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Direct Imaging Processes

Electrophotography | Ionography | Magnetography | Ink-Jet | Thermal

Electrophotography

Electrophotography is the most widely used of the plateless printing technologies. It is common feature for electrophotographic equipment to have the ability to print different information on every printed impression. Every page in a book, for example, can be printed in a single production run using a digital press or desktop printer. Electrophotographic equipment can produce booklets and manuals at high speeds and four color items directly from the original artwork or from a computer file. The two types of electrophotography are xerography and laser.

 

Xerography

Original artwork is placed face down on the flat glass plate of a xerographic device. A light source beneath the glass plate travels the distance of the image and the image is reflected onto a photoreceptive drum. The drum is given an electrostatic charge as it passes along a corona wire assembly. Light is reflected from the original document onto a mirror and from there the light is reflected onto the charged drum where an image is formed. The image on the drum, which is invisible, is known as an "electrostatic latent image". After exposure to light, the photoconductors discharge from the non-image areas of the drum. The drum passes along a toner roller and liquid or dry toner particles are attracted to the charged image areas. The paper receives a charge from a corona transfer assembly and when the drum makes contact with the charged paper, the toner is deposited onto the paper. The toners are fixed onto the substrate with heat and pressure rollers. The drum is then cleaned of any remaining toner particles and the latent image is removed from the drum with an erase corona. The main components of a xerographic device are shown in the illustration below.

Laser Printing Systems

A Laser Electrophotographic printing device combines a scanner and imagesetter into one system. The original artwork is digitally scanned and the digital information is imaged onto an electrostatic drum with the use of a laser light or LED (Light Emitting Diode). A document or image already stored on a computer can also be retrieved for imaging on the laser printer. Toner is attracted to the laser imaged areas of the drum, which is then deposited onto the substrate.

The toners can be a dry mixture or liquid. Dry toner images are fixed onto the substrate with heat and liquid toner images dry shortly after the liquid toner makes contact with the substrate. A minimum of 1800 dots per inch is required for high-quality print applications when using laser systems. The main components of a laser printer are shown in the illustration below.

Ionography

Ionography is also known as "ion deposition" or electron "charge deposition printing". The ionographic process creates an image with the use of an electron cartridge which creates a negative charge on a nonconductive surface. The nonconductive surface consists of a drum with a dielectric surface of aluminum oxide which attracts a magnetic toner. The toner is then fixed to the substrate with a cold fusion process. Ionography uses a static electric charge to draw the toner particles from the drum onto the substrate. A high pressure roller fuses the toner to the substrate. A scraping device removes any excess toner from the drum and an erasing rod removes the latent image from the drum so that the drum will be ready for the next copy.

Ionography is used only for one color printing because the high pressure cold fusion process can slightly distort the substrate, which means that multiple colors may not line up correctly. It is useful for high volume applications and for variable information printing, which allows for changes in the content of the print application during the press run. Variable applications such as checks, statements, letters, tickets, and tags, are printed with the ionographic process. Applications printed with the ionographic process do not hold up to rough handling as well as applications printed with other processes.

 

Magnetography

Magnetography is similar to ionography except that the drum that is used is magnetic. The electronic image is converted to a magnetic charge on the drum, which attracts a toner containing iron particles. The toners are very opaque so the process is best suited for spot colors rather than four color process printing, which requires transparent colors in order to work properly.

Magnetography is used for applications, which require variable imaging such as labels, business forms, direct mail pieces, tickets, and barcoding, which is the main product created with the process. Because the toners are so opaque, they are well suited for barcode printing, which requires a thick black colorant.

The print resolution is lower with magnetography than with some of the other processes, but the printing speed is very fast. Press speeds as high as 460 feet per minute can be attained.

 

Ink-Jet Technology

Ink-jet technology creates printed documents with streams of ink drops that are deflected to the substrate based on information in digital files. It does not require an image carrier, or plate, and it does not require equipment like a Xerographic device or a printing press. The same information can be printed throughout a print job or variable information can be printed based on the requirements of the application. The main types of ink-jet technologies are continuous jet and drop-on-demand.

