Attention! All materials of the given publication have only information and fact-finding character. The author does not responsible for the harm put to equipment, people or animal and also for the half-received profit or losses at usage of the materials from given publication in practical activity.
The given article is dedicated to modern ways of computer printing. Naturally the article does not pretend to entirety or the true in the last instance. Therefore everybody can write to me, set questions, signal about the noted errors, offer the additional information or give the information on other sorts of printing. The article is permanently processed and added.
Now the main part of the article is dedicated to xerography and laser printing. Besides thermoprinters, daisy-wheel printers and duplicators are described there.
As soon as possible I plan to locate the description of dot-matrix printers and hardprinting printers of the corporation Xerox-Tektronix.
Laser printers and copy machines
Now laser printers gradually turn from expensive apparatuses accessible only large enough and moderate-sized firms in apparatuses to high-quality and high-speed printing at home and at a small office.
In the given article I shall try to explain what is the modern laser printer, how it works and how to select a laser printer or a copy machine.
The process of dry xerography underlies of operation both a copy machine and a laser printer. This process is based on electrostatic photo.
Xerographic process was invented by the American engineer Chester Karlson in 1938. In November, 1940 he got the patent for the invention. In 1947 the American company "Haloid Company" purchased the given invention for development of the first copy machine which was produced in 1950. Later this company was reorganised several times and now we know it under the name Xerox.
Electrostatic photo is based on the ability of some semiconductors to decrease specific resistance under light action. Such semiconductors are named photoconductors and are used for photoreceptor manufacture.
The basic photoconductors characteristics are named below.
These photoconductor characteristics are carefully analysed at choice it as a photoreceptor for a copy machine or a printer.
Photoreceptors production process
Photoreceptors are usually put on an aluminium cored cylinder. Selenium and its combination or organic combination are served as photoreceptor.
Organic photoreceptor is two-ply. The first layer is the layer where the charge carry is effected, under it there is the layer where a charge is generated. Then there is a thin layer of oxide film which prevents the charge leak to the wafer. A wafer is the last aluminium layer.
The selenium photoreceptor consists of a "trap layer" which is a natural oxide layer. This layer reduces dark leak charge rate. Then there is a photoconductivity layer, an aluminium oxide layer and a wafer.
There are two sorts of photoreceptors: tape and cylindrical. First of them are usually used in apparatuses with a very high speed, as they allow to provide higher exposing speed.
Photoreceptor charging is the process of uniform charge plotting on the photoreceptor surface. The charging is effected by the corotron. There are some sorts of them which we shall examine below.
The high potential with the help of the high-voltage power supply unit is given on the corotron for charging. Between corotron and photoreceptor there is the voltage in several kV that leads to the shock air ionisation (corona discharge), and ions are stored on the surface of a photoreceptor. The part of electrons leak on the ground from the grounded wafer, thus the excessive charge opposite to the charge on the surface of photoreceptor arises in the material of wafer, close to the border of photoconductor. The corotron screen is grounded not to reduce the potential difference between photoreceptor and corona wire , as this difference should exceed the threshold voltage of the corona (the corona discharge doesn't arise at the voltage lower than this).
Sorts of corotrons:
Corotron is a source of characteristic ozone scent outgoing from the copy machine in the operating time. It is necessary to mark that at good filters usage and their timely replacement the scent is not felt. Now corporations - manufacturers transfer on ozone free technology.
After charging the image which is lit by a powerful light source and projected through the system of mirrors in copy machines is given on the photoreceptor. Usually the carriage with a lamp is used for lighting the original as in scanners, however in the machine Xerox 1075 (with a tape photoreceptor) the lamp - flash is used and it lights the whole original at once. The lens with the changed focal length serves for image increase and decrease. The drum rate and the carriage rate should be matched. The image from the exposing glass is lit by a lamp and through the system of mirrors it is projected on the photoreceptor. The places on the photoreceptor where light falls lose their potential. Thus on the photoreceptor there is the design of the original as the charged areas.
The apparatuses can be divided into three groups: apparatuses with moving table where the original transfers together with the exposing glass about the light source, nonmovable table where there is a carriage and the system of mirrors (or a scanner) and apparatuses with a lamp - burst in which the whole original is lit at once. In wide-form copy machines the broach of the original about exposing glass and and the light source is used.
