Melt mass-flow rate (MFR) determination is one of the most widely performed physical tests for thermoplastics, providing a fast and reproducible measure of flowability that reflects average molecular weight and processability. Conducted with an extrusion plastometer in accordance with ISO 1133-1, ASTM D1238, and GB/T 3682.1, the method is essential for incoming material inspection, grade identification, and lot-to-lot quality control. This article explains the test principle, apparatus, sample requirements, operating procedures, and typical applications of MFR measurement.
Technical Overview
Melt mass-flow rate (MFR), formerly known as melt flow index (MFI) or melt index (MI), is defined as the mass of a thermoplastic material, in grams, extruded through a standardized die over a period of 10 minutes under a prescribed temperature and piston load. The result is expressed in g/10 min. A companion quantity, melt volume-flow rate (MVR), is expressed in cm³/10 min and can be converted to MFR using the melt density at the test temperature.
The measurement is carried out with an extrusion plastometer (melt flow indexer), which consists of a heated vertical barrel, a weighted piston, and a calibrated die with a bore of 2.095 mm and a length of 8.000 mm. Under the combined effect of temperature and load, the molten polymer is extruded through the die, and either the mass of the extrudate (Method A) or the piston displacement (Method B) is recorded over a defined time interval.
MFR is inversely related to melt viscosity and therefore to average molecular weight: a higher MFR indicates a lower-molecular-weight, more easily flowing resin, while a lower MFR indicates a stiffer, more viscous melt. Because the test is quick, inexpensive, and highly standardized, it serves as the primary flow-quality parameter on most thermoplastic datasheets and purchase specifications worldwide.
- Rapid indicator of melt viscosity and average molecular weight
- Primary acceptance criterion in incoming-material and release testing
- Applicable to most thermoplastics, including polyolefins, styrenics, engineering resins, and recycled grades
- Standardized in ISO 1133-1/-2, ASTM D1238, and GB/T 3682.1/-2
Testing Samples
Compatible Material Types
MFR determination is applicable to the majority of thermoplastic materials that can be melted without rapid degradation at the test temperature. Typical material families include:
- Polyolefins: LDPE, LLDPE, HDPE, PP, PB, and their copolymers
- Styrenics: PS, HIPS, EPS, ABS, SAN, ASA
- Engineering thermoplastics: PC, POM, PMMA, PBT, PET, PA6, PA66
- Others: EVA, TPU, PPS, blends, filled or reinforced compounds, and recyclates
Sample Forms and Preparation
Samples may be supplied as pellets, granules, powder, or small strips cut from film, sheet, or molded parts. Powder and film samples should be charged carefully to avoid bridging or air pockets in the barrel. The required charge mass is typically 3 to 8 g per determination, depending on the expected flow rate: low-flow materials require more mass to obtain measurable extrudate within the timing window.
Hygroscopic resins such as PA, PC, PET, PBT, TPU, and PMMA must be dried in accordance with the material supplier's recommendation before testing. Residual moisture causes bubble formation in the extrudate, hydrolytic degradation, and falsely elevated flow values. Heat- or shear-sensitive materials and materials with significant time-dependent thermal history should be tested in accordance with ISO 1133-2, which imposes stricter control of preheat time and temperature exposure.
Testing Items
Melt Mass-Flow Rate (MFR)
The primary reported result, in g/10 min, always quoted together with the test condition, for example "MFR (190 °C / 2.16 kg) = 12.3 g/10 min". Comparison of values is only valid when the temperature, load, and test method are identical.
Melt Volume-Flow Rate (MVR)
Measured in cm³/10 min by piston-displacement measurement. MVR is independent of cut-timing errors and is preferred in automated instruments and for materials where the extrudate is difficult to cut or weigh accurately. When both quantities are measured simultaneously, the melt density at the test temperature can be derived from MFR divided by MVR.
Flow Rate Ratio (FRR)
The ratio of MVR (or MFR) values measured at the same temperature but different loads, for example 190 °C / 21.6 kg versus 190 °C / 2.16 kg. FRR provides an indication of shear sensitivity and is a qualitative marker of molecular-weight distribution: broad-distribution resins tend to show higher FRR values.
Typical Specified Test Conditions
- PE: 190 °C / 2.16 kg; high-load melt index for HDPE: 190 °C / 21.6 kg
- PP: 230 °C / 2.16 kg
- PS: 200 °C / 5.0 kg
- ABS: 220 °C / 10 kg
- PMMA: 230 °C / 3.8 kg
- POM: 190 °C / 2.16 kg
- PC: 300 °C / 1.2 kg
- PET: 280 °C / 2.16 kg
The applicable condition is defined by the material specification or the relevant product standard and must never be changed arbitrarily, because MFR values obtained under different conditions are not interchangeable.
Testing Methods
Apparatus Requirements
The extrusion plastometer comprises a vertically mounted steel barrel with an internal diameter of 9.550 ± 0.025 mm, a piston fitted with a guided head and displacement reference marks, a die with a bore of 2.095 ± 0.005 mm and a land length of 8.000 ± 0.025 mm, a set of calibrated load masses, and temperature control capable of holding the set point within a fraction of one degree along the working zone. Instrument calibration includes barrel and die dimensional verification, temperature mapping, and load verification, and should be performed periodically by accredited third-party laboratories or qualified metrology personnel.
