Electrophotography Overview

Howard Mizes, Dan Hays, and Nancy Goodman Xerox Corporation

History and Overview of Electrophotography

The electrophotographic process was invented by Chester Carlson in 1938. The process is also referred to as xerography, from the Greek words for dry and writing and from which the name of Xerox Corporation was derived. It is the creation of a visible image using electrostatic latent images in the form of surface charge patterns on a photoconductive surface. The visible images consist of fine particles called toner. Early development work was performed at Battelle Memorial Institute and Haloid Corporation. Haloid become Xerox, which commercialized the copier.

Electrophotography is based on two natural phenomena: materials of opposite electrical charge attract and some materials become better conductors of electricity when exposed to light. Carlson invented a six-step process to transfer an image from one surface to another using these phenomena, illustrated in figure 1. First, a photoreceptive surface is given a negative electrical charge, represented by the gray areas (a). The photoreceptive surface is then exposed to the image of a document (b). Because the illuminated sections (the image areas) become more conductive, the charge dissipates in the exposed areas. Negatively charged toner particles spread over the surface adhere to the discharged image areas (c). A sheet of paper positively charged on the backside is placed over the image and the negatively charged toner is attracted to the paper as it is separated from the photoreceptor (d). Heat fuses the powder image to the paper, producing a copy of the original image (e). The last step is to clean any residual toner from the photoreceptor so the process can be repeated (f).

The first commercial xerographic copier, the Xerox 914, made seven 9"×14" copies per minute in black. Higher end copiers now produce images more than twice that size at over 100 pages per minute in full color with a digital image and multiple sensors and actuators to control the xerographic process. Xerography is now a business with revenues over $100 Billion per year and employs over 500,000 people.

Photoreceptor

In contrast to ink jet printers where the image is written directly to paper, in xerography, the image is first written as a pattern of charge on a photoreceptor. A photoreceptor is a thin organic material coated on a conducting substrate and is shown in a side view in figure 2. The simplest photoreceptor consists of three layers: a metallic ground plane, a charge generation layer, and a charge transport layer. For this type of photoreceptor, the top surface is coated with a negative charge as described in the next section. The metallic surface is held at a ground potential and forms positive countercharges. This creates a strong electric field across the charge transport layer.

The charge transport layer is a polymer matrix that contains a high concentration of molecular additives that can transport charge in a high electric fieldx. The charge generation layer contains molecules that will ionize when exposed to light. When the charge generation layer molecule is ionized, the electron will move into the ground plane. The hole (the absence of an electron) will jump from molecule to molecule through the charge transport layer until it combines with charge from the surface and both disappear. In this way, the surface charge can be controlled by the amount of light exposure.

Image Source

All modern copiers capture the image digitally, by sweeping a linear array of photodiodes across the paper. The charge collected by each photodiode is proportional to the reflectance of each area of the paper. A reflectance map of the image on the paper is built up in a single sweep. Color images can be captured by exposing the image with colored light or by using colored filters in the linear array. The fastest color scanners have three rows of photodiodes with each row covered with a red, green or blue filter. The captured digital image can be manipulated with image enhancement software to improve the image quality. Because the image is captured digitally, modern copiers also can operate as printers and faxes in which case the digital image is obtained from a computer or phone, respectively, rather than from a scanner built into the machine.

Charging Subsystem

The purpose of the charging subsystem is to uniformly charge the photoreceptor. The voltage from the charge is on the order of 500 volts. This voltage level is required to obtain a strong enough electric field to attract the toner particles used in xerography.

One technique to apply a uniform sheet of charge to a photoreceptor is to generate the charge from the air. One example of a device that uses air ionization is a corotron. The corotron consists of a long wire surrounded by a metallic shield, shown from the side in Figure 3. The wire is the red dot in the middle and the shield is the blue structure surrounding it. The photoreceptor is shown below the corotron. A negative high voltage of a few thousand volts is applied to the wire. A strong electric field forms between the wire and the walls of the corotron. The electric field lines also extend to the surface of the photoreceptor.

