Parallel Plate Geometry for the DHR/AR Rheometer

General Information

Parallel Plates are used to test polymer melts, soft solids, and higher viscosity fluids. Disposable plates are used for testing thermosetting resins and materials that cure. The range of geometry sizes, the variable gap, and the ease of loading make parallel plates very versatile geometries that can be used to test materials with a wide range of viscosities. As demonstrated by the equation below for the strain constant, high shear rates can be achieved by using a small gap setting.

Testing Samples

The Parallel Plate geometry is used to test a wide variety of materials including polymer melts, suspensions, and emulsions. Because of the constant gap between the plates, there is a velocity gradient across the plate radius during testing. For the Parallel Plate geometry, the normal force, N, corresponds to the first normal stress difference minus the second normal stress difference, N₁-N₂.

Sample Loading

The recommended gap setting for parallel plates is between 0.5 and 2 millimeters.

1. Place the sample on the lower plate. Ensure that the sample is centered on the tool.
2. Adjust the gap such that the upper plate is close to contacting the specimen. Set the gap by using the Set Gap function in TRIOS or by using the Touch Screen. For specimens that are regularly shaped and do not flow, the gap may be manually set by continuing to lower the upper plate until only a slight force is generated. The initial gap should be set approximately 0.05 mm above the final desired gap to facilitate sample trimming.
3. If the specimen is a gel or a material that flows, lowering the upper plate onto the sample will result in the specimen being distributed across the lower plate into a regular cylindrical geometry. For this type of sample, the normal force limit helps to avoid damaging the sample. If the sample is compressed too rapidly, with the sample structure may be damaged as there may be insufficient time for sample relaxation. The gap should be approximately 0.05 mm higher than the testing gap.
4. Trim the sample flush with the edges of the plates.
5. Lower the stage to the final gap setting. Again, this can be done manually, or using the Set Gap function, changing the entered gap to the desired final value. The sample should bulge slightly, as shown in the figure below. The figure below shows loading a sample with a parallel plate geometry.

6. Allow samples to relax before beginning testing by monitoring the normal force and waiting for it to decay to close to zero.
7. Enter the sample dimension on the Geometry Experimental panel. Note that the actual sample gap is measured automatically at the start of the test.

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Cleaning the Geometry

After you have finished testing your sample, open the oven (if used), raise the upper geometry and brush or wipe off the sample. You can remove the geometry, if needed. An alternative to cleaning would be to use disposable plates and discard them after usage.

Equations

Strain Constant	Stress Constant	Normal Stress Constant
Variables R = Radius of plates (mm) H = Gap between plates (mm)

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