Development of the HP DeskJet 1200C print cartridge platform - includes related articles on ink cartridges for large-format color plotters and of design of print cartridges for environmental responsibility - design of ink cartridges for HP DeskJet 1200C and HP DeskJet 1200C/PS printers and HP DesignJet 650C plotter - Technical

Hewlett-Packard Journal, Feb, 1994

For both portions of the frame, manufacturability in high volume at low cost were driving objectives. Plastic injection molding is the technology of choice for print cartridge bodies because it meets the objectives of low cost and high volume and is capable of producing complex geometries in engineering materials. The desire to integrate two materials with two sets of design objectives presented an interesting challenge for the plastic injection molding process. Two-shot or "two-color" molding and insert molding have both been demonstrated as feasible.

The final design of the frame was driven not only by the two sets of design features for the external and internal frames, but also by the molding process. General plastic design rules regarding wall thickness and coring of thick sections were combined with the special tooling constraint of the insert and two-color molding processes to arrive at the final design.

The objective of building a thin print cartridge that delivers the most ink for a given overall volume meant that the walls of the reservoir had to be as thin as possible. The two side covers represent the majority of the surface area of the cartridge, which means that the amount of ink that can be contained within the structure is very sensitive to the thickness of the covers. Another key design objective is to keep ink off the customer. If the covers are too thin, a customer might force ink out of the printhead simply by squeezing it too hard during the normal handling expected for a routine cartridge replacement. This consideration led to the concept of "effective thickness," which is the sum of the actual material thickness plus the deflection of the covers under an applied squeezing load. To prevent ink from being forced out the printhead, the design rule stated that the inside surface of the covers could never touch the ink containment bag within the structure. When the covers were thin they could be squeezed more, which meant that more dead space had to be included between the covers and the bag to keep the deflected covers from touching the bag. Adding more thickness to the cover material made the covers stiffer, so the effective thickness was reduced and more ink could be held within the structure. The optimum cover thickness was the point where any additional increase in material thickness caused the effective thickness to increase also.

The amount that the covers deflect is primarily a function of the applied load and the material for a given wall thickness. After consulting reference material on ergonomics and performing some quick reality tests, it was determined that an average user would apply less than 10 pounds of force to the covers during normal handling. Covers of varying thickness were made from different thermoplastics and different metals were tested for deflection under the simulated maximum squeezing load. The side cover material that provided the lowest effective thickness and consequently the highest deliverable ink volume from the cartridge was determined to be steel. Prepainted, low-carbon steel was chosen because it meets the environmental requirements and is a standard material in the metal stamping industry.