Foil balloons leaking... defective or damaged?
If you've experienced foil balloons leaking shortly after filling them you're not alone. Foil or "mylar" balloons are made from very light and thin, high density plastic film with metal coating on one side. The materials used in first generation foil balloon material were heavy by today's standards. This made them tough enough to withstand being filled using a simple latex inflator which is typically supplied with a rental helium bottle. You could eyeball a fairly safe fill volume just by looking at the balloon. Beginning nearly a decade ago (even before the helium shortage), all US manufacturers began transitioning to higher performance plastic films available due to technological breakthroughs. These films proved to provide better helium retention while also being lighter in weight (and probably less expensive). This allowed some to even make slightly smaller balloons that would still float, thereby saving even more helium.
A full helium bottle is under very high pressure, between 2000 and 2400 psi depending on the size. Using a latex inflator with newer foil balloons can cause a couple of different issues. First, the initial burst of pressure against a folded over neck can blow a hole in the valve (sleeve) internally, causing it to leak from the opening. It seems as though the balloon is just defective. The foil balloon valve is just a sleeve, two plastic strips of material welded together lengthwise with a channel in the middle for gas to pass. On the inside of the balloon, the internal pressure of the filled balloon collapses this sleeve, preventing gas from seeping out. A hole in the valve inside the balloon will prevent the valve from working. The second issue is that the latex inflator is comparatively fast and has no automatic stop. The combination of too much speed and high pressure will cause those little creases along the seams to become pin holes. It's easy to think you're eyeballing the correct fill size but this newer film cannot tolerate as much pressure as the older materials.
With an auto-shutoff foil balloon filler/regulator you will be assured that the bulk of your balloons will fill perfectly. Partly because it's speed is regulated down dramatically. But more importantly, it utilizes a diaphragm device which closes the flow to the balloon when a certain pressure is detected by the device. You will not be able to overfill a balloon using one of these regulators.
Your helium supplier may offer one of these with cylinder rentals if you ask. You may need to pay a little more to rent one. You can get one from us. But if you've been in the position where lot's of your custom printed or birthday foil balloons are leaking, you can and should invest in a proper auto-shutoff helium regulator.