I agree with this guy, and I have to say it… username checks out.
Comment on I need to vent about plastic milk jugs
Machinist@lemmy.world 2 weeks ago
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.
CalmChaos72@lemmy.world 2 weeks ago
Professorozone@lemmy.world 2 weeks ago
I agree with you.
SoleInvictus@lemmy.blahaj.zone 2 weeks ago
Believe it or not, this person is correct. 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, winter 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.
Machinist@lemmy.world 2 weeks ago
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.
SoleInvictus@lemmy.blahaj.zone 2 weeks ago
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.
Professorozone@lemmy.world 2 weeks ago
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.
m0darn@lemmy.ca 2 weeks ago
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.
RattlerSix@lemmy.world 2 weeks ago
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.
www.usatoday.com/story/news/…/5458044001/
kogasa@programming.dev 2 weeks ago
This is supported by the patent documents for at least one type of milk jug, which I found thanks to Snopes:
www.snopes.com/news/…/milk-jug-indentations/
patents.google.com/patent/WO1999022994A1/en
SoleInvictus@lemmy.blahaj.zone 2 weeks ago
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.
Professorozone@lemmy.world 2 weeks ago