There’s a post I saw on reddit that points to the dimple on the side of a milk jug, and makes fun of all the people who don’t know what that’s for. In the comments are thousands of people giving dozens of different explanations, and all of them are wrong.
It is not there to indicate that the milk has spoiled by popping out due to gasses produced by spoiled milk. If there was enough gas to pop out the dimple, the whole jug would look like a balloon.
It is not there to provide structural integrity, like lateral support to prevent the bottles from crushing. The contents are under pressure, so if there was enough force on the jug from any direction, then the cap would pop off regardless of the shape in the sidewall.
The actual answer is that the dimple is added to ensure that all of the jugs contain the same volume of milk. Plastic jugs are blown into molds, and minor manufacturing variations over time would create jugs that hold different amounts of milk. Larger jugs would hold more than a gallon. They could just fill by volume, but consumers are wary of purchasing a bottle if it appears to be less full than the others. So they add the dimple to make it so that the level of milk is all the way at the top with minimal air between the milk and the cap.
You can verify this yourself by finding different jugs from the same supplier with dimples of different depths, or even no dimple at all. None of those other explanations would explain dimples of different sizes or jugs without dimples.
TLDR everybody is wrong. The milk jug dimples are added to ensure the jug contains the correct volume of milk.
Believe it or not, @themeatbridge@lemmy.world is correct, just not about why. It’s to adjust for differences in jug size caused by temperature.
Plastic jugs are made by blow molding, where a tube of plastic is warmed, then inflated within a mold using compressed air to create its shape. In winter, the air and environment are cooler so the plastic is also cooler and accordingly a bit less elastic while getting blown. This results in jugs that contract a bit more while cooling and are a bit smaller. To compensate, cool weather jugs have a shallower dimple. The alternative is either warming the air or warming the molds more, both of which cost more, while this actually slightly saves money by using a bit less plastic. The converse is true for summer jugs - bigger dimple, warmer air - as the warmer plastic molds more easily.
The dimple also adds a bit of structural stability, so the jugs can be made of slightly thinner plastic. These factories pump out millions of jugs, so even a $0.005 saving per jug adds up.
I actually did some work for a company that makes plastic containers, so I got it straight from them. Otherwise I’d provide a source. What I could find online that corroborates is low quality local reporting, so I didn’t bother with URLs.
That makes more sense. Nothing to do with wear. I guess the dimple would be a removable insert. You could have a selection of them and swap when calibrating the line.
I would think that blow mold is happening right before washing and bottling. Tube blanks are probably supplied in Gaylord’s coming from the plastic producer. Transporting semis full of empty jugs doesn’t make sense.
I’m suprised there is that much variation in volume, I would expect the temps to be more consistent. I guess the compressed air temp is the main variable, mold temps should be pretty consistent. Ambient air temp when the bottle is cooling probably also plays a role, more or less shrink before it “freezes”. Not sure if they’re made from LDPE or HDPE but those are both really stretchy, so I guess they very well could jump all over on size.
Most of my mold experience is in automotive, which is going to be a tighter process.