The oily substance at the end of the neutral refill is called neutral pen tail oil lithium ester, and its function is as follows:
1. Keep a good moisturizing seal to prevent the ink from evaporating or overflowing.
2. When the gel pen is writing, the lithium ester can follow up at any time like a piston, and use the atmospheric pressure to press the ink down to keep the gel pen writing smoothly until the ink is written.
