Threads on neck of plastic drink bottles

I was consuming a 20 oz bottle of <carbonated carmelized soft drink> today and noticed the threads on the neck and in the cap.

The threads on the neck are not continuous. They are broken into (on this bottle) two different length arcs, same size arcs are lined up top to bottom. Inspection of the cap reveals a similar phenomenon, except the arcs are different that the ones on the neck.

This is probably best explained by looking at a plastic drink bottle.

Why the different length arcs in the thread? What purpose would that serve?

To relieve pressure before the cap is totally removed instead of spraying soda all over your clothes.

These bottles are typically blow-molded, with air under pressure forcing the plastic against a mold with the shape of the bottle, threads included. Since it is then necessary to remove the bottle, the mold has to separate into 2 sections. The mold halves have to be carefully matched so that there is minimal “parting line” evidence left on the surface of the bottle, but this is almost impossible to eliminate completely. Just look along the sides of a bottle, in line with the break in the threads, and you might see some discontinuity. It’s easy to hide under a label, though.

But in the threads, the parting line mismatch could make it difficult to get the cap on. The torque required to do so by the cap-installing feature on the bottle-filling machine would be variable, and it could even happen that some number of caps wouldn’t go on tightly enough to seal at all. Breaking the threads at the parting line so that there can be no “flash” seems to be the preferred approach.

I hate them because when you take a drink and pull you lip away from the threads, a little bit trickles out of the slot between the arcs…modern day " dribble glass " I guess.

No one has gotten around to why the thread-arcs the OP described were different lengths. If Elvis’ flash reduction post is correct, and I think it is, that’d mean that the two pieces of the mold are asymmetrical. Is that true, and why?

You hear that fizz as you open the bottle? that is relieving the tension inside, the reason it is shaped “weirdly” is so you cant take it off so quick, and to keep the bottle cap from being able to accidently be opened

Elvis was correct as far as he went. However, there’s more to it. First, there’s the matter of mold slides around the bottle neck. When a mold is made for a part with some tricky bits such as threads or the 5-lobed foot of the bottle, it has to be made with sliding inserts. That is, before the mold opens to spit out the finished bottle, at least three parts of the mold pull back from the bottle. The foot part pulls down, the inside-the-neck plug pulls up, and one or two neck thread inserts pull sideways from the neck. If you look carefully, you can see the lines on the bottle.

Then there’s the cap. The threads are internal, so the mold parts for the inside of the cap are probably made to back out in pieces, like a wooden puzzle. The seal insert is probably popped in after molding.

I had always assumed that the gap in the threads was there to aid in drainage. In other words, when you tip the drink back, the excess liquid drains straight down instead of spiraling down. This would keep the threads from becoming a sticky mess (the liquid drains down under the broken-off plastic ring around rim). It also helps drain the last bit of soda into your mouth as you tip the bottle.

I tend to discount the theory that the slots aid in reducing the fizz blow-out when you open the bottle, since my water bottle here has those same slots and it doesn’t fizz at all.

But, these are just my guesses.

maybe bottling plants dont want to change molds like the braille drive thru atm thing

Since many blow-molded bottles are made with continuous threads, I don’t buy any of the molding explanations.

If I recall, during the first few years of PET bottles the threads were continuous. The slots were a later addition.

Open a well-shaken cola bottle and you’ll see a possible function. The stuff sprays downwards as well-organized jets, rather than spewing out sideways in a random direction.

On the other hand, perhaps the soft drink companies were trying to avoid lawsuits: without the slots, the pressure would leak off more slowly along the threads, and if you unscrewed the cap quickly it could blow past your hand and strike you in the eye. Better to relieve the pressure as quickly as possible by making the threads act as less of a barrier.

On further inspection of a 20 oz Pepsi bottle, I have made the following additional observations:

The flash lines ElvisL1ives mentioned actually go right through the threads. Specifically through the longest arcs. Flash on the bottle is worst at the threads, though registration seems quite good. This results in both halves of the top of the mold having bilateral symmetry. Oddly, the flash lines don’t extend all the way down the bottle. There is the top (threads and first inch or so of bottle), the middle (from the top section to just above the 5 ‘feet’), and the bottom (last inch or so. Just the feet) Middle and top lines don’t match, bottom doesn’t appear to have any. Just in case you wanted to know. :slight_smile:

The arc lengths of the bottle are approximately 120 degrees, and 60 degrees. (closer perhaps to 110 and 50, with the remainder as open areas. They are arranged long, short, long, short.

AskNott seems to have the correct idea about the cap since there are flash lines associated with each arc of thread. lengths on the thread are approximately 60 degrees and 30 degrees (again, shorter due to open spaces) and are arranged long, long, long, short, short, short, short, short, short…

The spaces one the cap and the bottle are the same size. On this bottle the cap is translucent enough that with a good back lighting, the holes line up differently depending on the relation between cap and bottle. Either 0, 2, or 4 channels are created during uncapping, which supports the pressure relief theory presented above. The first hiss the bottled produced coincided to the start of alignment of the channels. It seems logical that this would also work to dirrect any spray from an unsettled bottle.

Thanks all. This was a wonderful diversion before finals.

pandamonium, awesome detective work. Goes to show what actually investigating stuff for yourself can do.

Welcome to the boards! Finally, a smart one.

Somebody told me the grooves are called “lawyer grooves” and are to demonstrate a due diligence in trying to avoid caps that pop off and injure eyes due to unrelieved pressure.

This makes sense to me, because lawyers are always trying to make us look like drooling idiots.