Melt index determination (also known as melt flow index, MFI, or melt mass-flow rate, MFR, testing) is one of the most widely performed quality-control tests for plastic pellets, providing a rapid and economical measure of how a thermoplastic flows in the molten state under standardized conditions. This article explains the technical principles, applicable standards such as ISO 1133 and ASTM D1238, test methods, sample requirements, and industrial applications of melt index measurement for plastic granules, helping material engineers and quality professionals interpret and apply test results correctly.
Technical Overview
Melt index determination quantifies the flowability of a molten thermoplastic by measuring the quantity of material extruded through a standardized die over a period of ten minutes under a prescribed temperature and piston load. The result is expressed either as melt mass-flow rate (MFR) in grams per ten minutes (g/10 min) or as melt volume-flow rate (MVR) in cubic centimeters per ten minutes (cm³/10 min). Because the measured flow rate correlates inversely with melt viscosity and average molecular weight, the melt index is widely used as a quick indicator of a resin grade's processability and batch-to-batch consistency.
The test is performed on a melt flow indexer, also called an extrusion plastometer. The instrument consists of a heated cylindrical barrel with an internal diameter of approximately 9.550 mm, a weighted piston, and a calibrated tungsten carbide or steel die with a bore of 2.0955 mm and a length of 8.000 mm. A pre-weighed charge of plastic pellets is loaded into the barrel, allowed to melt and thermally stabilize, and then forced through the die by the loaded piston. The extrudate is collected either by timed cut-offs or by automatic piston displacement measurement, depending on the selected procedure.
Melt index determination for plastic pellets is generally carried out in accordance with the following standards:
- ISO 1133-1 and ISO 1133-2 (determination of melt mass-flow rate and melt volume-flow rate of thermoplastics)
- ASTM D1238 (standard test method for melt flow rates of thermoplastics by extrusion plastometer)
- GB/T 3682.1 and GB/T 3682.2 (Chinese national standards, technically aligned with ISO 1133)
- JIS K 7210 (Japanese industrial standard for melt flow rate of thermoplastics)
Testing Samples
Melt index testing is applicable to virtually all thermoplastic resins supplied in pellet or granule form. Typical sample categories include the following:
Polyolefin Pellets
Low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), and polypropylene homopolmers and copolymers are the most frequently tested materials. Polyethylene is commonly evaluated at 190 °C with a 2.16 kg load, while high-load conditions such as 21.6 kg are used for high-molecular-weight HDPE grades. Polypropylene is typically tested at 230 °C with a 2.16 kg load.
Styrenic Resins
General-purpose polystyrene (GPPS), high-impact polystyrene (HIPS), acrylonitrile butadiene styrene (ABS), styrene acrylonitrile (SAN), and ASA pellets are routinely characterized. Polystyrene is commonly tested at 200 °C with a 5.00 kg load, and ABS at 220 °C with a 10.0 kg load.
Engineering Thermoplastics
Polycarbonate (PC), polyamide 6 and 66 (PA6, PA66), polyoxymethylene (POM), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polymethyl methacrylate (PMMA), polyphenylene sulfide (PPS), and polyetheretherketone (PEEK) require higher test temperatures and, for hygroscopic resins, careful predrying. Corrosion-resistant barrel and die materials are recommended for fluoropolymers and chlorine-containing resins.
Compounds, Masterbatches, and Recycled Pellets
Glass-fiber or mineral-filled compounds, color masterbatches, and post-industrial or post-consumer recyclate pellets are also tested to verify flow consistency, assess degradation during reprocessing, and confirm suitability for the intended conversion process.
Testing Items
Depending on the selected standard and procedure, the following quantities can be reported for plastic pellets:
- Melt mass-flow rate (MFR), expressed in g/10 min, determined by collecting and weighing timed extrudate cut-offs
- Melt volume-flow rate (MVR), expressed in cm³/10 min, determined by automatic measurement of piston displacement
- Melt density at the test temperature, in g/cm³, obtained by combining displacement and mass measurements
- Flow rate ratio (FRR), the ratio of flow rates measured at two different loads, which characterizes the shear sensitivity of the resin
- Repeatability assessment across multiple charges to evaluate pellet homogeneity and measurement precision
Testing Methods
Two principal procedures are defined by ISO 1133-1 and ASTM D1238, and both are routinely performed by accredited third-party laboratories and in-house quality-control laboratories.
Procedure A (cut-and-weigh method): the operator collects extrudate segments during a defined time interval, weighs them on an analytical balance, and calculates MFR from the collected mass and the interval time. This classical method does not require knowledge of melt density and remains the reference approach for most material specifications.
Procedure B (automatic displacement method): an optical or magnetic displacement transducer tracks the piston travel, and MVR is calculated from the volume extruded over the measured time. When melt density is known or concurrently determined, MFR is derived from MVR. Procedure B eliminates operator cutting errors, shortens test time, and improves repeatability, particularly for high-flow resins.
