Why Doesn’t Ballpoint Pen Ink Drip Out, Yet Still Flow When You Write?

A Tiny Ball Carries the Ink From the Pen to the Paper

Look closely at the tip of a ballpoint pen and you will see a small metal ball held in a socket. It can turn freely, but it cannot fall out. The ball does something you might not expect: it does not squeeze ink out. Ink clings to the ball, and as the ball rolls against the paper, that ink is carried onto the page [1].

That already hints at an answer to our question. If ink only leaves when the ball turns, then a pen lying still in a pocket has no reason to let any out. The next question is what kind of ink can live in a pen like that.

László Bíró Wanted Newspaper Ink in a Pen, but It Was Too Thick to Flow

László Bíró was a Hungarian journalist, born in Budapest in 1899. He noticed that the ink in his fountain pen smudged because it took time to dry, while the ink used in newspaper printing presses dried much faster and left the page clean [2] [3].

So he tried newspaper ink in a fountain pen. It was too thick to flow through the tip [3]. A thinner ink would have flowed, but it would have smudged again. His way out was to stop asking the ink to flow down a narrow tube by itself, and to give it a ball that would carry it. The design was “a ball that freely rotated in a socket” [2].

Bíró’s Patents Describe a Ball That Seals the Tip and Thick Ink That Stays Wet

Two patents show how the idea took shape. The British patent GB 498,997, filed on June 15, 1938 and titled “Improved fountain pen,” describes a rotatably mounted ball as the marking element, fed by “a soft pulpy dye” that a piston presses against the ball. When the pen is not in use, the ball closes the opening at the front of the barrel; it opens only when the ball is pressed against a surface [4].

A US patent, filed June 17, 1943 and granted December 11, 1945, takes a different route. It points out that an open fountain pen lets its ink touch the air, so evaporation “cannot be avoided.” It asks instead for a thick ink that clings in a thin layer to the ball and can sit in the pen “without drying and without oxidizing in contact with the air.” The ball sits on three bearing points, with channels for ink running between them so that the ball stays covered [5].

So the thickness was not a problem Bíró put up with. It was part of the solution: thick ink does not dry out quickly in the pen and does not run on its own.

The pen did not stay a private project. According to the National Inventors Hall of Fame, the British government bought Bíró’s patent and had pens made for the Royal Air Force [2]. Britannica adds that the pens were wanted for use at high altitudes [3].

Thick Ink Stays Put in a Pocket Because It Barely Moves Unless the Ball Drags It

How thick is thick? Scientists measure it as viscosity, in units called mPa·s. A bigger number means a liquid that is slower to flow. A US patent on ballpoint ink gives 1,000 to 20,000 mPa·s for oil-based ballpoint ink, and 10 mPa·s or less for the low-viscosity water-based inks used in some other pens [1]. By those numbers, oil-based ink is at least a hundred times thicker.

That patent also explains the two ways ink reaches the paper. Oil-based ink is held in a narrow reservoir and is carried out by the turning ball; because it is so thick, it does not blob out of the tip. Water-based ink is thin enough to be fed to the ball by capillary action, the same pull that draws water up through a bundle of fine fibers [1]. So a thick ink and a rolling ball fit together: the ink stays in place when nothing is moving, and the ball’s turning is what moves it.

Gel Pens Use Ink That Is Thick at Rest and Thinner While You Write

Some water-based inks escape the thick-or-thin choice. The same patent describes a “shear thinning” ink: one with a very high viscosity when it is still, so it hardly flows, but whose viscosity drops when force is applied. In a pen, the force comes from the turning ball, so the ink in the tip becomes thin enough to write smoothly. When you stop, it thickens again, which helps prevent leaks [1]. This patent is about one particular ink design, so check a pen’s own information if you want to know about a specific brand.

The two designs answer our title question in different ways. Oil-based ink stays in because it is thick all the time. Gel ink stays in because it is thick when still. In both, the rolling ball is what makes the ink move.

Try It Yourself: Does a Pen Still Write on Slick Paper or When Held Upward?

This is my own suggestion, not a claim from the sources, and results will differ from pen to pen. Take a ballpoint pen and some scrap paper. Write a line on ordinary paper. Then try the glossy cover of an old magazine or a flyer, and see whether the line gets lighter or skips. Next, hold the pen so the tip points upward and write a few words. Does it keep going, or stop?

If a line breaks up, look at the ball: it may have stopped turning, and by this note’s story, a ball that does not turn does not carry ink. Use scrap paper only, and keep the pen away from clothes, walls and furniture.

Sources

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