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Comment by tryonenow | Hacker News Reader

They didn't really indicate why the end is sealed with a fluid as opposed to something physical. I suspect it's because flowing ink with a solid stopper would create a vacuum and quickly cease to function. A stopper is fluid and so can flow along with the ink. The space between the stopper and the shell is probably vented.

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Yes, it's such an obvious question that's not addressed in the article. I was thinking about why they don't pinch the plastic tube closed at the point where the ink stops being filled. The following statement in one of the patents[1] seems to confirm your explanation:

The aims of the arts are to make the ink follower follow the ink smoothly.

[1] https://patents.google.com/patent/EP1002663A1/en

Not vented so much as just not air tight-- no reason to make it air tight, and the inside of the pen is by no means sealed, as you can tell if you've ever taken one apart (or had it explode inside your bag). The parts just don't fit that well together. Which I guess is a vent in a sense, although that's probably not the main design consideration. The (obvious) exception being the space pens, which do have some pressure chambers. They solve the vacuum issue by being pressurized, which has the nice benefit of letting them write upside down.

I imagine they've made it viscous/non-Newtonian/whatever (not sure on the technical fluid mechanics term) enough such that gravity pushes/pulls it down towards the tip but prevents it from flowing the other way.

Edit 1 (just before getting in the shower): Removed references to air pressure, as I don't think that makes a difference.

Edit 2 (post shower): Air pressure might a difference after all, it depends on where the seal is. If there's an air tight seal on the tip end (which there might be, if that's how it's kept from leaking) then air pressure might add to the "only goes one direction" effect.

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