This is an idea that has been revitalized for molding internal threads & undercuts. As illustrated below, a collapsible core is made of a central core pin surrounded by a slotted sleeve. The slotted sleeve is a spring steel that is cut form the impression of threads or undercuts on the inside of the plastic part. When the mold is closed, the center pin is pushed in, expanding the slotted sleeve to make its impression on the part. As the mold opens, the center pin extends, retracting the slotted sleeve. As the sleeve retracts, it clears the part's internal cuts or thread. The part is then ejected normally. Collapsible cores can produce many parts that used to be impossible. Machines with collapsible cores can run faster since there is no waiting for a cam to slide or a threaded core to unscrew from the part. However, due to lack of cooling some material may run slower.