 

Continuous Jet

With continuous jet technology, drops of ink are continuously produced and applied to the substrate to produce the image. A pump sends ink drops through a nozzle at the rate of over a million per second which can produce an image of nearly the same quality as a continuous tone image such as a photograph. There are three types of continuous jet technologies: charged drops for printing, uncharged drops for printing, and electronic deflection.

  • Charged Drops for Printing: Drops of ink are given a charge and are deflected to the substrate to produce the image. The ink drops that are uncharged are recycled through the system to be reused.

  • Uncharged Drops for Printing: This type of technology also applies a charge to the ink drops except that the uncharged drops are used for the actual printing and the charged drops are recycled.

  • Electronic Deflection: This type of technology applies a charge to all of the ink drops and the application of the drops is determined by deflection, which is controlled electronically.
 

Drop-on-Demand

Drop-on-demand is a type of ink-jet technology in which the ink drops are formed and then applied as a response to a digital signal. There are two types of drop-on-demand printer systems: piezoelectric and thermal ink-jet.

  • Piezoelectric: A piezoelectric crystal is given an electric charge, which produces a pressure pulse in the imaging head. This produces the emission of an ink droplet onto the substrate.

  • Thermal Ink-Jet Systems: There are two types of printer systems using thermal ink-jet technology: liquid thermal/bubble jet and solid ink-jet:
 
  • Liquid Thermal/Bubble Jet: Heat produced from an electrical resistor vaporizes the moisture in the ink which causes an ink bubble to form. The expanding bubble creates pressure inside the ink nozzle which propels the ink to the paper. The ink bubble then contracts which lowers the pressure causing more ink to be drawn into the printing head. The entire process occurs very rapidly in the printing device. In fact, the process is repeated thousands of times per second, producing high quality results.
  • Solid Ink-Jet: A solid ink-jet printer is also known as a "phase change ink-jet printer". The ink begins as a solid and is heated to convert it to a liquid state. The ink is propelled as drops onto the substrate from the impulses of a piezoelectric crystal. Once the ink droplets reach the substrate, another phase change occurs as the ink is cooled and returns to a solid form instantly. The print quality is excellent and the printers are capable of printing on almost any type of paper and transparency substrates.

Thermal Technology

The three main types of thermal technologies are thermal transfer, thermal dye sublimation, and thermal wax transfer.

 

Thermal Transfer

With thermal transfer technology, an image is transferred to the substrate from a plastic ribbon that is impregnated with colored pigments. A thermal head heats the ribbon, transferring the pigment to the printing surface. The process produces very high quality results, but it is slower and more costly than other technologies, such as ink-jet. Some types of products printed with the thermal transfer process include barcoded applications, labels, and colorful logos.

Thermal Dye Sublimation

Thermal dye sublimation is similar to the thermal transfer method except that dye pigments are used rather than thermoplastic ink. The dyes are contained in a transfer ribbon made of plastic film. The printing head contains thousands of heating elements which are each capable of producing as many as 256 different temperatures. The varying temperatures of the heating elements cause varying amounts of pigment to be deposited from the transfer ribbon onto the printing surface. The result is different densities of color that are created in the image. When the dye pigments are heated, they go through a process called "sublimation", in which a solid changes directly into a gas. When the dye, in the form of a gas, makes contact with a specially coated paper, it changes back into a solid. The individual spots of dye created with the thermal dye sublimation process blend together to make an almost continuous tone image similar to an actual photograph.

Thermal Wax Transfer

Like the dye sublimation process, many heating elements are contained within the printing head, which control the amount of pigment that reaches the substrate. Instead of using a dye, a wax-based ink is used. The heating elements melt the wax-based ink and it is deposited onto the printing surface. It is necessary with many wax thermal transfer printers to send the substrate through the equipment multiple times in order to print an image in full color. One pass is necessary for each of the process colors, cyan, magenta, yellow, and black. Even with this drawback, thermal wax transfer systems produce a very intense color image and the printed piece is resistant to moisture and ultraviolet light. Systems using thermal wax transfer technology are typically used for color proofing, presentation applications, and T-shirt printing.

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