What is Melt Flow Rate? Everything You Need to Know

Inconsistency in material flow is a common problem in polymer processing. It can result in defects, irregular product standards, and minimized efficiency. The solution lies in accurately measuring and controlling MFR, ensuring that polymers flow within the specified range for particular applications.

The melt flow rate is a critical property that defines the fluidity and ease of processing polymers. It directly affects the functionality of manufacturing processes such as molding and shaping or material forming. This article will explore the MFR definition, calculation method, standards, and importance of optimizing polymer processing.

What is Melt Flow Rate? 

The Melt Flow Rate is a measure of how easily a plastic material melts or flows under specific conditions when it’s melted. It also evaluates how much of the material can pass through a small opening in a specific amount of time (10 minutes) under heat and pressure. MFR is useful in material selection, quality control, process optimization, etc.

A higher MFR means the plastic flows more easily, while a lower MFR indicates it flows more slowly. This helps manufacturers understand how the plastic will react during processes such as molding or shaping and ensures the right material is chosen for the right application.

MFR/ MFI Definition

A standard measure of how a thermoplastic deforms and moves under processing conditions. It serves as an indirect indicator of molecular weight, where a higher MFR/MFI corresponds to a lower molecular weight and improved ease of flow.

Melt Flow Index Formula

The Melt Flow Index, also known as the Melt Flow Rate measures how easily a polymer melts and flows under specific conditions. It is the measure of the flowability of the thermoplastic materials, indicating their fluidity and their molecular weight indirectly.     

Melt Flow Index (MFI) Formula:

                           MFI= W/t

Where:

• W= Weight of extruded polymer (grams)

• T = Time duration (10 minutes)

MFR Calculation Method

MFR/MFI is calculated by measuring the polymer mass that flows through a standard die within 10 minutes under controlled temperature and load. 

• MFI = Weight of extruded polymer (in grams) / 10 minutes 

• MFR = MVR x ρ

Here, MFR: is the MFR value in g/10 min

MVR: is the melt volume-flow rate in cc/10

ρ: defines the apparent melt density of the polymer at the test temperature.

What is melt flow rate vs melt flow index? 

Melt Flow Rate and Melt Flow Index are nearly equivalent in assessing melt flow characteristics by analyzing thermoplastic flow behavior. MFI is the flow of extrudate expressed in g/10 mins, whereas the MVR (melt volume flow rate) is the volume of extrudate that flows and is expressed as cm3 /10 mins.

MFR has become the industry standard and refers to the amount of polymer that flows through a die, measured in grams per 10 minutes (g/10 min). 

What is Difference Between MFR and MVR?

Melt Flow Rate (MFR) and Melt Volume rate (MVR) are key parameters used to evaluate the flow properties of thermoplastic polymers in their molten state. 

• MFR (Melt Flow Rate): Measures the mass of polymer extruded through a standardized die in grams per 10 minutes (g/10 min) under a specific temperature and load.
• MVR (Melt Volume Rate): Measures the volume of polymer extruded in cubic centimeters per 10 minutes (cm³/10 min) under the same conditions.

Difference between MFR & MVR: MFR vs MVR

What is the Use of Melt Flow Rate (MFR)?

The Melt Flow Rate (MFR) is a critical parameter in the polymer industry, measuring the ease with which a thermoplastic polymer flows when melted under specified conditions of temperature and load. It plays an important role in various aspects of polymer selection, processing, and quality control. Below is a detailed explanation of the MFR uses:

1. Polymer Grade Selection

• MFR helps in choosing the right polymer grade for different manufacturing processes.
• Low MFR polymers (high molecular weight) are ideal for applications requiring high mechanical strength, such as blow molding and extrusion.
• High MFR polymers (low molecular weight) are preferred for easy-flow applications like injection molding.

2. Processability Assessment

• MFR is used to determine whether a polymer can be processed efficiently in specific molding or extrusion techniques.
• A polymer with an appropriate MFR ensures smooth flow during processing, reducing defects such as warping, incomplete filling, or surface imperfections.

3. Quality Control and Batch Consistency

• Ensures uniformity between different production batches by verifying that the MFR remains within acceptable limits.
• Variations in MFR can indicate issues like polymer degradation, contamination, or inconsistencies in raw material properties.

4. Prediction of Molecular Weight and Flow Behavior

• Since MFR is inversely related to molecular weight, it provides an indirect estimation of polymer chain length.
• Polymers with higher molecular weight typically have lower MFR, offering better strength but lower flowability.

5. Identifying Processing Issues and Defects

• A significant change in MFR between batches can indicate polymer degradation due to excessive heat, oxidation, or contamination.
• Helps detect variations in additives, fillers, or moisture content affecting polymer flow behavior.

Importance of Melt Flow Rate in Plastic Processing

Melt Flow Rate (MFR) is a key factor in plastic processing, affecting material choice, manufacturing performance, and product consistency. It guarantees that polymers achieve the necessary flow properties for various applications, optimizing manufacturing processes while maintaining consistency. 

