Ethan Clarke
Melt Flow Index (MFI), also referred to as Melt Flow Rate (MFR) or Melt Flow Number (MFN), is a measure of the ease of flow of melted thermoplastic polymers. It is often used in the plastic industry for quality control of thermoplastics and to determine how a polymer will process. The MFI value is quoted on many data sheets and refers to the amount of polymer that is extruded through a given orifice (die) and is expressed as a quantity in g/10 mins.
| Characteristics | Values |
|---|---|
| Full Form | MFI (Melt Flow Index) |
| Synonyms | Melt Flow Rate, Melt Index |
| Abbreviations | MFI, MFR, MI |
| Purpose | Measure of the ease of flow of the melt of a thermoplastic polymer |
| Testing | Performed at one temperature, under very low and uncontrolled shear rate conditions during a steady-state isothermal condition |
| Testing Sample | About 5 grams of plastic heated above its melting or softening point |
| Testing Duration | 10 minutes |
| Testing Units | g/10 mins or cm3/10 mins |
| Testing Standards | ASTM D1238, ISO 1133 |
| Applications | Quality control of thermoplastics, determining processing and performance characteristics of plastic materials in manufacturing |
| Inverse Relationship | Higher MFI corresponds to lower material viscosity and lower molecular weight |
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What You'll Learn
Melt Flow Index (MFI) and Melt Flow Rate (MFR)
The Melt Flow Index (MFI) and Melt Flow Rate (MFR) are measures of the ease of flow of melted plastics or thermoplastic polymers. They are used to determine the processing and performance characteristics of plastic materials in manufacturing. MFI and MFR are important parameters as they provide the necessary data of mass flow rate per 10 minutes. The higher the MFI, the lower the viscosity of the melted plastic.
The MFI 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. The MFI value quoted on many data sheets refers to the amount of polymer that is extruded through a known given orifice (die) and is expressed as a quantity in g/10 minutes. The test conditions of MFI measurement are normally expressed in kilograms rather than any other units.
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 in diameter is inserted into the apparatus. The material is packed properly inside the barrel to avoid the 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 minutes at 190 °C for polyethylene and 6 minutes at 230 °C for polypropylene. After the preheating, a specified weight is introduced onto the piston. Examples of standard weights are 2.16 kg, 3.2 kg, 5 kg, and 3.8 kg. 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.
The 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 two common standards used worldwide for MFI testing are ASTM D1238 and ISO 1133.
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How MFI/MFR is measured
The Melt Flow Index (MFI) or Melt Flow Rate (MFR) is a measure of the flow characteristics of melted plastic, or how 'runny' the plastic is. It is defined as the mass of polymer, in grams, flowing in 10 minutes through a capillary of a specific diameter and length by a pressure applied via prescribed alternative gravimetric weights for alternative prescribed temperatures. The MFI is an important metric for determining the processing and performance characteristics of plastic materials in manufacturing.
The MFI is typically measured using a melt flow indexer or extrusion plastometer. The process involves heating a polymer sample to a specific temperature, applying a standard weight to force the melted polymer through a die, and measuring the extruded material over a 10-minute period. The standard bore die size is 2.095mm in diameter. The test conditions depend on the type of polymer, with temperature being a key factor. A temperature that is too high may cause the polymers in the plastic to decompose.
The MFI is often used in conjunction with other rheological measurements to gain a comprehensive understanding of polymer behaviour and make informed decisions in material selection and processing. For example, a higher MFI indicates better processability but may result in reduced mechanical strength.
There are several standard methods for determining the MFI, including ASTM D1238 and ISO 1133. These standards ensure consistency and reliability in MFI measurements across different laboratories and applications. The ASTM D1238 method specifies the piston travel distance or section interval at which measurements are made, while ISO leaves this largely up to the operator.
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MFI/MFR and viscosity
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. The MFI is also known as the Melt Flow Rate (MFR) and is important for determining the processing and performance characteristics of plastic materials in manufacturing.
MFI/MFR is used to indicate a measure of a given polymer's flow characteristics, also known as the rheological properties in the molten state under a known applied pressure. The MFI value quoted on many datasheets refers to the amount of polymer that is extruded through a known given orifice (die) and is expressed as a quantity in g/10 mins. The test equipment is diagrammed in Fig. 3.22. The conditions of the test depend upon the type of polymer. One does not want a temperature so high that the polymers in the plastic decompose.
