The electrophotography , xerography or electric facsimile method is a photo-electric printing processes for duplicating documents. For this purpose, a photoconductor is exposed to the optical image of an original, which creates a latent image from electrical charges. Ink in the form of a toner adheres to the charged areas, which can then be used to print a copy of the original.
In everyday language, the term copy (or photocopy ) is used synonymously, although electrophotography is not the only photographic copying process.
Method of electrophotography
A distinction is made between direct and indirect as well as wet and dry electrophotography. The wet process uses a suspension of an aliphatic solvent with a low dielectric constant and the toner as a developer (see also wet peeling process ), while the dry process uses a powder.
The direct method uses a photoconductive layer on the substrate itself (for example zinc oxide on paper); development takes place wet with a suspension of toner in an insulating liquid (e.g. light kerosene) or with dry ink.
Like the xerography machines, the indirect wet process uses a photosensitive drum; however, this is wetted with a toner suspension. The adhesive toner is applied directly to the paper and only needs to dry to fix.
The indirect, dry process, which is exclusively used today, uses photosensitive drums or tapes, the toner image of which is transferred in powder form to the carrier (paper, copier foil) after development and is thermally fixed there. The process works with dry toner; it is therefore also called xerography (Greek for "dry writing").
Matrix process with liquid paint
In risography , a matrix is first exposed electrophotographically. The resulting fine holes then transfer the printing ink to the paper. The process allows inexpensive mass printing with large editions, high speed with up to 180 pages per minute in A3 format, in full color printing or with 16 monochrome colors and paper weights in the range 40–400 g / m². The copies or prints can be produced with differentiated gray values generated by screening . Risography is characterized by the fact that the color is applied to the paper without the use of chemicals or heat. The ecological advantage is accompanied by low consumption costs.
The xerography is a method for dry copy of mostly plain paper documents (eg. As files) that common in all today copiers and laser printers is used. The results are superior to those of the inkjet printer or processes using thermal paper ( thermal copier , thermal printer ) in terms of resolution, lightfastness and durability.
The patent was applied for on October 27, 1937. The first successful attempt took place on October 22, 1938 with the help of a metal plate electrically charged with a cloth, sulfur powder, dust-fine bear moss spores and a wax plate. The first photocopy (dry copy) read “10. – 22.-38 ASTORIA”. These were the date of the first photocopy, October 22, 1938, and the location, Astoria, New York .
The Haloid Company bought the patent in 1947 and launched the first commercial copier in 1949. In 1961, the Haloid Company was renamed Xerox . In Germany, the license was given to the English Rank Group , from which the Rank Xerox company emerged. The name Xerox Machine is also used for the copier itself in English-speaking countries.
The central element in xerography is the drum, which is provided with a light-sensitive coating, hereinafter referred to as the active layer or photoconductor . It has the property of being electrically non-conductive in the dark, while allowing electricity to conduct when light falls. Amorphous selenium was used until about 1975 ; today amorphous organic semiconductors , amorphous silicon or arsenic triselenide (As 2 Se 3 ) are used.
Corona charging of the active layer
A series of thin stainless steel or tungsten wires is charged positively with respect to the active layer by means of a voltage of usually 5 kV . Due to the high voltage, the ambient air is ionized , negative ions are drawn to the positively charged active layer, where they are deposited and have a negative charge because it is not conductive in the dark. The roller is negatively charged, layer by layer.
The active layer on the roller or the flexible belt is exposed:
- With the photocopier (or analog copier) the original is illuminated by a strong light source (e.g. halogen lamp ). The original is imaged on the active layer via an objective.
- With a laser printer or digital copier, the reflected light is first recorded by a line sensor , comparable to a scanner . If necessary, after image processing, the digitized print image is written on the photo semiconductor with a laser or an LED line (see laser printer ).
The incidence of light generates charge carriers in the active semiconductor layer (internal photoelectric effect ). The charge carriers discharge the positive surface charges to the electrically conductive rear side (aluminum drum) at the exposed areas - the latent image consists of charge-free zones.
