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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What is the Melt Flow Index (MFI)?

The Melt Flow Index (MFI), also known as the Melt Flow Rate (MFR), is a fundamental property used to characterize the flow behavior of thermoplastic polymers during the extrusion process. MFI measures the rate at which a molten polymer flows through a standardized die under specified conditions of temperature and load. The resulting value is expressed in grams per 10 minutes (g/10 min), indicating the mass of polymer that flows through the die within that time frame.

MFI is one of the most commonly used parameters in polymer science and the plastics industry, as it provides insight into the processability and viscosity of a polymer. A high MFI value indicates a low-viscosity polymer that flows easily, while a low MFI value corresponds to a high-viscosity polymer with more resistance to flow.

Purpose and Importance of Melt Flow Index

The Melt Flow Index is a critical property for the following reasons:

• Processability: MFI provides manufacturers with an indication of how a polymer will behave during processes such as injection molding, extrusion, blow molding, and thermoforming.
  
• Quality Control: Consistent MFI values are essential for maintaining uniform product quality, especially in mass production.
  
• Material Selection: Engineers and designers can use MFI values to select suitable polymers for specific applications based on their flow characteristics.
  
• Comparative Analysis: MFI allows the comparison of different grades of the same polymer or different types of polymers in terms of their flow behavior.
  
• Predicting End-Use Properties: Since MFI is related to the molecular weight of a polymer, it can help predict the mechanical properties of the final product, such as tensile strength, impact resistance, and elongation.
  

Measurement of Melt Flow Index

The MFI is measured using a standardized instrument called a Melt Flow Indexer, which consists of a heated barrel, a piston, and a die with a specific orifice diameter. The test procedure is standardized by several international standards, including:

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

Test Procedure

The basic steps involved in measuring the Melt Flow Index are as follows:

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• Sample Preparation
  A sample of the polymer, typically in pellet form, is loaded into the heated barrel of the Melt Flow Indexer.
  
  
• Temperature Stabilization
  The barrel is heated to a specific temperature, depending on the type of polymer being tested (e.g., 190°C for polyethylene, 230°C for polypropylene). The sample is allowed to melt and reach thermal equilibrium.
  
  
• Application of Load
  A specified load, usually in the form of a weight, is applied to the piston to exert pressure on the molten polymer.
  
  
• Extrusion through the Die
  The molten polymer is forced through the die under the influence of the applied load. The extrudate is collected over a period of 10 minutes.
  
  
• Weighing the Extrudate
  The mass of the extrudate collected over the 10-minute period is measured, and the MFI is calculated using the following formula:
  
  
  Code Select Expand
  MFI (g/10 min) = Mass of extrudate (g) / Time (min)
[/ol]
  
  Factors Affecting Melt Flow Index
  
  Several factors can influence the MFI of a polymer:
  
  
  • Molecular Weight:
    Polymers with a high molecular weight tend to have a lower MFI because their long chains create more entanglements, increasing resistance to flow.
    
    
  • Temperature:
    MFI increases with temperature, as higher temperatures reduce the viscosity of the polymer melt.
    
    
  • Applied Load:
    Higher loads result in higher flow rates, increasing the MFI value.
    
    
  • Additives and Fillers:
    The presence of additives, such as plasticizers, and fillers, such as glass fibers or talc, can affect the flow behavior of the polymer and alter its MFI.
    
    
  • Polymer Type:
    Different types of polymers have inherently different viscosities and flow behaviors, leading to varying MFI values under the same test conditions.
    
  Significance of MFI in Polymer Processing
  
  MFI is a crucial parameter in various polymer processing methods:
  
  
  • Injection Molding:
    In injection molding, polymers with a higher MFI are easier to mold into complex shapes because of their lower viscosity.
    
    
  • Extrusion:
    For extrusion processes, a consistent MFI ensures uniformity in the cross-sectional dimensions of the extruded product.
    
    
  • Blow Molding:
    In blow molding, the MFI affects the ease with which the molten polymer can be inflated into the desired shape.
    
    
  • Film and Sheet Production:
    A suitable MFI ensures uniform thickness and surface quality in film and sheet production.
    
  Relationship between MFI and Molecular Weight
  
  MFI is inversely related to the molecular weight of a polymer. This relationship can be expressed as follows:
  
  
  Code Select Expand
  MFI ∝ 1 / MW

  Where:
  - MFI = Melt Flow Index
  - MW = Molecular Weight of the polymer
  
  Polymers with high molecular weight exhibit low MFI values due to their high viscosity, while low molecular weight polymers have high MFI values due to their lower viscosity.
  
  Advantages and Limitations of Melt Flow Index
  
  
  • Advantages:
    
    • Simple and quick test method
    • Provides a good indication of polymer flow behavior
    • Useful for quality control and material selection
    • Standardized across the industry
    
    
  • Limitations:
    
    • MFI provides a single-point measurement and does not capture the complete rheological behavior of a polymer.
    • It is not suitable for polymers that degrade at high temperatures.
    • The test does not account for shear rate and shear stress, which are critical in real-world processing conditions.
  Applications of MFI in Different Industries
  
  MFI is widely used in various industries, including:
  
  
  • Automotive Industry:
    In the automotive industry, MFI is used to select polymers for components such as dashboards, bumpers, and interior panels.
    
    
  • Packaging Industry:
    In packaging, MFI is used to choose suitable polymers for film production, bottles, and containers.
    
    
  • Electrical and Electronics Industry:
    Polymers with specific MFI values are selected for wire insulation, connectors, and housings.
    
    
  • Medical Devices:
    In the medical field, MFI is used to select materials for disposable syringes, tubing, and diagnostic equipment.
    
  Conclusion
  
  The Melt Flow Index (MFI) is a vital parameter for understanding the flow behavior of thermoplastic polymers. It plays a crucial role in polymer processing, material selection, and quality control across various industries. While MFI provides valuable insights into the processability of polymers, it should be used in conjunction with other rheological measurements for a comprehensive understanding of polymer behavior under different processing conditions. By ensuring consistent MFI values, manufacturers can produce high-quality products with reliable performance and durability.