Melt flow index (MFI) testing measures the rate at which a molten thermoplastic flows through a standard die under prescribed temperature and load conditions, making it the most widely used indicator of polymer processability and batch-to-batch consistency. This article provides a complete determination procedure covering apparatus requirements, sample preparation, ISO 1133-1 and ASTM D1238 test conditions, Procedure A and B operating steps, result calculation, and validity criteria for manufacturers, quality engineers, and accredited third-party laboratories.
Technical Overview
The melt flow index (MFI), also expressed as melt mass-flow rate (MFR) or melt volume-flow rate (MVR), quantifies the flow behavior of a thermoplastic material in the molten state under a standardized load and temperature. In principle, a measured quantity of polymer is loaded into a heated cylindrical barrel, a piston with a defined dead weight applies force, and the molten material is extruded through a short capillary die of precise dimensions. The mass or volume extruded over a fixed time interval is reported per ten minutes.
Because melt viscosity correlates strongly with average molecular weight and the degree of degradation or crosslinking, the melt flow index serves as a fast, sensitive, and economical quality-control parameter. Higher melt index values indicate lower melt viscosity and easier flow, while lower values indicate stiffer, more viscous melts. The principal standards governing this determination are ISO 1133-1 (general method), ISO 1133-2 (materials sensitive to time-temperature history and moisture), ASTM D1238, and the Chinese national standard GB/T 3682. Results obtained under identical conditions of temperature, load, and procedure are comparable between laboratories.
Testing Samples
Thermoplastic Pellets and Granules
Pellets and granules are the most common sample form. A typical charge of 3 to 8 grams is sufficient for a single determination, depending on the expected flow rate. Pellets should be free of dust, fines contamination, and foreign matter. Virgin material samples should be taken from sealed packages or conditioned storage to prevent moisture pickup.
Powders and Fine Materials
Powdered thermoplastics, such as rotomolding-grade polyethylene or some polymer additives masterbatches, may also be tested. Powders trap more air during charging, so careful compaction with a packing rod in several increments is required to eliminate voids, which would otherwise produce bubbles in the extrudate and invalidate the measurement.
Finished Parts, Film, and Regrind
Samples cut from molded parts, extruded profiles, film, or regrind material must be reduced to pieces small enough to charge the barrel freely, typically a few millimeters in size. Metallic inserts, pigmented surface layers, or contamination should be removed before cutting. Results from finished parts reflect the processed resin state and may differ from virgin pellet values due to processing history.
Conditioning and Pre-Drying
Hygroscopic polymers such as polyamides, polycarbonate, PET, and ABS must be dried before testing in accordance with the relevant material specification, since residual moisture causes hydrolytic degradation and bubbles, leading to falsely high and scattered flow values. General conditioning guidance includes:
- Store and test samples under standard laboratory atmosphere, typically 23 °C and 50% relative humidity, unless otherwise specified.
- Dry hygroscopic resins (for example 2 to 4 hours at 80 to 120 °C in a dry-air or vacuum oven) and test promptly after drying.
- Keep polyolefins and other non-hygroscopic resins in closed containers to avoid surface dust and static pickup.
- Record the drying and conditioning history in the test report, as required for materials covered by ISO 1133-2.
Testing Items
The melt flow determination protocol encompasses the following measurable outputs:
- Melt mass-flow rate (MFR), expressed in g/10 min, determined by Procedure A.
- Melt volume-flow rate (MVR), expressed in cm³/10 min, determined by Procedure B.
- Melt density at the test temperature, derived from paired MFR and MVR measurements.
- Flow rate ratio (FRR), the ratio of flow rates measured at two different loads at the same temperature, used to characterize shear sensitivity.
Typical standard test condition codes are written as temperature/load, for example 190/2.16 for polyethylene and 230/2.16 for polypropylene. Commonly applied conditions include:
- PE (LDPE, HDPE, LLDPE): 190 °C with 2.16 kg load; high-load 21.6 kg for very low-flow grades.
- PP: 230 °C with 2.16 kg or 5.00 kg load.
