Viscosity by Capillary Rheometry ASTM

A capillary rheometer (Fig. 6.8) is similar to a melt indexer. The main difference is that instead of by gravity, the piston is driven by a variable speed motor. Also, a load cell in-line with the piston measures ram force in real time. This configuration allows tests to be run at a controlled shear rate, and even to vary the rate over a large range during a single test run. The piston force and the orifice geometry provide the data necessary 

One of the reasons for the popularity of melt index data is that rheometer data is significantly more expensive. Because of the higher cost of rheometry equipment and the advanced training needed by operators, most plastics manufacturers do not have a rheometer on site. Nonetheless, the data obtained from a rheometer is quite valuable to processors, so a technique has been developed to use an extruder to obtain more complete rheological data than is available from the melt index test. 

A paper entitled Obtaining Flow Properties Directly from an Extruder [47] details a technique for measuring rheometer-type data using extruders on the plant floor. It describes how extrudate samples, collected from an extruder in a matter of minutes, are used to measure throughput. These measurements are then used along with processing values such as screw speed and head pressure to calculate the polymer s viscosity and power law index. 

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