Why Do Ink Pens Explode?

Okay, “explode” might be a little bit of an exaggeration. We’ve all had a run-in with a pen that leaked, though, right? 

The pen that leaks into your pocket right before a presentation. The pen that leaked in the bottom of your purse. The pen that leaked in your car’s glove compartment during the summer. That pen that leaked in your luggage when you flew. 

So, why do pens explode? Are there pens that are more likely to cause an issue? Less likely? That’s what we’re covering today. First, though, we’re going to talk about how a pen works.

How does a pen work?

Every pen out there is a controlled leak. Ink is a liquid, and liquids want to flow. The trick to a pen is controlling that flow in a way that only puts the ink where you want it to be: on the page. Ballpoints have a refill full of dye mixed with an oil and some thickeners. The ball at the end of the pen blocks the tip until you put it to the page and spin the ink-covered ball. Rollerballs do the same trick, but with a less viscus ink. Fountain pens control the flow by using very thin channels in the feed coupled with surface tension and air pressure. Fiber tips like sign pens, Sharpies, and highlighters are sort of like an extra-stiff paintbrush that puts up enough resistance to keep the liquid ink from falling out, but not so much that it keeps it in the “brush.” Some of these have an absorbent core from which they draw ink, and we’ll talk about that in a bit. 

What causes leaks?

It’s a delicate balance for all writing instruments, and a leak occurs when some force overcomes the resistance of the system or when something gives the ink a more convenient path out of the pen (like the fabric of your pants). Let’s break down some of the most common causes of leaks.


This is one of the two most common causes of a leak in any pen. A pen refill of almost any kind contains ink and air. When your pen heats up, that heat affects both of those components. The ink inside of your ballpoint becomes less viscous. At the very same time, the air in the refill behind your extra-runny ink is expanding from the heat. That combo will push the ink out of your pen and into your pocket, onto your shirt, or into the cap of your pen. Yuck. The same kind of thing happens with rollerballs and fountain pens. Their inks are already a thin fluid, so the expanding air in those pens has an even stronger effect. 

So, leaving them in your car can be a gamble. I usually leave a ballpoint in my truck, and it hasn’t exploded yet. I’m rollin’ the dice all summer!

Why Do Ink Pens Explode?

Air Pressure

Air pressure changes can occur without heat, and this can also cause a leak. Moving quickly from high to low altitudes can do it. This is what happens in airplanes. The cargo area of a passenger plane is pressurized just like the cabin is, but there are pressure changes at takeoff and landing in both areas that can cause a pen to leak. I’ll keep my pens in my carry-on to avoid the jostling that checked luggage takes, and I put my fountain pens in an air-tight bag to protect them from pressure changes or contain any leaks that do occur. 


Force is the next culprit in a leaky pen. Shaking a pen vigorously or continuously can disrupt the controlled-leak system that is your pen. A pocket while you’re running. A hard surface in a car. Extreme gesticulating. All of those could make your pen leak. 

Why Do Ink Pens Explode?

A New Path

Remember, the liquid ink in your pen loves an easy path to freedom. That ball of lint in your jeans pocket is an express ticket out of its confinement, and it’ll absolutely soak into your pants or shirt just in time for your big presentation. Rollerball pens are especially liable to do this, and rollerballs without caps have ruined a least a couple of bits of my wardrobe. Ballpoints and fountain pens can leak this way, too, but the ballpoint’s thick ink and the fact that fountain pens generally have caps makes them less likely to try this trick.

Air Leaks

Fountain pens are the most delicate of these systems, and sometimes your pen will seem to leak for no good reason at all. That is almost always the result of an air leak. Check that your converter or cartridge is seated well in your pen, and then make sure that your nib and feed are pressed all the way back into the grip section of your pen. There are other reasons your pens might leak from the nib, but those are by far the most common reasons. 

Why Do Ink Pens Explode?

Which pens are most likely to leak?

As a fountain pen lover, it’s hard to says this: Fountain pens are the most likely to leak. The ink is in a liquid state, the systems are more sensitive to outside influences, and so they’re more likely to leak than most other pens. Pens that are eye-droppered are especially likely to do this. Since the body of the pen is filled with ink, there is more volume and more volatility. What I mean is that the weight of ink pressing down on the feed and nib is greater than when you’re using a converter or a cartridge, and the heat from outside of the pen is more likely to heat up the air inside the pen. This makes the air expand and push ink out of the pen. Sometimes even the heat of your hand can do this. 

Rollerballs are the second most likely to leak. The ink is watery and it can be affected by heat, pressure, and motion just like a fountain pen. The ball in the tip is an extra barrier, though, and that helps. 

Ballpoints and gel pens can leak, too. The ink is thicker, though, so it is less likely to happen than in their liquid-ink cousins. 

Which pens are least likely to leak?

Okay, the part you’ve been waiting for. What pens are least likely to leak? I have two types of pen on this list. The first is something with a fiber refill. Highlighters, Sharpies, and other felt tipped pens often use an absorbent media inside their barrels which holds the ink but allows it to flow through the fibers of the tipping. Those pens aren’t nearly as subject to the whims of heat, pressure, or force. I’ve never had one of these leak on me. The ink will still soak into whatever touches the tip, but it’s unlikely to leak as other liquid inks will.

The second pen type is the Fisher Space Pen. Those refills are sealed, pressurized, and built to take on a vast number of conditions. Heat, cold, pressure, and water don’t have much of an effect on these pens, and that’s awesome. They don’t put down my favorite line, but the advantages sometimes outweigh the aesthetics for me.

How can you keep your pens from leaking?

The easiest answer is: Don’t subject them to things like heat and extreme motion. 

Keep your pens in a pen case and on your person if you can. If you’re keeping them in a bag, try and keep the fountain pens nib-up, and the ballpoints tips retracted or capped. The less they’re thrown about the less likely they are to leak. 

If you’re carrying them in your pocket, I suggest a pen sleeve of some sort. I almost always carry a two-pen sleeve in my pocket with a fountain pen and a ball point of some sort. The sleeve keeps them tip-up and it protects my pants in case I have some sort of leak. 

If you’re leaving them in your car? You’re rolling the dice. I roll those same dice. We’re in it together. My truck-pen is a BIC ballpoint in whose cap I’m placing a lot of faith. 

Why do pens explode? Knowing your pens, treating them well, and avoiding those inky fingers and wrecked pants can mitigate much of the risk associated with pen leaks.

Why Do Ink Pens Explode?

About the Author

Mike Matteson is an educator, gardener, and video game player when he isn’t creating stationery content on YouTube, Twitch, or his blog.

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