- PS and HIPS: 200 °C with 5.00 kg load.
- ABS: 220 °C with 10.0 kg load.
- PC: 300 °C with 1.20 kg load.
- POM: 190 °C with 2.16 kg load; PMMA: 230 °C with 3.80 kg load; PET: 285 °C with 2.16 kg load.
The exact condition shall always be taken from the material datasheet, purchase specification, or the tabulated conditions in ISO 1133-1 or ASTM D1238, and shall be reported with the result.
Testing Methods
Apparatus Requirements
A melt flow indexer (extrusion plastometer) consists of a vertically mounted steel barrel, a temperature control system, a guided piston with removable weights, a standardized die, and cutting or displacement-measuring accessories. Key dimensional requirements are:
- Barrel inner diameter of 9.550 mm with a close tolerance and a smooth polished bore.
- Die of 2.095 mm inner diameter and 8.000 mm length, made of hardened steel with a smooth finish.
- Piston head diameter slightly smaller than the barrel bore, fitted with insulating guide lands; piston with reference marks defining the measurement zone.
- Test loads from a certified weight set, typically 0.325, 1.00, 1.20, 2.16, 3.80, 5.00, 10.0, and 21.6 kg, including the piston mass.
- Temperature control maintaining the set point within approximately ±0.5 °C along the measurement zone after stabilization.
- An analytical balance readable to 0.001 g or better, a timer accurate to 0.1 s, and a displacement transducer or encoder for Procedure B.
Sample Charging and Preheating
Bring the instrument to the specified temperature and allow it to stabilize fully. Insert the die and verify the barrel temperature with a calibrated sensor. Then proceed as follows:
- Charge the barrel with the sample in one to three increments, compacting each increment with the packing rod to exclude trapped air.
- Insert the piston (with the weight support but without the load) and allow a preheating period, commonly 5 minutes per ISO 1133-1 or approximately 6 minutes per ASTM D1238, so the entire charge reaches thermal equilibrium.
- Limit the total elapsed time between charging and the start of measurement (on the order of 12 minutes maximum per ISO) to prevent thermal degradation of the melt.
- Optionally apply a small pre-load or floating weight on materials prone to oxidation, and purge the barrel with nitrogen where specified.
Procedure A – Cut-Off (Mass) Method
Procedure A determines MFR directly by mass and requires no melt density assumption:
- Apply the specified load to the piston at the end of the preheating period.
- Allow extrusion to begin and discard the initial extrudate segment, which may contain bubbles.
- Collect successive cut-off segments by shearing the extrudate at the die face with a cutting tool at equal, pre-selected time intervals, while the piston remains within its marked measurement zone.
- Choose the time interval so each segment is large enough for accurate weighing, typically at least 0.1 g, and collect at least three acceptable segments.
- Weigh each segment individually on the analytical balance after cooling, discard any segment containing visible bubbles, and compute the average mass.
Procedure B – Displacement (Volume) Method
Procedure B determines MVR by monitoring piston travel and is preferred for automated instruments and low-flow materials:
- Apply the load and record the time required for the piston to travel a defined distance (for example 6.35 mm or other instrument-defined interval) within the measurement zone, or record distance over a defined time.
- Repeat the measurement at least three times during a single charge where the charge volume allows, or perform multiple charges.
- Calculate MVR from the piston cross-sectional area and travel distance; where melt density is known or simultaneously measured, convert to MFR.
Calculation of Results
For Procedure A, the melt mass-flow rate is calculated as MFR = 600 × m / t, where m is the average mass of the cut-off segments in grams and t is the time interval in seconds, giving a result in g/10 min. For Procedure B, the melt volume-flow rate is calculated as MVR = 600 × A × l / t, where A is the effective cross-sectional area of the barrel and piston in cm², l is the piston travel distance in cm, and t is the corresponding time in seconds, giving a result in cm³/10 min. When both rates are measured on the same material, melt density equals MFR divided by MVR. The flow rate ratio is computed by dividing the flow rate at the higher load by that at the lower load at the same temperature. Report the result rounded to two significant figures together with the condition code, procedure used, and any deviations.
