Rotating cores are the most common solution to this problem. This is one of the most common setups. The rotating core features a gear, bearing journals and a retracting thread, in addition to the molding thread.

The basic operation is to rotate the core and retract it at the same time and at the same rate as the core is unscrewing from the cap on the other end. This requires several other components. Usually a rack is used to rotate the gear and an adjustable lead nut is used to time the retraction. The rack requires an air cylinder or some other means of drive. The core will require bearings of some type to allow free movement and guidance of the core during the retraction. The mold must have a feature to keep the cap from turning while the core is turning and retracting from inside. This is usually done with anti rotation lugs, which are cut into the ejection sleeve or into the stripper ring.

  As you can see there are quite a few parts and some very critical timing involved in order to make a simple thread. It’s called a simple thread because the threads on a rotating core must also run all the way off the end of the core. If not the core would strip the threads off as it retracted from the cap.                  

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