If you’ve ever shopped for tennis balls, you have noticed that some of them are sold in pressurized cans.
Why the pressurized containers, though? Wouldn’t the tennis balls do just as fine if stored on a shelf before someone buys them?
Well, no. There is a reason why some tennis balls are sold in sealed containers. You don’t need to know this reason to become a better player, but if you want to spend less money on tennis balls in the long run, read on!
Why Do Tennis Balls Come In Pressurized Cans?
If you go shopping for tennis balls, you will quickly realize that there are actually two types of tennis balls – pressurized and pressureless.
Pressurized tennis balls are hollow and contain air at a pressure of about 26.7 psi (though some balls may be pressurized much lower). Pressurized tennis balls are designed this way to reduce weight and increase bounce.
If you’ve played with both pressurized and pressureless tennis balls, then you know how lifeless pressureless balls can feel.
The ITF regulations also require that tennis balls bounce from 48-53 to 53-60 inches, depending on their type, and this is achieved via the right combination of weight and internal air pressure.
But what does this all have to do with pressurized tennis ball cans?
The ambient air pressure is about 14.7 psi – a whopping 12 psi lower than inside the ball. Due to this big pressure difference, air will slowly leak out of a tennis ball, lowering internal air pressure and weakening the ball bounce. And as you hit the ball with your racket, you are exacerbating the leak even further.
Aside from that, after manufacturing, tennis balls may sit in warehouses for weeks before being purchased. To ensure that the balls lose little to no air while waiting for their new owner, they are stored in pressurized cans.
Tennis ball cans are pressurized so that there is little to no difference between the air pressure inside and outside the ball. The pressure inside the can is why you hear the distinct pop when opening it for the first time.
Inside a pressurized can, tennis balls can maintain their bounce for years, but their shelf life is still finite.
How Are Tennis Balls Pressurized?
Popular Mechanics has outlined how tennis balls are pressurized on the example of Wilson balls:
- First up, the rubber shell of the tennis ball is manufactured. The rubber used in Wilson balls is sourced from rubber trees in Thailand. The rubber compound is thinned, heated, and cut into sections.
- After being cut, the rubber compound is molded into half-shells. To solidify the shells, the molds are exposed to high temperatures and pressure.
- The seams of the half-shells get buffed and covered with adhesive.
- The half-shells are placed in a hydraulic press, with the ambient air pressurized at 18 psi. As the shells are combined into a tennis ball core, the pressurized air is trapped inside.
- Once the adhesive cures, the shells are cooled to at least 127 degrees Fahrenheit to prevent the core from exploding. By the time the core is cool enough, the internal pressure reaches 15 psi.
- The core is then covered with the exterior felt, which I won’t go into since this isn’t what we are interested in today.
- By the end of the process, when the balls are ready to be put in the can, the internal pressure is 12 to 14 psi. The cans get pressurized at two psi more because they hold more air than the balls.
Here, you might have noticed some discrepancies – earlier; I said that the internal pressure of a tennis ball is at 26.7 psi, whereas Popular Mechanics claims 12-14 psi.
This may be because the psi given in the guide is actually gauge pressure, which is relative to the atmospheric pressure. 12-14 psi gauge pressure is the same as roughly 26.7-28.7 psi absolute pressure. If this is what the article meant, then the numbers match.
But since Popular Mechanics doesn’t specify anywhere what kind of pressure figures are given in the post, I’m not entirely sure.
However, what I am sure about is that tennis balls are pressurized higher than atmospheric pressure – otherwise, they wouldn’t retain their shape.
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How Long Do Pressurized Balls Last?
Once you take pressurized tennis balls out of their can, they will last 1 to 4 weeks. Depending on how frequently you play, balls may last more or less.
But no matter the intensity of training, tennis balls will start leaking air once they are out of the sealed can. So you better start hitting the ball immediately because the more you wait, the worse the ball bounce will get.
With all that said, let’s also not forget that tennis balls wear physically when you hit them. A ball with perfect internal pressure will become utterly useless if you damage the core. If this happens, all the pressurized air will escape.
And what this means is that low-quality tennis balls may last much less than what you would expect.
Do Unopened Tennis Balls Expire?
Yes, tennis balls expire even when unopened, although balls’ shelf life in their sealed tube tends to be two years.
No matter how well-sealed a tennis ball can is, it also leaks air. Manufacturers most likely can make sure that no air makes it out of the container, but if they did this, you probably wouldn’t have been able to open it up. Plus, the manufacturing costs would most likely increase.
What Do You Do With Old Tennis Balls?
Old tennis balls that have lost their internal air pressure are no longer suited for training or competitive play. But you may actually be able to find a use for them!
Many soccer players actually use tennis balls for juggling. Tennis balls are tiny and very hard to hit compared to regular-sized soccer balls, which makes them great for training coordination and accuracy.
If you have a dog, then you could give the tennis ball to him too! However, make sure to keep an eye on the pup during play because he may accidentally swallow the felt – it comes off easily.
Finally, you could use your tennis balls as a massage roller of sorts, if you don’t have better equipment for this. Foam rolling is known to help with muscle soreness and improve flexibility. A tennis ball isn’t as easy to use as a foam roller, but it could have the same benefits.
Are Pressureless Tennis Balls As Good?
Although pressurized tennis balls are the industry standard when it comes to competitive play, pressureless tennis balls should not be ignored.
Pressureless balls don’t suffer from the air leakage problem – as long as the ball is physically sound, you will be able to use it for play. Pressureless balls do wear down with use as their rubber core softens, but they remain usable for much longer than pressurized balls.
On the other hand, pressureless balls are heavy and thus require more force to hit. These balls generate less spin too. Due to this, they are typically used with tennis ball machines or for lessons.
As a bottom line, tennis balls are sold in sealed cans to maintain their internal pressure. Once you take the balls out, the clock starts ticking, and the balls start slowly losing their internal air.
This knowledge won’t magically turn you into a high-level tennis player, but it may help you save money in the long run. Don’t open up your tennis ball can unless you intend to spend a few weeks actively hitting the balls. This tip will help you maintain the bounce longer and hopefully spend less money on new balls. That’s all you need to know about why are tennis balls pressurized.