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The threads are formed inboth halves of the mold. If you look closely you should see a tiny thin raised line on either side. That is called the parting line.
Since the pen cap is one piece the threads are done using slides or lifters that are a sub-feature within the mold.
As Mr. Plastic said, the external threads are created in two halves of the mold and when the mold opens these two halves are seperated in two opposite directions giving way for the component to come out.
When the thread is inside the part, the component is ejected from the core in many number of ways. It depends on the shape, depth and material of the thread. If thread is not deep, with filleted cross section and if material is flexible like Polypropelene(PP) then it is directly stripped out or pushed out. If this is not possible the the core insert with threads can be rotated and componentcan be ejected out. The rotation can be given mechanically with attachment of gears. With my experience I have seen moulds of such kind with 64 cavities. ie. 64 components at one shot.
Molds of this type are definately not for the inexperienced. Unscrewing molds, as they're called, are very complex... Especially in high cavitation molds, such as ink pens, deoderant bottles, etc....
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