The ISO melt flow index test, governed by the ISO 1133 standard series, is the internationally recognized method for measuring the melt mass-flow rate (MFR) and melt volume-flow rate (MVR) of thermoplastic polymers. This technical guide explains the test principle, apparatus, sample requirements, standard test conditions, and typical applications of ISO melt flow index determination in polymer quality control and materials development.
Technical Overview
The melt flow index (MFI), formally designated as the melt mass-flow rate (MFR) or melt volume-flow rate (MVR) under ISO 1133, is a fundamental rheological parameter that quantifies the ease of flow of a molten thermoplastic under specified conditions of temperature and applied load. MFR is expressed in grams per 10 minutes, while MVR is expressed in cubic centimeters per 10 minutes. Together, these values provide a rapid, standardized indication of melt viscosity and, by extension, the average molecular weight of the polymer.
The test is performed using an extrusion plastometer, in which a measured quantity of polymer is loaded into a heated cylindrical barrel and forced through a standard capillary die by a piston carrying a specified mass. In Procedure A, timed segments of the extrudate are cut and weighed; in Procedure B, the displacement of the piston over a timed interval is recorded electronically. The mass or volume extruded in a reference time of 10 minutes is then calculated.
ISO 1133 is published in two coordinated parts: ISO 1133-1, which specifies the apparatus, the two measurement procedures, and the calculation rules, and ISO 1133-2, which tabulates the official temperature and load combinations assigned to specific material families. Because the melt flow index correlates inversely with melt viscosity and molecular weight, the method is universally used for grade specification, incoming resin inspection, batch-to-batch consistency control, and processability assessment in injection molding, extrusion, blow molding, and compounding operations.
Testing Samples
Virgin Thermoplastic Resins
Pellets and granules of virgin resin are the most common test samples. They can be charged directly into the plastometer barrel, provided they are free from contamination and, where applicable, dried beforehand. Typical charge masses range from 3 g to 8 g depending on the expected flow rate.
Powder and Recycled Materials
Powdered materials, regrind, flakes, and recycled feedstocks can also be evaluated. Powder grades must be loaded carefully to avoid voids in the barrel, and the charge should be compacted gently with the packing rod. Testing recycled materials is particularly valuable for detecting molecular degradation, which appears as an elevated flow rate compared with the virgin reference.
Finished Parts and Molded Articles
When the original resin is no longer available, finished products such as films, sheets, pipes, and injection-molded components can be tested. Specimens should be cut or ground into small pieces of approximately 2 mm to 3 mm so that the barrel can be filled uniformly. Care must be taken to exclude painted, printed, adhesive-contaminated, or multi-material zones unless the objective is to characterize the material as received.
Materials routinely characterized by ISO melt flow index testing include:
- Polyethylene (LDPE, LLDPE, HDPE)
- Polypropylene (PP homopolymer, random and impact copolymers)
- Polystyrene (PS), high-impact polystyrene (HIPS), and EPS
- ABS and ASA terpolymers
- Polycarbonate (PC) and PC blends
- Polyethylene terephthalate (PET) and PBT
- Polyamides (PA6, PA66)
- Polyoxymethylene (POM), PMMA, EVA, and thermoplastic elastomers
- Glass-filled and mineral-filled compounds
Testing Items
An ISO 1133 test campaign can include the following measurable outputs, depending on the procedure selected and the client's specification requirements:
- Melt mass-flow rate (MFR), in g/10 min, determined by Procedure A or derived from Procedure B
- Melt volume-flow rate (MVR), in cm³/10 min, determined by Procedure B
- Melt density at the test temperature, calculated as MFR divided by MVR when both values are obtained
- Flow rate ratio (FRR), the ratio of flow rates measured at two different loads at the same temperature, indicating shear sensitivity
- High-load melt index (HLMI), for high-density polyethylene measured under the 21.6 kg condition
Testing Methods
Apparatus and Test Setup
The core instrument is an extrusion plastometer consisting of a vertically mounted, thermostatically controlled steel barrel, a close-fitting piston with detachable weights, and a standardized capillary die. The standard die has a bore of 2.095 mm diameter and 8.000 mm length; an optional small-bore die of 1.050 mm is available for very low-flow materials. Barrel temperature is maintained within ±0.5 °C and verified with a calibrated temperature probe before testing. The piston and weight set provide nominal total masses such as 0.325 kg, 1.00 kg, 1.20 kg, 2.16 kg, 3.80 kg, 5.00 kg, 10.00 kg, and 21.60 kg.
Sample Preparation and Conditioning
Hygroscopic polymers such as PET, PBT, polycarbonate, and polyamides must be dried before testing, because moisture causes hydrolytic degradation and bubble formation, both of which distort the measured flow rate. The conditioned charge is introduced into the barrel within one minute, compacted to eliminate air pockets, and preheated for the standard period, typically four to five minutes, before the load is applied and measurement begins.
