HP DeskJet 1200C printer architecture - includes related articles on use of solid modeling CAD system in design, on how reusable packaging and rugged design can minimize environmental impact, and on a new model for product development - HP DeskJet 1200C and HP DeskJet 1200C/PS ink-jet printers - Technical

Hewlett-Packard Journal, Feb, 1994 by Kevin M. Bockman, Anton Tabar, Erol Erturk, Robert R. Giles, William H. Schweibert

Airflow Considerations. In laying out the Deskjet 1200C architecture, special provisions had to be made for airflow. To keep costs low, the project team decided to use a single fan to accomplish the tasks of heat dissipation and vapor removal.

Cooling the power supply board became a major consideration. The 180-wattt supply runs at 140 watts steady-state with an efficiency of approximately 70%. This means that there are about 40 watts of heat to dissipate. Several power supply vendors recommend that to cool a power supply of that size, a minimum airflow rate of 20 ft3/min is required.

Initial testing of a mockup indicated that with a single fan we would only have about 8 ft3/min. The team decided to take parallel paths, that is, pursuing the single-fan option and leaving the option of a second fan open by including the required support. Space for the second fan was included in the left chassis and additional grills were added. At the same time, the cooling airflow was designed to move directly across the power supply area and the board layout was optimized to best use the available airflow to cool the critical components. A few iterations resulted in a design that has ample thermal margin with less than half the original conservative airflow estimate. The additional grills are still there, unused, in the final product.

The second requirement of the fan system is to remove excess vapor from the writing area and excess vapor and heat from the heater enclosure. To accomplish this, a manifold was designed to sit directly behind the print cartridges, extending the full width of the writing area. This design also pulls air from the heater enclosure.

During development, many of the variables that affect the airflow were changing. We had not selected a fan supplier and wanted to keep that selection open. The print cartridges and the heater system were also under development so the actual requirements were still unknown. To meet these changing requirements, a simple wall was designed into the vapor removal snout that would allow the system to be tuned to any combination of fan supplier, print cartridge design, and heater design.

Filtering was another consideration for the airflow system and planning ahead allowed an optimal system to be designed and implemented. Fig. 5 shows the airflow paths in the Deskjet 1200C.

Manufacturing Considerations. As explained earlier, the product architecture changed from the early prototype to a revised prototype (see Fig. 1). This was partly because of a desire for a lower-risk permanent tooling set. Splitting the two plastic structural pieces into three helped reduce part complexity which made the tools easier to debug. A two-piece enclosure with a front and rear case as opposed to a single-piece, over-the-top enclosure also made the tooting easier to put together.

Extensive use of rapid prototyped parts also smoothed the tooling debug phase because our designs were more complete before releasing for tooling.

The design team stressed the use of sheet metal, which allows quick prototype turnaround. The team was trained and understood that product forecast volumes were high enough to support tooting (i.e., progressive dies) of sheet-metal parts.