Melt Flow RateTesting/ASTM D 1238
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Melt flow index or MFI is the flow measurement of a thermoplastic melt. It is defined as the mass of polymer, in grams, flowing in ten minutes through a capillary of a specific diameter and length via pressure applied by gravimetric weights. The method is described in the standards ASTM D1238 and ISO 1133.
Melt flow rate or MFR is an indirect measure of molecular weight, with high melt flow rate corresponding to low molecular weight. Melt Flow rate is an assessment of average molecular mass and is an inverse measure of the melt viscosity; in other words, the higher a MFR, the more polymer flows under test conditions. Knowing the MFR of a polymer is vital to anticipating and controlling its processing. Generally, higher MFR polymers are used in injection molding, and lower MFR polymers are used with blow molding or extrusion processes.
Many factors affect polymers' flow properties. Molecular weight distribution, the presence of co-monomers, the degree of chain branching and crystallinity influence a polymer's MFI as well as heat transfer in polymer processing.
Ratios between two melt flow rate values for one material at different gravimetric weights are often used as a measure for the broadness of the molecular weight distribution. MFR indicates "melt flow ratio", the ratio between two melt flow rates at different gravimetric weights. More accurately, this should be reported as FRR (flow rate ratio), or simply flow ratio. FRR is commonly used as an indication of the way in which rheological behavior is influenced by the molecular mass distribution of the material.
In addition to its importance to natural polymers and plastics, the MFI of a masterbatch is also important because the MFI indicates the relative ease with which a masterbatch can be distributed during a compounding operation or in injection molding machines, film extruders, etc.
MFI is vital knowledge for polymer processing, please refer to the following references:
- ASTM Standard D1238 "Standard Test Method for Flow Ratios of Thermoplastics by Extrusion Plastometer"
- ISO1133 "Plastics - Determination of the Melt Mass-Flow Rate (MFR) and Melt Volume-Flow Rate (MVR) of Thermoplastics".
- V. Shenoy, D. R. Saini: Melt Flow Index: More Than Just a Quality Control Parameter. Part I., Advances in Polymer Technology, vol 6, No. 1, pages 1 - 58.