Strange Imaging Problem on the HP4500's9 Oct, 2001 By: Steve Geishirt imageSource
Strange Imaging Problem on the HP4500's
calls and says their 4500 prints are missing colors in certain areas of the
page. Your first thought is user error, but you decide to inquire further. The
customer continues by telling you the left side of the page is in monochrome but
the right side is in color – but it’s all supposed to be in color. They also
tell you this occurs on a configuration page. Now they have you wondering. When
you show up at the customer site, sure enough, they have a monochrome print on
one side of the page and color on the other side, and it’s printing that way
on the configuration page (see figures 1,2). Does this scenario sound
far fetched? This happens out in the real world, so lets discuss this further.
|Figure 1||Figure 2||Figure 3||Figure 4|
1 & 2 - Notice how only color drops out on the left side of figure 1,
compared to a good print in figure 2.
3 & 4 - Reflect the same characteristics.
1 – Problem Resolved
have been a couple of printer problems that have caused this type of print, and
it’s not a supply item causing it. What boggled all of the technicians I spoke
with was why color dropped out on one side but the other side of the page is in
monochrome. Most of them started thinking it was a logic problem, how could the
printer just drop out color from one side of the page? By the time they called
looking for clues, they had replaced the formatter, DC controller; one even
replaced the laser scanner. What the repairs turned out to be, were definitely
surprise to the technicians working on them.
problem in scenario 1 was that one of the two lift levers that lift the ITB
(Intermediate Transfer Belt) up to the drum unit had fallen off its shaft (see figure
5). The E-clip somehow came off and the arm eventually wobbled its way off
the shaft and dropped into the bottom of the printer. This caused one side of
the transfer belt to drop slightly away from the image drum. When the arm was
replaced, full color returned to the page.
another scenario, a streak down the page was printing little or no color; this
streak just went monochrome down the page. The rest of the page had good color.
When the technician ran an engine test, the black lines appeared, but the color
lines did not print in that area.
2 – Problem Resolved
resolve this problem, the technician replaced the ITB unit and the transfer
roller, but the monochrome streak continued. As it turned out, the problem was a
dirty mirror in the laser scanner assembly. Once it was cleaned, full color was
The Toner Differences
question begging an answer here is why did just the color toner drop out and the
black toner still print? Good question. The answer is, black toner is different
from the color toners. Black is your traditional mono-component toner composed
of black plastic resin bonded with iron oxide. The color toners are made of a
different composition; their inventor – Canon, knows them as S Toners. The S
Toner is not mono-component, but a different formulation. It has a core made of
wax, which is chemically coated with a pigmented toner. This combination of
toner and wax was created for a reason. On previous color models, toner was the
standard mono-component type in different colors, of course. When printing in
color, different pigments of toner are mixed together at the fusing process to
create other colors. In melting them together, they need an agent to help them
mix and keep the toner from sticking to the fusing rollers. Using the
mono-component color toner, the mixing agent is silicon oil, but this mixing
agent can also be a detriment. The oil is a consumable, which means it needs to
be replenished over time, and it can get quite messy in some machines leaving
streaks of oil on desk tops, printed pages and sometimes cloths. The S toner
does not need silicon oil; it uses the wax in the core of the toner particles as
the mixing agent. This wax also makes the print look shiny on standard paper –
an extra added advantage.
is also important to note that color fusers are different from the standard
monochromes; this boils down to the fusing rollers specifically. Monochrome
printers typically have an aluminum core, Teflon Ò
coated upper fuser roller with a silicon rubber lower pressure roller to create
enough heat and pressure to press and melt toner into the fibers of the paper.
In the color process, most of the fusers have lamps in both the upper and lower
rollers – consistent heat is very important. Both rollers have a special
rubber coating and together they create a lot of pressure. The 4500 printer
actually has a process of removing pressure in the event of a paper jam so the
end user can pull the paper out of the fuser. If this process were not in place,
the customer would tear the paper due to the pressure between these two rollers.
Yet, pressure is very necessary to mix the colors together in the fusing
To The Scenario
get back to the point of why color dropped out, remember how we looked at the
different properties of toner earlier in the article. The black toner, still a
mono-component, is heavier with the iron oxide. It also responds better to the
electrostatic properties that move and hold the toner on the imaging devices in
the printer. The color S toner is much lighter as many technicians have told me,
the color toners float all over these printers, but not the black. This floating
of the color toners is the down side to S toner. It is also important to note
that since the toners are not the same, the charges used to create these images
are not the same. This would make sense considering the different toners used.
This would also make sense when it comes to the image problems at the start of
as in the scenario 1, when the ITB had dropped away from the image drum on one
side, the black toner was able to migrate across the gap to the belt, while the
color toner with its lighter weight did not. The resulting print was a half page
of color and a half page of monochrome only. In scenario 2, the laser beam was
partially weakened by specific area of build up on the mirror that reflects the
beam down onto the drum to write the image of the color in process. When it came
to developing black, the toner was able to jump the gap to create an image due
to its mono-component properties, while the lighter color S toners did not. The
resulting print was a color page with a line of monochrome print from the top,
to the bottom of the page.
of this is important to keep in mind when working on either the 4500 series or
8500 series of printers, as they both use the Canon S toner. We must remember
that a color printer is just another laser printer. We must also remember that
the original monochrome printer is a springboard to the new concepts of color
that we need to learn and understand. The world is indeed turning to color and
the question is who is going to be there to service it.