Mechanical design of a color graphics printer

Hewlett-Packard Journal, August, 1988 by Chuong Cam Ta, Lawrence W. Chan, P. Jeffrey Wield, Ruben Nevarez

Mechanical Design of a Color Graphics Printer

FROM THE MECHANICAL DESIGN VIEWPOINT, the PaintJet Color Graphics Printer offered several challenges. Among the more interesting ones were the development of the print cartridge latching mechanism, the electrical interconnect to the print cartridge, the media drive design, and the primer design.

Cartridge Latching

There were several objectives for the design of the cartridge latching mechanism. Since there are two print cartridges (also called pens), one black (Fig. 1) and one three-color (Fig.2), there has to be a way to identify the correct placement of each type of cartridge in the print cartridge carrier (Fig. 3), which is called the carriage. The latching mechanism must be robust so that it won't be broken, and more important, so that it won't damage the print cartridge even if wrong placement of the cartridge occurs. The print cartridge loading and unloading procedure should be simple and obvious to the user.

Since the repeatability of print cartridge registration to the carriage directly affects the print quality, it must be controlled as tightly as possible. The 30 repeatability goal is [puls-or-minus]12 micrometers. When the mechanism is latched, the print cartridges should not lose registration with respect to the carriage after a shock or other disturbance. This made it mandatory to design with stability in mind and choose registration surfaces carefully.

Finally, the latching mechanism must be manufacturable. This means a simple design, a reliable process, low part count, and low cost.

Implementation

Everyone knows one should read the instructions before attempting to operate a new product. However, many users skip this step and are frustrated if their attempt isn't a success. More seriously, the user may blame the product's design if something breaks during the attempt.

To help the user find the correct place for each print cartridge, color coding is used. After priming and wiping a new pen, the user turns to the carriage. The obvious place to put the print cartridge must be one of the two openings on the carriage. The question is which one. Color matching the colored dots on the pen label with those on the carriage lid lable (Fig.4) will lead the user to the right place. Even if the cartridge is placed incorrectly, there will be no damage to either the pen or the carriage. The mechanism will refuse to latch to let the user know there is something wrong. Try again.

Loading and unloading the print cartridge is simple, as illustrated in Fig. 4. First, load the print cartridge to the correct opening on the carriage. Gravity will hold the cartridge in the right position to be clamped. Next, pull the green latch up to clamp the cartridge in place. To unload, reverse the procedure.

The keys to satisfying the repeatability and stability objectives are picking the best set of plastics for the print cartridge body, the carriage, and the latch to minimize the coefficient of friction between them, and picking the right locations for loading points and reaction points to minimize problems with frictional moments that would prevent the cartridge from moving freely to where it is supposed to be. We were faced with a narrow set of plastic options for the print cartridge body because of constraints including compatibility with the ink and dimensional stability after molding and after trigger curing (a process for quickly bonding the printhead to the pen body). Nor did we have a great variety of plastics to choose from for the carriage, because that plastic, aside from being a best match with the cartridge body plastic, should be a good bushing material, have high dimensional stability after molding, and be conductive to protect the electronics from electrostatic discharge. The easiest material to choose was the latch material, which needs to match the cartridge body material and have low wear characteristics. We picked 20% glass fiber filled Noryl (modified polyphenylene oxide) for the print cartridge body, polycarbonate with 10% glass, 10% carbon, and 15% Teflon for the carriage, and polycarbonate with 20% glass and 15% Teflon for the latch. Picking the right locations for loading and reaction points required some careful judgments and verification of the chosen set of locations by a free-body-diagram computer model of the print cartridge body.

The print cartridge body has some uncertainty in dimension. This is partly because of material shrinkage after coming out of the mold and partly because the body tends to deform after the trigger cure process, which allows the stress residue in the plastic to relax even more. To minimize this effect, the x and y registration surfaces are chosen as close to the printhead as possible. For the most positional stability, the z registration surfaces are chosen as far from the printhead as possible. Of course, these objectives have to be balanced with the physical siz constraints.

Final Design