HP 1200C User Manual download pdf (Page 2)

February 1994 Hewlett-Packard Journal 73

• 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 ad-

justable plate, the drive rollers, and the pinch rollers. While

driving the media these components work together to en-

sure 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 al-

ways 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 manu-

facturing 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 opti-

mize 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 me-

dia 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 roll-

ers 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 grit-

coated 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.

Fig. 2. A grill and three shims hold the media flat in the print zone.

Center Shim

Right Shim

Grill

Left Shim

The media path must also hold the print media flat. The en-

vironment 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 indepen-

dence, but at the same time, the heater and ink wreak havoc

with the fibers that make up the paper. The fibers, predomi-

nantly 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 con-

strains the right edge of the media and acts as the zero refer-

ence 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 car-

tridge. 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

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