Free ions that are normally present in air are attracted to the wire while the free electrons are repelled from the wire. The rapidly moving electrons and ions inside the shield collide with air molecules, ionizing them. This creates more charged particles and a corona rapidly forms. Electrons continue to follow the electric field lines to the photoreceptor until a uniform charge builds up. As the photoreceptor passes under the corotron, the uncharged areas acquire a charge which results in a uniformly negatively charged surface as the photoreceptor exits from under the corotron.

Exposure Subsystem

One technique for writing an electrostatic image is to use a laser raster output scanner or ROS shown in figure 4. The simplest ROS consists of a single light emitting source. The light beam is focused onto a polygon mirror. A polygon mirror consists of a 6 or more flat surfaces or facets arranged around a cylinder. When the ROS is operating, the polygon is rotating rapidly at speeds on the order of 20,000 revolutions per minute. The laser beam is reflected off the polygon and onto the photoreceptor. As the polygon rotates, the laser beam moves from one sideof the photoreceptor to the other. A lens system (not shown) ensures that the beam remains in focus as it traverses the photoreceptor. When the laser goes past the edge of the photoreceptor, a new facet rotates in front of the laser and the beam starts sweeping again across the photoreceptor. During this time, the photoreceptor has moved slightly so the new beam sweeps along a different but parallel path. A video signal from the digital image controls turning the laser on and off, and in this way the photoreceptor is exposed to form a latent electrostatic image. For higher speed imaging, two or more lasers can be simultaneously imaged on the photoreceptor and can sweep in parallel to write the image. Alternate methods of exposing the photoreceptor include using a full-width array of LEDs.

Toner

The electrostatic image on the photoreceptor is developed with a development subsystem that contains charged toner particles. Figure 5 shows high magnification images of typical toner particles (about 5-10 micron diameter) used in xerography. Toner particles are small enough so that individual particles cannot be perceived in an image, but large enough so that they can be controlled to move from surface to surface with electric fields.

Conventional toner particles can be created by grinding clear polymer pellets with colorants and chemical additives to give the desired color and triboelectric charging properties. An alternative way of producing toner is to use a chemical process of emulsion aggregation, where toner particles are grown from a liquid solution. Emulsion aggregation can produce particles ranging from potato shape to quite round particles with a more uniform size distribution.

The clear polymer of the toner particle is chosen so that the toner will triboelectrically charge to the desired polarity when contacting the surfaces it encounters. The polymer is also chosen so that it melts at the desired temperature during fusing. Internal waxes can be added to lower the toner's adhesion to the fuser roll and allow the toner to fix to the paper. Charge control agents are molecules added to the toner which control the sign, level, and the rate of triboelectric charging. Surface additives are nanometer sized particles that adhere to the toner surface. They improve the flow of toner by decreasing its adhesion to surfaces and are also used to control the toner triboelectric charge. Toner is typically triboelectrically charged by mixing it with larger carrier beads. The mixture is called developer. In some cases the toner is triboelectrically charged against one or more rolls in the development subsystem without the use of carrier beads.

Development Subsystem

Development occurs due to an electrostatic attraction between the toner and oppositely-charged areas on the photoreceptor. The development subsystems electrostatics can be adjusted so that development can take place in either the charged areas or the discharged areas of the photoreceptor. Discharged-area development is preferred for digital copiers and printers since power dissipation in the light source is much less since the image areas typically comprise only 6% of the total document area.

For digital copiers and printers, areas of the photoreceptor that are discharged by light exposure are developed by toner with the same polarity as the charged photoreceptor (typically negative). By setting the voltage on the development system at a level near the charging potential of the photoreceptor, toner from the development system is attracted to the photoreceptor in the image (exposed) areas.

