Comment on I need to vent about plastic milk jugs
Machinist@lemmy.world 1 hour agoThat 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 1 hour 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 30 minutes 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.