The toner is brought as evenly as possible to the exposed roller, namely everywhere, both to the exposed and to the unexposed areas that have remained charged. This is done by means of another roller called a “brush”, which is magnetic, whereby the toner (two-component toners contain iron particles, one-component toners are themselves magnetic) adheres to it, whereby the particles stand up like bristles due to the direction of the magnetic field lines. The toner particles (diameter 3–15 µm) are transferred to the contact point between the two rollers due to electrostatic attraction and stick to the unexposed, i.e. charged areas of the photo layer ( black writing or charged area development ). If the toner is charged with the same name, they can also stick to the previously exposed, i.e. discharged areas ( white writing or Discharged Area Development ).
In the so-called jump development, the toner is only transported near the photoconductor with the help of a roller. The remaining air gap skips (engl. To jump ), the toner then due to the electrostatic attraction. The advantage of this method compared to the brush development is that the brush does not smudge the already developed toner and that the image drum wears less. One-component toners are used in almost all inexpensive cartridge systems; the particles land completely on the drum and the carrier. With two-component toner, the magnetic component remains; only the toner particles are used.
The toner image has to be transferred from the drum onto the medium to be printed (paper or copy transparency). For this purpose, a second charge source (drum or belt) is used, which is more heavily charged (usually at 15 kV) than the drum and accordingly attracts the toner. If the printing material is passed between the two at this point, the toner transfers to it.
To make the picture durable it needs to be fixed, i.e. H. Usually passed through two heated rollers (with some devices also through a heating chamber without pressure), whereby the toner particles melt and bond firmly to the printing material.
To prevent the toner from sticking to the fuser rollers, they are either made of a special material (e.g. Teflon) or are covered with a very thin layer of fuser oil (usually silicone oil). The latter method was mainly used with full color systems, as these devices can lead to a thick application of paint and elastic rollers (rubber) have to be used. In addition, the gloss left by the fixing oil was quite desirable for some printed products. In newer devices, an elastic plastic is used, which makes the fixing oil superfluous. The fixing by means of strong flashes of light, which is used in older SW devices, is an ideal contactless method, but is no longer used today because it is not suitable for color devices. The most common method uses a heat roller and a press roller. The fixing temperature is between 165 and 190 ° C. The fixation is decisive for the durability of the copy. The service life of the heating source (e.g. a halogen incandescent lamp in the form of a rod inside a roller) can be 50,000 to 500,000 copies.
Full discharge and cleaning
After the toner has transferred to the medium, the remaining charge on the drum must be removed before the next charge and exposure. This is done through full exposure (rod-shaped light source) and electrical stripping of the charges.
Finally, the drum is cleaned of any toner residues with a scraper or a brush. The waste toner is disposed of in a container built into the device. In some devices, the toner is also recycled and fed back into the development process.
The requirements for the active layer of the drum are very high: it must have a low level of conductivity in the dark together with a high level of light sensitivity. During exposure, it must briefly have high conductivity in the area of small lateral distances, otherwise the resolution or sharpness would be lost. It must be mechanically stable and withstand the effects of ultraviolet radiation and ions or radicals .
Drums can be made from various materials; Common are OPC ( Organic Photo Conductor , English for organic photoconductor) or a-Si ( amorphous silicon ). However, the service life of these drums is limited. The manufacturers give an approximate number of possible deductions. While OPC drums, which are mainly used in office equipment, have a life of 25,000 to 60,000 pages, a-Si drums can have a life of one million to five million pages, making them suitable for large systems such as the Example telecommunications companies use to print their bills.
However, the number of prints is only an ideal value - the age and, above all, the type of use are also decisive: If a copier or laser printer is only switched on when needed and only a few prints are made, the number of possible copies decreases. Abrasive fillers in the paper or other mechanical damage (e.g. when jammed pages are pulled out) can permanently roughen or scratch the photosensitive layer.
In contrast to inkjet or thermal prints, xerographs are very durable and, above all, lightfast. However, the toner only adheres to the surface and can be removed from the carrier. Sometimes this happens by itself over the years. The toner can also peel off at kinks. There are now polymer toners with finer and more uniformly shaped particles. This no longer flakes off at the folded edges.
Papers printed on both sides can adhere to one another; Sometimes a part of the toner sticks to transparent sleeves.
Various reports certify that the xerographs and laser printers are archived for over fifty years. Wet copies supposedly last longer than xerographs - these products, which are no longer in use, have not shown any service life restrictions.
