High vs. Low Flow Materials: 4 Solutions for More Accurate Melt Flow Index Testing
Unveiling the Secrets of Plastic Behavior for Enhanced Product Development
When plastic materials are processed — whether through injection molding, extrusion, or film production — their ability to flow under heat and pressure determines everything from manufacturing efficiency to the quality of the finished product. Understanding and controlling that flow behavior starts with one key measurement: the Melt Flow Index.
Rooted in the science of Rheology, Melt Flow Index testing gives engineers and quality control teams a reliable, standardized way to evaluate how a polymer will behave under real processing conditions — and to ensure that every batch of material meets specification before it ever reaches the production line.
What is Rheology?
Rheology is the science of how materials deform and flow under an applied force — and its influence extends well beyond industrial settings. Many of the products we use every day are the result of careful rheological study. The viscosity of toothpaste is engineered so it stays on the brush rather than slipping through the bristles. Shampoo is formulated to sit in the palm of your hand without running off. Even the ink in a ballpoint pen is precisely tuned to flow smoothly and consistently under the pressure of writing.
In the plastics industry, these same principles apply at a much larger scale. Understanding how a polymer flows under heat and pressure — and how that behavior changes with different formulations or processing conditions — is fundamental to developing materials that perform reliably in manufacturing.
Melt Flow Index (MFI)
The Melt Flow Index is a decisive parameter in the polymer industry. It is primarily used to evaluate the quality and characteristics of plastic materials. It measures the performance of a plastic material in its molten state to flow under a given pressure, offering insight into the polymer's processability and behavior during manufacturing processes.
The Melt Flow Index (MFI) is determined by having molten polymer flow through a nozzle with dimensions conforming to ISO 1133 and ASTM D1238 test standards while under the weight of a specified mass. The amount of material flowing out of the nozzle is then measured, usually in grams every ten minutes, to provide an estimate of the flowability of the material at that particular temperature and pressure.
Where is the Melt Flow Index found?
The value of a material's Melt Flow Index can be found in datasheets and technical information sheets provided by plastics manufacturers. These documents contain detailed information on the physical, chemical, and mechanical properties of the polymer, giving engineers and lab technicians a comprehensive reference point for understanding how a material is expected to behave under processing conditions.
Critically, datasheets also include the standard temperature and applied mass conditions used to measure MFI, allowing laboratory tests to be replicated to confirm the values provided by the supplier. This is an important step in quality control — verifying that the material received matches the specification on the datasheet helps prevent processing problems and ensures consistency in the final product.
When manufacturer datasheets are not available, specialized online databases that collect information on a wide range of plastic materials can be consulted. These databases indicate reference values for the MFI alongside the corresponding test conditions, making them a useful resource for research and development, material selection, and cross-referencing supplier data.
It is worth noting that MFI values are only valid under the specific test conditions at which they were measured. Temperature, applied mass, and material preparation can all influence the result, which is why replicating the exact conditions stated in the datasheet is essential for meaningful comparison and accurate quality verification.
Application areas of the MFI
Quality Control
The MFI is critical to ensure that plastic resins meet the required specifications before being used in manufacturing processes. Manufacturers use the MFI to verify the consistency of material received from suppliers, preventing production problems and defects in finished products.
Research and Development
In research and development, the MFI helps users understand how changes in the chemical composition of the polymer affect the material's flow properties. This information makes it possible to develop new materials with optimal properties for specific applications.
Recycling Plastics
The MFI is important for evaluating recycled plastic materials, determining their suitability for reuse in manufacturing processes and ensuring that recycled materials can be combined with virgin materials without compromising the quality of the final product.
How is it measured? MFR and MVR
The Melt Flow Index can be measured in two ways:
- Melt Mass-Flow Rate (MFR): measures the grams of material flowing through a nozzle in 10 minutes.
- Melt Volume-Rate (MVR): measures the volume of material in cubic centimeters flowing in the same time interval.
The relationship between MFR and MVR is derived from the formula:
MFR = MVR x material density at test temperature
Classification According to Flow Rate
- High Flow Materials: They have a high MFI of up to 1600 g/10 min. They are characterized by low viscosity and high fluidity and are ideal for applications such as injection molding.
