More food for thought... Has anyone seen these caps for kits out in the wild?
View attachment 345556
This article was written by Richard Binder who was one of the best known fountain pen repair persons in North America. Here's a link to a National Geographic video about him:
The drawing depicts a typical lever-filling fountain pen. For years, this was the most common design from the invention of transportable fountain pens until it was replaced by cartridges and converters. Ink was contained in a rubber sac inside the barrel; to fill the pen, one would pull back on a lever to force a metal plate to squeeze the sac, and when the lever was released, the resulting vacuum would draw ink through the nib and into the sac. One of the shortcomings of the design was that the rubber used to make the sac would dry out and crack, and much of Binder's work involved replacing old sacs. There aren't many lever fillers being made any more, but you will see many of them in pen show collections. I presume that some of them may have the inner cap arrangement show in the sketch, but its not possible to know for sure without disassembling the cap. However, I do know that none of the screw-cap fountain pens currently residing on my desk has an inner cap.
The text accompanying the image includes this description: "The inner cap, also shown cut away, is pressed permanently into the cap, and the lip of the inner cap mates with the flat face of the section to form an airtight seal when the cap is screwed on tightly." That means that the air seal was created when the end of the inner cap was pressed against the face of the section as the cap was screwed onto the body. However, the potential weakness of this arrangement is that it relies on the two mating surfaces being absolutely flat, and on the cap threads assuring that they are perfectly parallel so that they seal tightly against each other.
I would think another way to accomplish the same effect would be to incorporate a rubber o-ring in the seal. However, o-rings periodically need to be replaced, and that introduces a new set of problems.
Snap cap pens have a semi-flexible plastic inner cap that works by capturing a ridge at the end of the section in a recess inside the cap. That works well as a way to hold the cap onto the body, but because the ridge is only loosely held in the recess, it's not airtight. Screw cap pens can seal better than snap-caps, but require that the cap be screwed onto the body, and if the cap isn't tightened enough, the seal won't be effective, and also the cap can work its way off the body. The Japanese pen manufacturer Platinum uses a patented 'slip and seal' design that supposedly overcomes the shortcomings of both snap can and screw-cap designs.; this design appears to employ a spring inside the cap to maintain pressure on the mating surfaces of the air seal.
Fountain pens are notorious for leaking. Hence, those of us who were considered nerds in high school typically used plastic pocket protectors to avoid ink stains on our shirts.