It is mainly used to replace the ink core of supporting pen.

Pen ink bag is the space behind the pen tip and tongue for storing ink. There are two kinds of ink bags: one is transparent rubber capsule, soft. By squeezing out the air inside, and then inhale the ink as the rubber slowly returns to its original state.

The other is a hard transparent plastic pipe with a rod similar to a piston and a rubber ring as a sealing ring. The piston is pushed to the nib to exhaust the air in the pipe, and the piston is pulled to the tail of the pipe to suck in the ink.