocarbon-based carrier and is charged by means of the electrical double layer that is produced when the toner is taken into solution. Typically, the liquid toner is brought into contact with the photoconductor via a roller. Upon contact, particle transport mechanisms, such as electrophoresis, supply toner to the image regions. Fluid carryout is a major challenge
for these printers. To date this has meant commercial use where complex fluid containment systems can be employed. The technique is capable of competing with offset lithography and has also been used for color proofing.
4. The transfer and fuse stage imposes yet more demands on the toner and photoconductor. The toner must be released to the paper cleanly and then fixed to make a durable image (fusing). The majority of fusing techniques employ heat and pressure, although some commercial systems make use of radiant fusing by means of zenon flash tubes. The toner particles must be melted sufficiently to blend together and form a thin film, which will adhere firmly to the substrate. The viscosity of the melted toner, its surface tension, and particle size influence this process. The design challenge for this process step is to avoid excessive use of heat and to limit the pressure so as to avoid smoothing, that is, calendering and/or curling of the paper. Hot-roll fusing passes the toned paper through a nip formed by a heated elastomer-coated roller in contact with an unheated opposing roller that may or may not have an elastomer composition. Some designs also apply a thin film of silicone oil to the heated roller to aid in release of the melted toner from its surface. There is inevitably some fluid carryout under these conditions, as well as a tendency for the silicone oil to permeate the elastomer and degrade its physical properties. Once again materials innovation plays a major role in electrophotography.
5. The final phase involves removal of any remaining toner from the photoconductor prior to charging and imaging for the next impression. Common techniques involve fiber brushes, magnetic brushes, and scraper blades. Coronas to neutralize any residual charge on the PC or background toner are also typical components of the cleaning process. The toner removed in this step is placed in a waste toner hopper to be discarded. The surface hardness of the PC plays a key role in the efficiency of this step. Successful cleaning is especially important for color laser printers since color contrast can make background scatter particularly visible, for example, magenta toner background in a uniform yellow area.
Dot Microstructure
With respect to image microstructure, the design of the toner material, the development technique, and the properties of the photoconductor play key roles. It is desirable to have toner particles as small as possible and in a tightly grouped distribution about their nominal diameter. Composition of toner is the subject of a vast array of publications and patents. Fundamental goals for toner are a high and consistent charge-to-mass ratio, transparency in the case of color, a tightly grouped distribution, and a minimum, preferably no, wrong-sign particles. The latter are primarily responsible for the undesirable background scatter that degrades the print. Recent developments in toner manufacture seek to control this by means of charge control additives which aid in obtaining the appropriate magnitude of charging and its sign. Grayscale in laser printers is achieved by modulating the pulse width of the diode laser. The shape and steepness of the transfer curve, which relates exposure to developed density, is a function of photoconductor properties, development process, and toner properties. It is possible to produce transfer curves of low or high gradient. For text, a steep gradient curve is desirable, but for images a flatter gradient curve provides more control. Since the stability of the development process is subject to ambient temperature and humidity, the production of a stable grayscale color laser printer without print artifacts is most challenging.