Rheology application cone-plate geometry

All the major manufacturers of viscometers and rheometers have Internet sites that illustrate and describe their products. In addition, many of the manufecturers are offering seminars on rheometers and rheology. Earlier lists of available models of rheometers and their manufacturers were given by Whorlow (1980), Mitchell (1984), and Ma and Barbosa-Canovas (1995). It is very important to focus on the proper design of a measurement geometry (e.g., cone-plate, concentric cylinder), precision in measurement of strain and/or shear rate, inertia of a measuring system and correction for it, as well as to verify that the assumptions made in deriving the applicable equations of shear rate have been satisfied and to ensure that the results provided by the manufecturer are indeed correct. 

Numerous variants of this technique have been created to adapt it for more practical applications. The most popular of these is the parallel-plate apparatus. Here the melt is placed between two parallel disks, 1-2 mm apart. The shear rate varies from the center to the circumference of the disk and corrections need to be performed [1]. The parallel-plate geometry is less sensitive to errors in gap and is also more able to handle filled materials. In contrast, cone-and-plate geometries are not useful in cases where the filler dimension is of the same order of magnitude as the gap. Parallel plates are also recommended in situations where rheology is measured as a function of temperature, where tool thermal expansion would otherwise affect the accuracy of the measurement. 

Experimentally, the dynamic shear moduli are usually measured by applying sinusoidal oscillatory shear in constant stress or constant strain rheometers. This can be in parallel plate, cone-and-plate or concentric cylinder (Couette) geometries. An excellent monograph on rheology, including its application to polymers, is provided by Macosko (1994). 

The application of the theoretical treatment depends on the ability to measure the extrusion force and rate. Most commercial extruders do not allow for these types of measurement. Normal rheological equipment, such as cup-and-bob or cone-and-plate, do not have a suitable geometry or instrumentation to handle materials of the consistency normally used. A ram extruder is a suitable experimental design. 

FIGURE 4.6 Geometry of cone and plate shear flow. (Adapted from Middleman, S. (1975). Advances in polymer science and engineering applications to food rheology, in Theory, Determination and Control of Physical Properties of Food Materials, Rha, C., Ed., Reidel D. Publishing Company, Dordrecht, Holland.)... 

---