See the picture below.
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 as some 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 they have 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.

1 Answer 1


Your reasoning actually comes quite close.

I found a quote through Quora, credited to John Lienhard from the University of Houston after having contacted Pilot:

"The trick is to keep just the right amount of ink in contact with the wick that feeds the delicate roller ball tip. The discs are baffles that balance the forces of surface tension against air flow resistance. Regardless of the angle, just the right amount of ink stays in contact with the wick.

The pen's "containers need complex baffles that let surface tension position the liquids", which makes sure that "the pen writes beautifully in any orientation."

In Pilot's words, as used to promote their pens on various commercial websites:

"The V System ink regulator is a direct result of Pilot's fountain pen technology. If excess ink flows from the ink reservoir while writing, it is temporarily stored in the special pen core by capillary action. In that way, amount of ink flowing to the pen point always stays constant. This mechanism ensures that the ink supply remains stable to the last drop."

This disc-system is similar to those found on fountain pens, here called a "finned ink buffering structure":

enter image description here

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