Melt Flow Index (MFI), also known as Melt Flow Rate (MFR) and Melt Index (MI), is a measure of the ease of flow of the melt of a thermoplastic polymer. It is defined as the mass of polymer, in grams, flowing in ten minutes through a capillary at a prescribed temperature by a pressure applied.
Melt flow rate is an indirect measure of molecular weight, with high melt flow rate corresponding to low molecular weight. At the same time, melt flow rate is a measure of the ability of the material's melt to flow under pressure. Melt flow rate is inversely proportional to viscosity of the melt at the conditions of the test.
The test method is described in ISO 1133 and is similar to ASTM D1238.
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