The collapsible core offers some unique advantages to the traditional rotating core. The concept of the collapsible core is to first collapse the core to a small enough size to be able to clear all the molded features of the piece part, and then retract the core from the molded part. It sounds and looks a lot like a magic trick.

The basic core uses two components, an actuator pin and a collapsing sleeve. The sleeve is made up of several fingers of two types: (1) inner fingers and (2) outer fingers. The fingers are arranged in a cylindrical formation and all the fingers are bent toward the center of the cylinder. T

he inner fingers are narrower and bent further toward the center than the outers.
The actuator pin is a cylindrical shaft used to hold the fingers in place and to form the top section of the molding surface. When the actuator pin is inserted from the back of the core, the inner fingers are forced outward against the outer fingers. As the pin is further inserted, the inner and outer fingers move to their molding position and form a solid sleeve around the actuator pin.

When you look at the core from the molding end of the assembly, the fingers look like pie sections placed next to each other in a circle around the pin. The way it works is that when the pin is removed the smaller (inner) pie sections will move to the area that was occupied by the pin. This produces gaps between the bigger (outer) pie sections. The outer sections move toward the center until the gaps are taken away.

The actions all happen at the same time due to the spring loading of the fingers against the pin, and the corresponding angles on the flanks of the fingers allow them to slide against each other as the core is expanded or contracted. A tapered area inside the bore area of the sleeve allows the core to open at a controlled rate.

So a collapsible core has on average 12 fingers and a pin to form the inside section of a threaded cap. Each finger has three surfaces that need to seal out the plastic during the molding cycle. That’s 36 surfaces that need to come together within tenths! A pretty daunting task considering all of the fingers are bent to produce the spring action needed to close the core.

This brings us to the main drawbacks of collapsible cores. They may flash. Plastic moves into any gap that is large enough. The resulting flash can make the molded parts unacceptable. Plastic can build up on the flanks of the fingers and require cleaning. Spring pressures seem to be inconsistent.

Current finishing processes use complicated fixtures and detailed instructions on how to finish the molding details without affecting the sealing abilities of the core. Some say that the cost of the core and its unreliable sealing ability and complicated finishing process make them a last resort.