Why Do Pens Need Caps?

With the exception of pens that allow the tip to recede into the implement itself, most pens have caps. But have you ever wondered why? Why do pens need caps? Most people assume it has something to do with keeping air away from the ink but is that true?

As a matter of fact, it is, but these caps aren’t as important as you might think. Their primary purpose is to slow down the ink drying process, but they don’t prevent it entirely. In fact, all pen ink will dry out eventually, even if the cap never comes off the pen at all.

So do pen caps really serve any special purpose? And what about the ones with holes in them? How could they possibly prevent ink from drying with a big hole for the air to get in? We’ll talk about all of this and more in this article.

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Why Does Pen Ink Dry Out?

Most people are aware that keeping ink exposed to the elements is bad, but they probably don’t understand the science behind it. Why does exposure to air cause ink to dry faster? Why does it dry at all? Well, it’s because pen ink has volatile solvents in it, and that evaporates over time. Once they evaporate, the ink is dried out.

Some inks will naturally take longer to dry out than others because some inks are oil-based while others are water-based. For instance, ballpoint pens, which often have caps with holes in them, almost always use oil-based ink, which is not as quick to dry out as water-based ink, which is often used by fountain pens and rollerballs.

Ballpoint pens also function in a way that prevents ink from unnecessarily leaking out onto the ball, meaning less ink is exposed to the elements when not in use. This is a big reason ballpoint pens can afford to have a cap with a hole in it: there isn’t much ink being directly exposed anyway, so it doesn’t matter that much.

Even so, having ink exposed to air speeds up the evaporating process, and having a cap helps prevent that. A cap does not completely stop the ink from drying, but it does extend how long it takes for it to dry, and that is the sole reason pens have caps on them.

Why Do Ballpoint Pens Have Holes in Their Caps?

As mentioned previously, ballpoint pens don’t have to worry about airflow as much as other pens do, because their ink is not only oil-based, but the tip mechanism allows most of it to not be exposed in most scenarios. But just because it can afford a little more airflow doesn’t mean it should. Why do the caps have holes at all?

Well, this probably comes as a surprise to most people, but the hole in the cap of ballpoint pens actually serves a function that has nothing to do with the pen itself. The sole reason those holes exist is to reduce the chance of suffocation if someone were to swallow a pen cap accidentally.

After all, pens are used by many children in the world, and sometimes, accidents happen. If a pen cap were to get lodged in someone’s throat, they would have a higher chance of being able to breathe if there was a hole in it that would prevent the cap from obstructing airflow. 

Naturally, it would be better if all pen caps had this kind of functionality, but unfortunately, only certain types of pens can afford to. Like we said before, this is mostly reserved for ballpoint pens since they don’t have to worry about drying out as much.

Which Pens Need Full Caps and Why?

Most pens that use some sort of water-based ink must have a cap on whenever they are not in use, and those caps can’t be the kind that has holes in them. Water-based ink evaporates quickly when exposed to the elements, which will leave behind little more than residue (if that) from the dye itself. Rollerballs, felt tips, and fountain pens are in this category.

On top of using water-based ink, these types of pens have tips that lead to more exposure for their ink. Felt tip pens, for example, constantly wick ink from within the pen to the tip, meaning that, even as the ink evaporates, more of it will be drawn forth to get wasted if a cap is not on the pen.

Again, having a cap will not prevent these pens and their ink from drying out eventually, but considering that water-based inks can evaporate in mere minutes when left exposed, the greatly extended amount of time they can last when a cap is used makes it worth it.


Lo and behold, pen caps serve the exact purpose most everyone assumed they did: help prevent the ink from drying out when a pen is not being used. Naturally, there’s a lot of complicated science behind why this helps, but all that you need to know is that a cap reduces or eliminates airflow, which reduces the speed of evaporation significantly.

This is especially vital for water-based inks, which can evaporate in minutes if they are left exposed to the elements. Such inks must use a cap with no holes in it while they are not in use. Pens with tips that constantly draw more ink are very vulnerable to this kind of problem as well.

Ballpoint pens use oil-based ink, which does not evaporate nearly as quickly as water-based ink. Furthermore, the ballpoint mechanism prevents the entire ink reservoir from drying out even if it is exposed. That’s why they can afford to have caps with holes in them.

As for that hole, its only purpose is to reduce the risk of suffocation if someone were to accidentally swallow the pen cap and get it lodged in their throat. The hole increases the odds of the airway remaining unobstructed. That’s everything you need to know about pen caps and why they’re used!

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