Meaning
Constant mechanical forces are applied to sensor assemblies to establish a stable operating baseline. For piezoelectric cavity pressure sensors, the static preload force ensures that the transducer remains in tension or compression during the entire moulding cycle. This continuous contact allows the sensor to measure both compression during packing and decompression during cooling.
Preload Action
Applying a specific mechanical force shifts the operating point of the sensor into its linear response region. Below this threshold, the piezoelectric crystal displays a non-linear behavior, which leads to inaccurate pressure readings. The static preload force must be high enough to exceed the maximum expected melt pressure to prevent the sensor from losing contact with its seat.
This configuration ensures that even the minor pressure drops during the cooling phase are tracked with high accuracy. It also prevents high-frequency vibrations from the injection machine from introducing noise into the pressure signal.
Installation Rule
Precision torque wrenches are required to apply the correct mechanical load during tool assembly. If the static preload force is too low, the sensor will be loose, which leads to signal loss or sluggish response to pressure changes. Conversely, excess force can damage the sensor or cause premature fatigue of the piezoelectric element.
This installation step must be completed before the mold is loaded into the press.
Temperature Impact
Thermal expansion of the tool steel can alter the preloaded tension during operation. Because the mould operates at high temperatures, the steel around the sensor expands, which can increase the load on the transducer. This change must be accounted for in the system calibration to prevent baseline drift during long production cycles.