The principle of the carriage action will not be described here because our article is dedicated to the theory and practice of xerographic printing.
At the stage of exposing on the surface of the photoreceptor the hidden electrostatic image is formed. Let's examine this process in detail.
Before the beginning of the exposing the surface charge of the photoreceptor is held on the place by the interaction with the opposite charge which is situated on the border of the ground wafer and the photoreceptor.
Before light gets on the photoconductivity layer the quantity of free charge carriers in it is small but the resistivity is great. Actually electrons in the photoconductor after charging are displaced from the balance position, but they are still are in their molecules. Such displacement of positive and negative charges in the molecule is named as polarisation.
Let's examine the simplified model of the process which occurs at photoreceptor lighting. Let's consider that the photoreceptor is charged by the positive charge.
When light is got on the photoconductor there is a generation of charge free carriers in it. An electron of that molecule which is posed closer to the surface of the layer is transferred in the direction of a positive ion on the surface. This relocating neutralises a part of positive ions on the surface. At the same time a molecule in the upper layer remains positively charged. Absence of electrons in the molecule is named "a hole". The type of conduction when the main charge carriers are holes is named hole conduction. At hole conduction electrons relocating from one atom to the next one takes place. The result is relocating positive charges - holes - in the direction opposite to electrons motion.
After light gets on the photoreceptor, the electrostatic field on the surface of the photoconductor is changed. It doesn't operate between the charge on the surface of the photoreceptor and the wafer. It operates between the "upper" molecule and the wafer.
The electrons which are situated below the "upper" molecule immediately react to the positive charge and start to transfer to the "upper" molecule to neutralise a part of the arisen charge. Because of the electrons migration the positive charge from the "upper" molecule transfers to the molecule from the following, "second" layer of photoconductor molecules.
Thus the electrostatic field arises between the molecule of the "second" layer and the wafer. The hole is transferred from the "upper" molecule to the molecule from the "second" layer. The process repeats until the hole moves to the molecule of the photoconductor nearest to the wafer. In this case electrons are transferred from the wafer to the photoconductor to neutralise the positive charge.
Development is a process of image generation on the photoreceptor by toner.
Toner is a powder which fractions consist of polymer or rubber and dye (soot is used for black toner).
There are two variants of development - one-component and two-component. Let's first examine two-component way.
The two-component way is used only in the case of photoreceptor negative charging.
Toner from the bunker through the special dosing device moves in the bunker with developer. Developer is the fractions of magnetic material coved with polymer.
The adherence of toner to developer is taken place by friction electrization. During friction the fragments of toner and developer get different charges and toner evenly covers developer.
Developer sticks to the magnetic shaft, which is a hollow shaft with permanent magnets inside. The shaft covered by developer with toner directly contacts with the photoreceptor, therefore the fragments of toner having the charge opposite to the photoreceptor charge are attracted to its charged areas.
The pure developer with the rest of toner gets in the bunker again. Developer is mixed with toner again and gets on the magnetic shaft. Developer is not spent in the development process. However as the result of friction developer loses the polymeric layer that leads to its disability to attract toner. Besides such developer can cause mechanical failure of the photoreceptor.
The bias voltage about 100-500V of the same sign with the photoreceptor charge is given on the magnetic shaft not to give toner be transferred on slightly charged areas of the photoreceptor. The force of attraction is increased by this and toner is not transferred on slightly charged areas. It is possible to regulate copy saturation by regulation of bias voltage value for example to create a good copy from the bad original. The modern apparatuses usually regulate copy quality well enough and the operator's interference is not practically required.
One-component development is usually used in apparatuses of the small class and laser printers. Toner of another composition is required in this case. Naturally such toner is more expensive. One-component development does not foresee developer availability. In this case toner is produced from the mixture of magnetic material, polymer and dye fragments.
From the bunker toner gets on the magnetic shaft. The charging blade (Rachel) is situated above the shaft, at the way out of the bunker it fulfils two functions:
Toner particles friction about the blade leads to toner charging by the sign opposite to the sign of the photoreceptor.
