Detailed Determination of Plastic Melt Flow Index (MFI): Standards, Methods, and Applications

Finished and Semi-Finished Products

Molded parts, films, fibers, pipes, and other finished goods can be evaluated by first cutting or granulating them into pieces of roughly uniform size. Test personnel should note that the thermal and mechanical history of a processed part can alter its MFI relative to the original resin; nevertheless, this measurement is extremely valuable for failure analysis, comparing suspect and reference parts, and verifying reprocessing effects.

Testing Items

Typical deliverables from a melt index determination include:

• Melt Mass-Flow Rate (MFR), expressed in g/10 min, at a specified temperature and load.
• Melt Volume-Flow Rate (MVR), expressed in cm³/10 min, obtained by piston-displacement measurement.
• High-Load Melt Index (HLMI), commonly measured on polyethylene at 190 °C with a 21.6 kg load for low-flow materials.
• Flow Rate Ratio (FRR), the ratio of MFR values obtained at two different loads at the same temperature, which indicates shear sensitivity and reflects the breadth of the molecular weight distribution.
• Derived melt density at the test temperature, calculated when both MFR and MVR are measured.
• Multiple-condition testing, where the same material is measured at two temperature/load combinations specified by the material specification.

Testing Methods

Apparatus and Reference Test Conditions

The standard instrument is an extrusion plastometer consisting of a vertically mounted heated barrel (nominal inner diameter 9.550 mm), a tempered steel die with a bore of 2.095 mm diameter and 8.000 mm length, a piston with a guided head and detachable weights, a precision temperature controller maintaining the set temperature within ±0.5 °C in the measuring zone, a timer, and an analytical balance readable to 1 mg. ISO 1133-1 defines Method A (mass measurement) and Method B (volume measurement); ASTM D1238 defines analogous procedures, including Procedure A (cut-and-weigh) and Procedure B (automatic piston-displacement instruments with a sealed die during preheating).

The test condition—temperature and total piston load—must be specified by the material standard or grade datasheet, because results obtained under different conditions are not comparable. Commonly used conditions include:

• LDPE / LLDPE: 190 °C, 2.16 kg
• HDPE (standard): 190 °C, 2.16 kg; HDPE (high-load HLMI): 190 °C, 21.6 kg
• PP (polypropylene): 230 °C, 2.16 kg
• PS, HIPS, EPS (polystyrene family): 200 °C, 5.00 kg
• ABS: 220 °C, 10.0 kg
• PMMA: 230 °C, 3.80 kg
• POM (polyoxymethylene): 190 °C, 2.16 kg
• PC: 300 °C, 1.20 kg
• PET: 285 °C, 2.16 kg (thorough drying required)
• PA (polyamide): 235 °C with 1.00 kg or 2.16 kg, or 275 °C with 0.325 kg, depending on grade specification

For moisture-sensitive and time-temperature-sensitive polymers, ISO 1133-2 defines modified procedures with shortened preheating and controlled sample conditioning, and such materials should be tested according to that part of the standard whenever applicable.

Method A — Timed Cut-and-Weigh (Mass Measurement)

In Method A, the barrel is brought to the set temperature and allowed to stabilize with the die and piston in place. The sample is charged into the barrel in small increments within about one minute, compacted with a packing rod to eliminate air pockets, and the preheated piston is inserted. After a defined preheat period—typically 5 minutes for test temperatures up to 300 °C and longer for higher temperatures or moisture-sensitive grades—the specified load is applied to the piston. Initial extrudate, which may contain bubbles, is discarded. Successive extrudate segments are then cut off at the die face at equal, timed intervals, with the timing interval selected so that each segment is large enough for accurate weighing (typically at least 0.05 g).

Valid measurements are confined to the piston travel between the two reference scribe marks (nominally 30 mm apart), which excludes the initial and final portions of the charge where the flow rate is unstable. Segments containing visible bubbles are rejected; if the masses of individual segments differ from their mean by more than approximately 15%, the determination should be repeated with a fresh charge. The MFR is calculated from the mean segment mass:

MFR (g/10 min) = (m × 600) / t, where m is the mean mass of the cut segments in grams and t is the cut interval in seconds. A minimum of three valid segments should be collected, and the mean value is reported.

Method B — Piston Displacement (Volume Measurement)

Method B determines the Melt Volume-Flow Rate by automatically measuring either the time required for the piston to travel a calibrated distance or the distance traveled in a fixed time, using an optical sensor or displacement transducer. The MVR is calculated as:

MVR (cm³/10 min) = (A × l × 600) / t, where A is the mean effective cross-sectional area of the piston and barrel (nominal value 0.711 cm²), l is the measured piston travel in centimeters, and t is the elapsed time in seconds.

