This may be a stupid question but...

[ndw76]

This may be a question that belongs with the others like is super glue good for pens and plumbers tape for tread sealant, but I have to ask. Yesterday I bought my daughter a pack of balloons for making balloon animals. I got to thinking they're made from latex, they're cheap, and can be cut to any length. Would they be any good as ink sacs? Don't worry, I'm not intending to do this. Just thinking aloud.

[Tony Rex]

"There are no stupid question, only stupid answer." Here's mine:

Why not? But aren't they too floppy to bounce back to original position? Because a sac needs to have memory with force strong enough to suck ink. I haven't seen any kids balloon good enough for this purpose, tbh.

[Jeph]

I have to disagree with Tony, and I think that this is the first time. There are, in fact, stupid questions. This was not even close to being one of them.

Tony's answer, however, is correct. I have seen some heavy duty balloons that would probably have the toughness to survive countless fill cycles, but not the resiliency to also draw ink reliably. And those would be near the price of actual ink sacs and far too large to fit within most pens.

[mhosea]

Latex fountain pen sacs need a metal inhibitor added to prevent the latex from breaking down where it is in contact with metal. Without it, latex will slowly degrade. Most sac'd pens have at least metal pressure bars. As has been noted, latex sacs need to be able to draw ink. If the sac can do that, then you might be able to use it in bulb filler.