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*.
- 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.
Machinist@lemmy.world 1 week 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.