That is true. And there are several examples around on similar solution: you get a cartridge, cut it, fit a large (as much as the body allows) sac and fix it with shellac. Much like any sac filled pen from yonder.
What I would rather see is a twist converter working on the principles of Arqhimedes' screw: turning a knob connected to the screw would move up and down the piston to fill/empty the converter. This would do away with waste space needed for the axis when the piston goes up. What it would move up and down would be the piston, and the piston by creating vacuum, would draw the ink.
Why? Because then you do not need any space for the piston axis, which is what makes current converters waste half of the available space. That would mean (almost) doubling the capacity of any converter size.
Why not? One may link the screw to the twister, so that as you twist it, the screw turns. For it to make the piston go up and down you would need at least one "guide" on the wall of the converter that fixes the piston orientation, so that when the screw turns it forces the piston to move. This requires a container that is not a simple, straight tube. And the piston should have a shaped, non-cylindrical hole adapted to the screw (like a nut). I can see why with old machinery and design media this would be discarded in the old times. But this should be easily doable with modern technology; yet, properly producing it would likely require new machinery and to justify the cost of this machinery one would need to guarantee a large enough demand, which with modern (lack of) use of refillable writing instruments may be difficult to justify,
On the other hand, plastic is becoming an ever larger concern. Fountain pens are a great solution to reduce the huge amount of "bic" and bic-like plastic pens being discarded and thrown away every year. And converters to reduce use of cartridges. A more conscious, waste-reducing society should reconsider going back to refillable devices like fountain pens, specially now that carbon copy paper is no longer needed. That makes for a strong argument to address contamination-conscious users and appeal them to the FP world. So may be at one point it may make sense to redesign converters to profit from new advances in technology.
Sorta, something like this very gross draft:
Note that many current converters do use a screw but it attached to the piston and rotation draws -by a similar principle- the screw axis and piston up and down. What I propose is that the screw is attached to the know and not the piston, so that it is the piston that "screws in/up - out/down" when the knob (and attached screw) is turned. Just as current converters need a fixed nut to move up/down the screw/piston unit. the piston would act as a nut, fixed in the X-Z plane by the "tongue" in bass relief in the inner part of the sleeve cylinder and would be free to move on the Y direction (up/down) as the screw is turned.
Come to think of it, an easier design would be to have an hexagonal reservoir (instead of a cylindrical one as it is now) so that the piston would be actually a normal nut and the axis a bolt, this would only require a (more or less) normal/standard nut and bolt/screw and would not require a specially shaped cylinder with an inner tongue, which may be more prone to failure. We see a similar design in many mechanical items indeed.
Edited by txomsy, 18 June 2019 - 15:26.