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The Value of Melt Flow Index Testing

Melt Flow Index TestingThe melt flow index (MFI) is a measure of the mass of polymer that is extruded through a capillary die at a certain temperature and force. The sample can be in the form of virgin resin pellets or ground material from a finished thermoplastic product. A sample is loaded into the preheated barrel of the instrument, together with a piston and weight is placed on the piston to apply force. When the right flow conditions are met a timed specimen is collected. After cooling, the specimen is weighed. The collection and the mass of the specimen are used to calculate the MFI value, which is the amount of sample that flows in ten minutes (g/10min). We offer GMP compliant MFI testing in accordance with ASTM D1238 1 and ISO 1133.205 2 standards.

The MFI test, also known as melt mass-flow rate or MFR, has been used since the early days of polymer science and material characterization. Although the basic parts of the instrument are rudimentary, the method provides data for quick and direct comparison between samples.  MFI is listed on specification sheets for many polymer resins that date back to the 1940s and ‘50s.  Some processors still value the comparison of MFI data from new lots of resin to the conventional specifications. Our customers often include MFI with USP3 testing for polymers that are intended for use in medical devices and food and pharmaceutical packaging. Polymer producers and manufacturers of polymer products use the results to ensure that a material will perform as intended during processing. The MFI of a polymer compound is particularly valuable information for estimating how the material will behave during extrusion or injection molding.

Relative differences in several material properties can be inferred from MFI data. The melt viscosity at the test temperature is directly related to the MFI as more viscous samples will flow more slowly. MFI data also correlates to the molecular weight of the sample. Polymers with longer chains or greater branching take longer to push through the capillary and therefore flow more slowly. Differences in average molecular weight can be inferred from a single test. Tests at the same temperature with different load weights can provide information on the molecular weight distribution. Finally, MFI can provide a rough estimation of the shear viscosity of the sample through an inversely proportional relationship. The MFI test is significantly quicker, and generally less costly, than the tests that specifically evaluate these properties.

Another interesting application for MFI is in support of failure analysis. A sample from a failed product can be tested for comparison to the material specification. The MFI analysis will easily identify if the part has been manufactured with the wrong polymer type. The data will also provide an understanding of the properties that were previously described. Molecular weight is correlated to the mechanical strength of a material. A faster flow than expected can be indicative of abuse during processing. If processing temperatures or pressures are too high, the polymer will be degraded and some of the chains will be sheared. The mechanical properties of the product will be affected and the MFI will increase. Commercial plastic parts typically contain fillers, additives, and colorants along with a polymer blend. Imbalances in the recipe can adversely affect the mechanical properties and melt flow results.

While Melt Flow Index testing may appear to be rudimentary and is often outshined by more complex and modern testing techniques, it can provide tremendous analytical value depending on the testing need. It should not be overlooked as a great first step—or perhaps the only step—of a testing protocol. Furthermore, while this testing technique is inexpensive and relatively basic it should still be done by a reputable testing lap with appropriate qualifications that speak to the use of great science, thereby producing results that can be trusted and used as actionable data.

1ASTM D1238-10 “Standard Test Method for Melt Flow Rates of Thermoplastics by Extrusion Plastometer.”  This method is on our scope of LAB accreditation.

2ISO 1133.2005 “Plastics – Determination of the Melt Mass Flow Rate (MFR) and Melt Volume Flow Rate (MVR) of Thermoplastics”

3USP 37 <661> Plastic Containers