Procedure A – Cut-Off (Mass) Method
A pre-weighed charge of 3 to 8 g is introduced into the barrel within about 1 minute, the piston is inserted, and the assembly is allowed to thermally stabilize for a preheat period, typically around 5 minutes per ISO practice or 7 minutes per ASTM practice, extended for temperatures above 300 °C. The load is then applied, an initial purge extrudate is discarded, and successive timed extrudate segments are cut off at the die exit with a cutting tool. Each segment is weighed, and the MFR is calculated as the extruded mass multiplied by 600 and divided by the cutting interval in seconds. At least three valid cut intervals are required, and the scatter between values must fall within the repeatability limits of the standard.
Procedure B – Displacement (Volume) Method
Instead of cutting and weighing extrudate, the instrument measures the time required for the piston to travel a defined distance between reference marks, using a mechanical flag or an optical/electronic encoder. MVR is calculated from the piston cross-sectional area, the travel distance, and the measured time, and MFR is obtained by multiplying MVR by the melt density. Method B eliminates operator cutting errors, enables fully automatic testing, and allows continuous monitoring of flow stability over the measurement window.
Procedure C – Automatic and Half-Height Method
ASTM D1238 defines Procedure C for automatic timed-flow measurement, including the half-height (half-die) technique for very high-flow materials above approximately 75 g/10 min. In this variant, a die of half the standard length is used to extend the measurable flow range, and results are reported with the appropriate half-height designation to distinguish them from standard-die values.
Key Parameters Affecting Accuracy
- Temperature accuracy and stability along the barrel, verified by calibrated sensors
- Die and bore cleanliness and freedom from deposits, scratches, and polymer residue
- Correct load selection and load-mass calibration
- Adequate drying of hygroscopic resins and prevention of oxidative degradation during preheat
- Consistent charging technique, timing precision, and careful cutting of extrudate segments
- Extrudate integrity: segments containing bubbles must be discarded
It should be recognized that MFR is measured at a single, low-shear condition and does not describe the full viscosity curve. For materials intended for high-shear processing, or when flow behavior across a shear range is needed, MFR should be complemented with capillary or rotational rheometry.
Applications
MFR determination supports decisions across the entire polymer value chain, from resin production to finished-part verification:
- Incoming material inspection: verification that purchased resin matches the datasheet grade and contractual specification
- Manufacturing quality control: monitoring of lot-to-lot consistency in polymerization, compounding, and masterbatch production
- Grade selection: distinguishing high-flow injection molding grades from low-flow extrusion, blow molding, and pipe grades
- Degradation assessment: comparison of MFR before and after processing, drying, regranulation, thermal aging, or sterilization to detect chain scission or crosslinking
- Recycled material characterization: flow verification of regrind and recyclates, often combined with other property tests in accredited third-party laboratories
- Research and development: screening of formulations, stabilizer systems, peroxide visbreaking, and reactive modifiers
- Regulatory and specification compliance: supporting material declarations and product standards referencing flow limits
FAQ
What is the difference between MFR, MFI, and melt index?
These terms refer to essentially the same property. MFI (melt flow index) and MI (melt index) are legacy designations, while MFR (melt mass-flow rate) is the standardized terminology of ISO 1133 and ASTM D1238. Modern test reports should use MFR together with the exact temperature and load condition.
Does a higher MFR mean better quality?
Not necessarily. A higher MFR simply indicates a lower-viscosity, lower-average-molecular-weight melt that fills thin-wall molds more easily but may offer lower mechanical strength and melt strength. The correct MFR is the one specified for the intended process and application.
Which method should be chosen, Method A or Method B?
Method A is direct, suitable for routine manual testing, and does not require melt density data. Method B is faster, fully automatable, and more precise for very high or very low flow rates. When the two are compared, agreement within the reproducibility of the standards is expected, provided identical conditions are used.
Are ISO 1133 and ASTM D1238 results directly comparable?
The instruments and principle are the same, but the standards differ in details such as preheat time, temperature tolerances, and data-handling rules. For critical comparisons, the report should state which standard was followed, and laboratories should avoid mixing values obtained under different protocols without qualification.
Can powders, films, or finished parts be tested?
Yes. Powders and cut film strips are charged with care to prevent bridging, and parts are cut into small pieces before charging. The source form of the sample should be recorded in the report, as processing history may influence the measured flow rate.
Why must hygroscopic resins be dried before testing?
Moisture in resins such as PA, PC, PET, and PBT generates bubbles in the extrudate and can trigger hydrolytic degradation during heating, producing fragmented chains and falsely high flow values. Controlled drying according to supplier guidelines is a prerequisite for valid results.
What information should a complete MFR test report contain?
A compliant report identifies the standard and method used, the test temperature and load, individual and mean flow values, sample identification and conditioning, any deviations, and the date of test, ensuring traceability and comparability of results between laboratories.