HP 1200C User Manual
Media Path for a Small, Low-Cost,
Color Thermal Inkjet Printer
The DeskJet 1200C media path is heated for media independence,
requiring development of a new grit drive roller and pinch wheel
combination. A new stepper motor was developed to attain the target
speed and accuracy. Media flatteners and precise gearing with an
antibacklash device contribute to accuracy.
by Damon W. Broder, David C. Burney, Shelley I. Moore, and Stephen B. Witte
The media path is the part of a printer that moves media
past the print cartridges, stopping and accurately locating
the media for printing. The media path also constrains the
media in a plane a fixed distance from the print cartridges. A
media path should advance media accurately and quickly, be
quiet, be inexpensive, hold the media flat, and keep the media
the correct distance from the print cartridges.
Fig. 1 shows an overview of the media path of the HP DeskJet 1200C printer. Media stacked in the input tray (1) is individually picked by a media pick roller (not shown) and
driven around the curved preheat zone (2) where it is preconditioned (moisture is driven off and the temperature is
raised). When the page reaches the pinch/drive rollers (3),
the main drive system (4) takes over from the pick roller
drive (not shown). Once in the print zone (5) the media is
heated further and ink is sprayed onto the page. The heating,
soaking, and drying causes the media to move out of its
Paper Control Shims
6
Preheat Zone
2
3
Pinch/Drive
Rollers
Main Drive System4
Input Tray
1
plane, but the media control shims (6) help hold it flat for
better print quality. The page is then incrementally advanced
and printed upon until the entire page has been printed.
Finally, the page is fed out into the output tray (7) and the
process is ready to repeat.
Design Approach
For the office printer market, the DeskJet 1200C is designed
to support a wide variety of plain papers, to be HP LaserJet
printer compatible, to print text very quickly, to print highquality graphics, and to be cost competitive. These characteristics forced the design team to face the following challenges:
• Constrain plain papers flat even though plain paper tends to
cockle and curl in various directions when ink is sprayed on
and heated.
• Print to 50-dot row margins for LaserJet compatibility, even
though such small margins allow very little control over
media flatness.
Fig. 1. Overview of the media
path of the HP DeskJet 1200C
printer. Media stacked in the input
tray (1) is individually picked by
a media pick roller (not shown)
and driven around the curved
Print Zone
5
Media Direction
7
Output Tray
preheat zone (2) where it is preconditioned (moisture is driven
off and the temperature is
raised). When the page reaches
the pinch/drive rollers (3), the
main drive system (4) takes over
from the pick roller drive (not
shown). Once in the print zone
(5) the media is heated further
and ink is sprayed onto the page.
The heating, soaking, and drying
causes the media to move out of
its plane, but the media control
shims (6) help hold it flat for better print quality. The page is then
incrementally advanced and
printed upon until the entire page
has been printed. Finally, the
page is fed out into the output
tray (7) and the process is ready
to repeat.
72 February 1994 Hewlett-Packard Journal
• Move media very quickly through the print area while main-
taining high placement accuracy for good graphics print
quality.
• Keep the price low.
The parts of the media path (Fig. 1) that drive and meter the
media include the stepper motor, the drive pinions, the drive
gears, the antibacklash device, the shaft bushings, the adjustable plate, the drive rollers, and the pinch rollers. While
driving the media these components work together to ensure fast, accurate movement of the print media, which in
turn provides the best possible throughput.
The stepper motor provides the fastest response available in
an inexpensive motor. Although stepper motors aren’t always the most accurate type of motor, the accuracy of the
system can be optimized by designing the system so that the
motor always moves in multiples of four half steps. Moves
of four half steps cancel out all of the error caused by manufacturing variation except the locations of the stator teeth
which are formed out of the sheet-metal case of the motor.
Used in this way, the stepper motor can provide very fast
and accurate moves and is well-suited for driving the media
in an inkjet printer.
The gearing system in the DeskJet 1200C is designed to optimize the accuracy of the media drive. A single-reduction drive
is used because there are fewer components in the gear train,
so this type of drive is more accurate. However, accuracy is
not the only reason to use a single reduction. The size and
spacing of the drive rollers relative to the print cartridges and
heater also pushed the design towards a single-reduction
gear train. The DeskJet 1200C relies on a heated media path
to dry the ink as it is being printed. To make room for the
heater while maintaining control of the print media, the drive
rollers had to be much smaller than other inkjet printers had
typically used. By using smaller drive rollers, we were able
to use a single-reduction gear train.
The spacing of the drive rollers was a critical issue in the
design of the DeskJet 1200C. The spacing is constrained by
the size of the print cartridges and heater. Thus, we had to
trade off room for the heater and print cartridges against
control over the leading edge (the star wheels hold the media down against the output drive roller) and print quality in
the bottom margins (the main drive roller is more accurate
than the output drive roller). As the space between the rollers increases, there is a longer span of media at the top of
the page to control with the star wheels and a longer space
at the page bottom where the media is driven by the output
roller.
A major problem in reducing the shaft spacing was reducing
the size of the drive roller. Drive rollers in inkjet printers
have typically been rather large elastomer-coated shafts, 40
to 70 mm in diameter. To shrink the roller to a size we could
use (20 mm) we had to find a different process, one that
was new to inkjet printing. We decided to try the grit drive
system used in several HP plotters, which consists of a gritcoated metal drive wheel and an elastomer pinch wheel. By
using a grit system, we could minimize manufacturing errors
associated with the size and shape of elastomer rollers and
reduce the size to one we could use. Of course, we then faced
many problems adapting the grit drive system to a heated
media path system.
