Print cartridge fixturing and maintenance in the HP DeskJet 1200C printer - HP DeskJet 1200C and HP DeskJet 1200C/PS ink-jet printers - Technical

Hewlett-Packard Journal,  Feb, 1994  by Michael T. Dangelo,  Reza Movaghar,  Arthur K. Wilson

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The carriage assembly locates and transports the four print cartridges precisely. The service station provides capping, wiping, and priming functions for print cartridge maintenance and recovery.

One of the main goals for the HP Deskjet 1200C printer was a narrow footprint so that the printer could be placed on a desk or existing desk stand. To achieve this goal, the carriage assembly and print cartridges are designed for minimum space between parts and for minimum overall width.

The overall width of the printer is dependent on the width of the paper plus two times the width of the carriage assembly. This is necessary to allow all of the print cartridges to print to the edge margin of the paper on each side.

The compact design makes it possible to register the four print cartridges accurately with respect to each other and with respect to the paper to ensure high print quality and high definition.

Design for manufacturing and assembly was considered from the beginning of the project. Areas of importance were low part count, modular construction, no adjustments, and ease of assembly. All parts either snap on, are nested inside other parts, or are held on with a minimum number of fasteners. The carnage assembly is modular in construction which allows subassemblies to be built and tested before being installed on the carriage.

Carriage Assembly Description

The HP Deskjet 1200C printer carriage assembly is built around a single-piece print cartridge housing, with all other assemblies and parts attaching to it. The carnage assembly includes a modular interconnect assembly, a paper sensor assembly, print cartridge bias and loading springs, slider rod bushings and preload pads, and the cover/trailing cable assembly, as shown in Fig. 1.

Print Cartridge Housing and Parts

The one-piece approach was taken to minimize tolerance buildups, maximize stiffness, and lower cost. This approach does have its drawbacks, however. To get a complex-shaped one-piece part with many features on all surfaces requires a very intricate and complex plastic injection molding tool.

To ensure the closest possible alignment of the print cartridges, the print cartridge datums in the print cartridge housing are machined relative to the slider rod axis to match actual operating conditions.

The print cartridges are located in the housing with biasing springs that push the print cartridges into position against the datums. Three sets of springs are used for biasing the print cartridges in the three respective axes. Of particular interest is the Z bias spring, which serves several functions. It pushes the print cartridge down to the bottom of the housing (Z direction), it clamps the top of the print cartridge in the housing, and it pushes the top of the print cartridge in the X direction against the X stops. The spring assembly nests in the housing and is held in place by the interconnect assembly.

Attached to the side of the print cartridge housing is the paper sensor. This is used to tell where the edge of the paper is and identify whether paper or transparency media is being used. The sensor assembly snaps onto the housing and plugs into the interconnect assembly.

The slider rod bushings, slide pads, and preload pads all snap or slide onto the housing and require no other attachment.

Interconnect Assembly

The interconnect assembly (Fig. 2) is modular in construction and is installed in the print cartridge housing as a complete unit. The backbone of the interconnect assembly is a one-piece frame with alignment pins and holes for obtaining proper alignment of parts and for positioning in the print cartridge housing.

Attached to the frame is a one-piece flexible circuit that makes electrical contact with all four print cartridges and with the carriage print cartridge driver board (carriage board).

The flexible circuit interconnect pads are positioned against the print cartridge contacts with an elastomeric pad. This pad is backed up with a spring-loaded plate, which supplies the force required for electrical interconnect. This system allows a long interconnect distance and also gimbals to align the face of the interconnect with the print cartridge contacts.

Also included in the frame are encoder strip guides. These guides align the encoder strip to an encoder for determining carriage position and velocity, which is attached to the carnage board.

The carriage board has a number of functions, the primary one being to drive the print cartridges. On this board are the encoder, connectors for the trailing cable and paper sensor, and electrical interconnect pads for connection to the flexible circuit.

Pressure is used for the electrical interconnect of the carriage board to the flexible circuit. An elastomer strip is nested in the frame and forces the flexible circuit against the carnage board contact pads. The carnage board is clamped to the frame along the contact area with three screws.

The carriage board is aligned to the frame and flexible circuit with two locating pins. This provides for proper positioning of the flexible circuit to the carnage board and aligns the encoder in the frame. The carnage board is positioned vertically in the carnage assembly. This allows the trailing cable to attach directly to the top of the board using minimum space.