Toner carrying from the shaft on the photoreceptor is effected with the help of the bias voltage applied to the magnetic shaft. In this case bias voltage is alternating voltage with constant component which corresponds to the photoreceptor charge sign. During the period with the sign opposite to the photoreceptor charge the toner is transferred on the photoreceptor. During the period with the sign corresponding to the photoreceptor charge toner is returned from background areas to the magnetic shaft.
The control of copies quality is effected by constant component variation.
Carrying is the process when toner is transferred on paper.
The paper is passed between corotron and photoreceptor where toner design. Carrying corotron gives the paper the charge corresponding to the photoreceptor charge. In the wafer of the photoreceptor there is a charge opposite by sign to the paper charge. The paper is attracted to photoreceptor by this.
To transfer toner to the paper the force of attraction between it and toner should be more than the force of attraction between toner and photoreceptor. Not all toner is transferred on the paper. Therefore its remains are deleted in the photoreceptor purification process.
For improvement the image quality and tone consumption reduction in some apparatuses the preliminary carrying is effected and during this process the photoreceptor charge is weakened. For this purpose either the photoreceptor is previously lit or the alternating voltage is given on carrying corotron.
Paper separation from the photoreceptor is effected by both mechanical and electric way.
In the first case either separation tips situated close to the photoreceptor or separation belts installed from one of the photoreceptor edges are used. The edge of the paper slips on the belt and then is easily separated form the from the photoreceptor.
In the second case the separation corotron is used together with mechanical means. For paper separation from the photoreceptor the alternating voltage is given on the separation corotron. It generates positive and negative ions. A part of them weakens paper force of attraction to the photoreceptor, and another part provides toner adherence to the paper.
After carrying a copy is already ready. But the image obtained on the paper can be erased by any mechanical effect (for example by slight friction). Naturally such copy is not suitable for practical usage. To increase toner cohesion with paper the mechanism of fixing is used.
There are several ways of fixing. Most widespread way is thermomechanical when a copy is exposed to heating and mechanical pressing.
Fixing mechanism is called a fuser. The mechanism consists of the heated teflon shaft with a quartz lamp inside and rubber pressure shaft. Sometimes instead of the teflon shaft the special ceramic thermoelement is installed and it is separated from the paper by thermofilm. Such copy machines have a smaller warming period and smaller energy consumption, however considerably smaller quantity of copies and it is much more easy to damage it by incorrect paper extract.
In a part of apparatuses heated shaft smearing by silicon grease is foreseen. It allows to avoid toner adherence to the cylinder. Besides a special towel for deleting the remainder of toner or other mud adhered to the shaft can be used.
The separation tips are used to separate the paper from the shaft.
The mechanism with a quartz lamp is more expensive but also more reliable and it is usually used in high-productivity computers. They are for example laser printers Xerox N24/32/40, HP 4plus, Xerox 5405 and so on. The mechanism with thermofilm is used in printers and copy machines of the small class. They are copy machines Canon a FC/PC series, printers HP 5L, 6L, 5P, 6P, 1100
Clearing is a process of toner remainder deleting from the photoreceptor after transference on paper.
Directly ahead of clearing preclearing can be used with the help of the photoreceptor exposure or corotron preclearing which generates positive and negative ions.
The remained toner particles are deleted with the help of the rakel knife which is in the direct contact with the photoreceptor. The rakel is produced and exactly taken up a position relative to the photoreceptor to not damage it. The waste toner gets to the bunker. Its repeated usage is not recommended as the toner particles are stuck together and polluted.
It is also possible to remove toner by a soft brush with the vacuum rollout system installed inside.
The last stage of clearing is residual charge removing which is effected with the help either a light source or corotron which voltage sign is opposite to the charge sign of the photoreceptor.
The general scheme of the copying process is given in the following figure:
The practical side of xerography
After we have cleared up physical bases of copy machine operation, we shall examine the concrete working of xerographic technology in these or those apparatuses.
