Xerography is typically a fairly high gamma process, which is suitable for applications such as document copying.
It is an imaging process that is based on a photoconductive substance whose electrical resistance decreases when light falls on it. Xerographic printing is the basis of the most commonly used document copying machines.
Xerographic printing is a process used very often in all different commercial and institutional environments.
It is a highly technical process that uses many applications or principles developed in electromagnetic field theory.
How does xerographic printing process?
The first commercial use was the manual processing of a flatbed photosensor (an electrostatic component that detects the presence of visible light) with a copy camera and a separate processing unit to produce offset lithographic plates.
Today this technology is used in photocopiers, laser printers and digital presses. They are slowly replacing many traditional offset presses in the printing industry for shorter runs.
The UV printing can be used after the first xerographic pass when it cools down.
- The first step in the Xerography process is to charge a photoconductive metal drum evenly by rotating it on its surface and applying an electrostatic charge.
- The document is then passed over the surface of the photo-sensor drum and illuminated by a laser, this illumination creates the latent image by simply passing through sections of the document without text.
- The image produced on the drum is then bound with magnetically charged toner.
- The toner particles are then transferred to a print medium by a corona device generating an electric field with the ability to overcome the magnetic field of the toner, thus attracting the toner to the substrate.
- Before the new document is ready, the toner must fuse with the paper in some way. This is done by heated steel rollers that generate the necessary combination of heat, pressure and radiant energy.
In the following we will name the steps of the process applied on a cylinder, as in a photocopier. We will only describe a little bit of each step of the xerographic printing process, but obviously the physics of the xerographic process is discussed extensively in a book, of course.
1 Charge
An electrostatic charge is distributed over the surface of the drum by a current discharge, with the output limited by a control grid or screen.
2 Exposure
The document to be copied is illuminated by flash lamps on the platen and passes over a lens or is scanned by a moving light and lens. Its image is projected and synchronised with the surface of the moving drum.
3 Development
In high-volume xerographic copiers, the drum is presented with a slowly turbulent mixture of toner particles and larger, reusable iron carrier particles.
The toner is a powder.
The carrier particles have a coating that generates a kind of static electricity.
The mixture is also manipulated with a magnetic roller to present a toner brush to the surface of the drum or belt. The charge attracts the toner to form a visible image on the drum. A bias voltage is applied to the developer roller to control the amount of toner transferred and counteract the attraction between the toner and the latent image.
A negative image is required because when printing from a microform negative, the toner has the same polarity as the corona in step 1.
Electrostatic lines of force pull the toner particles away from the latent image towards the uncharged area, which is exposed to the negative.
Early colour copiers and printers with xerography used multiple copy cycles for each page output, using colour and toner filters. Modern units use only one scan to four separate miniature processing units, which operate simultaneously, each with its own crown, drum and developer unit.
4 Transfer
The paper passes between the drum and the transfer belt, which has an opposite polarity to the toner charge. The toner image is transferred from the drum to the paper by a combination of pressure and electrostatic attraction.
5 Separation or detachment
The AC partially neutralises the electrical charges on the paper from a second corona, usually built parallel to and immediately after the transfer corona. As a result, the paper, complete with most (but not all) of the toner image, separates from the drum or belt surface.
6 Fix or merge
The toner image is fixed to the paper using a heat and pressure mechanism (hot roller fuser) or a radiant fusing technology (oven fuser) to fuse and bind the toner particles into the media being printed. They are also used to provide “offline” steam fusers.
7 Cleanup
The drum, which has already been partially discharged during the separation process, is further discharged by the light. The remaining toner not transferred in step 6 is removed from the drum surface by a rotating brush under suction, or a squeegee known as a cleaning blade.
This “waste” toner is usually sent to a waste toner compartment for later disposal and reuse.
Some systems have abandoned the separate developer (carrier). These systems, known as mono-component systems, operate in the same way as the above but use a magnetic toner or fusible developer. There is no need to replace the worn developer, as the user effectively replaces it along with the toner.
An alternative development system, developed by the KIP company from a line of research abandoned by Xerox, completely replaces the handling of the magnetic toner and cleaning system with a series of computer-controlled variable biases.
Toner is printed directly onto the drum, by direct contact with a rubber development roller. It removes all unwanted toner and returns it to the developer unit for reuse by reversing the trend.
What are the advantages and disadvantages of xerography?
Advantages:
- These are affordable machines for small, medium and large offices.
- The ease of reproducing text and graphics in a matter of seconds.
- Variable image resolution.
- They feed paper, cardboard and acetates.
- Easy to operate.
- No CPU required.
- Can do inserts, stapling and double-sided printing.
- No drying required.
Xerographic printing, together with inkjet printing, has the main advantage of not needing plates, which significantly reduces your space, economic and time requirements. All subsystems necessary for the acquisition, engraving, checking, storage and destruction of the plates are eliminated.
Drawbacks:
- Tonal scale is lost.
- Toner leaves a white smudge if it is running out.
- For printing quality images or photographs, photo printing is best.
- Some prints come out extremely hot, impossible to be taken by the operator.
- You cannot print on textile, for that there are screen printing, sublimation and other techniques.
Brief history of xerography
Xerography was invented in the late 1930s by an American patent attorney named Chester Carlson.
At first, engineers considered the idea useless and it took several years before the potential of the invention was appreciated by industry.
During those years, IBM, Kodak, General Electric and RCA were among the companies that turned Carlson down.
The Battelle Memorial Institute, a non-profit organisation, invested in Carlson’s research and eventually signed a licensing agreement with a company called Haloid. Battelle and Haloid collaborated on research and demonstrated the technique in 1948. Haloid subsequently became Xerox.
We have more interesting articles on the types of printing, such as rotogravure, printing 3D metal or printing metal.