xerography

Xerography (or electrophotography) is a dry photocopying technique invented by Chester Carlson in 1938, for which he was awarded U.S. Patent 2,297,691 on October 6, 1942. Carlson originally called his invention electrophotography. It was later renamed xerography—from the Greek roots xeros (dry) and graphos (writing)—to emphasize that, unlike reproduction techniques then in use such as cyanotype, this process used no liquid chemicals.

Although Georg Christoph Lichtenberg invented a dry electrostatic printing process in 1778,^[1] Carlson's innovation combined electrostatic printing with photography. Carlson's original process was cumbersome, requiring several manual processing steps with flat plates. It was almost 18 years before a fully automated process was developed, the key breakthrough being use of a cylindrical drum coated with selenium instead of a flat plate. This resulted in the first commercial automatic copier, released by Haloid/Xerox in 1960. Xerography is used in most photocopying machines and in laser and LED printers.

The xerographic process

Schematic overview of the xerographic photocopying process.

The first commercial use was hand processing of a flat photosensor with a copy camera and a separate processing unit to produce offset lithographic plates. Today this technology is used in photocopy machines, laser printers, and digital presses such as Xerox iGen3 and Xeikon presses which are slowly replacing many traditional offset presses in the printing industry for shorter runs.

By using a cylinder to carry the photosensor, automatic processing was enabled. In 1960 the automatic photocopier was created and many millions have been built since. The same process is used in microform printers and computer output laser or LED printers.

The steps of the process are described below as applied on a cylinder, as in a photocopier. Some variants are described within the text. Every step of the process has design variants.

A metal cylinder is mounted to rotate about a horizontal axis. This is called the drum. The end to end dimension is the width of print to be produced plus a generous tolerance. The drum is manufactured with a surface coating of amorphous selenium (more recently ceramic or organic photo conductor or OPC), applied by vacuum deposition. Amorphous selenium will hold an electrostatic charge in darkness and will conduct away such a charge under light.

Laser printer photo drums are made with a doped silicon diode sandwich structure with a hydrogen doped silicon light chargeable layer, a boron nitride rectifying (diode causing) layer that minimizes current leakage, as well as a surface layer of silicon doped with oxygen or nitrogen, silicon nitride is a scuff resistant material

The drum rotates at the speed of paper output. One revolution passes the drum surface through the steps described below.

In place of the drum there may be a flexible, oval or triangular belt.

Step 1. Charging An electrostatic charge is uniformly distributed over the surface of the drum by a corona discharge from a Corona unit (Corotron), with output limited by a control grid or screen. The complete unit is correctly called a Screened Corotron or Scorotron for short. This effect can also be achieved with the use of a contact roller with a charge applied to it. The polarity is chosen to suit the Positive or Negative process. Positive process is used for producing black on white analogue copies. Negative process is used for producing black on white from negative originals (mainly microfilm) and all digital printing and copying. This is to economise on the use of laser light by the Blackwriting or Write to Black exposure method.

Step 2. Exposure The document or microform to be copied is illuminated and either passed over a lens or is scanned by a moving light and lens, such that its image is projected onto and synchronised with the moving drum surface. Alternatively, the image may be Flash Exposed, using a Xenon strobe, onto the surface of the moving drum or belt, fast enough to render a perfect latent image. Where there is text or image on the document, the corresponding area of the drum will remain unlit. Where there is no image the drum will be illuminated and the charge will be dissipated. The charge that remains on the drum after this exposure is a 'latent' image and is a positive of the original document.

In a laser or LED printer, modulated light is projected onto the drum surface to create a latent image. The modulated light is used only to create the positive image, hence the term Blackwriting.

Step 3. Development The drum is presented with a slowly turbulent mixture of toner particles and larger, metallic, carrier particles. The carrier particles have a coating which, during agitation, generates a Triboelectric charge (just one form of static electricity), which attracts a coating of toner particles. The mix is manipulated with a magnetic roller to present to the surface of the drum/belt a brush of toner. By contact with the carrier each neutral toner particle has an electric charge of polarity opposite to the charge of the latent image on the drum. The charge attracts toner to form a visible image on the drum. To control the amount of toner transferred, a bias voltage is applied to the developer roller to counteract the attraction between toner and latent image.

