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.
SoleInvictus@lemmy.blahaj.zone 8 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 8 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.
SoleInvictus@lemmy.blahaj.zone 5 hours ago
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*.
Machinist@lemmy.world 6 hours 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.