Information on the most widely used ASTM standards within the materials testing industry
Melt flow index (MFI) : The determination of melt flow rate (MFR) and melt volume rate (MVR) is described in various international standards.
The most common ones are ISO 1133 and ASTM D1238. These two standards describe similar tests, but slightly different. This article aims to provide a brief overview and demonstrate the differences between them.
The ISO 1133:2005 has been split and replaced with ISO 1133-1:2011, which describes general testing methods. In addition, ISO 1133-2:2011 has been published for materials that are sensitive to time temperature history and/or humidity. According to the different testing materials, appropriate standards must be used.
ISO 1133-1 basically describes two different procedures. In program A, the extruded wire is cut off according to the defined time steps during the measurement period. MFR is determined based on the weight of the wire section. In program B, measure the piston speed and calculate MVR based on it. The melt density can be used to convert between MVR and MFR. The measurement is always conducted within a measurement range of 50-20 millimeters in front of the capillary, with a melting time of 5 minutes. For low viscosity materials, a capillary tube of half size (half diameter, half length) can also be used. The determination of flow rate ratio (FRR) is also described in the standard.
ISO 1133-2 is a further refinement of ISO 1133-1. The main difference is that for moisture absorbing materials, pre drying and determination of residual humidity are required. There are stricter regulations on the time sequence of filling and measurement. More precise temperature control is also required.

ISO 1133-1 basically describes two different procedures.
In program A, the extruded wire is cut off according to the defined time steps during the measurement period. MFR is determined based on the weight of the wire section. In program B, measure the piston speed and calculate MVR based on it. The melt density can be used to convert between MVR and MFR. The measurement is always conducted within a measurement range of 50-20 millimeters in front of the capillary, with a melting time of 5 minutes. For low viscosity materials, a capillary tube of half size (half diameter, half length) can also be used. The determination of flow rate ratio (FRR) is also described in the standard.
What's the ifference of ISO 1133-1 and ISO 1133-2, ASTM D1238 of Melt Flow Index Test?
ISO 1133 has been split and replaced with ISO 1133-1:2011, which describes general testing methods. In addition, ISO 1133-2:2011 has been published for materials that are sensitive to time temperature history and/or humidity. According to the different testing materials, appropriate standards must be used. ISO 1133-1 basically describes two different procedures. In program A, the extruded wire is cut off according to the defined time steps during the measurement period. MFR is determined based on the weight of the wire section. In program B, measure the piston speed and calculate MVR based on it. The melt density can be used to convert between MVR and MFR. The measurement is always conducted within a measurement range of 50-20 millimeters in front of the capillary, with a melting time of 5 minutes. For low viscosity materials, a capillary tube of half size (half diameter, half length) can also be used. The determination of flow rate ratio (FRR) is also described in the standard. ISO 1133-2 is a further refinement of ISO 1133-1. The main difference is that for moisture absorbing materials, pre drying and determination of residual humidity are required. There are stricter regulations on the time sequence of filling and measurement. More precise temperature control is also required. |
ASTM D1238: The American standard ASTM D1238 also describes two procedures A and B which correspond to the basic measuring procedure of ISO 1133-1 procedures A and B. The biggest differences to ISO 1133-1 are: The melting time is 7 min instead of 5 min. The standard measuring distance is between 46-20.6 mm (1 inch length) in front of the capillary and there is a shortened measuring distance for highly viscous materials of 46-39.65 mm (1/4 inch length). In addition, slightly different requirements are placed on the temperature distribution in the test channel. There is no equivalent to the more stringent ISO 1133-2. However, the standard directly mentions, that appropriate pre-conditioning of sample is necessary. The measurements with the “half-die” (capillary with half diameter and length; same dimensions as in ISO 1133) are described as procedure C. With the exception of the capillary, the procedure corresponds to Procedure B. Procedure D describes an multi-weight measurement for determining the FRR with only one filling of the barrel. |
Related Standard
ISO 1133: Plastics -- Determination of the melt mass-flow rate (MFR) and the melt volume-flow rate (MVR) of thermoplastics.
ISO 1133 standard specifies two procedures for the determination of the melt mass-flow rate (MFR) and the melt volume-flow rate (MVR) of thermoplastic materials under specified conditions of temperature and load. One is a mass-measurement method. The other is a displacement-measurement method. Normally, the test conditions for measurement of melt flow rate are specified in the material standard with a reference to ISO 1133. The test conditions normally used for thermoplastics are listed in annexes.
Deep guide for the MFI Test
The melt flow index (MFI) is a measure of the ease of flow of the molten mass of a thermoplastic polymer. It is defined as the mass of polymer, in grams, that flows in ten minutes through a capillary of specific diameter and length under a pressure applied by prescribed alternative gravimetric weights for prescribed alternative temperatures. Polymer processors often correlate the MFI value with the polymer grade they need to choose for different processes, and in most cases, this value is not accompanied by units, as it is assumed to be g/10 min. Similarly, the test conditions for measuring the MFI are usually expressed in kilograms instead of any other unit. The method is described in similar standards ASTM D1238 and ISO 1133.
The melt flow index is an indirect measure of molecular weight; a high melt flow index corresponds to a low molecular weight. At the same time, the flow index of the molten material is a measure of the material's ability to flow under pressure. The melt flow rate is inversely proportional to the viscosity of the molten mass under test conditions, although it should be noted that the viscosity of any material depends on the applied force. The relationships between two melt flow index values for a material with different gravimetric weights are often used as a measure of the breadth of the molecular weight distribution.
The melt flow index is very commonly used for polyolefins; polyethylene is measured at 190 °C and polypropylene at 230 °C. The plastic engineer must choose a material with a melt flow index high enough for the melted polymer to be easily shaped into the intended article, but low enough for the mechanical strength of the final article to be sufficient for its use.
The ISO 1133-1 standard regulates the procedure for measuring the melt flow index.
The procedure for determining the MFI is as follows:A small amount of polymer sample (around 4 to 5 grams) is taken in the specially designed MFI apparatus. A die with an opening of approximately 2 mm in diameter is inserted into the apparatus. The material is properly packed inside the barrel to prevent the formation of air pockets. A piston is introduced that acts as the means to cause the extrusion of the molten polymer. The sample is preheated for a specified amount of time: 5 minutes at 190 °C for polyethylene and 6 minutes at 230 °C for polypropylene. After preheating, a specific weight is placed on the piston. Examples of standard weights are 2.16 kg, 5 kg, etc. The weight exerts a force on the molten polymer, which immediately begins to flow through the die. A sample of the melt is taken after the desired amount of time and weighed accurately. MFI is expressed in grams of polymer per 10 minutes of test duration. Synonyms for melt flow index are melt flow rate and melt flow index.
The most commonly used are their abbreviations: MFI, MFR, and MI.
Confusingly, MFR can also indicate "melt flow ratio," the ratio between two melt flow rates with different gravimetric weights. More accurately, this should be reported as FRR (flow rate ratio), or simply flow index.
The FRR is commonly used as an indication of how the rheological behavior is influenced by the molecular weight distribution of the material.
previously: (MFI = Melt Flow Index) → currently: (MFR = Mass Flow Rate of Melt)
previously: (MVI = Melt Volume Index) → currently: (MVR = Melt Flow-Volume Rate)
before: (MFR = Melt Flow Ratio) → currently: (FRR = Flow Ratio)
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