Where a negative image is required, as when printing from a microform negative, then the toner has the same polarity as the corona in step 1. Electrostatic lines of force drive the toner particles away from the latent image towards the uncharged area, which is the area exposed from the negative.

Early color copiers and printers used multiple copy cycles for each page output, using colored filters and toners. Modern units use only a single scan to four separate, miniature process units, operating simultaneously, each with it's own coronas, drum and developer unit.

Step 4. Transfer Paper is passed between the drum and the transfer corona, which has a polarity that is the opposite of the charge on the toner. The toner image is transferred by a combination of pressure and electrostatic attraction, from the drum to the paper. On many color and high speed machines, it is common to replace the transfer corona with, one or more, charged Bias Transfer Rollers (BTRs), which apply greater pressure and a higher quality image.

Step 5. Separation or Detack Electric charges on the paper are partially neutralized by AC from a second corona, usually constructed in tandem with the transfer corona and immediately after it. As a result, the paper, complete with most (but not all) of the toner image is separated from the drum or belt surface.

Step 6. Fixing or Fusing The toner image is permanently fixed to the paper using either a heat and pressure mechanism (Hot Roll Fuser) or a radiant fusing technology (Oven Fuser) to melt and bond the toner particles into the medium (usually paper) being printed on.

Step 7. Cleaning The drum, having already been partially discharged during detack, is further discharged by light and any remaining toner, that did not transfer in Step 6, is removed from the drum surface by a rotating brush, under suction, or a squeegee, known as the Cleaning Blade. In most cases, this 'waste' toner is routed into a waste toner compartment for later disposal; however, in some systems, it is routed back into the developer unit, for reuse. This process, known as Toner Reclaim, is much more economical but can possibly lead to a reduced overall toner efficiency through a process known as 'toner polluting' whereby concentration levels of toner/developer having poor electrostatic properties are permitted to build up in the developer unit, reducing the overall efficiency of the toner in the system.

Note: Some systems have abandoned entirely the use of a separate developer (carrier). These systems, known as Mono Component, operate as above but use either a magnetic toner or fusible developer (however you wish to view it). This results in the complete removal of the need to replace worn out developer as the user effectively replaces it along with the toner. An alternative developing system, developed by KIP from an abandoned line of research by Xerox, completely replaces magnetic toner manipulation and the cleaning system, with a series of, computer controlled, varying biasses. The toner is printed directly onto the drum, by direct contact with a rubber developing roller which, by reversing the bias, removes all the unwanted toner and returns it to the developer unit for re-use.

The development of xerography has led to new technologies that some predict will eventually eradicate traditional offset printing machines. These new machines that print in full CMYK color, such as Xeikon, use xerography but provide nearly the quality of traditional ink prints.

Xerography in animation

Ub Iwerks adapted xerography to eliminate the hand-inking stage in the animation process by printing the animator's drawings directly to the cels. The first feature animated film to use this process was One Hundred and One Dalmatians (1961). At first only black lines were possible, but in the 1980s, colored lines were introduced and used in animated features like The Secret of NIMH.

References

Footnotes

1. ^ Schiffer, Michael B.; Hollenback, Kacy L.; Bell, Carrie L. (2003). Draw the Lightning Down: Benjamin Franklin and Electrical Technology in the Age of Enlightenment. Berkeley: University of California Press. pp. 242–44. ISBN 0-520-23802-8. http://books.google.com/books?id=9TuH6Lg8IasC&dq=electrophorus+volta. 

Sources

• Owen, David (2004). Copies in Seconds: How a Lone Inventor and an Unknown Company Created the Biggest Communication Breakthrough Since Gutenberg. New York: Simon & Schuster. ISBN 0-7432-5117-2. 

• Schein, L.B. (1988). Electrophotography and Development Physics, Springer Series in Electrophysics. 14. Springer-Verlag, Berlin. 

External links

• Encyclopedia.com's Xerography Definition