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.
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.