Melt Flow Index (MFI): Understanding Polymer Processability
What Is Melt Flow Index?
Melt Flow Index (MFI), also called Melt Flow Rate (MFR), is a measure of the ease of flow of a molten thermoplastic polymer under defined conditions of temperature, load, and time. It is expressed as the mass of polymer extruded through a standardized capillary in 10 minutes (g/10 min) under a specified piston load and temperature.
MFI is one of the most widely used quality control parameters in the plastics industry. It provides a rapid, economical indication of the average molecular weight and processing viscosity of a polymer, enabling incoming material inspection, lot-to-lot consistency verification, and basic processability screening across the packaging, automotive, pipe, fiber, and compounding industries.
The MFI Test Procedure (ASTM D1238 / ISO 1133)
Equipment
A melt flow indexer (melt indexer) consists of a heated barrel, a capillary die (standard dimensions: 2.095 mm diameter × 8.000 mm length), and a loaded piston that applies a defined force to the polymer melt.
Procedure
- The barrel is preheated to the test temperature specified for the polymer (e.g., 190°C for polyethylene; 230°C for polypropylene; 300°C for polycarbonate)
- A measured quantity of polymer pellets or powder is loaded into the barrel
- A preheat period (typically 5–10 minutes) allows the polymer to melt and reach thermal equilibrium
- The specified load (e.g., 2.16 kg for the standard PE and PP condition; 5.0 kg, 10 kg, or 21.6 kg for other conditions) is applied to the piston
- Cut-off extrudates are collected at defined time intervals and weighed
- MFI (g/10 min) = mass of extrudate × (600 / collection time in seconds)
Two Test Methods
Method A (Cut and Weigh): Standard method for most polymers; extrudates are cut at timed intervals and weighed.
Method B (Displacement Rate): The piston displacement rate is measured automatically; MFR is calculated from the displacement and piston/barrel geometry. Faster and preferred for automated quality control systems.
Interpreting MFI Values
|
MFI Range (g/10 min) |
Typical Processing Method |
|
<0.5 |
Extrusion, pipe, sheet—high viscosity |
|
0.5–5 |
Blow molding, extrusion—medium viscosity |
|
5–20 |
Injection molding—standard grades |
|
20–100 |
Injection molding of thin-wall or complex parts |
|
>100 |
Fiber spinning, very-thin wall injection |
Higher MFI → lower molecular weight → easier flow → generally lower mechanical properties (lower tensile strength, lower impact resistance). Lower MFI → higher molecular weight → more difficult to process → generally better mechanical properties.
MFI vs. Melt Viscosity
MFI provides a single-point viscosity indication at a specific shear stress (determined by the applied load). It does not characterize the full flow curve of the polymer. For accurate process simulation and die design, a complete viscosity-shear rate curve measured by capillary rheometry (ASTM D3835) or rotational rheometry is required. MFI is a QC tool, not a substitute for full rheological characterization.
Factors Affecting MFI
- Molecular weight: Primary determinant—higher MW → lower MFI
- Molecular weight distribution (MWD): Broad MWD increases flow at low shear rates
- Moisture: Hygroscopic polymers (PA, PC, PET) must be dried before MFI testing—moisture causes hydrolytic degradation during the test, increasing apparent MFI
- Stabilizers and additives: Lubricants increase flow; fillers reduce flow
Why Choose Infinita Lab for MFI and Polymer Testing?
Infinita Lab offers MFI testing per ASTM D1238 and ISO 1133 for all standard thermoplastics and engineering polymers. Our accredited laboratory network provides fast, accurate MFI results for incoming material inspection, production quality control, and material development programs.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you. Request a Quote
Frequently Asked Questions (FAQs)
What is the difference between MFI (Melt Flow Index) and MFR (Melt Flow Rate)? MFI (Melt Flow Index) is the older term, used predominantly in North America per ASTM D1238. MFR (Melt Flow Rate) is the ISO 1133 term used in international standards. They describe the same measurement—grams of polymer extruded through a standardized die per 10 minutes under defined conditions. The terms are used interchangeably in practice.
Why must MFI test conditions (temperature and load) always be stated with the result? MFI is extremely sensitive to both test temperature and applied load. A polymer tested at 190°C/2.16 kg gives a completely different MFI value than the same polymer tested at 230°C/2.16 kg or 190°C/5.0 kg. Reporting MFI without the specific conditions makes the result meaningless—always report as "MFI = X g/10 min (190°C/2.16 kg)" or equivalent per ISO 1133 notation.
Can MFI detect polymer degradation during processing? Yes. Thermal degradation (chain scission) during processing reduces molecular weight and increases MFI. Measuring MFI of pellets before processing and regrind or product after processing reveals degradation. An increase in MFI from virgin to processed material indicates excessive thermal exposure. Maintaining MFI change below 20–30% is a common processing quality criterion.
What is the Melt Flow Ratio (MFR ratio) and what does it indicate? The MFR ratio is the MFI measured at a high load (e.g., 21.6 kg) divided by the MFI at a lower load (e.g., 5.0 kg) at the same temperature. It indicates the shear-thinning behavior of the polymer—a proxy for molecular weight distribution breadth. Higher MFR ratio = broader MWD = more shear-thinning = better processability at high shear rates (injection molding) but lower impact strength.
What is the typical MFI specification for HDPE used in blow molding? Blow molding HDPE grades typically have MFI values of 0.1–1.0 g/10 min at 190°C/2.16 kg. The low MFI reflects the high molecular weight needed for parison sag resistance and adequate top-load strength in the blown container. Very low MFI grades (<0.1 g/10 min) are used for large industrial containers; higher MFI grades (0.5–1.5 g/10 min) are used for small personal care bottles.
