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The hole in the pen barrel is there to prevent ink leakage. If there wasn’t such a little hole, as the ink was consumed, a vacuum would form in the place where there was ink before and it would tend to leak behind. Think of the reservoir tube. It has two outlets for the ink: one on the writing tip and one on the top end, right? In the manufacturing process, the air is removed from the tip used for writing, so that the space is filled by the ink. On the other side, the air that enters through the little hole prevents the ink from going the other way and leaking through the back. About 90% of pens have some opening to prevent leakage. More expensive models, which do not have vented bodies, do not leak because they contain sealed, pressurized ink cartridges. As the ink is used up, the compressed air expands and occupies the free space in the reservoir. But there are two occasions when there is no hole to protect against leaks. The first one is kind of absurd: if we are using the pen in a spaceship! In this situation, the reduced atmospheric pressure cannot hold the ink in the reservoir. “At the request of ME, I did an experiment that revealed that a pen with a hole can leak when the atmospheric pressure is 35 times lower than that of the city of São Paulo. This would only happen far from the earth’s surface”, says physicist Cláudio Furukawa, from USP. Second, on hot days or when someone takes a whiff of the reservoir, some pens leak because the heat thins the ink’s pasty consistency, making it more runny. There, it’s dirt.
Without the little hole, ink output would create a vacuum. Big leaks would roll!
pen with hole
With the little hole, the ink that comes out through the tip of the pen is replaced by the air that enters through the little hole. The molecules that form the ink remain grouped inside the reservoir and do not leak
pen without hole
Without the hole, the ink that comes out of the pen is not replaced by anything. Result: there is a vacuum in the upper end of the tube, which ends up “pulling” the molecules that form the ink out of the pen
hole function
The hole serves to allow air from the environment to enter the pen. It equalizes the atmospheric pressure and the pressure inside the ballpoint pen, filling the empty space left by spent ink.
How the pen tip works
The tip of the pen has no air – so the ink comes out that way. A half-millimeter ball of metal, made of tungsten, a super-resistant metal, spins on the way out, pulling a tiny bead of ink from the tube and passing it onto the paper you’re writing on.