Meaning
A rotational rheometer setup utilizes two flat, co-axial discs to measure the flow behavior of polymer melts and highly viscous liquids. This testing arrangement, known as parallel plate geometry, accommodates thick samples and allows technicians to adjust the gap height between the plates to match the polymer type. It is particularly useful for measuring the dynamic viscosity of thermoplastics at processing temperatures.
The configuration provides a clean method for characterizing resin properties before injection moulding and helps pre-empt melt fracture issues during extrusion.
Testing Configuration
Setting the gap between the plates represents a critical step in the measurement procedure. A typical gap ranges from one to two millimeters, which prevents edge fracture while ensuring a uniform sample temperature. Technicians load a solid polymer disc or pellet between the heated plates and compress it to the target gap height.
Shear Distribution
The shear rate within this system varies from zero at the center of the plate to a maximum value at the outer edge. Because the shear field is non-uniform, mathematical corrections are required to calculate the true viscosity from the torque measured by the rheometer. This non-uniform shear represents the main difference between this setup and a cone and plate configuration.
Melt Measurement
Sample degradation can occur if the polymer is exposed to high temperatures for too long during the test. Nitrogen purging is often used to prevent oxidation of the resin during long frequency sweeps. Protecting the melt from oxidation ensures that the measured viscosity values reflect the true molecular weight of the raw material.
