I was pretty excited this morning to see the result of an experiment I ran overnight.
I replaced a printhead channel with a single glass capillary tube, filled refillable cartridges with colored distilled water, pressurized the cartridges at 5 psi. I then drew the colored water through the capillary tube and depressurized the cartridge thereafter.
An actual printhead contains 360 ‘capillary tubes’ about 50 x 50 microns, creating a suction force of about 0.5 PSI. The dampers open at approximately 0.05 PSI, so the capillary force is plenty enough to suck ink from the slight vacuum created by the dampers. Note that a single printhead nozzle also ‘sucks’ with a pressure of about 0.5 PSI.
The smallest glass capillary tube I could put my hands on was 100 micron in diameter by 100 mm in length. The suction created by the glass tube is about half of a printhead nozzle: 0.25 PSI.
I also experimented with a 400 micron plastic tube that exhibited essentially zero capillary forces.
When I drew water through the plastic tubing alone, the water would recoil back towards the damper as soon as suction was removed. This demonstrates neatly that there is indeed a slight vacuum in the damper and printhead.
When I drew water through the 100 micron capillary tube, the water would fill the glass tube on its own even without any suction being applied. Suction was needed to draw ink to the entrance of the tube, but once the liquid made contact with the tube, capillary force alone was sufficient to draw the liquid from the damper and into the remainder of the tube. Even with the cartridges depressurized the water would remain in the glass tube, at least for the first few hours…
When I checked the glass tube this morning, about 15 hours after filling it, it was empty. The water was sucked back in the damper! Why?
The low pressure chamber of the damper happened to contain about 50% of trapped air. Overnight, the temperature in the room dropped by about 4 degrees Celsius and the atmospheric pressure increased by about 5 hPa. Could it be why?
It could be a fluke - I need to repeat the experiment with and without trapped air a few times. But if the glass capillary tube is a good proxy for how the printhead behaves, and if further test runs confirm the first result, I think this is a good candidate for the main mechanism causing Epson x900 nozzles to go missing.