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.
YOUR MOMS BLOWN INTO MOLDS!
Yeah? Well I blew into your mom’s mold last night!
How dare you! She’s been growing that slime mold for weeks! Her project is ruined! She’s gonna get fired from the research lab you fiend!
The really clever solution is to bag milk:
Absolute savages.
All the milk bag haters here. You’re a bunch of hosers!
This makes me irrationally uncomfortable. It feels like a direct attack on my worldview. You can’t just put milk in a bag y-you just can’t!
I have vague memories of my childhood visits to my parent’s native country where I drank milk from a bag.
… And shatting my brains out which was probably not because of the milk.
Exactly it was because of the bag
100% because of the bag. It personally wanted revenge from being drunk from.
We still have bagged milk made by some smaller dairies , its less expencive to pacage in bags than bottles
Heretic
Udderly foolish!
deleted by creator
America fuck yeah
Comin’ again to save the motherfucking day, yeah
I’m in Western NY, the “fancy” milk comes in glass bottles while the “store brand” is in these plastic jugs.
I buy non-dairy milk though which comes in cartons.
I’ve never seen a glass bottle of milk outside of old movies, around here it’s plastic jugs and the “fancy” stuff comes in cartons
So this is how I find out I’m poor…
A lot of the glass bottle companies also recycle the bottles, so that you can swap your existing glass bottle for a new bottle and they’ll take your old one back. It adds a little bit of logistical complication to the stores that deal with it, but it could be useful if you’re really trying to reduce plastic usage and you drink a lot of milk.
We have one particular brand here that does “Milk, ice cream & more” that has their milk in glass bottles. It’s this ridiculous super premium thick milk, I get some of the chocolate sometimes.
I think it’s about $6per (half gallon/4pint) which is considerably more than average but it’s like drinking chocolate ice cream.
We had those in Vancouver, too, which came from a dairy in the Greater Vancouver Area. I used to get fancy milk on payday and keep the bottles for crafts, away from the recycling bandits. They love them because you can get like a dollar back, per each.
I’ve been using plastic bags for a while now. The difference in use is minimal, but I think the plastic difference is big.
Okay but I’ve been in plastic bag land for a while and every goddamn time I make the first cut, I spill milk everywhere. Is there a trick? I already know the “both sides” method and put it in the jug before I cut the corner. Right now I just have a glass standing by for my obligatory shot every time I open one.
I cut only a small hole from one of the corners, and when I pour, I prop up the other corner as I pour to keep the bag steady. There are probably better ways of doing it but this has worked for me so far.
This worked! Thank you. My two-hole method was fucking me over.
Glad to hear!
We have plastic bags
Europa has as far as I know only 1 litre Tetra Pak milk (non see through) or 1 litre glass bottles (reusable).
They found milk on one of Jupiter’s moons?! Pretty cool, I guess.
I think they’re talking about Europa, the water-nymph and consort of Zeus.
I did not know she was that into milk packaging, but that’s cool.
It’s going to depend on your country and the stores you visit. If I wanted to, I could buy milk in a plastic bottle in most stores in Belgium. Tetra pak also exist in various sizes, 200 ml is the smallest I’ve seen and 2l the largest.
This also describes hiding shrinkage.
Cool fact, makes sense and it is absolutely trivial but interesting. Thanks!
No I never thought to ask about it. But it’s good to know, I guess?
I always believed that it was to allow for expansion of the product when it freezes. I often freeze similar jugs full of water, to use in coolers, and that big dimple always ends up pushed out after it’s frozen solid. If it hadn’t been for that expansion dimple, the jug would have split.
Milk is ruined if it is frozen.
I hate milk, so I never drink it today, but as a kid, I drank a lot of thawed milk. My mom always bought two gallons, and froze one while we used the first. It might not please a milk connoisseur, but it always tasted fine. It certainly wasn’t spoiled, or anything like that.
It causes the fat to separate. Maybe she got skim milk?
1-2%, so I never noticed any separation. I always shook up the milk whenever I used it on my cereal anyway. Probably still would, if I ever was to use milk for something.
