On Sun, 22 Mar 2009 11:03:22 -0700 (PDT), engineman
wrote:



I don't think that these threads were designed for pressure relief,
they seem to be very strong.
The bottle seems to have 3 parting lines and the parting lines are all
on the lands of the thread.
Also it appears that the length and positioning of the lands would not
permit removal of the thread from the mold without unscrewing.

I think it's quite likely the threads are designed for fastest
handling by the molding and filling automation. In particular, the
blow molding machinery operates at very high rates. The preforms for
the blow molders are injection molded in multi-cavity molds, so the
cycle rates there are not too high. But I've seen blow molders running
at something like 20 bottles per *second*. These were continuous
motion machines, which means the preform, which looks like a little
capsule attached to a bottle neck and thread, have to be screwed into
a station on a carousel on the fly. The carousel on the fastest
machines I've seen were 8-10ft in diameter and spinning so fast the
individual stations were a blur. That interrupted thread would allow
the preform to be mated up in a fraction of a turn rather than the
couple turns required to remove the cap.

An animation of a single station:
http://www.youtube.com/watch?v=vSabFFQUr9E

There's a continuous motion blow molder in the middle of this video,
but it's only running at 3 bottles/second.
http://www.youtube.com/watch?v=T01i_vp2mJE

--
Ned Simmons