failed plastic thread 1

[brettac]

Hi - I designed a closure for a PET bottle with a 3 start thread (30mm od , 3mm pitch ), the cap was designed for a pop off type mould ( stripper rings rather than unscrewing )and the cap was moulded from HDPE.
the problem now is that it is possible to overtighten the cap and cause the threads of the cap to ride up over the bottle threads ie: the cap can be forced to turn endlessly without tightening.
Only the ends of the threads shows signs of being "rolled over" after overtightening, could non-uniform shrinkage cause the angle of the thread helix to change so that the flanks of the respective threads do not mate properly.
Perhaps HDPE is the wrong material ?

regards, Brett

 

[TheAnswerGuy]

brettac,

As long as your tool design had included the shrink factor for the threads, you should be fine.
Polyethylene is a material which typically has a lower durometer than Polypropylene and is subject to deformation as you are witnessing.
The caps such as used on soda bottles are molded in Polypropylene and bumped off the tool.
I am hoping that your thread design is proper for needs.
There are quite a few other factors which can also cause this propblem.
I am only offering an educated guess here.

theanswerguy@tr-usa.com

 

[rnd2]

you may have been better going for PP as answer guy says & 2 start thread