Plastic melt flow index (MFI) testing is a fundamental laboratory method for evaluating the flow behavior and processability of thermoplastic materials. This article provides a comprehensive technical guide covering testing principles, applicable standards such as ISO 1133 and ASTM D1238, sample requirements, test procedures, and typical applications across the plastics industry.
Technical Overview
The melt flow index (MFI) test, also known as the melt flow rate (MFR) test or melt index test, is one of the most widely performed quality control and material characterization tests in the plastics industry. The test measures the ease with which a thermoplastic polymer melts and flows under a specified temperature and load, expressed in grams per 10 minutes (g/10 min) for melt mass-flow rate (MFR) or cubic centimeters per 10 minutes (cm³/10 min) for melt volume-flow rate (MVR).
During the test, a sample of thermoplastic material is loaded into a heated barrel of an extrusion plastometer, and a piston with a specified dead weight applies a controlled force to extrude the molten polymer through a standardized die. The mass or volume of extrudate collected within a defined time interval determines the flow index value. A higher melt flow index indicates lower melt viscosity and easier flow, while a lower value indicates a more viscous material that is harder to process.
The melt flow index provides valuable insight into the average molecular weight of a polymer, since melt viscosity is strongly correlated with molecular chain length. It is therefore used extensively for incoming material inspection, batch-to-batch consistency verification, grade differentiation, process troubleshooting, and specification compliance. The principal international and national standards governing this test include ISO 1133-1, ISO 1133-2, ASTM D1238, and GB/T 3682.
Testing Standards
Plastic melt flow index testing is performed in accredited third-party laboratories in accordance with recognized international standards. The following standards are most commonly applied:
- ISO 1133-1: Determination of the melt mass-flow rate (MFR) and melt volume-flow rate (MVR) of thermoplastics, Part 1: Standard method
- ISO 1133-2: Determination of the melt mass-flow rate (MFR) and melt volume-flow rate (MVR) of thermoplastics, Part 2: Method for materials sensitive to time-temperature history and/or moisture
- ASTM D1238: Standard Test Method for Melt Flow Rates of Thermoplastics by Extrusion Plastometer
- GB/T 3682: Chinese national standard for the determination of melt mass-flow rate and melt volume-flow rate of thermoplastics
- JIS K 7210: Japanese standard for the determination of the fluidity of thermoplastics using extrusion plastometers
Testing Samples
Sample Forms
Melt flow index testing can be performed on thermoplastic materials in a variety of physical forms, including virgin resin pellets, regrind or reprocessed material, powder, granules, and molded or extruded parts that have been cut or ground into small pieces. The test is applicable to virtually all melt-processable thermoplastics.
Applicable Materials
- Polyolefins: polyethylene (PE), polypropylene (PP), polymethylpentene (PMP)
- Styrenics: polystyrene (PS), high-impact polystyrene (HIPS), acrylonitrile butadiene styrene (ABS), styrene-acrylonitrile (SAN)
- Engineering plastics: polycarbonate (PC), polyamide (PA/nylon), polyoxymethylene (POM/acetal), polybutylene terephthalate (PBT), polyethylene terephthalate (PET)
- Specialty polymers: polymethyl methacrylate (PMMA), polyetheretherketone (PEEK), thermoplastic polyurethane (TPU), thermoplastic elastomers (TPE/TPV)
- Fluoropolymers: polytetrafluoroethylene (PTFE) melt-processable grades, fluorinated ethylene propylene (FEP), PVDF
Sample Requirements
A typical test requires approximately 5 to 15 grams of material, depending on the expected flow rate and the selected cut-off interval. Materials that are hygroscopic, such as polyamide, polycarbonate, PET, and PBT, must be pre-dried according to the material manufacturer's recommendations before testing to prevent moisture-induced hydrolysis and bubble formation in the melt. Materials sensitive to time-temperature history, such as PVC and certain polyolefins containing volatile additives, should be tested in accordance with ISO 1133-2 with shortened preheating times.
Testing Items
Accredited third-party laboratories typically offer the following test items related to melt flow measurement:
- Melt mass-flow rate (MFR), expressed in g/10 min, determined by Method A (cutting and weighing of extrudate segments)
- Melt volume-flow rate (MVR), expressed in cm³/10 min, determined by Method B (piston displacement measurement)
- Melt density at test temperature, calculated from paired MFR and MVR results
- Flow rate ratio (FRR), the ratio of MFR values measured under two different load conditions, used to characterize shear sensitivity
- Melt flow index under customer-specified non-standard temperature and load combinations
Testing Methods
Instrumentation
The test is conducted using an extrusion plastometer consisting of a heated steel barrel with an internal diameter of approximately 9.5 mm to 10 mm, a temperature control system accurate to within ±0.5°C of the set point, a close-fitting piston guided within the barrel, a set of calibrated dead weights, and a die of hardened steel with a length of 8.000 mm and an internal diameter of 2.095 mm, in accordance with ISO 1133 and ASTM D1238 dimensional requirements. Method B instruments additionally incorporate an automatic piston displacement transducer and timing system.