When the melt density at the test temperature is known, the MFR can be derived as MFR = MVR × ρ. Typical melt densities used for conversion include approximately 0.763 g/cm³ for polyethylene at 190 °C and approximately 0.739 g/cm³ for polypropylene at 230 °C; values for other materials should be taken from validated tables or determined experimentally. Method B is faster, less operator-dependent, and well suited to production quality control and to high-flow materials where manual cutting is impractical; Method A remains the reference approach when melt density is uncertain or when direct mass verification is required.

Key Factors Affecting Accuracy

MFI results are highly sensitive to test execution, and accredited third-party laboratories control the following variables closely:

• Temperature accuracy and stability, including thermometer calibration and thermal recovery after charging the sample.
• Load accuracy and piston friction; the piston must slide freely, and the total mass must match the specified load within tolerance.
• Die and barrel condition, including cleanliness, freedom from carbon deposits, and dimensional conformance after extended use.
• Sample moisture content, particularly for PC, PA, PET, ABS, and PMMA; inadequate drying is a leading cause of erroneously high and scattered results.
• Air entrapment during charging, caused by too-rapid loading or insufficient compaction.
• Thermal degradation or crosslinking during the preheat or measurement period, especially for materials tested near their degradation temperature.
• Consistent timing and cutting technique in Method A, and correct sensor calibration in Method B.
• Correct designation of the test condition, since even small deviations in temperature or load make results non-comparable.

A complete test report should identify the material and lot, the test standard and method, the temperature, load, and die configuration, any sample conditioning or drying, individual and mean results, and the date of determination. Well-controlled laboratories typically achieve within-lab repeatability within a few percent, while interlaboratory reproducibility is usually on the order of 5 to 10%.

Applications

Melt index determination supports a broad range of industrial and scientific objectives:

• Incoming resin inspection and lot acceptance against datasheet or purchase-specification limits.
• Grade identification and differentiation, for example distinguishing injection-molding, film-extrusion, and blow-molding grades of polyolefins.
• Processability assessment: higher MFI favors thin-wall injection molding and high-speed filling, while lower MFI indicates higher molecular weight suited to pipe, blown film, and blow molding.
• Monitoring of degradation during processing, reprocessing, or recycling; an increase in MFI signals chain scission, while a decrease may indicate crosslinking or contamination.
• Compounding and additive quality control, verifying the consistency of masterbatches, blends, and reinforced compounds.
• Failure analysis of molded parts, comparing the MFI of failed components with reference material to detect degradation or grade substitution.
• Research and development studies of molecular weight effects, stabilizer performance, and reaction processing.
• Support for supplier certifications and regulatory submissions generated by accredited third-party laboratories.

FAQ

What is the difference between MFI, MFR, and MI?

These terms are largely synonymous. "Melt Index" (MI) and "Melt Flow Index" (MFI) are traditional names, while ISO standards use the formal term "melt mass-flow rate" (MFR). All refer to the mass of polymer extruded in 10 minutes under a specified temperature and load, expressed in g/10 min.

What is the difference between MFR and MVR?

MFR is a mass-based result in g/10 min obtained by weighing extrudate (Method A), whereas MVR is a volume-based result in cm³/10 min obtained from piston displacement (Method B). The two are related through the melt density at the test temperature: MFR = MVR × melt density.

How much sample is needed and how long does a test take?

A single determination typically consumes 4 to 10 g of material, and duplicate determinations are recommended. Including instrument stabilization, sample conditioning, and measurement, a complete test usually takes 30 to 60 minutes per condition.

Can MFI values be used to compare different polymer types?

No. MFI is a single-point measurement valid only for comparing materials of the same polymer family tested under the identical temperature and load. Different polymers have different test conditions, melt densities, and shear responses, so cross-material comparison requires full rheological characterization.

Why did my material show a higher MFI after processing or recycling?

An increase in MFI usually indicates a reduction in average molecular weight caused by thermal or hydrolytic degradation during processing. Common root causes include excessive residence time, elevated barrel temperatures, residual moisture in hygroscopic resins, or repeated regrind use.

Are all thermoplastics suitable for MFI testing?

Most thermoplastics can be tested, including PE, PP, PS, ABS, PMMA, PC, POM, PET, PA, and EVA. Materials that degrade rapidly at the test temperature, strongly crosslink, or are highly filled may require modified procedures or capillary rheometry instead; PVC, for example, is generally characterized by other methods due to its thermal sensitivity.

How should filled or reinforced compounds be handled?

Glass-fiber and mineral-filled compounds can be tested, but results should be interpreted with care because fibers may orient, break, or partially plug the small die bore. Consistent charging technique and verification of die cleanliness are especially important, and the filler loading should always be stated alongside the MFI value.

What does flow rate ratio (FRR) tell me?

FRR is the ratio of MFR values measured at two loads at the same temperature. A higher ratio indicates greater shear thinning and generally a broader molecular weight distribution, which is useful information for predicting behavior in high-speed injection molding versus extrusion processes.