Omar Khalid
The Melt Flow Index (MFI) or Melt Flow Rate (MFR) is a measure of the ease of flow of melted plastics, specifically thermoplastic polymers. It is often used in the plastic industry for quality control of thermoplastics and to determine the processing and performance characteristics of plastic materials in manufacturing. The MFI value quoted on many datasheets refers to the amount of polymer extruded through a known orifice (die) and expressed as a quantity in grams per 10 minutes. The MFI is inversely proportional to the viscosity of the polymer, meaning a higher MFI indicates a lower material viscosity.
| Characteristics | Values |
|---|---|
| Full Form | MFI (Melt Flow Index) or MFR (Melt Flow Rate) |
| What it measures | Ease of flow of the melt of a thermoplastic polymer |
| Unit of measurement | Grams/10 minutes (g/10 min) |
| Correlation with polymer grade | Higher MFI indicates lower viscosity and ease of flow |
| Use in quality control | Used as a quality control measurement for thermoplastic polymers, blends, and composites formulation |
| Impact on productivity | Variations in MFI can affect productivity and quality; higher MFI can cause flashing, while lower MFI can result in incomplete part filling |
| ASTM standard | ASTM D1238 – Standard Test Method for Melt Flow Rates for Thermoplastics |
| ISO standard | ISO 1133 – Determination of Melt Mass Flow Rate and Melt Volume Flow Rate of Thermoplastics |
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What You'll Learn
Melt Flow Index (MFI)
The Melt Flow Index (MFI) is a measure of the flow characteristics of a thermoplastic polymer. It indicates how easily a thermoplastic material can flow under a specified pressure and temperature. MFI is widely used in the plastics industry to assess the processability and quality of different materials. It is also used to determine how a polymer will process.
MFI is measured by using a testing machine called a melt flow indexer or extrusion plastometer. The device has a heated barrel with a die at one end and a piston to act on the sample material. A known mass of plastic is placed in the barrel and heated to a specified temperature. Then, a constant load is applied to the piston to force the molten plastic through the die. The amount of plastic that flows out of the die in 10 minutes is recorded as the MFI value. The test conditions (temperature, load, and die diameter) vary depending on the type and grade of the plastic.
MFI is important for several reasons. Firstly, it affects the molding and extrusion behaviour of the plastic. A higher MFI means a lower viscosity and a faster flow rate, which can improve the filling and dimensional stability of the molded or extruded part. However, too high an MFI can cause problems such as sagging, flashing, or poor mechanical properties. Secondly, MFI reflects the polymer’s molecular weight and molecular weight distribution. A lower MFI means a higher molecular weight and a higher degree of polymerization, which can enhance the plastic’s strength, toughness, and heat resistance.
MFI also has some limitations. It is not a direct measure of viscosity or molecular weight but rather an empirical value that depends on the test conditions. Therefore, MFI values are only comparable for materials tested under the same conditions. MFI does not account for other factors that affect the flow behaviour of thermoplastics, such as shear rate, shear stress, temperature gradient, pressure drop, or rheological properties. As such, MFI may not accurately predict the actual flow behaviour of thermoplastics in complex molding processes or under varying operating conditions.
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Melt Flow Rate (MFR)
MFR is a measure of the ease of flow of melted plastic or the flowability of a polymer. It represents how easily a polymer can be processed through different methods. The MFR is defined as the mass of polymer, in grams, flowing in ten minutes through a capillary or die of a specific diameter and length by a pressure applied by a given weight at a given temperature. The unit of measurement for MFR is typically grams per ten minutes (g/10 min).
The MFR value is crucial for selecting the appropriate resin and processing methods. For example, a high MFR indicates a higher flow rate but weaker mechanical properties, while a low MFR results in lower flow rates but stronger mechanical properties. Inconsistent MFR can adversely affect productivity and quality, leading to issues such as flashing or incomplete part filling.
MFR is also used to compare batches of the same material or estimate flow properties of different materials. It is commonly used for polyolefins, with polyethylene measured at 190°C and polypropylene at 230°C. The standard designation for MFR is Melt Mass-Flow Rate, expressed in SI units as g/10min. An alternative quantity is the Melt Volume-Flow Rate or MVR, expressed in cm3/10min.
The most globally recognized standards for MFR testing are ASTM D1238 and ISO 1133. These standards help ensure consistent and accurate measurements of MFR, which is critical for polymer processing efficiency and quality control in the plastics industry.
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Rheological properties
Rheology is the science of flow or deformation of complex fluids. Rheological properties of plastic materials measure their resistance to creep and deformation in solution, dispersion, and molten state. The rheological properties of polymers define how a material will deform when certain stresses are applied. This characteristic is important in the plasticizing zone and the die-forming regions of the process.