I sometimes did the same, but I grew up on a dairy farm and had milk that wasn’t homogenized.
We used these old one gal apple juice bottles that had a narrow neck, so the cream would float to the top and collect in the narrow opening.
When I got older, I actually liked getting that first dollop of cream on my cereal.
Now though, anything over 2% just tastes too heavy. Growing up on unpasteurized milk, I am amazed at what I got away with.
The title of this post led me to believe it was about something else entirely
It’s so the sides don’t sag or bulge when full. It’s like corrugations in metal roofing/siding. Lets them get away with thinner walls.
If that were true, they would be on every jug of milk, and they would all be the same size. I have a gallon jug in my fridge right now that doesn’t have a dimple, but does have a circle where it should be.
Fuck Reddit and Fuck Spez.
k
You are correct that there is a connection to ensuring fill levels, but incorrect on it not being intended to provide structural integrity. It is both.
You can disable javascript to get around snopes adblock block: https://www.snopes.com/news/2023/01/07/milk-jug-indentations/
You can also view the original patent here: https://patents.google.com/patent/WO1999022994A1/en
Notable excerpt from patent:
When the horizontal ribs are not provided completely around the container, the face panels may be provided with indentions of preferably a circular configuration. The size and depth of the indentations may be varied to control fill level of a given volume of contents in the container in addition to further stabilizing the sidewalls.
Thanks for including this excerpt. I’m losing my fucking mind that OP was so detailed in his explanation yet left such a rudimentary question unanswered like “do they vary the depth of the dimple depending on fill level”?
I don’t think this is correct and would need to see a source before I believe it. I doubt the dimple is adjustable in the way you’re describing.
The amount of wear needed to change the volume by a noticable margin would be quite significant. Surface finish of the mold would be degraded enough that they would probably scrap the mold before using an adjustment like this as the mold would have sticking problems.
It might be volumetric compensation, but I doubt it’s directly wear related.
The mold is going to be at least two parts that split to get the blown jug out. The jug feedstock probably starts as a molded tube blank with the threads already in it. Would look like a test tube with a milk jug mouth.
Thinking about it, and I suppose you could actually call it wear compensation. Machine the mold with max dimple present. As your parting faces/lines take damage, you reface, and take some off the dimple to compensate for reduced volume. Maybe. That’s my best guess if it isn’t structual. Usually the rest of the mold has taken enough damage/wear that you’re scrapping the entire thing.
Believe it or not, @[email protected] 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.
You’ve got it! My work was about sustainability, but that includes plastic consumption, so I learned about the factors that affect the amount used. You’re right on the process - they’re gross immediately after molding, so washing is next. The molds are water cooled, so they’re pretty consistent, it’s just heating the tube and the temperature of the compressed air that’s affected the most.
The volume change is unintuitively high. Jugs have a high SA:V ratio, being a curvy semi-rectangle with a hollow handle. A 1% surface area reduction results in a >5% drop in volume, about 7 fluid ounces per 1%, or 0.875 cups. Manufacturers really only see <1% reductions, but if they stuck with the same mold through the summer, they’d end up with a jug that looks to be about 0.5-0.75 cups low after filling. That’s pretty conspicuous for customers, especially since the top portion tapers, making the level drop even more dramatic.
I’m sorry, I’m just not buying this explanation. I’d need more evidence.
What surface area? It’s volume we’re talking about. You mean if the plastic gets thicker, thereby reducing the interior surface area there is a corresponding decrease in volume? And it’s 5 to 1 ratio? So if the plastic is thicker by 20% there is no room for milk?
Winter bottles, summer bottles? Like the temperatures aren’t controlled because it costs money? They just compensate with a plug, what every season? Like it costs money to control the temperature of a process but it doesn’t cost money wasting plastic.
Hey, I’m not an expert on this subject and I could be wrong but from my perspective you’re just some guy on the Internet that sounds like he knows what he’s talking about.
I actually think they’re correct. It explains most of it and jives with my experience.
The amount of plastic used is fixed. Here is a bottle blank I have for a 2 or 3 liter soft drink:
We’re assuming that milk jugs are blow molded from a similar blank at the bottling plant just before washing and filling.
