The easiest way to refill a fountain pen cartridge is with a syringe. The syringes used for this purpose have skinny, long metal tips that are blunt at the end. The blunt end will product you as well as your pens and their components.
Many, many companies make appropriate syringes for refilling pens and cartridges, though none of them are made expressly for refilling fountain pens so these syringes might have to be purchases through office suppliers, industrial suppliers, scientific testing equipment dealers, or even electronics resellers.
Some fountain pen stores — online or otherwise — will resell these plastic syringes but expect to pay a hefty price premium for the same quality of parts.
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Fountain pen syringes are used for filling fountain pen cartridges in a safe, and non-messy manner.