Cycle time is the mold's most valuable resource, and the thermocouple's contribution to cycle time optimization is through the cooling time's boundary. The cooling time is set to ensure the gate has frozen and the part has cooled to ejection temperature, and both conditions are measured by thermocouples. A probe that reads inaccurately causes the cooling time to be set conservatively, and the conservatism is the cycle time that the mold never recovers.
The first mechanism is the gate freeze. The cooling time must extend beyond the gate freeze, and the freeze time is measured by the near-gate probe. A probe that reads high causes the controller to believe the gate is cooler than it is, and the cooling time is extended to compensate. The extension is a cycle time cost that is paid on every shot, and it is invisible because the controller shows the setpoint rather than the freeze time.
The second mechanism is the ejection temperature. The part must cool to a temperature at which it can be ejected without distortion, and the part's surface temperature is measured by a probe or an infrared sensor. A probe that reads low causes the controller to believe the part is cooler than it is, and the ejection occurs too early, producing distortion; the response is to extend the cooling time, and the extension is again a cycle time cost.
The third mechanism is the thermal uniformity. A part with a temperature gradient cools unevenly, and the coolest region determines the ejection time while the hottest region determines the distortion risk. The probes' readings across the part's surface show the gradient, and the gradient's reduction-through cooling circuit changes or gate timing-allows the ejection time to be reduced. The probes are the instrument that shows whether the gradient has been reduced.
The fourth mechanism is the hot spot. A local hot spot-at a thick section, at a gate, or at a rib-determines the cooling time, and the probes that monitor the hot spot's temperature show when it has cooled. A probe placed at the hot spot is the instrument that allows the cooling time to be set from the hot spot rather than from the average, and the difference is often several seconds per cycle.
The practical use is to log the near-gate and part-surface readings across a range of cooling times and to identify the cooling time at which the gate freeze and the ejection temperature are both satisfied. The optimization is a measurement exercise, and the probes are the measurement. A mold whose cooling time was set by trial and error is a mold whose cycle time includes a margin that the probes could have eliminated, and the margin is paid on every shot for the mold's life