Limitations of the procedure
Due to the optical scanning of the original, the image quality and the resolution of the line sensor or the drum are decisive for the resolution. In the range of gray values with less than ten percent color coverage, even high-quality devices show weaknesses in the form of noise or so-called dirt effects . The homogeneity, the graduation and the color fidelity have been improved in the last few years, but are inferior to other reproduction processes.
The color pigments used set limits, especially with color systems, since the different colored toners all have to meet the same high requirements, which are not necessarily compatible with a good print result.
With digitization, it is possible to prepare halftone images in such a way that, like other printing processes, finely structured surfaces are printed. In this way, areas with poor ink coverage can be more reliable at the expense of resolution.
Xerographic copiers, like laser printers , use dry toner , which contains a black pigment soot and, in the case of certain types, heavy metals such as lead and cadmium , so it can be harmful to health.
The problem with this is not only the toner composition, but also its (desired) fineness. Although the toner powder has particle sizes above the respirable fine dust , it is still deposited in the bronchial tubes because it cannot simply be removed by coughing up: Toner changes its condition at body temperature and may stick to the mucous membranes. This means that toner pollutants can have a permanent and direct effect on the mucous membranes, particularly the respiratory tract or the skin.
Toner is not only inhaled, but also accidentally swallowed due to contamination. Especially those employed in the service, refill and recycling sectors are naturally exposed to the pollutants over a long period of time.
The corona discharges (corona wire) used to generate charges generate ozone : In the area of high field strengths, the ambient air is ionized. a. Ozone is created. Ozone is harmful to health and in turn generates harmful free radicals from other substances. Most of such devices, however, have ozone filters made of activated carbon , which remove a large part of the pollutants.
Printing units that charge the imaging drum with the aid of a charging roller are completely ozone-free. This is in direct contact with the imaging drum; no corona discharge is required and therefore no ozone is produced.
Copying of documents or banknotes
The making of copies of certain documents or valid banknotes is prohibited if there is a threat of punishment. Some manufacturers have implemented functions that prevent or make such copies more difficult.
After the image data has been prepared for printing ( RIP ), it is examined again for certain patterns , such as are only used on banknotes or certain documents. If such a pattern is discovered, there are various ways of reacting. Many devices print a black surface instead of a copy, distort the colors or cover the document with the clear imprint "Copy". Other devices simulate a device fault and request customer service.
Clear identifiability (assignment of each copy to the copier used)
Some manufacturers of copy machines store electronic identifications (e.g. the machine identification code ) on the copies. This is done, for example, by applying a defined bit pattern over a wide area in the color yellow for color devices and for black and white devices as a weak tint on the carrier.
One manufacturer has the serial number of the device etched almost invisibly on the back of the glass plate and is recorded with every copying process.
These measures enable manufacturers and investigative authorities to infer the copier itself, the location and possibly even the person making the copy. Data protectionists see constitutionally guaranteed fundamental rights at risk (e.g. through the simple possibility of exposing press informants).
Photocopiers are very reliable, but not maintenance-free. Due to the fine toner powder, a large part of the failures can still be attributed to soiling. For technical reasons, the devices are not completely hermetically sealed, so that toner powder is often deposited on the exposure unit.
The transport system of a copier consists of rubber rollers, which can age and then either be regenerated or replaced.
Analog and digital copying technology
Copiers can be divided into analog and digital copiers. Until around the mid-1980s, only analog copiers were manufactured. Since then, more and more digital copiers have been developed. The analog copier has been supplanted by digital copiers since around the year 2000; analog copiers are no longer manufactured. Exceptions to this are small A4 copiers for personal use with a speed of around four A4 copies per minute. These are still produced by some manufacturers (as of April 2011).
In analog copiers, the drum is developed directly from the original using a lens and mirror; the image of the original is optically reproduced on the drum. Exposure and development run synchronously in one device. The digital copier, on the other hand, consists of two separate units, the scanner and the printer. As a rule, however, these units are housed in one device like an analog copier. With a digital copier, the template is digitized with the scanner and temporarily stored in a memory ( RAM or hard disk ). The image of the template saved here is then electronically transferred to the printer (laser printer) and printed out.
