Electrophotography

The electrophotographic printing process can be broken down into five steps:

1. Imaging

Imaging is carried out by charging a suitable photoconductive surface (creating a homogeneous

charged surface) with subsequent imaging via a controlled light source (this may be scanning laser

light or light emitted by an LED array [light-emitting

diodes]). The print image corresponds to the

positioning of the light signals on the photoconductor

drum. The homogeneous charged image on the surface is discharged in parts as a result of exposure and changed in accordance with the desired print image. (Since imaging in electrophotography can be done both by laser light and by light given off by light-emitting diodes, the frequently-used term “laser printer” instead of the term “electrophotographic printer” is misleading.)

2. Inking

Special inks are used for electrophotography. These

may be powder or liquid toners, which may vary in

structure according to their composition, and contain

the colorant in the form of pigments. The ink is the fundamental and decisive element for the impression.

Inking is done via systems which transfer the fine toner particles, approximately 8 µm in size, without contact to the photoconductor drum. The toner charge is configured in such a way that the charged areas of the photoconductor surface accept the toner. Imaging has, therefore, been done with a negative image because the

positive charges have been discharged by exposure.

Therefore, after inking, the latent image on the photoconductor drum becomes visible where the toner is applied.

3. Toner transfer (printing)

The toner may be transferred directly onto the paper,

although in some cases it may also be transferred via intermediate systems, in the form of a drum or a belt. Transfer mostly takes place directly from the photoconductor drum to the substrate. To transfer the charged toner particles from the drum surface to the paper, electrostatic forces are generated via a charge

source (corona) in the nip and it is these forces, supported by the contact pressure between the drum surface and the paper, that transfer the particles to the paper.

4. Fixing the toner

A fixing unit is required to anchor the particles of toner on the paper and create a stable print image. This is usually designed so that melting and consequent anchoring of the toner on the paper takes place by heat application and contact pressure.

5. Cleaning

Residual charges and individual particles of toner remain on the drum after the print image has been transferred from the photoconductor drum to the paper. To prepare the drum so that the next image can be printed, both mechanical cleaning and electrical cleaning of the surface are necessary. The mechanical cleaning, which removes particles of toner, can be done by means of brushes and/or suction, while the electrical “cleaning” (neutralizing) is accomplished by homogeneous illumination of the surface, after which the surface is electrically neutralized and is free from

toner particles. The photoconductor drum is now charged again with a homogeneous, charged image

via the corona, with subsequent imaging in accordance

with the desired print image (as described in step one).

From the procedural steps described, it is easy to see

that electrophotography with a fixed, engraved print

image operates without a master. A different charged

image can be applied to the photoconductor drum after

each rotation. This means that it is possible to generate

a completely different print image rotation-byrotation,

print by print.

Unlike printing methods that use a fixed image carrier

(e.g., a plate), this process (taking a print run of one hundred identical impressions, for example) requires

that the same print image be created again and again, rotation by rotation, in the form of a latent charged image. This repetition could lead to process-related

variation in the print image. Such fluctuations may be caused by tolerances employed to create the charged image and/or by the tolerances (involved in the technical process) produced during the inking of the

photoconductor with toner and the subsequent, electrically-supported transfer of the image to paper.

Therefore non-impact printing technologies may produce greater variation within a print run than is the

case with technologies requiring a master.

On the other hand, the fascination and the advantage

of this technique lie in the fact that a completely different print image can be created rotation by rotation

(variable printing), thus it is not necessary to generate

a mechanical master for each printed sheet. Extremely

short print runs (even just one copy) can be produced

economically in this way (print on demand). Furthermore, each page of a brochure, from the first to the last page, can be produced in succession; the pages for the second copy are not printed until the first book has been completed (book on demand). Also, personalizing a part of the print images, for instance by inserting an address or recipient-specific additional information, can be directly carried out copy by copy (personalization, customizing).

This system is based on the unit design principle like

sheet-fed offset presses.

