Why does the ink come out of the refill when it is kept cap-less?

When a pen refill is kept cap-less, the ink can dry out due to exposure to air, leading to a blockage in the nib or tip. Additionally, the absence of a cap allows for evaporation of the solvent in the ink, which can cause the ink to thicken or dry completely. In some cases, gravity can also cause the ink to flow out uncontrollably if the pen is positioned downward. Overall, keeping a pen cap-less disrupts the controlled environment necessary for the ink to remain usable.