In big devices toner, photoreceptor, developer, rakel knife and corotron are substituted separately, after passing the certain quantity of copies. In small printers and in copy machines all these parts are united in one cartridge (super-cartridge). Such cartridge is divided into two ones: the copy - cartridge (photoreceptor with the purification and charging system) and the toner - cartridge (toner with the magnetic shaft). Under the service regulations all such cartridges have the certain lifetime and should be substituted after its completion. In particular the toner cartridge is substituted after toner is over.
In some apparatuses the replacement of the certain expendable materials such as a copy - cartridge should be fulfilled by the service engineer, because the device counts the quantity of copies after a new catridge installation and is stopped after the certain quantity of copies is passed. The service engineer at replacement of such expendable materials should reset the counter. However practice shows that practically all such expendable materials except toner can be used for 1,5-2 periods of their real service especially it concerns to devices Xerox. For example the printer Xerox N40 fuser is recommended to be changed after 300 thousand copies. However it can really pass 600 thousand copies, if it is periodically served.
Therefore a service engineer just resets the copies counter without its replacement. It is cheaper for users. The information about reset counter codes is kept in a secret by the corporations - manufacturers. The modern devices beforehand notify about the necessity of such detail speed replacement.
The action principle of the laser printer differs from operation principles of the copy machine a little. A laser is a light source here and it reduces the potential in the certain areas of the photoreceptor. Thus the photoreceptor background areas remain charged. Toner is charged by the opposite charge. At contact toner is attracted by the wafer in the areas of low potential punched by the laser.
Laser exposure is effected by the following way: a laser shines on the mirror which is rotated with a high speed. The reflected beam through the system of mirrors and prism gets on a drum and by the turn of the mirror knocked out charges on all length of the drum. Then there is a turn of the drum on one step (this step is measured in lobes of inch and it determines the printer resolution on vertical) and a new line is drawn. In some printers except the drum turn the mirror turn on vertical is used which allows to draw two lines of points on one step of the drum turn. In particular first printers Lexmark with resolution 1200 dpi used this principle.
The mirror rotation rate is very high. It is about 7-15 thousand RPM. To increase the printing speed without increasing mirror rate the mirror is fulfilled as a polyhedral prism.
In LED printers (OKI, Panasonic) LED panel works instead of laser.
The general laser scheme is given below:
The beams of blue and red colour correspond to various mirror positions. At the moment A the mirror is turned under one corner (red mirror position). In the next moment corresponding to the laser rate the mirror is turned and occupies the blue position. The reflected beam gets already in another point of the photoreceptor. Naturally in reality there are some more additional mirrors, prisms and light-guides responsible for focusing and beam veering.
Laser printers except mechanical part include serious enough electronics engineering. In particular in printers the large memory size is installed not to load the computer and to store the jobs in memory. On the part of printers the hard disks are installed. The electronic stuffing of the printer also contains various data description languages (Adobe PostScript, PCL and so). These languages are intended to take a part of operation from the computer and to transmit it to the printer.
Some words about network printers. There is a big difference between the network printer and the usual printer in network used by the several users. In particular network printers are usually faster, are planned for large load, they contain more trays for paper, often have the duplex device (for printing from two sides of a page), own hard disk for certain documents storage (for those which are often typed). The modern printers have own HTML-site that allows to work with them through Internet and Intranet. For example I can be connected with the office by modem and send my file for printing sitting at home. Besides there are various safety means and a lot of program complexity. But the fundamental difference between a network printer and a usual one is that a network printer has a built - in network card. It does not need to be connected to the computer. It works autonomously. It gives a lot of advantages:
Therefore if you have several workplaces in the corporation (more than 5) or large printing size (more than 10000 copies per month), you should think of the network printer. Now such printers are made by all laser printer manufacturers. It is necessary to say that the fastest printer in this class is Xerox N40 with the printing speed 40 pages per minute.
There are also apparatuses of the class printer - copier or digital copier. They can fulfil laser printer and copier functions. Digital copier at the beginning scans the image in memory and then types by the laser way. Those are practically all colour copiers, however the cost of the computer interface for colour copier is comparable to the cost of the apparatus. Besides digital copiers provide higher quality of shades transmission even at black-and-white printing. The modern line is the gradual analogue copiers replacement (where a lamp is a light source) by digital ones. Advantages of digital printing are:
The colour printing is provided by multi-coloured toner (CMYK model) usage. Thus toner of different colours is sprayed on the copy. As the result of powders mixing the colour copy is got. The toner of each colour is stored in the separate bunker with the own magnetic shaft and developer.
