Specifications

Caution
Ozone is a noxious and corrosive gas that should be avoided in closed, unventilated spaces. Although ozone is used to
deodorize air and purify water, working in close proximity to laser printers for extended periods of time without a sufficient
fresh air supply can cause health problems.
Many laser printers have replaceable ozone filters that should be changed after several thousand pages have been
printed. Check your printer documentation to determine when the ozone filter should be changed. Use the self-test feature
on the printer to print a page showing the number of pages the printer has produced to help you determine how many
more pages you can print before you change the filter (or if you’re overdue).
HP’s Web site has detailed information on which of its laser printers require ozone filter changes and the relevant part
numbers.
The drum is sensitive to any type of light, but a laser can produce fine enough dots to support the
high resolutions required for professional-looking documents. Every spot the laser light touches on
the drum is electrically discharged, leaving the pattern of the page’s characters and images on its sur-
face. The laser in a printer discharges the areas of the drum corresponding to the black parts of the
page—that is, the characters and images that comprise the document’s content. This is known as
write-black printing. By contrast, copiers discharge the background areas of the page—a process called
write-white printing.
Toner Application
As the photoreceptor drum rotates, the portion of its surface the laser has discharged next passes by
the developer unit (see Figure 3). The developer is a roller coated with fine magnetic particles that
function as a “brush” for the toner. Toner is an extremely fine, black plastic powder that actually
forms the image on the printed page. As the developer roller rotates, it passes by the toner container
and picks up an even coating of the particles on its magnetic surface. This same developer roller is
also located right next to the photoreceptor drum. When its surface passes by the roller, the toner par-
ticles are attracted to the areas that have been discharged by the laser, thus forming the image of the
page on the drum using the toner particles as a color medium.
As the drum continues its slow rotation, it next passes close to the surface of the paper. The printer
has an entirely separate mechanism for extracting one sheet of paper at a time from the supply tray
and passing it through the print engine so that its flat surface passes underneath the drum (without
actually touching it) at the same speed that the drum is rotating. Beneath the sheet of paper is
another corotron (called the transfer corotron) that charges the paper, causing it to attract the toner
particles from the drum in the exact pattern of the document image. After the toner is transferred to
the page, the continued rotation of the drum causes it to pass by a discharge lamp (usually a row of
LEDs) that “erases” the image of the page by completely discharging the surface of the drum. By this
time, the drum has completed a full revolution, and the entire charging and discharging process can
begin again for the next page of the document.
As you might imagine, these processes leave little margin for error when it comes to the proximity of
the components involved. The drum must pass very close to the corotrons, the developer roller, and
the paper surface for the toner to be applied properly. For this reason, many print engines (including
Canon and HP) combine these components into a single integrated cartridge that you replace every
time you replenish the printer’s toner supply. This increases the price of the toner cartridge, but it also
enables you to easily replace the most sensitive parts of the printer on a regular basis, thus keeping
the printer in good repair.