Melt Index Tester

The melt index tester is essentially a simple capillary rheometer. The piston is pushed down by a weight see Fig. 6.20(a) [99]. 

The melt index is the number of grams of polymer extruded in a time period of 10 minutes. Details of the geometry and test procedures are described in ASTM D1238. 

The melt index tester is used in many companies to quickly test the polymer melt. Unfortunately, many times MI data is the only information available on the polymer melt flow properties. 

From the dimensions of the MI apparatus, the weight on the plunger, and the MI value, one can determine the approximate shear stress, shear rate, and viscosity. By using Eqs. 6.48 and 6.49, the shear stress at the capillary wall can be determined from  

The apparent shear rate is thus directly proportional to the MI value and inversely proportional to the polymer melt density. The apparent viscosity can now be determined from  

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ASTM D1238 standards were attempted when melt flow index testing the material and ground parts, but were found to be inadequate for the two polycarbonates and the ABS because the materials would not flow through the indexer. The materials were tested to find new temperatures to test the polycarbonates and ABS at so they would flow through the melt index tester at the desired speed. 

Simplified versions of capillary rheometers are melt flow testers, which are especially suitable for laboratory use since they are relatively easy to handle and represent a fast measure. However, they only give information about the fluidity at a certain shear rate which however for chemists in most cases is enough to judge the material s fluidity. Using a melt flow tester the measure of fluidity is not expressed in terms of the melt viscosity but as the amount of material extruded in a given time (10 min). The amount of extrudate per unit of time is called the melt index or melt flow index i (MFI). It is also necessary to specify the temperature and the shearing stress or load. Thus MF//2 (190 C) = 9.2 g/10 min means that at 190 C and 2 kg load, 9.2 g of polymer melt are extruded through a standard nozzle in 10 min. Today the expression melt flow index is often replaced by the melt flow rate (MFR)... 

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Figure 2.9 Melt flow index tester Constant shear rate and laminar flow.

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