Meaning
Specific crystallographic orientations of quartz plates determine their primary vibration mode under electrical fields. When used in high temperature rheological sensors, the thickness shear cut produces a quartz crystal that vibrates parallel to its plate surface rather than perpendicular to it. This vibration mode is essential for measuring the shear viscosity of adjacent polymer melts.
Vibration Mode
Applying an alternating voltage to the electrodes initiates a shearing action where the top and bottom surfaces move in opposite directions. The thickness shear cut ensures that this oscillation propagates purely as a shear wave into the polymer. Because shear waves do not travel far in liquids, they are heavily damped by the viscosity of the surrounding polymer.
This localized interaction allows the sensor to measure the properties of the boundary layer without being affected by the bulk flow. The shear strain generated by this high frequency vibration does not alter the molecular structure of the bulk polymer, ensuring non-destructive measurement.
Thermal Behavior
Excellent frequency stability across a wide temperature range is provided by certain cuts like the AT-cut. This temperature stability is essential in polymer extrusion where melt temperatures fluctuate and heaters cycle on and off. Without this crystallographic optimization, the frequency shifts caused by temperature would mask the viscosity changes.
Boundary Limitation
Accumulation of degraded polymer or contaminants on the crystal surface disrupts the shear wave propagation. If a layer of carbonized plastic builds up on the sensor, the thickness shear cut crystal loses its sensitivity to the flowing melt. This limitation requires regular cleaning of the sensor surface to maintain measurement accuracy during continuous extrusion runs.