Milk bottles are either High or Low Density Polyethylene. A notoriously elastic plastic. It also creeps all over with temperature, you can take a bowed 3" thick sheet of it, put it on the floor and it will usually be flat in the morning, especially if it’s above 75deg F or so.
Milk jugs aren’t a pressure vessel like soft drink bottles.
They’re saying that due to the large surface to volume ratio and thin walls, there is a lot of seasonal variation in final volume. This is primarily due to the compressed air used during blow mold, ain’t nobody paying to heat or cool it. Also, the ambient temps in the plant, in the blow mold area may see 40deg F swing, maybe more, over the course of a year. They aren’t going to pay to condition the air if it doesn’t affect final product. Fuck worker comfort.
This would be enough to show seasonal variation in milk level due to volume changes, especially since the jug necks up and exaggerates differences. Reduced headspace probably also keeps it fresh longer due to reduced oxygen. Mostly, if your competetior’s jug looks more full, you sell less milk. One producer does it, they all have to do it.
It’s a totally believable and logical explanation to me.
You’re welcome and encouraged to look into it yourself. You misunderstand what I’m saying and draw further conclusions based on that, though, so I can see why it doesn’t make sense. I’ll take a stab at explaining.
I did mean surface area, not thickness. As volume decreases, so do the dimensions of the object. The thickness of the plastic is already negligible and any change within that plane is a fraction of that, so even less pertinent here. The remaining two planes of the exterior, being several orders of magnitude larger, do experience functionally significant, easily measured change. Those two planes as they relate to volume are most succinctly explained as surface area.
I mentioned the SA:V change to illustrate that this size change isn’t visually apparent, so it’s important to adjust the volume via the dimple. This maintains a steady milk level so jugs can hold an entire gallon in the winter and ensures customers don’t think jugs are underfilled in the summer. In cold weather, the dimensions of the jugs reduce less than 1%, which means visually the change is difficult to notice, but the volume changes a fair amount, around 5%. A change in size imperceptible to most reduces the volume of the jug by about 1/20 without compensation*. By reducing the size of the dimple, less plastic can be used, which saves money.
- This is how manufacturers can so easily fly under the radar with shrinkflation. It’s hard to see, especially since they use shapes that obfuscate these changes, but they’re easy to calculate.
So I’m simpler terms I think what you’re saying is the bottle shrinks (surface area decreases). When it does, the volume decreases too and it’s a 5:1 ratio. Correct?
I did click on the link someone posted here and the article made sense to me. Number one was structural. Which is what I would think. Number two mentioned this winter summer think but not during manufacturing ( which makes sense to me), but during shipping which still doesn’t make sense to me. What milk or even orange juice is NOT refrigerated?
Bingo, you got it. It only shrinks so much in volume because there’s a ton of surface area. If it was something with less surface area in relation to volume, like a sphere, it’d be much less dramatic.
Yeah, the shipping thing doesn’t really make sense for exactly the reasons you stated. The issue is more about the temperature when the bottle is made. This is done by inflating a warm plastic tube in a mold. Cooler plastic has a bit more rebound, where it shrinks a little when the air pressure is removed. This still happens in the summer, but since the environment is a touch warmer, it shrinks just a bit less. Since a 0.5-1.0% overall change has a significant effect on the volume, you get the compensating dimple.
Ambient air temp when the bottle is cooling probably also plays a role, more or less shrink before it “freezes”
Yeah I agree with that. Also maybe inconsistency with the plastic batch.
But also milk jugs are blown directly from pellets, no threaded blank. Water bottles and pop bottles go from threaded blanks though.
I don’t remember seeing washing but I guess that would be on the filling side, (jugs are made on one side of the plant then go through a wall to the clean side) I don’t think I’ve seen that for milk.
This is supported by the patent documents for at least one type of milk jug, which I found thanks to Snopes:
https://www.snopes.com/news/2023/01/07/milk-jug-indentations/
https://patents.google.com/patent/WO1999022994A1/en (page 2 lines 7-10, page 3 lines 27-29).
This is awesome. Thank you! I think the collective efforts of everyone here may have made this the Internet’s definitive post about milk jug construction.
