Comment on I need to vent about plastic milk jugs
SoleInvictus@lemmy.blahaj.zone 3 hours agoBelieve 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.
Professorozone@lemmy.world 2 hours ago
Machinist@lemmy.world 3 hours 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.
m0darn@lemmy.ca 45 minutes 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.
SoleInvictus@lemmy.blahaj.zone 2 hours 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 hours 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.
Machinist@lemmy.world 29 minutes ago
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: Image
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.