- Low Flow Materials: They have a low MFI, generally less than 4 g/10 min. They are characterized by high viscosity and lower flow capacity, and are preferred for applications such as extrusion, which allows the production of tubes and foils.
What Solutions Can Be Adopted
Specific solutions can be adopted to carry out testing according to the time and manner required by international standards.
Light Piston
The 100-gram light piston is an ideal solution for testing high-fluidity materials that tend to flow rapidly under their own weight. Unlike the standard 325-gram piston, the lighter piston exerts less pressure on the material during the preheating phase, allowing better control of the material's rate of descent and reducing the risk of premature flow before testing begins.
This is particularly important for polymers with a high Melt Flow Index, where even small variations in preheating conditions can lead to inconsistent results. By applying less downward force during this critical phase, the light piston gives the material time to reach the correct test temperature uniformly — without being pushed through the die prematurely.
This piston is particularly advantageous for polymers that require gentle handling to prevent premature release of the material from the testing device. The result is a more controlled, repeatable test that better reflects the true flow characteristics of the material and supports compliance with ISO 1133 and ASTM D1238 testing standards.
Zero Gravity Piston
The Zero Gravity piston represents a significant innovation in high-flow materials testing. Equipped with a magnet at the top, it allows the piston to be suspended during the preheating phase — eliminating the risk of premature material spillage that can compromise test accuracy and waste material.
Traditional pistons apply weight to the material from the moment they are loaded, which can cause high-fluidity polymers to begin flowing before the test officially starts. The Zero Gravity piston solves this by holding the piston in place magnetically, ensuring the material preheats under controlled, weight-free conditions. When the test is ready to begin, the piston is automatically released, applying load consistently and at precisely the right moment.
This approach not only improves the reliability of results but also supports compliance with ISO 1133 and ASTM D1238 test standards, where consistent preheating conditions are essential for accurate Melt Flow Rate and Melt Volume Rate measurements.
Automatic Die Plug
The Automatic Die Plug is an accessory developed to ensure the accuracy of Melt Flow Rate tests, especially with high-fluidity materials. Without a die plug, high-flow polymers can begin escaping through the nozzle during the preheating phase — before the test has even started — reducing the amount of material available for measurement and introducing variability that can skew results.
This device functions as a shutter that blocks the material outlet during the preheating phase. Equipped with a sensor, the Automatic Die Plug remains closed throughout preheating, ensuring that no material can escape until the test begins. This is particularly critical for materials with a high Melt Flow Index, where even a small amount of premature flow can meaningfully affect the quantity of material remaining in the barrel and the consistency of the final measurement.
When the test is ready to begin, the system automatically releases the shutter at precisely the right moment, allowing material to flow through the nozzle under controlled, standardized conditions. This removes the variability associated with manual die plug removal, which can introduce timing inconsistencies and disturb the material just as testing begins.
The result is a more reliable, repeatable test that maintains a sufficient amount of material in the testing device, prevents leakage that could compromise accuracy, and ensures full compliance with ISO 1133 and ASTM D1238 testing standards.
Unit for Automatic Compacting and Purging
Low-velocity materials, characterized by a lower Melt Flow Rate, have a higher viscosity and reduced flow capacity than high-velocity materials. These polymers require special consideration during testing and processing, as their highly viscous nature implies unique challenges for accurate measurement and handling in manufacturing processes.
The extrusion of these materials takes place at lower deformation speeds to avoid the formation of defects such as bubbles or fractures, which could compromise the product's mechanical properties. Testing materials at low speeds requires that the polymer be completely free of air bubbles and well compacted, as any air entrapment could cause significant variations in the test results. Compaction of the material during the preheating phase is particularly critical. Using a unit for automatic compacting and purging can help apply a specified amount of pressure evenly and repeatably to the material, eliminating trapped air and ensuring optimal compaction.
This intelligent system can adjust the applied pressure according to the response of the material, keeping the pressure value constant throughout the preheating process. In addition, at the end of the test it will help purge the material left inside the barrel by promoting faster and easier cleaning of the barrel.