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Lesson

Printing Technology

Computer printing devices employ a number of technologies to create prints. The most common are

  • dot matrix (impact printing). An early technology, now used mainly for printing multipart forms for business purposes
  • inkjet (heat activated, and piezo-electric activated)
  • laser (laser beam, LED array, and laser engraving)
  • wax (solid wax and thermal wax)
  • dye-sublimation
  • plotter (multiple coloured pens draw on the media)
  • cutter (pen replaced by a blade to cut the outline)

Several printer technologies are described below

Dot Matrix

Dot matrix uses a print head with a row of pins, each driven by a small electromagnet. As the print head moves across the page, the pins strike the ribbon, creating the pattern. Paper advances after each pass. Resolution is very low. The max is around 24 DPI.

Figure Dot Matrix Printer technology

Inkjet

Inkjet printers use liquid inks. Some inks use dyes dissolved in the ink, and others use ground-up pigments suspended in the ink. Pigment inks tend to be more opaque and last longer. The ink is forced through the nozzle in very fine droplets, smaller than the diameter of a human hair. A print head can have 300, 600, or more nozzles, giving resolutions up to 1200 DPI. Each nozzle can be fired independently. The droplets hit the paper and are absorbed into the surface. There are two technologies for producing the droplets - thermal, and a piezo-electric crystal. Canon and HP use the thermal method which heats the ink, causing a bubble to form, pushing ink through the nozzle, which forms a droplet and sprays onto the paper.

Figure Thermal Inkjet technology

The heating - cooling cycle is a physical limit to how fast droplets can be expelled from the nozzle.

Epson uses the piezo-electric effect. Electricity is applied to a quartz crystal, causing it to flex or move, forcing the ink out of the nozzle.

Figure Piezo-electric Inkjet technology

With no heating - cooling cycle to slow things down, this technology can operate faster

Laser

A laser printer uses a laser to draw the image onto the surface of a rotating drum. The printer rasterizes the image into a series of dots. The laser switches on and off rapidly to draw the dots. A rotating mirror caused the laser beam to travel left to right across the surface of the drum as the drum rotates. Where the beam draws, the surface of the drum is positively charged with an electrostatic charge. The toner is negatively charged. As the +ve charged areas pass the toner, toner is attracted to the surface of the drum. It sticks only to the drawn image. The paper, which is given a larger -ve charge, is fed past the drum. The toner on the drum transfers to the paper. As the paper exits the printer, it passes through the fuser, which is a heating element. The toner (which includes waxes) melts and fuses to the paper.

Figure Laser Printer Technology

Other than the source of information to drive the laser beam, laser printers work like photocopiers. Inexpensive lasers are usually 600 DPI. Medium priced lasers are around 1200 DPI, which is approaching publishing quality.

There is another type of toner based printer that works like a laser printer, except it uses a row of LEDs (light emitting diodes) to create the image. for a resolution of 300 DPI, there are 300 diodes per inch. On a typical 8.5 inch wide printer, the print area is 8 inches, requiring 2400 diodes. For 600 DPI, there would be 4800 diodes. Each is turned on and off independently.

Figure LED Printer Technology

Regular and Large Format Printers

Regardless of printing technology, there are three categories of printers

  • regular printers (consumer and commercial qualities) ranging in size from 2 by 4 inch label printers up to 13 by 19 inch printers
  • large format commercial and industrial printers starting at 24 inches wide, capable of printing on continuous roll paper or fabric
  • image setters and printers designed to make printing plates for presses

Printer Description Language

Information has to get from the computer to the printing device in order for the device to produce the print. Printing devices have special processors and software, called page description languages (PDL), to handle the information. There are 3 basic types of PDL

  • Adobe's Postscript, now up to version 2
  • Hewlett Packard's Page Control Language (PCL), now up to version 6
  • Hewlett Packard's Graphics Language (HPGL) used with plotters/cutters (and similar Graphic Languages from other plotter/cutter manufacturers)

The PDL sends the page as a stream of vector data to the printer. Both Postscript and PCL then turn that vector data into a raster, or set of printer dots using the resolution of the printer. Printers can have resolutions of 300, 600, 720, 1200, or more DPI. HPGL also goes out as a vector data stream. Plotters and cutters use the vector data to drive the pens and cutters directly. There is no conversion to raster. The limit in quality is determined by the number of steps that the stepper motors driving the pen/cutter head can take in one revolution. More steps (400 per revolution, for example) means more precise drawing.

The publishing industry uses Postscript almost exclusively. It will print identical images at the best resolution of the printing device, regardless of printing device. This allows the use of less expensive, lower resolution printers to proof (check layout, colour, etc) before committing the production to very expensive production printers.

Because Adobe charges manufacturers for the use of Postscript in each printer, and HP does not charge for PCL, Postscript printers are much more expensive than PCL printers.

Further Information

For further information, check out these sources

Activity

Assigned activities

The purpose of this activity is to research printing technologies.

  • Research a printer technology (laser, inkjet, wax, dye-sub, etc), to determine the state of current technology
    • effective resolution
    • quality of images
    • speed of printing
    • how it works
  • Create a suitable entry in the course portfolio to include all your findings

Test Yourself

There is no self test for this lesson.