The function of the development subsystem is to present charged toner particles to the latent electrostatic image on the photoreceptor so the toners can selectively adhere to the discharged areas to form an image on the photoreceptor. There are many ways to perform this function, depending on the cost, size, and image quality required for the development subsystem. One option is two component magnetic brush development, shown in figure 6.

In figure 6, toner is added as needed from the toner dispenser into the development sump. An auger mixes the toner with larger (about 70 micron diameter) magnetic particles called carrier. The toners are charged by the phenomenon of triboelectricity (often referred to as static electricity) and adhere to the carrier.

Inside the development sump, a magnetic brush of developer is formed on a rotating sleeve surrounding permanent magnets. The developer mixture of toner and magnetic carrier beads is attracted to the magnets and picked up by the sleeve. The toned magnetic carrier beads form chains that are metered to the desired height by the doctor blade as the sleeve rotates. A magnetic brush of developer is rubbed against the photoreceptor. Toner is electrostatically attracted to the discharged areas of the photoreceptor, but repelled from the charged areas. In this way, the latent image is developed. Following development, the carrier is returned to the development sump where it acquires fresh toner.

Transfer Subsystem

The developed latent image is, at this point, ready to be transferred to a sheet of paper or other medium. The transfer process is illustrated in figure 7. The medium comes in intimate contact with the toned photoreceptor. The transfer corona unit behind the medium gives the medium a charge opposite that of the toner and strong enough to overcome the toner's adhesion to the photoreceptor. A second precisely controlled corona charge unit reduces the electrostatic adhesion of the medium to the photoreceptor to enable release of the medium, now containing the image, from the photoreceptor.

For black and white printers, the photoreceptor can transfer the image directly to paper. However, for most color printers the image is formed from four colors (cyan, magenta, yellow and black) of toner and the image is built up first on an intermediate surface. For example, the Xerox DocuColor 6060 digital color press contains four photoreceptors which develop cyan, magenta, yellow, and black latent electrostatic images. Each color separation is transferred to the transfer belt in sequence. Once the full-color image is on the transfer belt, then another transfer takes place where the full-color image is transferred to the paper. Different color printers can use a different sequence of events ultimately leading to a full-color image on the paper.

Fusing Subsystem

Unlike inkjet printing, the dry toner particles do not strongly adhere to the paper. If the paper was removed from the printer after transfer, the toners could be wiped from the paper with one's finger. The image must be fused to the paper to make it permanent.

In the fusing process, the toner is heated under pressure so that it coalesces and penetrates into the paper fibers. This is accomplished by passing the paper through a pair of rollers. A heated roll melts the toner, which is fused to the paper with the aid of pressure from the second roll. The gloss of the image can be controlled by the temperature, pressure, and the length of time the toner remains in the fuser nip. Coated papers control the amount of penetration into the paper fibers and can improve image quality. The fusing process is illustrated in figure 8.

Cleaning Subsystem

Toner transfer from the photoreceptor to the paper is not 100% efficient. Small toners and toners with a low charge may have a strong adhesion to the photoreceptor and can remain there after transfer. These particles must be removed from the photoreceptor before the next print cycle, or they will affect the printing of the next image.

One technique to remove the toner is with a compliant cleaning blade, which is shown in figure 9. The blade rubs against the photoreceptor and scrapes off any toner that attempts to pass under it. The toner falls into a waste sump. The photoreceptor is exposed with a bright light (erase lamp) which removes the residual latent image. Another technique to remove the toner which is used in most medium and high speed copiers is a rotating brush cleaner, which is more efficient at removing toner and less abrasive to the photoreceptor.

Having returned to its original state, the system is ready to print the next image. Since each page is imaged individually onto the photoreceptor, each can be unique. This enables economic printing of variable information data and other types of very short runs. In a traditional lithographic press a new set of plates must be created for each unique image, which is costly and time consuming, so runs of under about 1000 sheets are impractical.