Melt flow index (MFI) testing is the most widely used process test for characterizing the flow behavior of thermoplastic materials during melt processing. This article explains the technical principles, standard test methods, sample requirements, and industrial applications of melt flow index testing in accordance with ISO 1133, ASTM D1238, and related national standards. Whether you need incoming material inspection, batch quality control, or processing parameter optimization, this guide will help you understand how melt flow data supports reliable decision-making in plastics manufacturing.
Technical Overview
The melt flow index (MFI) test, also referred to as the melt mass-flow rate (MFR) test, is a standardized process experiment that measures the ease of flow of a molten thermoplastic under a specified temperature and load. The result quantifies how many grams of polymer are extruded through a calibrated die in ten minutes, expressed in g/10 min. Because melt viscosity correlates strongly with average molecular weight, the melt flow index serves as a fast, economical, and reproducible indicator of a material's processability and consistency.
The test is performed on an extrusion plastometer, which consists of a heated steel barrel, a piston with a detachable weight set, and a calibrated die with a bore of approximately 2.095 mm and a length of 8.000 mm. A pre-weighed charge of plastic is loaded into the barrel, allowed to melt and thermally stabilize, and then forced through the die by the loaded piston. The extrudate is either collected and weighed (mass-based method) or calculated from piston displacement (volume-based method).
Melt flow index testing is governed by internationally recognized standards, including:
- ISO 1133-1 – Determination of the melt mass-flow rate (MFR) and melt volume-flow rate (MVR) of thermoplastics, standard method
- ISO 1133-2 – Method for materials sensitive to time-temperature histories and/or moisture
- ASTM D1238 – Standard test method for melt flow rates of thermoplastics by extrusion plastometer (Procedures A and B)
- GB/T 3682 – Chinese national standard, technically aligned with ISO 1133
- JIS K 7210 – Japanese industrial standard for melt flow rate of thermoplastics
- ASTM D3364 – Flow rates for PVC and other thermoplastics requiring modified conditions
Test conditions (temperature and piston load) are material-specific and defined by the relevant standard. Commonly used conditions include:
- Polyethylene (PE): 190 °C with a 2.16 kg load (high-load 21.6 kg conditions for low-flow grades)
- Polypropylene (PP): 230 °C with a 2.16 kg load
- Polystyrene (PS): 200 °C with a 5.00 kg load
- ABS: 220 °C with a 10.0 kg load
- Polycarbonate (PC): 300 °C with a 1.20 kg load
- Polyoxymethylene (POM): 190 °C with a 2.16 kg load
- PMMA (acrylic): 230 °C with a 3.80 kg load
Testing Samples
Virgin Pellets and Granules
Virgin resin in pellet or granule form is the most common sample type. Pellets are charged directly into the barrel without additional preparation. A typical test consumes approximately 4 to 8 grams of material, and most standards recommend testing at least two charges per sample to verify repeatability.
Powder and Regrind Materials
Powdered resins, regrind, and reprocessed flake can be tested using the same procedure. Because these materials may contain moisture, volatiles, or fine particles that affect melt flow, pre-drying and careful charging are recommended. Powder materials may require gentle tamping to ensure a uniform charge without voids.
Filled and Reinforced Compounds
Glass-filled, mineral-filled, and pigment-loaded compounds can be evaluated for melt flow, provided that the standard condition appropriate for the base polymer is selected. The melt flow value reflects the combined effect of the polymer matrix and filler content, making the test useful for confirming compound consistency between production batches.
Finished Parts and Field Samples
Small cut pieces taken from molded parts, extruded profiles, films, or failed components can be tested to verify material grade, detect degradation, or compare suspect parts against reference material. Samples should be clean, free of contaminants, and cut into pieces small enough to fit freely into the barrel bore.
Testing Items
A melt flow index process test can include the following measurement items, depending on the selected method and client requirements:
- Melt mass-flow rate (MFR), expressed in g/10 min, determined by collecting and weighing timed extrudate segments
- Melt volume-flow rate (MVR), expressed in cm³/10 min, determined from piston travel distance and timing
- Melt density at the test temperature, derived by combining MFR and MVR results from the same charge
- Flow rate ratio (FRR), calculated as the ratio of MVR or MFR values measured under two different loads at the same temperature, providing an indication of shear sensitivity
- Multi-condition flow characterization for materials that may be processed under significantly different shear regimes
Testing Methods
Method A – Mass-Based Determination
In Method A (Procedure A of ASTM D1238), the operator cuts the extrudate at the die exit at uniform time intervals, typically ranging from 15 to 240 seconds depending on the expected flow rate. The collected segments are weighed on an analytical balance, and the melt mass-flow rate is calculated from the average mass extruded per unit time, converted to g/10 min. Cut intervals must be selected so that the extrudate segments are long enough to handle accurately and weighing uncertainty remains low.