Procedure A - Mass Determination Method
In the classic cut-and-weigh method, timing starts when the piston descends to the lower reference mark. The extrudate emerging from the die is cut off at uniform timed intervals, with the first cut containing start-up effects discarded. The collected segments are weighed to the nearest milligram, and the melt mass-flow rate is calculated. Cut intervals are selected according to the expected flow rate, from approximately 10 seconds for high-flow materials to several minutes for very low-flow grades. A minimum of three valid segments should agree closely for the result to be accepted.
Procedure B - Piston Displacement Method
In the displacement method, an optical or magnetic sensor tracks the distance the piston travels within a measured time window. Because the cross-sectional area of the system is known, the extruded volume is calculated directly, yielding the melt volume-flow rate without cutting or weighing. Procedure B is faster, less operator-dependent, and well suited to automated testing and to high-flow materials. If the melt density is known or simultaneously determined, MFR can be derived from MVR.
Standard Test Conditions
The temperature and load must match the conditions specified for the material in ISO 1133-2, and they must also match the conditions printed on the supplier datasheet for any comparison to be valid. Commonly applied conditions include:
- Polyethylene (PE): 190 °C / 2.16 kg, or 190 °C / 21.6 kg for the high-load melt index
- Polypropylene (PP): 230 °C / 2.16 kg
- Polystyrene (PS): 200 °C / 5.00 kg
- ABS: 220 °C / 10.00 kg
- Polycarbonate (PC): 300 °C / 1.20 kg
- PET: 250 °C / 2.16 kg (with mandatory drying)
- Polyoxymethylene (POM): 190 °C / 2.16 kg
- PMMA: 230 °C / 3.80 kg
Calculation and Reporting
The melt mass-flow rate is calculated as MFR = 600 × m / t, where m is the average mass of the extruded segment in grams and t is the cut interval in seconds. The melt volume-flow rate is calculated as MVR = 600 × V / t, where V is the extruded volume derived from piston travel. When both quantities are measured, the melt density follows from MFR divided by MVR. The test report must state the standard used, the procedure, the temperature and load, the die geometry, any preconditioning, the individual and mean results, and the units of expression.
Applications
ISO melt flow index testing supports decision-making across the entire thermoplastics value chain:
- Quality control and lot release against datasheet specifications during resin production
- Incoming inspection of purchased resins and compounds by converters
- Grade identification and differentiation when comparing candidate materials
- Process window definition for injection molding, extrusion, blow molding, and fiber spinning
- Qualification of recycled feedstocks and detection of thermal or hydrolytic degradation
- Research and development studies on compounding, additive packages, peroxide visbreaking, and polymer modification
- Failure analysis of returned or field-failed parts by comparing their flow rate with that of reference material
Because the measurement is fast, requires only a few grams of material, and uses relatively inexpensive equipment, it is often the first characterization test performed on a thermoplastic and serves as a screening tool before more sophisticated rheological analysis, such as capillary or rotational rheometry, is undertaken.
FAQ
What is the difference between MFR and MVR?
MFR is the mass of polymer extruded in 10 minutes expressed in grams, obtained directly by weighing the extrudate. MVR is the volume extruded in 10 minutes expressed in cubic centimeters, obtained from piston displacement. The two are linked by the melt density at the test temperature: MFR equals MVR multiplied by melt density.
How does ISO 1133 differ from ASTM D1238?
Both standards use a similar extrusion plastometer and the same conceptual approach, but they differ in details such as barrel and die tolerances, temperature calibration requirements, preheating periods, and data reduction rules. Results may deviate slightly, particularly for very high or very low flow materials, so ISO and ASTM values should not be compared interchangeably without verification.
How much sample is required for testing?
A single determination typically consumes 3 g to 8 g of material. Because the standard requires replicate measurements and possible repeat runs, laboratories generally request at least 20 g to 50 g of sample to guarantee a reliable reported value.
Which temperature and load should be used for my material?
The correct combination is dictated by ISO 1133-2 according to the polymer family, and it must correspond to the conditions under which the supplier's datasheet value was reported. Testing at any other condition produces a valid measurement of that condition only and cannot be compared with catalogue data.
What does a high melt flow index indicate?
A high MFI indicates lower melt viscosity, which generally corresponds to lower average molecular weight and easier processing at lower pressures. It is typically associated with easier filling of thin-walled moldings, while a low MFI indicates higher molecular weight, greater melt strength, and better mechanical performance in many applications.
Why must hygroscopic polymers be dried before testing?
Moisture in materials such as PET, PBT, PC, and nylon causes hydrolytic chain scission at test temperatures, artificially inflating the flow rate, and can also generate bubbles in the extrudate that distort mass and volume measurements. Drying to the recommended residue moisture level is therefore essential for accurate and repeatable results.
Can filled and reinforced compounds be tested?
Yes, glass-filled and mineral-filled compounds are routinely tested, provided the specified conditions for the base polymer are applied. However, abrasive fillers accelerate die wear, so die geometry must be inspected and recalibrated regularly to preserve long-term accuracy.
What repeatability can be expected?
Within a single laboratory, following ISO 1133 with calibrated equipment and trained operators, typical repeatability is within a few percent of the mean value. Between-laboratory reproducibility is somewhat wider, which is why consistent apparatus maintenance, temperature verification, and strict adherence to the specified procedure are critical for interlaboratory comparisons.