The photoconductor drum is imaged via an optical system in which the laser light is deflected onto the photoconductor drum by means of a rotating mirror and special optics. The laser beam is guided over the drum’s surface at high speed. Directed by a digitally-controlled modulator, the ray of light is turned on or off depending on the image, that is, the charge is discharged or remains on the previously charged photoconductor drum. The system shown is also known as an ROS imaging system (Raster Output Scanner).

The transfer of paper by means of a transfer belt where

no grippers are used to transfer the sheet. The fact that the paper is held only by electrostatic forces applied over the belt imposes limitations with respect to the accuracy of the color register – acceptable tolerances are usually two to four times higher than with a printing process using conventional

technology, such as offset printing. Printing on both

sides of a sheet (duplex printing) is also possible with

the printing system in which case the sheet is turned automatically after the front side has been printed and fed back to the printing units.

With printing systems of this kind, the content of the

print image may be produced in two ways. First, the

original can be scanned in with a scanner (that has either been integrated into the printing system or connected via an interface). Secondly, digital data can be used that has either been stored on data carriers or can be fed directly into the production system from a network.

The system has an integrated flatbed scanner which optically scans and digitizes the print copy. The printing process itself is then carried out. However, with conventional copiers, such as those installed in offices for single-color impressions, the copy image is not digitized, but exposed directly onto the photoconductor drum. Printing systems which receive information as analog copy (original) are usually referred to as “ copiers,” whereas the term “ printer ” is used if prepared job data is fed directly to the system in digital form.

There is a considerable difference between the printing

speed of a NIP printing system and the printing speeds of sheet-fed offset presses. The system can produce 1200A3 pages (impressions) an hour, whereas an A3 sheet-fed offset press normally produces between 10000 and 15 000 impressions an hour. This difference in productivity results primarily from the imaging technology used – each impression requires fresh imaging, even if the same image is to be printed. On the one hand, the imaging speed is determined by the different digital hardware and software components (which affect costs accordingly) and, on the other hand, the printing speed is also influenced by the physical processes and design of the ink transfer and paper transport systems.

With regard to the precision of modules and components (especially among image systems and cylinder groups), high requirements – like those expected for conventional presses – are absolutely vital. The bearings, for instance, are to be dimensioned similarly in respect to accuracy and resistance. The paper transport must also be supported by high-grade technical systems.

Due to a reduced printing speed, the dynamic requirements and loads of such machines are lower – a

fact that allows for a lighter construction needing fewer

materials.

Based on the availability of high-quality technological

components, the print quality generated by electrophotographic systems is at a high level. However, it is clearly lower than the high quality that can be achieved with conventional technologies. The quality relevant specifications of non-impact printing systems concentrate on addressability data (the number of dots/pixels per unit of length), number of gray values (or gray levels) per pixel and the toner technology used. The system has an addressability of 400 dpi (dots per inch).

When imaging one pixel it can generate different charges by varying the intensity of the imaging light ray (e.g., controlled by means of the ON period of the laser), meaning that it is possible to distinguish about ten gray values through the correspondingly varied toner transfer in the printed pixel.

The possible reproduction of very fine structures

is determined by the addressability and the possible reproduction of tonal values and gamut by the addressability and the gray values per pixel.

The print quality is also affected by the quality of the

toner, its particle size, geometric form, and chemical/

physical structure. In general, toners with particle sizes of only 6–8 µm and a narrow particle-size distribution

are used for high-quality printing so that good image reproducibility is possible.Using powder toners can lead to impaired print quality caused by dusting, that is, portions of the print image that should not carry any ink are inked with “stray” toner particles.

The decisive factor and the guarantee for continuous

high quality and optimal reproduction throughout a print run using conventional technologies requiring a

master is the fact that a stable image carrier is used that

represents the print image as a mechanical master (mechanical printing process).

In the case of electrophotography (electronic printing process) the constant need for re-imaging can produce system-related fluctuations from print to print.

Electrophotographic technologies may operate with powder or liquid toner. The use of liquid toners is still not very widespread, although they essentially have the advantage of assuring higher print quality with considerably smaller toner particles (approx. 1 to 2 mm).