In some apparatuses the paper is taken up a position on the carrying shaft and then contacts with the photoreceptor. The process is repeated 4 times. In some apparatuses toner at the beginning is transferred on the carrying shaft and then on paper.
The high price of colour apparatuses is caused by the fact that some details are given in four copies. Besides more serious drums for the improved shades transmission and also more exact than in usual apparatuses feed paper mechanism (as the paper passes on the drum four times) are used. Besides the photoreceptor of other structure is used, and the carrying shaft is fulfilled so that the length of its circle is equal to the length of maximum format paper.
Also in such apparatuses some innovations are included. The apparatus Xerox Majestic for example includes a touch-screen i.e. a sensor screen of the small size. Such screen substitutes a lot of control buttons for control though some buttons all the same are remained. In particular the given apparatus allows to fulfil colour replacement, creation of several copies from one original on one page, scaling, to increase or decrease luminance, figure saturation visibility, to stress or to weaken some colours, to increase the original, dividing it into some separate sheets which then stick together in the poster and so on.
Additional equipment for copy machines and printers
Documents autofeed. It is a device placed on the cover of the copy machine scanner which is intended for documents feed on the scanning desktop without opening the cover of the scanner that considerably speeds up operation. Autofeeds often can work with both one-sided and two-sided documents. Such devices can be purchased and to some scanners.
Duplex. This device is for duplexing. Certainly everybody can first print odd pages, then take a pile, invert it, put it in the tray and print even pages. But imagine the network printer placed in the other end of the office and a lot of people type on it. Duplex serves and for these purposes too. It allows to type from two sides at the same time. It does not mean that you purchase the second printer for yourself. Duplex just extends the paper by another side without your involvement.
Finisher. The finishing devices are different. It can be the sorters, staplers, the cutting torches etc. All are united by their aim to process the document after printing: sorting, joining or folding.
Additional trays. They serve to give you the possibility not to come up to your printer to provide it with paper too often.
Advantages of xerographic printing:
And at last a few words about what can be done and what can't be done with laser printers and copiers:
It is possible:
It is impossible:
Now about how to select the printer or the copier. At choice of such apparatus except the fundamental criteria, such as price, resolution or printing speed you should clarify the following moments:
In summary it would be desirable to say a few words about modern tendencies in the field of xerographic printing.
The fundamental tendency in the field of copy machines is their gradual refusal from analogue apparatuses and conversion to digital computers. In particular the corporation Xerox even has changed the trade mark in such way that it reflects the corporation conversion to copying digital technology.
Besides copy machines start to pass into printer - copier. At the moment it is possible to find both small apparatuses and high-duty digital computers.
In general it is possible to say that the modern copy machine passes into a êîícòðóêòîð. Let's examine Xerox DC214. At the beginning you purchase just a copier. Then you can purchase the computer interface and convert it into a network printer - copier, then in a scanner, then you can add a fax-modem card, finisher, additional trays, duplex and documents autofeed.
Now more service functions is fulfilled by a user. For example in the large printers and copiers (in particular in the printers Xerox N24/32/40) a user can change a fuser, a carrying shaft of carry and feed rolls (to say nothing of the cartridges).
The corporations gradually leave from separate parts of the development block and transfer to the united cartridge. I.e. the bunker with toner, the charge corotron, the rakel knife and the photodrum are united into one catrige.
The electronic parts of laser printers becomes more high-power. These are powerful processors, large hard disks, drives etc. A printer tries to take more operations for a computer.
It is modern now to build Web-site into printers. You can address them through the printer IP-address with the help of a usual browser. On the printer Web-site it is possible to find the complete information on the current printer condition and to make its configuration.
Thermoprinters are not practically used. Usually they are installed in faxes, however once they existed as separate printers and therefore we shall examine this class in the given article.
The thermoprinter action principle is very simple. The printing element is the panel with heated elements. Depending on the given image these or those elements are heated and they force special thermopaper to darken in the place of heating. An advantage of the given printer type is that the expendable materials except the special paper are not needed. A disadvantage is the special paper and slow printing speed.
