FROM MELT FLOW INDEX TO RHEOGRAM

Knowledge of the complete flow curve (rheogram) depicting the variation of the melt viscosity over the industrially relevant range of shear rates and temperature is essential in the design of polymer processing equipment, process optimization, and troubleshooting. 

The rheological data needed for constructing a rheogram are obtained on sophisticated scientific instruments, namely, rheogoniometers, capillary rheometers, mechanical spectrometers, and so forth. These instruments are very expensive and require trained operators. Thus, the collection of the necessary flow data is not always possible due to constraints of finance and the limited technical capabilities of most polymer processors. 

The flow parameter that is readily accessible to most processors is the melt flow index (MFI). The MFI is either specified by the thermoplastics raw material supplier or can be easily measured using a relatively ine nsive apparatus. It is a single-point viscosity measurement at a relatively low shear rate and temperature. Earlier, it was often said that MFI gives a dot when actually what is needed is a plot for the polymer processors. However, this is not true now because of a unique approach developed for estimating the rheogram merely from the knowledge of the MFI. This approach is discussed in detail in this chapter, and unified master rheograms for most polymers are presented. 

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Shenoy, A V, Chattopadhyay, S., and Nadkami, V. M., From melt flow index to rheogram, Rheol Acta, 22, 90-101 (1983). 

IMthin the melt flow indexer die, capillary entrance effects are important, as LID is equal to 3.8 for ASTM D1238 and 11.1 for ASTM D3364. These have not been accounted for during the derivation of Eqs. (4.16) and (4.17). However, from the above discussion, it is clear that MFI would be used merely as a normalizing flictor to obtain reduced viscosity versus shear rate curves. The MFI values used in generating the plots as well as those which would be used for obtaining the rheograms from the master plots would have the capillary entrance effects implicit in them and, hence, would annul each other. 

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