|MadSci Network: Engineering|
I didn't know the answer to this one, so I sent an e-mail to Alcoa's Closure division (I figured that Alcoa knows about anything associated with aluminum, including lids). I received the following answer: "In response to your question, the thread "cuts" or "gaps" in the threads on both the bottle and closure are vent slots which allow the CO2 gas to escape from the inside the package during opening. This escape of gas relieves the pressure on the inside of the bottle and allows the closure to be safely removed from the package by the consumer. The reason these threads are only found on soda bottles is because soda packages are normally the only packages pressurized with CO2. Non-pressurized packages (like bottled water) normally contain smooth, non-vented threads since there is no internal pressure to be relieved from the inside of the package. Please advise if you have other questions." Tony Smith Alcoa CSI I wondered whether beer bottles didn't have the cuts in the lids because they are under lower pressure. Again, Mr. Smith responded: "The internal pressures in a beer package are much less - the CO2 content of beer is roughly half the CO2 content of a carbonated soft drink. It also depends very much on the type of bottle and closure system being used for the beer package. In a traditional longneck beer bottle with a steel crown, the opening device bends the crown while holding it in place, which effectively vents the container safely. If the beer package contains a screw on closure, the bottles and closures are sometimes vented and sometimes not - it really depends on the preference of the company. Vented threads are not required at all for packages with this low a level of CO2, but some folks like to use them anyway. Tony Smith So there you have it. The vents are intended to relieve pressure as a cap is removed. You are a pretty keen observer to have noticed them. And thanks to Mr. Smith for the information.
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