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© Copyright Brian Brown, 1992-2001. All rights reserved.

Printers
There are two main classifications of printers.

• Impact
  In impact printers, the print head mechansim strikes an inked ribbon located between the print head and the paper. The general features of impact printers are
  • uses force by applying hammer pins to strike the paper
  • slow speed in characters per second
  • prints on most paper types
  • transperancies not supported
  • multiple copies may be printed at once

  The two main types of impact printers are Dot-Matrix and Daisy-Wheel.

• Non-Impact
  With non-impact printers, the print head does not make contact with the paper, and no inked ribbon is necessary. The general features of non-impact printers are
  • print head does not make contact with the paper
  • higher speed in characters per second is possible
  • prints on most paper types but better quality obtained with better paper
  • transperancies usually supported

  The three main types of non-impact printers are laserjet, ink-jet and thermal.

Dot-Matrix Printers
Dot matrix printers are suitable for draft copies and home use, where quality of the finished type is not critical. Dot matrix printers have tended to become cheap, but now are being quickly overtaken by cheap laserjet and inkjet printers, which offer higher printing speeds and superior quality, as well as good color.

• impact printing using an inked ribbon
• characters are formed from a matrix of dots
  
• the character is built up one column of the matrix at a time
  
• the print head consists of a number of wires formed vertically, creating one column of the dot matrix for every strike
  
  Fig 6.46: Dot Matrix Print Head
• the number of pins in the print head varies
• the more print head pins the better the printed quality
• the print head wires are made of tungsten and are connected to an electronic solenoid
  
  Fig 6.47: Dot Matrix Print Head Solenoid
• a large number of different characters or graphics can be created using a dot matrix pattern
• speeds up to 250 characters per second (cps) are possible

High speeds cause smudging of the characters, as the print head must stop, fire the pins, wait for the pins to retract, then move the print head to the next colum before firing again. The less time there is involved between firing the pins means it reaches a stage where the pins have not fully retracted from the previous firing, which causes smudging as the print head is moved to the next column position. The character printing speed restriction is thus based on the physical mechanism being used to print the characters.

Fig 6.48: Panasonic KXP-1150 Dot Matrix Printer © Panasonic

Daisy-Wheel Printers
Daisy wheel printers use a spoked wheel with characters placed at the end of each spoke. A print hammer is used to strike the desired character onto the ink ribbon and then the paper.

The spoked wheel of characters is rotated around until the desired character is under the print hammer, then the print hammer is fired which strikes the character, pushing it against the ink ribbon, and onto the paper, creating the character.

Fig 6.49: Daisy Wheel Print Head

Different fonts are available by changing the print wheels. Daisy wheel printers were commonly found in typewriters. Below is an image of the Brother electronic typewriter which uses a Dasiy Wheel print mechanism.

Fig 6.50: Brother WP-1700MDS Daisy Wheel printer © Brother

Laser-Jet Printers
Laserjet printers are very common today. Most laserjet printers have been based on the Canon Print Engine, which originally ran at 4ppm (pages per minute), with a resolution of 300dpi (dots per inch).

Refinements to the print engine has increased the speed of printing and resolution of the print quality.

How the LaserJet works

• the print information is sent from the computer to the printer
• the printer converts the print information into a series of off and on commands to the laser, creating corresponding pulses of light
• a spinning mirror deflects the laser beam creating a horizontal line across the surface of a light sensitive drum
• after creating one horizontal line, the drum rotates (normally about 1/300 of an inch) and the next horizontal line is drawn by the laser
• paper is drawn into the printer and electrically charged. In the canon based engine this is a positive charge.
• the drums surface was electrically charged negatively. Each point where the laser strikes the surface of the drum, it creates a dot of positive charge. Each positive charge on the surface of the drum represents a black dot which will be printed onto the paper
• the drum rotates past a container bin which contains a black powder called toner. The toner is negatively charged, and thus is attracted to the positive areas on the drums surface which was struck by the laser beam
• the drum is rotated a bit more and the paper is fed onto the drums surface via a pressure roller, which transfers the black toner onto the paper
• the drum rotates a little further round, the paper is fed past a heating element which fuses the toner onto the paper and then the paper is fed out of the printer
• the drum surface is then returned to a single charge using a corona wire, which removes all positive charges and returns the entire surface of the drum passing underneath it to a negative charge

The diagram below shows a simplified canon laser print engine.

Fig 6.51: Canon Laserjet Print Engine

The diagram below shows a laser scanner assembly from a canon based engine.

Fig 6.52: LaserJet scanner assembly

The following diagram below shows a Hewlett Packard Series 5 Laserjet printer.

Fig 6.53: HP LaserJet 5 Printer

Ink-Jet Printers
Ink-Jet printers have become the defacto standard for home computers and low cost printing. They offer good quality at an affordable price.

• conductive ink is forced through a small nozzle producing a small droplet of ink, which is propelled towards the screen surface
• the size and spacing of the ink drops is kept constant by vibrating the nozzle at a high frequency (100KHz)
• each drop of ink, after leaving the nozzle, is charged as it passes an electrode
• the drops are deflected using another electrode
• characters are formed from a dot-matrix, typically there are 10^3 drops per character
• print quailty is high, speed is slow, typically about 100cps
• good results may require special non-absorbing paper, overhead transperancies reqire special material

Fig 6.54: HP DeskJet 680c Printer

Thermal Printers
Thermal printers are generally used in low cost printers and fax machines.

• the print head contains high temperature heat elements arranged in a matrix
• when the print head is pressed againt the paper, the heat elements burn small holes in the paper, forming the character
• the paper is white and develops color (black or blue) when heated above 110 degrees C
• printing is generally slow
• the fumes generated are unpleasant
• a messy residue is left over by the heating process and can clog up the printer mechanism
• print quality is poor and fades with time
• special paper is required

Some types use a silvery grey paper, which is an aluminium surface coating. When the print head burns away this layer, it exposes a dyed layer underneath.

One application for thermal printers is the production of bar codes. The diagram below illustrates a bar code thermal printer.

Fig 6.55: Cognitive Solutions PD1520 Thermal Printer © Cognitive Solutions

Fig 6.56: Cognitive Solutions Thermal Printer Label © Cognitive Solutions

Summary
The two main classifications of printers are impact and non-impact. In impact printers, the print head makes physical contact, using force to create the characters.

Examples of impact printers are dot-matrix and daisy wheel printers. As new technology is developed, these mechanical type printers are becoming increasingly outdated.

Examples of non-impact printers are laserjet and inkjet. These printers offer superior quality and greater options (in terms of the number of fonts and quality of graphic pictures).

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© Copyright Brian Brown, 1992-2001. All rights reserved.