TnT Collapsable Cores

Collapsible Core Mold

Figure 1.

Molding internal threads or undercuts in plastic parts is a common challenge. Sometimes the solution is to rotate the core to unscrew the plastic part from the core. Mechanisms to do this and keep the part from rotating with the core can get complicated. Undercuts cannot usually be 'unscrewed'. Sometimes they can be stripped. One solution is to use some sort of collapsible core.

Collapsible cores are devices that become smaller during the ejection process. One commercially available device is shown in Figure 1 in the collapsed condition. The same core in the expanded condition is shown in Figure 2. The collapsible core is attached to the ejector plate and the center core to the back of the mold. The inner core is tapered allowing the threaded core to collapse as it moves forward.

Figure 2

Figure 3 shows a mold with the cores forward in the collapsed conditions. At this point, the plastic part can be stripped from the core with a secondary stripper plate near the center of Figure 3. In this case the stripper plate is operated by hydraulic cylinders. The sequence of operations is programmed with switches signaling the controller when the plates are in the desired position. Two of the switches are shown in Figure 3 the right side of the mold just below the centerline.

Figure 3

Figure 4 shows the mold halves separated with the stripper plate forward.

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