The MFI is a simple measurement of the quantity of polymer pushed through a die of specified dimensions at a set temperature within 10 minutes. It is frequently used to compare grades of the same polymer, as it gives an indication of the molecular weight present in that batch or grade. A low MFI polymer has a higher molecular weight than a high MFI grade of the same polymer.
MFI is often used to determine how a polymer will process. However, it takes no account of the shear, shear rate, or shear history and is therefore not a good measure of the processing window of a polymer. It is a single-point viscosity measurement at a relatively low shear rate and temperature. MFI is also used to measure melt viscosity under a constant load and low shear rates. The melt flow rate is inversely proportional to viscosity, with a higher MFI indicating lower viscosity.
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MFI/MFR in manufacturing
The Melt Flow Index (MFI) is a critical factor in manufacturing plastic products. It is a measure of the ease of flow of melted thermoplastic polymers. MFI is defined as the mass of polymer, in grams, that flows in ten minutes through a capillary of a specific diameter and length, under controlled temperature and pressure. This measurement is typically expressed in grams per 10 minutes (g/10 min).
MFI is important for determining the processing and performance characteristics of plastic materials in manufacturing. It helps manufacturers select the appropriate polymers for specific applications and processes. A higher MFI indicates lower material viscosity and better processability, allowing for faster production speeds and better filling of moulds. However, it may also result in reduced mechanical strength. On the other hand, a lower MFI indicates higher viscosity and better mechanical properties, such as strength and impact resistance, but may lead to flow issues and incomplete filling of moulds.
MFI is also used to assess batch-to-batch variations in materials. Variations in MFI can have costly implications. For example, a material with an MFI higher than expected can result in flashing of an injection mould, leading to increased reject rates and lost production. Conversely, a material with a lower-than-expected MFI can cause issues with parts not filling properly, again increasing the reject rate and quality costs.
The MFI is measured using a melt flow indexer or extrusion plastometer. The procedure involves heating a small sample of the polymer to a specific temperature, typically around 4 to 5 grams, and forcing it to flow through a capillary using a piston actuated by a specified weight. The weight of the melt in grams flowing through the capillary in 10 minutes is the MFI. This value is then used to calculate the MFR, which is the flow rate of the polymer.
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MFI/MFR shortcomings
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. MFI is often used to determine how a polymer will process and is important for determining the processing and performance characteristics of plastic materials in manufacturing.
However, MFI has its shortcomings. Firstly, MFI does not take into account the shear, shear rate, or shear history, and as such, it is not a good measure of the processing window of a polymer. This means that two polymers with the same MFI will not behave the same under any given processing conditions. MFI also does not account for long-chain branching nor the differences between shear and elongational rheology.
Another shortcoming of MFI is that it can result in batch-to-batch variations in materials, which can have costly implications. For example, a material with an MFI exceeding expectations can lead to flashing of an injection mold, resulting in increased reject rates and lost production. Conversely, a material with a lower than expected MFI can cause parts not to fill properly, again leading to increased reject rates and quality costs.
Furthermore, MFI testing must be conducted in compliance with specific standards, such as ASTM D1238 and ISO 1133, which govern the procedure for measurement. Failure to adhere to these standards can result in erroneous results. Samples must be pre-dried in accordance with the manufacturer's directions to avoid inaccurate outcomes.
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Frequently asked questions
Melt Flow Index (MFI) is a measure of the ease of flow of melted thermoplastic polymers. It is often used in the plastic industry for quality control of thermoplastics.
A small sample of about 5 grams is heated above its melting or softening point and forced to flow through a capillary using a piston actuated by a specified weight, usually 2.16 kg or 5 kg. The weight of the melt in grams flowing through the capillary in 10 minutes is the melt flow index.
MFI is important for determining the processing and performance characteristics of plastic materials in manufacturing. It is also used to assess batch-to-batch variations and to determine how a polymer will process.
The MFI test has several shortcomings. Firstly, it is a pressure-limited configuration with constant shear stress, which does not accurately represent the processing conditions of most plastics. Secondly, it does not take into account the effects of different velocities, shear rates, temperatures, and geometries, as well as heat transfer between the melt and the mold.