Method A: Mass Measurement
In Method A, the operator charges a pre-weighed quantity of sample into the preheated barrel, compacts it with a packing rod to eliminate air, and inserts the piston. After a specified preheating period, typically around 4 to 5 minutes under ISO 1133-1 for stabilized materials, the selected test load is applied. The extrudate emerging from the die is cut off at regular timed intervals, and at least three consecutive cuttings are collected after discarding the initial extrudate. The cut segments are weighed, and the MFR is calculated as the average extruded mass extrapolated to a 10-minute period, reported in g/10 min.
Method B: Displacement Measurement
In Method B, the melt volume-flow rate is determined by measuring the distance traveled by the piston over a defined time period or the time required for the piston to travel a defined distance, using an automatic displacement transducer. Because no cutting and weighing is involved, Method B reduces operator influence and is well suited to materials with very high or very low flow rates. The MVR is reported in cm³/10 min, and melt density can be derived when MFR and MVR are measured simultaneously.
Standard Test Conditions
The test temperature and load must be selected according to the material type. Commonly applied condition combinations include:
- PE: 190°C with 2.16 kg (or 21.6 kg for high-load conditions)
- PP: 230°C with 2.16 kg
- PS and HIPS: 200°C with 5.00 kg
- ABS: 220°C with 10.00 kg
- PC: 300°C with 1.20 kg
- PET and PBT: 250°C with 2.16 kg
- PA (nylon): 275°C with 5.00 kg, with pre-dried sample
- POM: 190°C with 2.16 kg
- PMMA: 230°C with 3.80 kg
- PSU, PEEK, and other high-temperature engineering plastics: elevated temperatures of 300°C to 400°C with appropriate loads
Factors Affecting Test Results
Several factors must be controlled to ensure accurate and reproducible results, including barrel and die temperature accuracy and uniformity, load calibration, sample moisture content, the presence of trapped air bubbles, thermal degradation or crosslinking during the preheat period, barrel and die cleanliness and wear, piston fit, and consistency of the cutting interval relative to the flow rate of the material.
Applications
Melt flow index testing serves a wide range of practical purposes across the plastics supply chain:
- Incoming material inspection: verifying that resin lots delivered by suppliers conform to purchase specification MFR ranges
- Quality control: monitoring production consistency in polymerization, compounding, and masterbatch manufacturing
- Material selection: matching resin flow grades to processing methods such as injection molding, extrusion, blow molding, film casting, fiber spinning, and 3D printing
- Grade identification: distinguishing between high-flow, medium-flow, and low-flow grades of the same polymer family
- Recycled material evaluation: assessing degradation and property retention in regrind and reprocessed resins
- Failure analysis: detecting molecular weight changes caused by excessive processing temperature, moisture exposure, or contamination
- Regulatory and specification compliance: supporting declarations of conformity for automotive, packaging, electrical, medical device, and consumer product applications
- Research and development: evaluating the effects of fillers, impact modifiers, stabilizers, and other additives on polymer rheology
FAQ
What is the difference between MFI, MFR, and MVR?
MFI (melt flow index) and MFR (melt mass-flow rate) refer to the same quantity expressed in grams per 10 minutes, measured by weighing the extrudate. MVR (melt volume-flow rate) is expressed in cubic centimeters per 10 minutes and is derived from piston displacement. The two are related through the melt density of the material at the test temperature.
Which standard should be used, ISO 1133 or ASTM D1238?
Both standards use similar apparatus, but they differ in procedural details such as preheating time, permitted flow rate ranges, and calculation methods. The choice depends on customer specification, the end market, and the material data sheet reference. Results obtained under ISO 1133-1 Procedure A and ASTM D1238 Method A are generally comparable for stabilized materials, but laboratories should always report the standard used.
How much sample is required for a test?
Typically 5 to 15 grams of material is sufficient for a single determination, although laboratories usually request additional material, commonly 30 to 50 grams, to allow repeat measurements and verification runs. Hygroscopic materials must be conditioned or dried before testing.
Why do my measured MFI values differ from the material datasheet?
Discrepancies can arise from different test standards, different temperature and load conditions, sample moisture content, thermal degradation during testing, contamination, or normal lot-to-lot variation. Confirming the exact condition code on the datasheet and using the same standard and conditions is essential for valid comparison.
Can filled or reinforced plastics be tested?
Yes, glass-filled, mineral-filled, and otherwise modified compounds can be tested, provided the melt flows within the measurable range of the instrument. The filler content should be noted in the report, since flow values of filled compounds are not directly comparable to unfilled base resins.
What does a high or low melt flow index mean for processing?
A high MFI indicates low melt viscosity, favoring thin-wall injection molding, high-speed filling, and fine detail reproduction. A low MFI indicates high melt viscosity and generally higher molecular weight, favoring extrusion, blow molding, structural strength, and impact resistance. Selecting the correct flow grade is critical to balancing processability with mechanical performance.
How long does a melt flow index test take?
A single determination typically takes 15 to 30 minutes of instrument time, including stabilization and measurement. In practice, laboratories complete testing within 1 to 3 working days when multiple samples and repeats are required, subject to sample preparation and queuing conditions.