When I started making 3D-printed fountain pens, initially for myself, I was intent on making a pen with a piston filling system. A noble objective. After all, piston filling systems are the acme of filling systems, aren’t they? The most expensive models from famous pen companies such as Pilot, Pelikan and Montblanc always have piston fillers. Surely that’s because they are better.
Well, piston filling systems are better than the bladders in my vintage Parker English Duofolds and Sheaffer Imperials. At least they’re better in some ways. Somehow. Pistons do not perish with age (or do they?), and they are not adversely affected by ‘difficult’ or ‘dangerous’ inks. Those things are either true or mostly true. However, in my opinion the push-button filling of the older of my Duofolds is less fiddly than a piston and the ‘Touchdown’ system of the Sheaffer is pretty cool. Probably the piston filler pens are more easily cleaned for ink colour changes, but they are certainly not superior in all ways.
I eventually gave up on the idea of making a 3D-printed piston filler because I was unable to make the inside of the ink cylinder smooth enough for a piston to seal (see my previous blog in praise of layer lines). I then fiddled around with printing ink bladders of various designs out of flexible TPE filament. They worked variably well but I never got them into a pen, for a variety of reasons that don’t matter here. (I may return to the printed bladder for a future series of pens. We’ll see.)

I gave up on pistons and printed bladders and bought some cartridge converters. Hey, hang on, this cartridge converter is a piston filling system!
(That might not be an entirely accurate account of the history, but it carries a good deal of verisimilitude.)
It turns out that, for me, cartridge converters are an optimal ink filling system. Optimal for me as a pen maker in the sense that it is easy for me to pair a converter with a converter-ready nib unit and get a pen that fills, empties, and writes reliably. And optimal for me as a pen user because cartridge converters facilitate ink changes. Changes of ink colour.
Coloured inks: the secret to fountain pen joy!
When I first used a fountain pen (don’t ask how long ago that was—I was in primary school) No-one I knew ever thought much about ink as we all used blue ink. I might have been forgiven if I had spelled the word ink with a Q because my dad was a Parker man (I wish I knew where his 51 went…) and so our house used Quink blue-black.

It turns out that whereas a fountain pen with one ink is a tool for writing, a fountain pen with its choice of many colours is a joy!
I have a collection of inks including the old Quink blue-black from my dad; that box is not empty. Most come from Australian ink makers. I’ve just counted the bottles and I’m a little embarrassed but the numbers. Fouteen from Van Dieman’s Ink, eight from Blackstone, a handful of sample vials from Robert Oster, but I also have inks from Waterman, Fountain Pen Revolution and Diamine. That’s a lot of ink, but I’m sure that I’ll buy more before too long. (Please don’t tell my family!)
With the large capacity of a piston filler I would have to hand write a novel each month to get more than a couple of inks into rotation, and that’s where cartridge converter comes into its own. The Standard International converters that I use hold just on 0.8ml if you get all the air out. It’s not difficult to write one dry, particularly with a broad and wet nib. It’s also easy to get a partial fill into the cartridge so that ink changes can be even more frequent. Lovely.
And to top it off, a cartridge converter is relatively easy to clean. If the ink is not readily rinsed out with a couple of fills and empties of the pen you can be remove the cartridge converter from the nib unit to allow flushing of the nib and feed under the tap or with a bulb. The converter can be flushed easily with a syringe and blunt needle and then you’re good to fill with the new ink in just a minute or two. Compare that to the sometimes tedious precess of cleaning out an ink bladder or fixed piston and I think you’ll see what I mean about the optimality of the cartridge converter.
All in all I am happy to make, use, and sell pens that come equipped with the ‘lowly’ cartridge converter.
Hi, I have used a syringe barrel and an acrylic tube for my own vacuum fill 3dprinted pen (Penny Gemstone)
However, you could also try drilling the inside of an undersized hole to create the smooth surface or try resin!