Method B – Displacement-Based Determination
In Method B (Procedure B of ASTM D1238), the instrument automatically measures the distance the piston travels over a defined time, using an encoder or optical sensor. The melt volume-flow rate is calculated from the piston displacement, the cross-sectional area of the barrel, and the timing. Method B eliminates cutting and weighing operations, offers improved precision at high flow rates, and enables direct determination of melt density when combined with Method A results.
Sample Preparation and Pre-Drying
Hygroscopic materials such as polycarbonate, polyamides, polyesters, and ABS should be dried before testing in accordance with the material specification or ISO 1133-2, because absorbed moisture can cause hydrolytic degradation and significantly distort the measured flow rate. For moisture- and oxidation-sensitive polymers, testing under a dry nitrogen atmosphere is recommended. Each charge should be tested promptly after loading to limit thermal exposure in the barrel.
Operating Procedure Summary
A typical melt flow index process test follows these steps:
- Stabilize the plastometer at the specified test temperature, with barrel temperature controlled within approximately ±0.5 °C
- Insert the piston and verify the cleanliness and calibration status of the die and barrel
- Charge the barrel with the prepared sample, insert the piston, and allow the material to melt and equilibrate during the specified preheating period
- Apply the selected test load so that the piston descends and molten polymer extrudes through the die
- Discard the initial extrudate segment containing potential air bubbles, then collect timed cut samples (Method A) or record piston displacement (Method B)
- Repeat the measurement on additional cuttings or charges as required by the standard
- Calculate MFR or MVR, report the test conditions, and clean the barrel, piston, and die with appropriate tools before the next test
Applications
Quality Control and Lot Release
Resin producers, compounders, and injection molders use melt flow index testing as a routine quality control tool. Every production lot can be checked against a specified MFR range to confirm grade conformity before the material is released for processing or shipment. Accredited third-party laboratories can perform these tests for supplier verification, lot certification, and dispute resolution.
Material Selection and Grade Differentiation
Melt flow values differentiate resin grades designed for different processes. Low-flow (high molecular weight) grades suit blow molding, extrusion of pipe and film, and high-strength applications, while high-flow grades are preferred for thin-wall injection molding and complex part geometries. Engineers use MFI data sheets to match material grades to part design and tooling requirements.
Processing Parameter Optimization
Melt flow results help processors establish initial molding temperatures, injection pressures, and cycle times. Materials with higher melt flow indices generally fill molds at lower pressures, while low-flow materials require higher melt temperatures or pressures. The flow rate ratio further indicates how a material will behave across the wide shear rate range encountered between extrusion and injection molding.
Degradation and Recycling Assessment
Repeated melt processing, thermal abuse, or hydrolysis changes the molecular weight of a polymer, which is directly reflected in the melt flow value. Comparing the MFR of virgin resin, regrind, recycled material, and processed parts reveals the extent of degradation. This makes melt flow testing an essential tool for recycling stream qualification and for validating drying and processing conditions.
Specification and Regulatory Compliance
Melt flow index limits are written into countless material specifications, purchasing agreements, and industry standards. Test reports issued in accordance with ISO 1133 or ASTM D1238 provide the objective evidence needed for regulatory submissions, customer audits, and product certification files.
FAQ
What is the difference between MFI, MFR, and MVR?
MFI (melt flow index) is the traditional term for the melt mass-flow rate (MFR), both expressed in g/10 min. MVR (melt volume-flow rate) is expressed in cm³/10 min and is measured by piston displacement rather than extrudate mass. The two values are related through the melt density at the test temperature.
How much sample is needed for a test?
A single barrel charge typically requires 4 to 8 grams of material, depending on the expected flow rate. A complete test, including repeat charges for repeatability, generally consumes 15 to 30 grams in total. Clients submitting finished parts should provide enough material to allow cutting, cleaning, and repeat measurement.
Why must hygroscopic resins be dried before testing?
Moisture in materials such as polycarbonate, polyamides, polyesters, and ABS causes hydrolytic chain scission at melt temperatures, which increases the melt flow rate and produces falsely high or irreproducible results. Drying according to the material specification, and testing in accordance with ISO 1133-2 where applicable, ensures representative data.
What does a higher melt flow index indicate?
A higher MFI indicates lower melt viscosity, which generally corresponds to a lower average molecular weight and easier flow. This is beneficial for filling thin-wall molds but usually comes at the cost of reduced mechanical strength, lower melt strength, and poorer creep resistance compared with lower-flow grades of the same polymer.
Should I test according to ISO 1133 or ASTM D1238?
Both standards describe an extrusion plastometer method and produce comparable data, but they differ in details such as preheating time, permitted temperature tolerance, and calculation of results. The choice is typically dictated by the customer specification, the end market, or the material datasheet. Accredited third-party laboratories can perform tests to either standard and advise on condition selection.
How is test accuracy and repeatability ensured?
Accuracy depends on instrument calibration (temperature, load, die dimensions, and balance), strict adherence to condition settings, proper drying of the sample, careful cleaning between charges, and the selection of appropriate cut-off intervals. Routine verification using a certified reference material and participation in interlaboratory comparisons further support result reliability.