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What is the Melt flow index (MFI) ?

The melt flow index (MFI) 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 of a specific diameter and length by a pressure applied via prescribed alternative gravimetric weights for alternative prescribed temperatures.[1] The method is described in the similar standards ASTM D1238[2] and ISO 1133.[3]

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, though it should be borne in mind that the viscosity for any such material depends on the applied force. 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.

Melt flow rate is very commonly used for polyolefins, polyethylene being measured at 190 °C and polypropylene at 230 °C. The plastics engineer should choose a material with a melt index high enough that the molten polymer can be easily formed into the article intended, but low enough that the mechanical strength of the final article will be sufficient for its use.

Measurement

Overview of the measurement of melt flow index (MFI)

ISO standard 1133-1 governs the procedure for measurement of the melt flow rate.[4] The procedure for determining MFI is as follows:

    A small amount of the polymer sample (around 4 to 5 grams) is taken in the specially designed MFI apparatus. A die with an opening of typically around 2 mm diameter is inserted into the apparatus.
    The material is packed properly inside the barrel to avoid formation of air pockets.
    A piston is introduced which acts as the medium that causes extrusion of the molten polymer.
    The sample is preheated for a specified amount of time: 5 min at 190 °C for polyethylene and 6 min at 230 °C for polypropylene.
    After the preheating a specified weight is introduced onto the piston. Examples of standard weights are 2.16 kg, 5 kg, etc.
    The weight exerts a force on the molten polymer and it immediately starts flowing through the die.
    A sample of the melt is taken after the desired period of time and is weighed accurately.
    MFI is expressed in grams of polymer per 10 minutes of duration of the test.

Synonyms of Melt Flow Index are Melt Flow Rate and Melt Index. More commonly used are their abbreviations: MFI, MFR and MI.

Confusingly, MFR may also indicate "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.

MFI is often used to determine how a polymer will process. However MFI takes no account of the shear, shear rate or shear history and as such is not a good measure of the processing window of a polymer. The MFI device is not an extruder in the conventional polymer processing sense in that there is no screw to compress, heat and shear the polymer. MFI additionally does not take account of long chain branching nor the differences between shear and elongational rheology. Therefore two polymers with the same MFI will not behave the same under any given processing conditions.[5]

For a detailed numerical simulation of the melt flow index, see [6] or.[7]





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Melt Flow Rate (MFR, MVR)

The Melt Flow Rate is a measure of the ease of flow of melted plastic and represents a typical index for Quality Control of thermoplastics. Originally called Melt Flow Index or Melt Index (typically for polyethylene, but applied to a variety of materials), the standard designation today is Melt Mass-Flow Rate or MFR, which is a mass flow expressed (SI units) in g/10min. An alternative quantity is the volume flow expressed (SI units) in cm3/10min, called Melt Volume-Flow Rate or MVR. MVR multiplied by the melt density (i.e. density of the material in the melted state) gives MFR.

Typical Melt Flow instruments are compact and easy to use. The basic principle is that a thermoplastic sample (originally in the shape of granules, powder or flakes) is made fluid by heating and forced to flow out of a cylinder through a capillary die. The extruding piston is loaded with dead weights, normally up to 21.6 kg. MFR (and MVR) are obtained under standard conditions of temperature and applied load, defined for each type of material, and normally using a fixed type of die (inner diameter 2.095 mm, length 8 mm). The result must always specify the test conditions because it's strongly dependent on those.

MFR and MVR are typically used in Quality Control labs and Production Control labs. The basic procedure foresees a manual timing, cutting and weighing of the extruded material, giving directly a value of MFR. Semi-automatic procedures allowed by modern instruments are based on direct measurements of piston displacement, hence they give directly a value of MVR. Melt density can be typed in or measured, and therefore MFR results are also obtained. Semi-automatic procedures can achieve a much higher accuracy and guarantee a wider range of measurable flow rates. More advanced methods foresee the application of several loads during the same test, each one giving a MFR (MVR) result. This is called a multiweight test and gives additional information on the sample, with some insight on the shear dependence of viscosity. A single-weight MFR test can be correlated with average molecular mass, while the shear dependence depends on the molecular mass distribution.

Melt flow rate is a quick tool to compare batches of the same material or to estimate flow properties of different materials (typically for extrusion processes), when a deep understanding of material properties is not required. A true rheological characterization can be carried out by means of rheometers. Melt flow rate is inversely proportional to (shear) viscosity, but is not used to determine the latter (can give just a rough estimation).
References

    ISO 1133:2005 "Plastics - Determination of the Melt Mass-Flow Rate (MFR) and Melt Volume-Flow Rate (MVR) of Thermoplastics"
    ASTM D1238-10 "Standard Test Method for Melt Flow Rates of Thermoplastics by Extrusion Plastometer"

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