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I'm trying to figure out how the heck some of these plastic bottle caps are injection molded. By having the thread on the inside of a cap, this obviously creates an undercut.

enter image description here

Now I've watched some videos on Youtube on the process, and I can see that a metal threaded core is twisted into place (automatically or manually by an operator). The mold is closed, plastic is then shot into it, the mold opens, and then the threaded core is twisted out of place to allow ejection.

Some caps have split threads (as seen below) and use a collapsible core.

enter image description here

And some caps use a long continuous thread (as seen below) which makes sense for ejection (one continuous spin).

enter image description here

But this other style cap uses a long but non-continuous thread (as seen below).

My question is, how is this achieved? How does the threaded core spin out of place without colliding into the threads since the thread has an ending. In my mind, when this spins out of place, the feature on the core called out with the arrow below would collide into the freshly molded threads on its way out.

enter image description here

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  • $\begingroup$ Looks two colours - could this be overmoulded? $\endgroup$
    – Jonathan R Swift
    Oct 31, 2018 at 21:30
  • $\begingroup$ It's actually a single color with heavy exposure so that you could see the black threads otherwise it would be hard to see. $\endgroup$
    – Davbog
    Oct 31, 2018 at 23:23

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the mold contains a rotary mechanism than unscrews the part off the mold core after the plastic is solid and the mold is split open. this is called an unscrewing mold and is used to make threads like this. the giveaway is the presence of engagement features around the part which are grabbed by mating features in that part of the mold that unscrews.

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  • $\begingroup$ This doesn't address the specific point of how the unscrewing core deals with the 'end of the thread'? $\endgroup$
    – Jonathan R Swift
    Nov 1, 2018 at 2:02
  • $\begingroup$ the core isn't the part that revolves; there is an annular ring surrounding the core that engages the part around its lip. that ring revolves and unscrews the part from the core, which stands still. that ring has threads in it down inside the mold whose pitch matches that of the pitch of the molded threads, so as the ring revolves, it rises up out of the body of the mold to follow the part as it is being screwed off the core. The other way this is done is if there's a rotating engagement feature in the other half of the mold that rotates the part as the mold splits. $\endgroup$
    – niels nielsen
    Nov 1, 2018 at 3:59
  • $\begingroup$ While that makes sense, I'm not sure that's right in this case. The other end of the thread also terminates like the picture with the arrow above. If the cap were screwed off, the freshly molded thread would now be colliding into this thread-end feature on the core. $\endgroup$
    – Davbog
    Nov 2, 2018 at 17:18

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