16 August 2019

About those dimples on the sides of milk jugs - updated


I had never paid attention to these dimples before; if I had I would have assumed they were to facilitate gripping the jug, but they are on the wrong side and seemingly superfluous for that purpose when a handle is present on the other side.

I have seen claims that these are pressure relief valves that pop out if the jug is dropped or the milk spoils and produces gases or freezes, or that they are structural supports for an otherwise smooth wall, or that they allow the size of the container to be varied without changing the mold,

As soon as this jug is empty I'm going to test the pressure-relief theory by dropping a water-filled one and/or freezing it.

Update:

It worked - sorta.   When the jug was empty I refilled it with water, adjusting the level to match the new unopened jug (airspace about 5cc under the cap).  I then tightened the cap and took the jug to the driveway, where I dropped it from waist level to simulate a shopper's misadventure...


Both dimples popped, one of them blowing out completely.  I'll plan to try once more, next time dropping from a lower height.

18 comments:

  1. They provide structural support. :)

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  2. we don't have them in Australia

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  3. Local store sells several milk brands; some have 'em, some don't, so while they may provide some structural support, it doesn't seem to be necessary.

    My best guess is the equivalent of the pop-up caps on juice / preserves / etc - indicator if the contents have gone bad. When I've left milk overlong, I've seen expansion to the point where the panels 'popped'.

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  4. Milk jugs are manufactured just before being filled and are almost always have more than a gallon volume. When 1.0 gallon of milk is metered in the jug, it appears the jug isn't "full" enough, so the dimples are added. Notice how the depth of the dimples vary from gallon to gallon.

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  5. I've always assumed they were there to create the illusion of more milk in the bottle.

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  6. The dimples provide the structural support allowing the bottle molder to use less plastic in production

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  7. We don't have them in New Zealand, but then we have grass fed cows and I guess that is why.
    Lol.

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  8. The indentations help to draw your attention to the Missing Person's Picture?

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  9. It's a conspiracy by Big Bovine.

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  10. I'm going with the "Oh, crap, we froze the milk" theory. Freezing causes expansion. In fact, my dad would fill empty milk jugs (without dimples, in those days, I suppose), then freeze them. If we needed additional ice (for a ice cooler, iced tea, or homemade ice cream, say), we'd take those jugs, bust them and--viola!

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  11. Of course, I'm still trying to work out what is the 15th month.
    Xenotober maybe ?

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    1. August 15, 2019

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    2. Anon, he's a Kiwi writing with his tongue firmly in his cheek...

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  12. Although the pressure relief aspect is a nice side effect, this white paper from a company that does blow molding of dairy containers, which was referenced on the Wikipedia page linked in the article above, indicates specifically that they are volume adjustment inserts to account for variations in container shrinkage due to changes in conditions, often seasonal: http://www.qenos.com/internet/home.nsf/(LUImages)/WP%20Understanding%20Dairy%20Bottle%20Shrinkage/$File/WP%20Understanding%20Dairy%20Bottle%20Shrinkage.pdf

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  13. my gallon water bottles do not have dimples.

    I-)

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  14. I don't understand. Without those dimples the jug would not break when thrown on the ground so this can not be the explanation. Who want's a broken jug?

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  15. Per Wiki, the dents are for volume control. https://en.m.wikipedia.org/wiki/Plastic_milk_container

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  16. I wonder if the blowout was not due to intentional design, but to the fact that the bending to form the dimple might have meant a weakened seam?

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