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.

enter image description here

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

  • $\begingroup$ Lamy use the same disks system in their fountain pen feeds (see Lamy Vista). The one I have in front of me has air bubbles between the disks but there's a solid section down the 'top' of the feed which I presume transports ink by capillary action - a new one will self prime when first inked for example. You might have some more luck reading into fountain pens, with them being more of a hobby item. $\endgroup$
    – Will.W
    Apr 9, 2021 at 20:04


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