Melt Flow Index / Melt Flow Rate - One Pointless Measurement
Mark Edwards
Gammadot Rheology Testing & Consultancy Services
The rheological behaviour of a material is the single most important property in the commercial processing of polymers. Fact: If the material cannot be deformed and made to flow, you can not process it! Many material generic types / grades have very similar flow behaviour at low shear rate but due to structural issues such as molecular weight / molecular weight distribution, chain branching, polymer / filler interaction, etc, have radically different flow characteristics at process relevant shear rates. Traditionally many processors have relied on the Melt Flow Indexer (MFI - or latterly known as the Melt Flow Rate, MFR) for pre-process quality control of incoming material batches.

Unfortunately because the MFI is a low shear, single point test, it is woefully inadequate at providing the full picture in relation to process-range data. Due to the pseudoplastic behaviour of polymer melts it is important to determine the magnitude of viscosity across as wide a shear rate range as possible, as the material will experience changes in flow geometries and hence shear rate during the process. Taking a typical Polypropylene having a MFI value of 2.4 g/10 mins (determined using a 2.16 kg applied mass @ 230°C), assuming the density is known, an approximate shear rate of 6.7 1/s can be calculated¥. (It is an approximation because another failing of the MFI is the fact that the rate is not controlled; there is an increase in velocity as the MFI barrel empties under the constant applied stress (load)). As polymer processing techniques can involve shear rates of ~10 1/s up to 100,000 1/s it can be seen that the MFI is a poor representation of the process. The figure below gives a visual comparison of the MFI to the main polymer processing techniques:

To determine the shear viscosity behaviour of polymers under processing conditions, a capillary rheometer is a fast, accurate means of providing flow behaviour over a wide range of shear rates at a number of temperatures and enables processing problems such as melt fracture, die swell, etc, to be quantified offline before product quality is affected. The data sets derived from testing can also be modelled mathematically to provide the input coefficients required by Finite element flow simulation software packages such as Moldflow and Sigmasoft 3D - to provide accurate mould filling simulations, accurate materials data is absolutely critical.
¥ Ref: Fleming, D.J, Melt Fracture and Elongational Viscosity via Convergent Flow Analysis , Procedings of PolymerTesting '96 , RAPRA (1996).
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