One advantage of digital technology is that pages can be copied multiple times from the buffer without having to expose the original again. In addition to pure copying, additional functions such as printing, faxing, scanning and sending the templates electronically by e-mail or in network directories can also be offered. Another advantage is the possibility of intermediate processing of a copy in the device. The most frequently used function here is the edge sharpening for fonts, which eliminates the problem of edge blurring known from analog systems and means a considerable improvement in quality, especially for documents.
The most important advantage, however, is the more compact and inexpensive design, since there is no need for optics, diaphragms and mirrors in the contaminated area between the exposure unit and the drum. In addition, the scanning does not have to run synchronously with the development.
Digital copying technology can make halftone reproductions more reliable by printing microscopic structures instead of a surface. This as well as the possibly insufficiently precise digitization and the sharpening of the edges can be undesirable or disruptive with halftone originals.
Additional functions of copiers
Copiers, especially those intended for professional use, usually have a large number of additional functions, mostly in the form of attachments:
- The automatic document feeder (ADF, Automatic Document Feeder ) allows automatic copying documents with multiple pages. The document feeder positions a page on the platen glass where it will be exposed. The page is then removed from the platen glass and the next page of the original from the document feeder is positioned on the platen glass. Recirculating Automatic Document Feeder (RADF ) feeders can also automatically position the back of a page of the original on the platen glass . An alternative method is to feed the pages fed in by the ADF past a fixed scanning line, where the original is scanned in as it moves. So-called dual-scan document changers have been manufactured since around 2005, which have a separate, integrated scan line for the back of the sheet and thus, together with the fixed scan line, can read both the front and the back of a sheet in one pass (without mechanical turning) . Regular maintenance of the ADF is necessary for reliable operation. The pad for separating the pages is usually made of rubber and cork. This rubber separator ages. If it is not cleaned and regenerated or replaced, feed errors will result.
- The duplex unit enables automatic printing on the back of copies. This means that paper consumption can be halved compared to single-sided copying.
Paper supply: Paper is kept in cassettes and magazines.
- Paper sizes from A5 to A3 or A3 + or SRA3 (oversize) can normally be held in cassettes . The cassettes are known as universal cassettes if they can be adjusted to the various paper sizes. The capacity of a cassette is approximately 500 to 1000 sheets of paper.
- Paper trays are usually designed for A4 size . A3 magazines are also available for production systems. As a rule, magazines cannot be set to a different paper size. The capacity of a magazine is around 2,500 to 4,000 pages.
- Finisher and sorter are used to receive the finished copies or prints. With digital copiers the output unit is called the finisher, with analog copiers it is called the sorter. In finishers and sorters, the copies are automatically sorted according to documents and pages. With many finishers and sorters, the sorted stacks can also be stapled and / or punched; the copy sets may contain up to 50 or 100 pages. Finisher can also create multi-page booklets. To do this, the printed pages are folded in the middle and stapled twice with wire. Possible formats are DIN A5 brochures (half a DIN A4 page) and DIN A4 brochures (half a DIN A3 page).
- The punch unit enables the copies to be punched. The copies are punched individually so that there is no limit to the number of pages (or thickness) of a copy set. In many copier models, the punch unit can be switched between the different standards, so that punching according to ISO standard 838 and according to the 4-hole expansion of ISO standard 838 is possible. The Swedish perforation is usually offered in a separate punch unit, which does not allow switching to the standards mentioned above.
- The print function enables documents to be printed from a computer workstation in the network or from a data carrier.
- The scan function enables the scanned original to be saved in the network, on a data carrier or sent directly by e-mail .
- The fax function works like a conventional fax machine. Documents are scanned in via the platen glass and transferred to the other end via a telephone connection. Faxes can also be received and printed out. In addition, received faxes can be sent directly to various destinations such as B. E-mail addresses or directories in networks are forwarded.
- R. Schaffert: Electrophotography. Focal Press, 1975. ISBN 978-0240507811
- R. Hoffmann: Modeling and Simulation of an Electrostatic Image Transfer. Shaker-Verlag, 2004, ISBN 3-8322-3427-6 .
- Detailed description, mathematical modeling and numerical simulation of the electrophotographic printing process ( Memento from October 22, 2005 in the Internet Archive ) (English, PDF, 7.3 MB)
- Xerox website: Our History
- Is Your Printer Spying On You? Electronic Frontier Foundation (EFF)