Validity and Repeatability Criteria
A determination is considered valid when the following checks are satisfied:
- Extruded segments are free of visible bubbles; if more than about 10% of segments contain bubbles, the entire determination shall be repeated with better compaction or drying.
- Masses of successive cut-off segments agree within approximately 15%; wider scatter indicates non-uniform temperature, trapped air, or in-flight degradation.
- All timed measurements are completed while the piston is within the marked measurement zone, away from the barrel top and the die.
- Replicate determinations on fresh charges should agree within a few percent; the mean is reported when the criteria of the governing standard are met.
Cleaning, Maintenance, and Calibration
- Clean the piston, barrel, and die immediately after each test using appropriate tools, lint-free cloths, and suitable solvents or a purging compound; never use abrasives that alter the die or barrel dimensions.
- Verify die and piston dimensions periodically against the certified drawings and replace components that are worn, scratched, or corroded.
- Calibrate the temperature sensors, load masses, balance, timer, and displacement transducer at defined intervals using equipment traceable to national metrology institutes.
- Check overall instrument performance at intervals using certified reference materials with known flow rates.
Safety Precautions
Barrel surfaces, dies, and pistons operate at temperatures up to 300 °C or higher and molten polymer can cause severe burns. Operators shall wear heat-resistant gloves and eye protection, never point the die outlet toward personnel during extrusion, allow components to cool before cleaning, and ensure adequate ventilation when testing polymers that may release decomposition products.
Applications
Melt flow index determination is applied throughout the plastics value chain:
- Incoming resin inspection and verification of supplier datasheet values at goods receipt.
- Batch-to-batch consistency control in resin production, compounding, and masterbatch manufacturing.
- Grade identification and differentiation of polyethylene, polypropylene, polystyrene, ABS, and engineering resins for specification compliance.
- Process tuning for injection molding, extrusion, blow molding, film blowing, and rotomolding, where flow rate governs filling behavior and die pressure.
- Evaluation of thermal degradation after processing, recycling cycles, or accelerated aging, detected as an increase in flow rate.
- Certification testing by accredited third-party laboratories for product data sheets, customer specifications, and regulatory submissions.
FAQ
What is the difference between MFI, MI, MFR, and MVR?
MFI (melt flow index) and MI (melt index) are traditional trade terms for the same property now formally defined as MFR, the melt mass-flow rate in g/10 min. MVR is the melt volume-flow rate in cm³/10 min measured by piston displacement. MFR and MVR are directly related through the melt density at the test temperature.
Should I test to ISO 1133 or ASTM D1238?
Both standards use essentially the same apparatus and produce comparable results when the temperature, load, and procedure match, but they differ in detail, such as preheating time, tolerances, and reporting requirements. Follow the standard referenced by your customer specification or product datasheet; where both are acceptable, state the standard and condition code with every result.
Why does moisture change the melt index result?
In hygroscopic polymers such as PA, PC, PET, and PMMA, residual moisture causes hydrolytic chain scission at test temperature, lowering melt viscosity and producing falsely high and scattered flow values, along with visible bubbles in the extrudate. Proper drying and prompt testing, per ISO 1133-2 where applicable, are essential for valid results.
What is the flow rate ratio (FRR) used for?
FRR compares flow rates at two loads, for example 5 kg versus 2.16 kg at the same temperature, and reflects how shear-thinning a resin is. It provides a simple two-point indication of molecular weight distribution behavior that is useful for distinguishing grades with similar single-point MFR but different processing behavior.
How much sample is needed and how many replicates are required?
A single charge typically requires 3 to 8 grams depending on the expected flow rate, and a complete determination with replicate charges generally consumes well under 50 grams of material. At least three valid measurements per charge or per fresh charge are recommended, with replicate charges used to confirm repeatability.
Does a higher melt index mean better quality?
Not necessarily. The melt index reflects viscosity suited to a process window: low values suit extrusion and blow molding, while higher values suit thin-wall or high-speed injection molding. Quality is judged as conformity to the specified flow range of the intended application, not by an absolute high or low value.