There is also one more type of thermoprinters. In it the thermoelement heats the special colouring tape which then the image is transferred on paper from. The unique model which I saw was Star Jet, besides I saw a fax Panasonic with such thermotape on the width of paper.
In our article we shall examine duplicators because some them them can be connected to the computer.
A duplicator (risograph) is intended for printing the large number of copies from one copy (from 50 copies).
The operation principle is the following: after the copy scanning the map is burned on the special master-film by the thermoprinting device. Then the master-film is spooled on the drum made of reticular material. The ink is given through the drum and they flow through the burned holes of the film and are transferred on the copy. It is possible to get up to 10000 copies from one master-film.
The low cost price of printing at large number of copies is caused by the low cost of ink.
For colour printing the removable drums are used. Thus each copy is run so many times how many colours are needed to be printed out. However the all colours printing can't be got on the given apparatus. It is possible to get 3-4 colour printing on good paper. At usage a lot of colours the quality of a copy is considerably worsened.
The shades transmission quality approximately corresponds to a usual copier.
The reason of the given apparatus usage only for printing of the large number of copies is the master-film high price and its impossibility of being used more than once.
The most widespread corporations in the market of the given devices are Duplo and Riso.
Daisy-wheel printers are similar to typewriters. Now daisy-wheel printers practically do not exist and the principle of printing by a daisy-wheel is used in electronic and mechanical typewriters.
Some time ago such printers were widespread, however with the appearance of more high-speed matrix shock apparatuses and also laser printers daisy-wheel ones practically have disappeared and now such way of printing is used only in typewriters.
Daisy-wheel printing devices do not form the image by the matrix of points.
The mechanism of printing is simple enough and is fulfilled as follows. In mechanical typewriters each key is just connected to the certain crandle and on the end of it there is the appropriate letter. At pushing a key there is a matrix strike on the colouring tape and through the tape on the paper.
In foreign machines the wheel similar to a daisy is used and the letters are put on its leaves. The quantity of leaves is equal to the quantity of possible symbols plus additional symbols for various ways of printing.
The daisy is put on the special wheel. The wheel through the drive is bridged to the stepper motor. Usually all this mechanism together with the wind tape drive, a cartridge with colouring and correcting tape are fulfilled on the carriage. At switching on the machine there is the initial positioning of the wheel. It is a very important moment in the machine operation because from the initial position there is a sample of each following letter. Usually for positioning the wheel is scrolled on the complete revolution and is locked by the mechanical way. After that the process of printing is very simple. The user pushes a key. The processor processes pushing and counts how many steps it is necessary to make up to the following letter. After that the stepper motor turns the wheel and stops it on the necessary letter. The electromagnetic hammer is used to strike a daisy-leaf. Through the colouring tape the leaf strikes on the paper. The common action mechanism is shown in the figure below.
The carriage is put perpendicularly to the cylindrical shaft and with its help the paper is given. The carriage goes along the shaft. Each following letter in the line is formed is formed in this way. For transferring to the following line the shaft is turned on one step. All used drives are step-by-step.
The change of daisy-wheels is possible and that allows to type by various prints or symbols sets.
There are three sorts of colouring tapes:
The tape of the second sort can be used as the correcting tape. In that case the white dye is put on it. Such tape allows to get a more clear copy, however after each strike the dye is completely transferred on the paper. After the tape is completely used, it needs to be replaced. Cloth tape is fulfilled as a ring that allows to use the same areas of the tape several times.
The correction is made as follows: the mechanism resets the carriage back. After that there is a replacement of the usual colouring belt tape on correcting for example by raising of the carriage mechanism or by raising of the stretched correcting tape. After that the letter which needs to be corrected is typed again but already through the correcting tape.
The most simple machines (for example Optima SP50) just type a letter after pushing a key, and some of them have functions of editing practically identical to text editors. However even the most simple machines can store the printed out letters with the purpose of saving the possibility of their further correction.
In modern typewriters the various fittings for the life simplification of a user are used. Line editing or text editing on LC display with the further printing is for example possible.
That is all for now. You can send your materials.
Write a comment below. No registration needed!