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

The print cartridge performed well at high altitude or temperature in part because the vent-free, collapsible, flexible design minimizes trapped air volume and provides compliance against air expansion forces. Numerous small problems were uncovered and fixed, too many to cover here. Briefly, they included ink level shift (showing partially out of ink when full) after drop, spring hangups on the frame (stranding excess ink), air intrusion through the ball-cork seal after temperature cycling (causing loss of backpressure), and side covers popping out of the frame after drop (causing print cartridge alignment errors).

The four biggest problems were WVTR, ink leakage from the bag-film-to-frame seal, ink leakage from spring-induced tears in the bag film, and ink wicking under the nozzle tape. The WVTR problem was made easier to solve by the ventfree design (vents are usually a large loss path), and by the low-water-loss second-shot plastic. But it was complicated by the large bag film area and the need for the film to be extremely flexible for reasonable backpressure and delivered volume performance. Most applications require low water loss or flexibility, not both. This required a custom film structure. Standard materials and assembly processes were used. After numerous iterations the nine-layer film structure shown in Fig. 4 was selected.

The second major problem was the bag film peeling away from the frame at the seal joint during drop testing, causing major ink leakage. During a drop, a lot of ink is pushing down on the film near the seal just as the frame is pulling the frame up at the seal edge, producing a severe reverse peel (the worst kind of load) on the seal. The original second-shot plastic material was HDPE, which is not a very good sealant. A burst test was developed to characterize seal strength (internally pressurize a sealed spring bag until the seal ruptures or the bag film breaks like a balloon). Optimization of the seal parameters (time, temperature, and pressure) and adjustment of the mechanical tooling tripled the burst pressure. At this point a single two-meter drop or four one-meter drops could be survived. This indicated that a single drop caused partial seal peel. While very few print cartridges were likely to see four drops, we hadn't tested millions of print cartridges, so the ultimate margin was not known. A material search was instituted and a new second-shot material was selected that has much higher seal strength. Burst strength tripled and the bag film breaks before the seal peels.

The third major problem was the metal spring piercing the bag film in drop or vibration testing, causing moderate ink leakage. The piston plate part of the spring is very thin and has a rough, burred edge that is right up against the bag film. Attaching the spring to the film eliminated the drop failures but aggravated the vibration failures (fixed the relative motion to one spot). The solution is a plastic shield over the spring piston plate (the shield is larger than the plate). Thus, no matter how rough the plate edge or how the bag film wraps the plate edge, direct metal contact is not possible.