See picture. In most rollerball pens there are those peculiar looking disks near the nib, the purpose of which I have not yet figured out, apart from being some kind of ink reservoir/flow control.

Non-uniform separation between the disks:

The further away from the tip, the smaller the separation. My guess would be that towards the top end of the "plates column"/bottom of the cartridge where the ink pressure is the highest, the liquid ink has more tendency to gush out, the denser plates serve as an "impedance" to avoid this from happening, and that a smaller "impedance" is required in the lower end to help the ink flowing when exiting the column

Flat edges:

Not all disks are circular with some that have asymmetrical flat edges on them, I'm not sure why this is apart from preventing the ink from jamming on the top?....

My reasoning seems very inadequate so I'd like some explanation as to why having such a specific design?

![enter image description here

Two flat disks, which clearly serve a purpose. Also, notice that there are rings of ink being formed around the disks.

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See Pilot V5/V7, Uniball Rollerpens, Sakura etc for reference, this is a common feature found in a lot of Japanese pens.

Please reconsider before closing my question. I admit that there may not be any truly essential physical principles attached to it, but I believe it's worth thinking about, given that the design has been used so widely. (Plus, this is going to be part of my undergrad summer project(!)). There is already a paper devoted to the study of rollerball pens, but not so much on the disks system. https://www.researchgate.net/publication/265598521_Modeling_and_Simulation_of_a_Rollerball_Microfluidic_Device

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    – Chris
    Commented Feb 5, 2021 at 3:45

1 Answer 1


For ink to flow out of the pen reservoir, air must be able to flow in, and for ink to not simply drool out of the nib via gravity, some means must be provided to develop a slight negative pressure inside the ink reservoir. Those ribbed features do both, as follows.

The air return path goes upwards past the fins, through a very small gap between the tips of the fins and the inside of the pen barrel. Loose ink is able to run out of the reservoir into that gap as well, and as seen in the photos it wicks into that gap and forms a meniscus there, sometimes several in series at adjacent fins.

Then, for ink to leave the reservoir, air must be bled into the reservoir and to do so it must flow through that same space that the ink has wetted. This requires the menisci to curve inwards to admit air, and the surface tension of the ink in the curved menisci resists that tendency until the meniscus breaks at some point and admits a little air before reforming. This process is called bubbling and establishes a slight negative pressure inside the reservoir which is just enough to counteract the positive pressure head generated by gravity, thus preventing the loose ink from drooling out through the nib.

The flats cut into the fin tips allow the establishment of a very precise capillary gap to be wetted by the ink, which then yields a precise value of capillary suction pressure required to pull return air into the reservoir.

As air gradually accumulates inside the reservoir as the ink is spent, that air is subject to expansion due to changes in ambient air pressure and temperature, and the backpressure is lost- and then the ink in the bubbler will get pushed into previously dry fins and populate them. At some point, when the pen is picked up again and used, the whole return path mechanism plays out anew, negative head is restored, and drooling is prevented.

The fin array inside the bubbler is long enough to accommodate pressure and temperature cycling for what is expected to be the useful lifetime of the ink in the reservoir, so before the time the entire bubbler is wetted, the pen has already run dry and hence will never drool over its lifetime.

By varying the spacing between the fins it is possible to sequester ink between the fins towards the bubbler lifetime as a backup means to prevent drool; this is why the spacing gets wider at the bottom of the fin array.

A very similar bubbler was used in the HP inkjet printheads used in the "classic" HP Deskjet printers manufactured in the 1990s. In this case, the bubbler was the annular space between a precision-ground stainless steel ball and a tapered bore into which the ball was pressed. Three interference ribs running down the insides of the bore then established the bubbler gap, and thereby set the nominal operating backpressure inside the reservoir.


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