topic of the day 11-9-01

Photocopiers and Laser Printers

An application of semiconductors and photoconductivity

Background

• Materials like Silicon are normally insulators
  • 4 valence electrons, 4 bonds
• Materials Changes if it absorbs light
  • light frees an electron
  • electron makes that region conductive

• The original Xerographic Copier
  • used thin layer of Selenium
  • coated on surface of a metal drum
• Modern devices use a plastic belt
  • the surface is a photoconducting polymer
  • the underside of the belt is metallized
  • (we will talk about using a modern belt)

• Step 1-- electrically charge the surface of the belt
  • (rub with cat fur? pass by high voltage electrodes?)

• Step 2-- Expose the charged surface to light
  • In laser printer, small laser
    • deflected by mirrors to where we want image
  • In photocopier, lamp and lens
    • no light around dark text
    • good light near white regions of page

• Step 2 consequences
  • Where illuminated,
    • the plastic conducts electricity
  • Where conductive,
    • charge leaks though the belt
  • You end up with the image as an electrical charge pattern on the belt

• Step 3-- The toner
  • A fine black Powder
  • Electrically charge it (other sign)
  • Dust it onto the belt (or drum)
    • attracted to charged areas of the image
    • collect there, not on rest of the belt
  • Now we have a black dust image
    • looks like the original B&W image
    • but it's just dust on the belt

• Step 4-- Transfer the charge to paper
  • Paper is not electrically conductive
  • Give paper a surface charge (opposite sign)
  • Roll paper across the surface of the belt
  • Paper attracts and picks up the toner
  • Now, we have toner on the paper
    • toner wherever the image was black

• Step 5 -- Sealing the image
  • (artists in charcoal would spray a sealer to preserve the image)
  • The toner is actually a polymer
  • Get it hot and it softens or melts
  • It sticks to the paper, permanently
    • that's why fresh photocopies come out warm
  • you can see that image is raised, sealed to top of paper
    • can scrape off with a razor blade w/o changing paper
    • (inks soak into the paper)
    • (erasing involves some paper removal)

Technique developed by Carlson around 1940's

• patented-- named it xerography (dry writing)
• found no interested companies
  • copies made by photography-- good enough
  • Xerox machine too complex to develop ?
• one small company : Haloid
  • (name from Halogen, Made Silver Halide photographic paper)
  • worked with Battelle Institute, Columbus OH
• put into production, 1959 was first model
  • changed name to Xerox
• in early 1970's production was limited
• by rabbit colonies in So. France
• Se drums scratched easily, needed cleaning
  • rabbit fur was only working material
• Modern machines (laser printers, photocopier)
  • treat drum and electrodes are disposable
  • (laser printers-- fixed toner supply)

nice step by step description with pictures