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Practical Analysis of correlation between Melt Flow Index and Capillary Rheometer – A case study of High Viscosity TPO material


Melt Flow Index (MFI), which is a measure of the ease of melt flow of a thermoplastic polymer. Thanks to the simplicity and cost, it is the primary choice for many thermoplastic processors. Although commonly used, the melt flow index is not beyond criticism。 On the other hand, Apparent Viscosity, which is a measure of a fluid of its resistance to deformation at a given rate. It can be obtained by dividing the shear stress at the wall, τ, by the apparent wall shear rate, γ̇. In practice, the word “apparent” is often omitted. The apparent viscosity can be measured using a capillary rheometer. This technique is widely used by thermoplastic R&D organizations and manufacturers. Both MFI and Viscosity can be referenced to determine the material flow property, suitable manufacturing process, and processability. However, due to the high cost of capillary rheometer, many resin end users do not have such privilege. Therefore, a lot of times the resin supplier and end-user are characterizing the resin flow property with different test methods.  In this posting series, we will study how to bridge the gap between MFI and Apparent Viscosity, establish the general calculation procedures in a practical way which would allow result obtained from one test method can be converted into the other one, and vice versa. The correlation between MFI and Viscosity is studied and analyzed with one high viscous TPO resin. The goal is to provide a reliable yet feasible conversion method so that the results measured by different methods can be compared on the same scale.

Keywords: Thermoplastic, Melt Flow Index, MFI, Rheology, Apparent Viscosity, Capillary Rheometer, Flow Property, Correlation

Table 1. Test method comparison between MFI and Viscosity

Melt Flow Index Viscosity
Instrument Melt Flow Indexer Capillary Rheometer1
Standard D 1238, ISO 1133 ASTM D3835, ISO 11443
Temperature Constant Constant
Applied Pressure Constant based on weight Varies based on program
Orifice Constant Various2
Shear Rate Constant Varies based on the applied force
Input Force/Pressure, Constant Force/Pressure, Variable
Output Melt Flow Rate, g/10 min. Apparent Viscosity, Pa·s



  1. Only the capillary rheometer is referenced and studied
  2. Only one orifice can be adopted in a single run

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