Standard Test Conditions
The temperature and load are specified according to the material grade and must be strictly controlled, since flow rate is highly sensitive to both parameters. Commonly used conditions include:
- Polyethylene (PE): 190 °C / 2.16 kg, or 190 °C / 21.6 kg for high-load melt index
- Polypropylene (PP): 230 °C / 2.16 kg
- Polystyrene (PS): 200 °C / 5.00 kg
- ABS: 220 °C / 10.0 kg
- Polycarbonate (PC): 300 °C / 1.20 kg
- Polyoxymethylene (POM): 190 °C / 2.16 kg
- PET and PBT: 250 °C / 2.16 kg
- PMMA: 230 °C / 3.80 kg
General Test Procedure
- Dry hygroscopic pellets (for example PA, PC, PET, ABS) according to the material specification to suppress moisture-induced degradation and bubbling of the extrudate
- Charge approximately 3 to 8 grams of pellets into the preheated barrel and compact the charge with the piston to eliminate air pockets
- Allow a defined preheating and thermal stabilization period, typically five to seven minutes depending on the standard and material
- Apply the specified load so the piston descends and molten polymer extrudes through the die
- Discard the initial extrudate segment and collect measurements over the valid piston travel region, excluding the beginning and end of the charge where flow instability occurs
- Calculate MFR or MVR, average at least two or three valid determinations, and verify that the repeatability meets the standard's tolerance
- Clean the piston, barrel, and die thoroughly with an appropriate cleaning tool after each test to prevent residue affecting subsequent results
It should be noted that the melt index represents a single low-shear flow point and cannot fully describe the shear-rate-dependent viscosity of a polymer. For comprehensive rheological characterization, melt index data are often complemented by capillary rheometry or rotational rheometry.
Applications
Melt index determination of plastic pellets supports critical decisions across the entire polymer value chain:
- Incoming material inspection: verifying that supplied pellets conform to the purchase specification and datasheet flow value
- Lot release and certification: providing standardized flow data for quality certificates and customer documentation
- Grade identification and differentiation: distinguishing resin grades that differ in molecular weight but appear visually identical
- Process window definition: guiding temperature, pressure, and cycle-time settings for injection molding, extrusion, blow molding, blown film, and fiber spinning
- Degradation monitoring: detecting molecular weight loss caused by excessive thermal history, reprocessing, or improper drying
- Recyclate characterization: assessing the flow quality of regrind and post-consumer recyclate before blending with virgin resin
- Research and development: evaluating the effects of additives, fillers, peroxide visbreaking, and compounding conditions on melt flow behavior
FAQ
What is the difference between MFR and MVR?
MFR (melt mass-flow rate) is the mass of extrudate collected in a given time, expressed in g/10 min, while MVR (melt volume-flow rate) is the corresponding volume, expressed in cm³/10 min. The two values are related through the melt density at the test temperature, so MFR equals MVR multiplied by melt density.
How do ISO 1133 and ASTM D1238 differ?
Both standards use the same basic apparatus, die dimensions, and load system, but they differ in details such as preheating time, piston design, permitted measurement range, die cleaning practice, and rounding of results. Because results obtained under different standards may not be directly interchangeable, the specification standard should always be stated when reporting melt index values.
What does a high or low melt index indicate?
A high melt index means the material flows easily, corresponding to lower melt viscosity and generally lower average molecular weight, which suits fast-cycle injection molding. A low melt index indicates higher viscosity and molecular weight, favoring extrusion, blow molding, blown film, and pressure-pipe applications requiring high melt strength.
How much sample is required for one test?
A single determination typically consumes about 3 to 8 grams of pellets depending on the expected flow rate. Laboratories generally request 30 to 50 grams to allow conditioning, multiple determinations for repeatability, and retesting if necessary.
Do hygroscopic pellets need drying before testing?
Yes. Resins such as PA, PC, PET, PBT, and ABS absorb moisture that can cause hydrolytic degradation and gas bubbles in the extrudate, leading to erroneous results. These materials should be dried according to the resin producer's recommendations before charging the barrel.
Why is the flow rate ratio (FRR) sometimes measured?
FRR is the ratio of flow rates determined at two different loads, for example 10 kg versus 2.16 kg, at the same temperature. It reflects how strongly the resin's viscosity responds to shear stress, which is valuable information for selecting processing conditions and for comparing polymer architectures such as linear versus long-chain-branched grades.
Can melt index results vary between laboratories?
Small variations can arise from differences in instrument calibration, temperature accuracy, load precision, operator technique, and sampling. Participation in interlaboratory comparisons, strict adherence to the cited standard, and use of calibrated, certified reference materials minimize variability and ensure comparable results.