Helical screw rheometer

If the die end of a screw extruder is closed off, fluid will recirculate between the fights in a complex shear flow pattern. Kraynik and co-workers (1984) have shown that this flow can be analyzed to give viscosity versus shear rate data from pressure rise over the screw and rotation rate. An advantage of this geometry is that it can keep 

Viscosity versus shear stress for a 5.5% sodium carboxy-methylcellulose in water solution. Solid points are for bob-in-cup geometry, open points are vane-in-cup. From Barnes and Camali (1990). 

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Another type of rotational viscometer is the helical-screw rheometer (176). This instrument is basically a screw-type metering pump that does not pump. The measure of force is the pressure difference resulting from the rotational motion. It is possible to use a bank of pressure transducers of different sensitivities to measure viscosity over a wide range. The instrument can be used for high temperature rheometry and to follow polymerization, shear and heat degradation, and other developments. 

An in-line measurement is performed in a process line an on-line measurement is performed in a bypass loop from the main process line and the food may be returned to the main process line after measurement is performed. A near-line measurement is performed on a sample taken from a process line which is often discarded after measurement. Because foods are complex materials (e.g., suspensions, emulsions, gels), stmctural changes may take place during sampling (e.g., flow through a valve) for on-line and near-line measurements (Roberts, 2003). Nevertheless, in principle, the previously described capillary flow, and rotational concentric cylinder, plate-cone, and mixer viscometers may be used for in-line, on-line, and near-line measurements. In this respect, Tamura et al. (1989) proposed a helical screw rheometer as an on-line viscometer. The empirical measurement methods described previously are used primarily in near-line measurements. 

Related to the helical screw devices are the instrumented batch mixers often called torque rheometers. They consist of a chamber or mixing bowl with two counterrotating blades. There are many different blade configurations. A common one is shown in Figure 5.6.5. These devices are primarily used as simulators of larger batch mixers and extruders and to prepare compounded samples. One of the rotating shafts has a transducer that records torque as polymers melt and mix with other additives. Blyler and Daane (1967) have shown that when the sample has reached steady state, one can obtain a good measure of n (but not m) for power law fluids fi om torque and screw speed. 

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