The melt indexer consists of a heated barrel and piston assembly to contain a sample of resin. A specified load (weight) is applied to the piston, and the melted polymer is extruded through a capillary die of specific dimensions. The mass of resin, in grams, that is extruded in 10 minutes equals the MFR, expressed in units of g/10 min. (This value is also commonly called the melt index, MI, or melt-flow index, MFI.) Some instruments can also calculate the shear rate, shear stress, and viscosity in centipoise.

The basic property measured by the melt flow test is the melt viscosity or flow resistance of the polymer at a particular shear stress (related to the applied load) and temperature. Polymer chains of short length and simple geometry “slide” past one another relatively easily and offer little flow resistance. In contrast, long chains of high molecular weight and more complex structure yield greater flow resistance or viscosity.

The MFR, then, is an indicator of average molecular weight and is inversely related to it. A resin with an MFR of 50 g/10 min indicates a lower molecular weight than one with an MFR of 10 g/10 min. While a higher MFR material may be easier to process, physical properties related to molecular weight, such as impact resistance, are often lower.

Thus, MFR is commonly used as a material-acceptance specification by processors and also as a means of comparing resins from different vendors so it also has many uses in quality control. Not only can variations in polymerization and compounding affect MFR of incoming resin, but it is also a valuable indicator of resin degradation caused by transport or storage conditions or improper drying. Regular MFR testing after molding or extrusion can help pinpoint improper processing conditions. It also provides a simple indicator of how addition of in-plant regrind or post-consumer reclaim may affect the virgin resin’s processability and end-use performance.

As an actual case study of how this information can be used: An injection molded polycarbonate part was found to crack when dropped. The virgin resin had an MFR of 28.5 g/10min. A sample from a known "good" lot of parts was ground up and tested in a melt indexer. It had an MFR of 27, well within established tolerances (+-30%). But a "problem" part showed an MFR of 90. From this evidence, it was discovered that the "bad" lot contained a large amount of regrind that had been thermally degraded in processing.

However, makers of melt indexers warn that MFR data have one fundamental limitation: The test is performed at a very low shear rate (about 0.5 to 200 sec-1). While this shear rate may accurately reflect how "stiff" a melt is in low-shear processing, such as in calendaring or compression molding, it is usually unrepresentative of melt behavior under the higher shear of injection molding (around 10,000 to 100,000 sec-1) or extrusion (around 100 to 1000 sec-1). To more accurately characterize processability under higher shear conditions, processors should consider buying a capillary rheometer. It measures multiple points on the shear rate/viscosity curve. While typically more expensive than a melt indexer, some of these instruments can be purchased for under $40,000.