HP (Hewlett-Packard) 1200c User Manual

       February 1994 Hewlett-Packard Journal

Fig. 3. Heated media path.

fed up into the main heater and the writing zone, it is
wrapped around the preheater. This contact allows the pre-
heater 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.

The DeskJet 1200C media advance is controlled by a high-
pressure 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 large-
diameter 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 low-
cost, 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 trac-
tion surface had to advance the media consistently a con-
stant 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 manu-
facturing. 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, proto-
types, and metrics simultaneously. We had to allow the de-
velopment 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.

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 con-
verted to linear media motion.
The surface must not slip or leave
tracks in the media. (4) Pitch di-
ameter is twice the radius from
the drive axle center of rotation
to the contact zone between the
media and the traction surfaces.