The ink of the syringe pen flows out of the tube as a result of air pressure and gravity. The air entering the pen tube comes from the tip of the pen during pen handling. Small bubbles float from the tip of the pen into the pen tube, and then press on the ink. The air duct in the nib assembly makes the air pressure inside and outside the pen tube even, so it can keep the ink flowing out evenly. If the airway is blocked, air bubbles cannot enter and ink cannot flow out. The blockage is due to the vibration of the pen, which causes the air to expand, allowing ink to enter the vent tube. In the tube pen is a thin metal wire tied to a heavy object. This is the only movable part of the needle pen. When the needle pen is tilted or vibrated, gravity moves the metal wire up and down to control the flow of ink and clear the pen tip. Any small debris.