Right Shim
Center Shim
Grill
Left Shim
Fig. 2. A grill and three shims hold the media flat in the print zone.
The media path must also hold the print media flat. The environment in the print region is somewhat hostile to paper.
The ink soaks the paper, and the heater boils the water out
of the paper and dries it out. The DeskJet 1200C is intended
to print on any office paper with nearly equal print quality.
The heater and the ink design help achieve media independence, but at the same time, the heater and ink wreak havoc
with the fibers that make up the paper. The fibers, predominantly cellulose, swell with ink and the paper expands.
Then, as the paper is heated and dried, the fibers shrink and
the media contracts. None of the expansion and contraction
is uniform from one side of the media to the other, so the
media tends to move out-of-plane. To print with high quality,
the distance from the print nozzles to the print media must
be accurately maintained. We hold the media flat against a
grill (which covers the heater) with three shims—center,
left, and right, as shown in Fig. 2. The fixed right shim constrains the right edge of the media and acts as the zero reference point for printing. The adjustable left shim adjusts to A
and A4 sizes while constraining the left edge of the media.
The center shim helps keep the media flat against the grill
and helps keep the media from crashing into the print cartridge. The center shim also allows printing very near the
bottom edge of the media—as close as 5 mm.
As the paper advances out of the print region, we also hold
it down with the star wheels. These systems provide good
control over the media, keeping it constrained in a plane at a
fixed distance from the print nozzles.
Heated Media Path
The DeskJet 1200C was envisioned to support a broad range
of media: not just A and A4 sizes, but also glossy paper,
transparency film, and virtually any paper on the market (at
one point we were even printing on paper bags). With such a
wide range of media as the goal, and the added challenge of
fast throughput, it became evident that the DeskJet 1200C
would need a heated media path to meet these goals with
any kind of reasonable print quality.
The heated media path of the DeskJet 1200C (see Fig. 3) is
composed of two main components: the preheater and the
main heater. The preheater is a flexible polyimide heater
that serves as the inner guide for the media. As the media is
February 1994 Hewlett-Packard Journal 73
Preheater
Main Heater
Fig. 3. Heated media path.
Writing Zone
fed up into the main heater and the writing zone, it is
wrapped around the preheater. This contact allows the preheater to precondition the media so that when the media
reaches the writing zone it is much more dimensionally
stable. Much of the paper fiber shrinkage that occurs with
heating happens before the paper reaches the writing zone.
The main heater consists of a Kanthal wire and a quartz
tube. Current drawn through the wire causes it to heat up
and emit infrared radiation. The radiation and convective
heat from the bulb help evaporate the water from the ink in
the writing zone. This increase in the evaporation rate of the
ink allows increased throughput and improved print quality
over a much broader range of media.
Drive Roller Development
The DeskJet 1200C media advance is controlled by a highpressure nip concept (drive roller and pinch wheel) located
near the left and right media margins (Fig. 4). As mentioned
earlier, this gives a clear advantage in space conservation
and design simplicity over other inkjet products using largediameter drive rollers and low-pressure nip concepts.
A traction surface designed to tolerate thermal shock with a
low thermal expansion needed to be developed. The lowcost, high-quality advance mechanism goals also required a
reliable manufacturing process that would produce 100%
in-specification parts.
The traction surface characteristics were initially defined
from a customer satisfaction viewpoint:
• The combination of nip pressure and drive roller roughness
could not mar transparencies or leave tracks in plain paper.
• The traction surface had to push the media with no slippage.
• Banding had to be controlled much more tightly than before
to meet print quality expectations. This meant that the traction surface had to advance the media consistently a constant distance for a given arc of rotation, that is, the pitch
diameter had to be as tightly controlled as technology would
permit. The printer’s performance could not be adjusted to
compensate for poor control of this specification in manufacturing. Print quality swath banding has a 1:1 correlation
with this assembly.
• The cost had to be low and the process compatible with
high-volume manufacturing.
• The materials selected had to survive with no degradation
of their properties within the thermal operating environment
of the printer.
A fundamental design goal for the Deskjet 1200C drive roller
assembly was to develop a roller surface that would not slip
on any media type. This was approached from a mechanical
friction and traction point of view. Lab tooling adequate to
characterize the traction surface had to be developed quickly.
The reality of our schedule required high-risk decisions with
data lagging by several months.
Concurrent development was started for lab tools, prototypes, and metrics simultaneously. We had to allow the development of the drive roller assembly to slip out of phase
with the rest of the project and get convergence by the time
of the production build.
1
74 February 1994 Hewlett-Packard Journal
3
2
Fig. 4. Drive roller and pinch
wheel. (1) Nip area (drive roller
and pinch wheel), one of two in
the printer. (2) Pinch wheel. The
pinch wheel rolls on the media
above the drive roller to apply
normal loading of the media into
the traction surface of the drive
roller. (3) Drive roller. Rotary
motion of the drive train is converted to linear media motion.
The surface must not slip or leave
tracks in the media. (4) Pitch diameter is twice the radius from
4
the drive axle center of rotation
to the contact zone between the
media and the traction surfaces.
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