Polymer rheology testing is the study of how the stress in a material or force applied is related to deformation and flow of the material. Rheology laboratory testing determines the rheological (flow) properties of polymer materials. Rheology tests are performed while the polymer is in the melt phase or while the polymer has been dissolved in a solvent for intrinsic viscosity and relative viscosity.
Rotational rheometers can be used to test viscosity at given shear rates and viscoelastic properties of melted polymers, pastes, and other fluids. Capillary rheometers can be used to test viscosity at a given shear of melted polymers. Extensional rheometers can be used to test melt strength and drawability.
The rheology data generated can help ensure that the polymer's properties meet specifications for end use (texture, product pour) and product quality: surface distortions, anisotropy, strength, and structure development. Rheology data can also be used to predict and troubleshoot polymer flow behaviour in many common forming processes such as extrusion, blow moulding, and injection moulding.
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. MFI is used as one of the quality control measurements for thermoplastic polymers, thermoplastic blends, and composites formulation.
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Quality control
The MFI test is conducted using specialised equipment, such as the Instron® Melt Flow Testers, which offer both manual and semi-automated testing procedures. The basic procedure involves manually timing, cutting, and weighing the extruded material to obtain the MFR value. On the other hand, semi-automated procedures utilise direct measurements of piston displacement to determine the MVR value. It is important to ensure that the testing equipment is properly maintained and calibrated to avoid inaccurate test results that can impact the quality of the final product.
The MFI value plays a significant role in determining the suitability of a thermoplastic material for a particular application. A high MFI indicates good flow properties, making it favourable for injection moulding as it can easily fill demanding flow paths. On the other hand, low MFI polymers offer higher melt strength, making them more suitable for blow moulding and extrusion processes. By understanding the MFI, manufacturers can optimise their production processes, troubleshoot issues, and select the right materials for specific end-use applications.
Additionally, the MFI is used to ensure lot-to-lot consistency in resin or thermoplastic materials. It is also an essential parameter for predicting several material properties, including processing behaviour, long-term viscoelastic behaviour, and stability during end use. By conducting the MFI test, manufacturers can make informed decisions about the suitability of a particular plastic granule for their intended application, ensuring that the final product meets the required quality standards.
There are two widely recognised international standards for MFI testing: ASTM D1238 and ISO 1133. These standards outline the equipment specifications and test methods to ensure reliable and precise results. By adhering to these standards, manufacturers can maintain consistency and accuracy in their quality control processes, making informed decisions about material selection and product development.
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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 expressed in grams of polymer per 10 minutes of the duration of the test, typically written as grams/10 minutes (g/10 min). The higher the MFI value, the lower the viscosity.
Melt Flow Rate (MFR) is a synonym for Melt Flow Index, and the two terms are used interchangeably. MFR is a measure of the ability of a material's melt to flow under pressure and is an indirect measure of molecular weight. A high melt flow rate corresponds to a low molecular weight. MFR is very commonly used for polyolefins, with polyethylene being measured at 190 °C and polypropylene at 230 °C.
The MFI value can be used to determine how a polymer will process and is correlated with polymer grade. It is used as a quality control measurement for thermoplastic polymers, thermoplastic blends, and composites formulation. A high MFI polymer is favourable for injection moulding as it is easier to fill demanding flow paths in a mould with 'high-flow' materials. Low MFI polymers are better for blow moulding and extrusion as they deliver higher melt strength.
The MFI test, however, was not intended to be used as a measure of viscosity, and it can sometimes be misleading. The viscosity of plastics changes with temperature and shear rate conditions, as well as the development of the frozen layer of plastic during the filling and packing stages of processing. The MFI test is performed at a single temperature and under very low and uncontrolled shear rate conditions. Therefore, the MFI test has little value when trying to relate it to the actual viscosity seen during a typical moulding process.
The MFI device is not an extruder in the conventional polymer processing sense, as there is no screw to compress, heat, and shear the polymer. It also does not account for long chain branching, shear, shear rate, shear history, or the differences between shear and elongational rheology. As a result, two polymers with the same MFI will not behave the same under any given processing conditions.
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Frequently asked questions
The Melt Flow Index (MFI) or Melt Flow Rate (MFR) is a measure of the ease of flow of the melt of a thermoplastic polymer. It is often used in the plastic industry for quality control of thermoplastics.
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 mins. The MFI value can give detailed information about the polymers.
The melt flow test plays an important role in the quality assurance of plastic products. Through regular melt index tests, deviations in the production chain can be detected and corrected early on in the process.