When comparing conventional printing technologies

to non-impact printing (especially technologies with a latent intermediate image), it should be noted that in NIP – for instance, with electrophotography – the circumferential length of the photoconductor drum need not be identical to the image length. In many cases the diameters of the drum are smaller than the maximum image length would require, which means that, in order to print a page, the drum has to be imaged over a 360° drum rotation. Therefore, even if the prints are identical, the latent image and the inking by the inking unit do not occur at the same point on the drum surface with every print. Regardless of powder or liquid toner, ink resplitting – such as that occurring (and automatically compensated for) in the sheet-fed offset printing process – is not acceptable in electrophotography.

Ink Jet

In principle, ink jet non-impact printing technology

does not require an intermediate carrier for the image

information the way a photoconductor drum does in

electrophotography. In the ink jet process the ink can

be transferred directly onto the paper.

Ink jet technologies can be classified as continuous ink jet and drop on demand ink jet. The ink used for ink jet printing is usually liquid. An alternative, however, is hot-melt inks which are liquefied by heating. The ink is sprayed onto the substrate where it solidifies after cooling.

The continuous ink jet technology generates a constant

stream of small ink droplets, which are charged according to the image and controlled electronically.

The charged droplets are deflected by a subsequent

electric field, while the uncharged ones flow onto the

paper. This means that the imaging signal for charging

the droplets corresponds to a negative print image.

Continuous ink jet printing usually feeds only a small proportion of the stream of droplets to the substrate. With continuous ink jet generally only a small part of the drop volume covering the sheet in accordance with the print information is applied to the substrate. The large part is fed back into the system.

With the so-called “ drop on demand ink jet ” technology, on the other hand, a droplet is only produced if it is required by the image. The most important “drop on demand”technologies are thermal ink jet and piezo ink jet printing.

Thermal ink jet (also known as “bubble jet”) generates

the drops by the heating and localized vaporization of

the liquid in a jet chamber. With piezo ink jet the ink

drop is formed and catapulted out of the nozzle by mechanically deforming the jet chamber, an action resulting from an electronic signal and the piezoelectric

properties of the chamber wall. Due to the technical

conditions, the possible droplet frequencies are lower

with thermal droplet generation than with piezo technology.

Taking a systematic view, ink jet printing represents

the most compact technology for transferring information to normal paper in the form of a printed image (comparable to light on photographic paper). It is only necessary to generate a droplet of ink on the basis of image-dependent signals and to spray this droplet directly onto the substrate without an intermediate carrier.

Printing systems based on the ink jet technology are

usually slow in comparison to conventional printing

technologies with master, that is, they operate at a lower printing speed, especially if imaging is carried out with individual nozzles.

An ink jet printing system in which a four-color impression is created by four ink jet systems (one for each of the four inks).To do so, the paper is fastened to a drum and the individual ink jet systems (for the process colors cyan, magenta, yellow, and black) transfer the individual color separations to the substrate. This occurs due to the axial scanning motion of the imaging head and rapid rotation of the drum. With

the system shown, an A3multicolor print is produced in

approximately five minutes (addressability 300 dpi, approximately 10 gray values per pixel). This type of system is, therefore, mainly employed to produce the proofs necessary in digital prepress before the computer to plate process (digital, filmless exposure of the printing plate) produces a printing plate. Such a proof allows the quality of the data file and the content and visual quality of the subsequent impression to be checked at a preliminary stage.

As previously mentioned, ink jet technologies, which

typically operate with a resolution between 300 and

600 dpi, can generate several gray levels per pixel, often depositing several droplets on one pixel. Up to

around 30 gray levels are possible with high-frequency

continuous ink jet systems.

To increase an ink jet printing system’s productivity,

nozzle arrays as wide as the printed page have to be used.

Using an ink jet system which prints across the whole width of the web with two ink jet heads (240 dpi) multicolor printing on the upper and lower side of the web is possible.

Ink-drying presents a particular problem in ink jet

printing, and the paper surface’s ability to carry ink deserves special attention. Special coated papers are usually required for high quality impressions, although

specially formulated inks used in conjunction with an

adapted drying process can greatly increase the range

of suitable papers. Hot-melt inks are primarily of interest because they dry rapidly and allow printing on avariety of papers.

Дата добавления: 2015-11-16