Mold Pressure Sensing Myths: Part II

In this second installment in the series kicked-off in November 1995, Brad C. Watkins, president of RJG Technologies Inc. (Traverse City, MI) continues in his efforts to dispel the five biggest "untruths" about mold pressure sensing.

A major myth about using ejector-pin-style mold pressure transducers, and one of the oldest, is that ejector pins stick, preventing pressure readings. This is nonsense, but is frequently brought up by mold pressure sensor system manufacturers who promote their own surface-mount technology, so-called direct sensing systems. In reality, it is an extremely rare case where a moving ejector pin sticks.

Sticking only happens if the pin is broken, or is severely bent. In such scenarios, the pin needs to be replaced anyway. Also, if sticking happens, it is immediately detected by observing, or interpreting, a drastic change in cavity-pressure readings or profiles, so it can be brought to someone's attention and repaired.

Some claim materials outgassing impedes the pin's ability to accurately translate force onto the transducer. In fact, a moving ejector pin cleans itself as it moves in and out each cycle, as shown in Figure 1. This prevents gases from building up around the pin. Experience shows that a transducer behind a moving pin is conducive to more repeatable readings than many surface-mount transducer styles. This is because there is an integral stationary pin found inside the surface-mount transducer's body (see Figure 2), which may be prone to sticking from gas buildup over time. The same holds true for placing a standard ejector pin sensor under a non-moving pin. Since the pin is stationary, a buildup of gases generally prevents repeatable pressure readings over time.

The tale of the sticking ejector pin is a phenomenon that holds true only for non-moving pins. Using standard ejector-style transducers under moving pins has proven to give the most convenient and reliable method for measuring cavity pressure in a production environment. This is evidenced in hundreds of applications, jobs still running millions of cycles without sensor replacement.