• It maintains consistency between production batches by controlling MFR within specified parameters.
• Identify inconsistencies in raw materials, polymer degradation, or contamination risks during processing.
• Higher MFR corresponds to a lower molecular weight, resulting in easier flow but potentially compromising strength.
• A lower MFR corresponds to a higher molecular weight, making the polymer thicker in flow but structurally stronger.
• Used to maintain reliability and reproducibility, MFR testing complies with established industry standards such as ASTM D1238 and ISO 1133.

Melt flow index Table of polymers

The Melt Flow Index (MFI) Table of Polymers provides the Melt Flow Index (MFI) or Melt Flow Rate (MFR) of different polymers, which indicates their flowability under a specific temperature and load.

How to Measure Melt Flow Index?

Melt Flow Index (MFI), also known as Melt Flow Rate (MFR), is measured using a Melt Flow Index Tester, following standards such as ASTM D1238 and ISO 1133. 

The process involves extruding molten polymer through a standardized die under a specified load and temperature.

1. Preparation of the Sample

• Select a dry and uniform polymer sample.
• Ensure it is free from contaminants or moisture, as this can affect results.

2. Setting Up the Equipment

• Preheat the Melt Flow Index Tester to the required temperature (e.g., 190°C for PE, 230°C for PP).
• Select the appropriate load (e.g., 2.16 kg for standard testing, 5.0 kg for high-flow polymers).
• Insert a standardized die (2.095 mm diameter, 8.00 mm length) into the barrel.

3. Loading the Polymer

• Place the polymer granules, powder, or pellets into the heated barrel.
• Use a piston to compact the material inside the chamber.

4. Applying the Load & Extrusion

• Place the appropriate weight (load) on the piston.
• Allow the polymer to melt and stabilize under heat.
• The polymer will flow through the die at a steady rate.

5. Measuring the Flow Rate

• Collect the extruded polymer over 10 minutes.
• Weigh the collected polymer in grams to determine the MFI (g/10 min).
• For automated machines, the system directly calculates the value.

What are the Standards for MFI Measurements?

The primary standards for the Melt Flow Index are: ASTM D1238 and ISO 1133, both standards outline methods for determining the melt flow behavior of thermoplastics under controlled conditions. 

1. ASTM D1238: Developed by the American Society for Testing and Materials, this standard is widely followed in North America and other ASTM-compliant regions. 

It outlines methods for determining the melt flow rate of thermoplastics using an extrusion plastometer.

2. ISO 1133: ISO 1133 is an international standard that specifies the methods for determining the Melt Mass-Flow Rate (MFR) and Melt Volume-Flow Rate (MVR) of thermoplastics.

Importance of Melt Flow Rate/ Melt Flow Index

Melt Flow Index (MFI), also known as Melt Flow Rate (MFR), is an essential measure in the plastic industry, indicating a polymer's flowability in its molten state. It is significant in processing efficiency, material selection, and product quality.

1. Assessing Flowability and Processability

The Melt Flow Rate (MFR) is essential for assessing the flowability and processability of thermoplastics. It determines how easily a polymer melts and flows under specific conditions, influencing material selection, mold filling, and extrusion efficiency.

2. Material Selection and Quality Control

Melt Flow Rate (MFR) plays a crucial role in material selection and quality control by determining a polymer’s processability. A consistent MFR ensures uniformity across batches, preventing defects in molding and extrusion.

3. Impact on Product Properties

The Melt Flow Rate (MFR) significantly impacts product properties by influencing mechanical strength, processability, and end-use performance. A higher MFR indicates lower molecular weight, enhancing flow but reducing strength, while a lower MFR suggests higher molecular weight.

4. Standardization and Testing

Melt Flow Rate (MFR) standardization ensures consistent polymer quality, enabling material comparison across suppliers. Testing under controlled conditions (temperature, load) guarantees reproducibility, aiding in quality control, process optimization, and regulatory compliance.

FAQs

What does a higher MFI mean?

A higher Melt Flow Index (MFI) means the plastic melts and flows more easily, like how honey flows faster when warm. This makes it quicker to shape and fill molds during manufacturing. 

What is the full form of MFR?

The full form of MFR is Melt Flow Rate which describes how easily a thermoplastic polymer flows when melted under specific temperature and load conditions. It is expressed in grams per 10 minutes (g/10 min) and is determined using a melt flow indexer (extrusion plastometer).

What is MFR in injection molding?

MFR (Melt Flow Rate) in injection molding defines how smoothly a polymer flows when melted, helping manufacturers select suitable materials and achieve uniform product quality.

What is MFR used for?

Melt Flow Rate (MFR) tells us how easily melted plastic flows. It helps manufacturers choose the right material for molding and shaping products. MFR also ensures plastic quality stays the same, checks for contamination, and helps detect any material damage or aging.

What does the MFR model mean?

The Melt Flow Rate (MFR) model describes how a polymer flows when melted under specific temperature and pressure conditions. It helps predict processability, material behavior, and quality, ensuring suitability for various manufacturing methods like injection molding and extrusion.

What is the ideal melt flow index?

The best Melt Flow Index (MFI) depends on how the plastic will be used. High MFI means the plastic flows easily, making it great for quick molding. Low MFI makes stronger plastics, ideal for pipes and tough products. Finding the right balance ensures good quality and easy processing.