I’ve now spent literal hours reading about and discussing jugs for milk, something I don’t drink. This isn’t even my area of expertise! I’m providing all of this to my therapist as further proof that yeah, I’m probably autistic.
I knew the OP couldn’t be right because how do you add a dimple after the jug is full?
But I’m not sure ambient air temperature during molding is the whole story either, although I expect that is a concern that is taken into consideration and the article below leaves it out.
The article says "*The high-density polyethylene plastic jugs are made of shrinks slightly over time. It also shrinks more in hot temperatures than cold temperatures…
Producers must make jugs slightly larger to offset inevitable shrinkage if they are exposed to summer heat in transport and/or go to long-term storage before being filled, but they want to keep the jugs smaller in cold months and if jugs go directly to be filled.*"
It’s saying the dimple is to adjust the size of the jug depending on what shrinkage it’s likely to see before being filled. So you can presumably have small dimples in summer if they go directly to be filled, and large dimples in winter if they’re going to be transported and stored first.
I agree with you.
I agree with this guy, and I have to say it… username checks out.
So how is the dimple added after filling? If it’s to ensure a common fill amount, it would have to be done post-fill.
At least, I can’t figure out how it would be done pre-fill without somehow precisely measuring the volume of the empty jug.
It’s assumed that all jugs from a mould hold the same volume, but not every mould holds the same volume, so by aiming high for the initial design of the mould and then adding spacers to the side of the mould as needed you can achieve a modified mould that stores the correct amount.
DISCLAIMER: I am not a milk jug engineer, that’s just how I interpreted it.
It’s added after molding the jugs and the volume is checked, but before the milk is filled.
So the mold is made, the volume checked, adjusted via an insert, and then they crank out jugs.
That’s plausible, but over-complicated in an age of digital CAD and mature molding techniques.
Sounds like milk bags are vastly superior.
Apart from when you want to use them.
Spoken like someone who’s never used them.
Milk specifically, no, other bagged liquids, yes. An absolute pain in the ass compared to a bottle.
If you have the right milk bag holder, it’s seriously a non-issue.
The only non-milk bagged liquids I’ve had were IV, and a nurse dealt with them, not me.
I mean your original argument was it was more convenient - everyone having to buy a milk bag holder already seems less convenient than one guy inserting a few shims in a few moulds every couple of months.
That’s a ton of manual labor on a disposable container. Manual labor is extremely expensive, and plastics manufacturing is extremely repeatable and extremely cheap at scale (that’s kind of the big reason plastic is so common, it’s cheap and reliable)
Granted I could imagine a machine which fills it with dyed water, uses a camera to determine the fill level then passes to another machine which adjusts by volume but that’s really going to slow down the production line doing all of that.
Ultimately plastics manufacturing is way more precise and consistent than that, you’ll basically end up with either a milk jug or a misshapen mess if there is a defect, and not really anything in between
Only the mold is being adjusted, not each individual plastic jug.
The dimple was there for stress releif:
Combined with the octagonal shape of the container, the circular, concave indent on the side of a milk container increases the stability of the plastic, allowing the internal pressure to disperse evenly. This improved structural support also allows jug manufacturers to reduce the amount of resin needed to make each container.
Read More: https://www.sciencing.com/1865028/milk-jugs-dimple-reason/
That very same article also does say however:
On top of that, the indentation allows the manufacturer to precisely control the volume that the jug can hold.
OP said it was not for integrity, which is wrong because it is for multiple things including integrity.
Right, but the person I replied to said it was for stress relief [only] - as if to invalidate by omission the original assertion that it’s for controlling fill level.
I pointed out the article supports both positions, and I did that for the benefit of people who might not read the article to see as such, and would otherwise take the comment to mean the article is in complete contradiction to the OP rather than partially contradicting and partially agreeing.
I don’t really have any personal stake or care about who as individuals are right or wrong in this, but I would like to get to the bottom of the milk dimple mystery. On that basis I’m not sure I know the truth even after reading the article, because it seems to be one of those things with a lot of myth behind it and not a lot of definitive sources.
