Determination of Melt Flow Index for Plastic Raw Materials: Principles, Standards, and Testing Proce

The melt flow index (MFI) test is one of the most widely used quality control methods for characterizing the flow behavior and average molecular weight of thermoplastic raw materials. Performed with an extrusion plastometer in accordance with ISO 1133, ASTM D1238, and GB/T 3682, melt index determination provides manufacturers with a fast, economical, and reproducible measure of processability for incoming resin inspection, batch consistency verification, and process optimization.

Technical Overview

The melt flow index, also referred to as the melt flow rate (MFR) or simply melt index (MI), quantifies the ease with which a molten thermoplastic flows through a standardized die under a specified temperature and load. The result is expressed as the mass of polymer extruded in ten minutes (g/10 min) or as the extruded volume per ten minutes (cm³/10 min, termed melt volume-flow rate or MVR).

Because melt viscosity correlates inversely with molecular weight, the melt flow index serves as an indirect but highly practical indicator of the average molecular mass of a polymer. A higher melt index indicates a lower molecular weight, lower melt viscosity, and easier flow—typically suited to injection molding. A lower melt index indicates a higher molecular weight and greater melt strength, which is preferred for extrusion, blow molding, and film applications.

Testing is conducted using an extrusion plastometer (melt flow indexer), which consists of a heated cylindrical barrel, a weighted piston, and a calibrated die of standardized dimensions. The instrument, the test conditions, and the calculation methods are strictly defined by international and national standards to ensure inter-laboratory comparability.

Testing Samples

Melt flow index determination is applicable to most thermoplastic materials. Typical sample forms include virgin pellets, granules, powders, reground materials, and samples taken from finished parts that can be reduced to a suitable size. Common material families submitted for testing include:

Polyolefins

Polyethylene (LDPE, LLDPE, HDPE), polypropylene (PP, including impact copolymers and random copolymers), and polybutene. Polyolefins represent the largest share of melt index testing volume worldwide, with well-established standard conditions such as 190 °C / 2.16 kg for polyethylene and 230 °C / 2.16 kg for polypropylene.

Styrenic Resins

General-purpose polystyrene (GPPS), high-impact polystyrene (HIPS), acrylonitrile butadiene styrene (ABS), and styrene-acrylonitrile copolymer (SAN), commonly tested at 200 °C with a 5.0 kg load.

Engineering Thermoplastics

Polycarbonate (PC), polyamide (PA6, PA66), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polymethyl methacrylate (PMMA), polyoxymethylene (POM), and polyphenylene sulfide (PPS). These materials typically require higher test temperatures, and moisture-sensitive polymers such as PA and PET must be dried before testing to prevent hydrolytic degradation and inaccurate results.

Recycled and Compound Materials

Regrind, regranulated post-consumer or post-industrial recyclates, and filled or reinforced compounds, where melt index verification is essential for confirming material identity and detecting degradation caused by reprocessing.

Testing Items

A comprehensive melt flow characterization program typically includes the following measurable parameters:

• Melt mass-flow rate (MFR), expressed in g/10 min, determined gravimetrically by cutting and weighing extrudate segments collected over defined time intervals.
• Melt volume-flow rate (MVR), expressed in cm³/10 min, determined from piston travel distance using an automated displacement transducer.
• Melt density at test temperature, calculated from paired MFR and MVR measurements.
• Flow rate ratio (FRR), obtained by testing the same material under two different loads (for example, 2.16 kg and 10 kg), which provides insight into shear sensitivity and molecular weight distribution.
• Verification of conformance to supplier datasheet values or purchase specification limits.

Testing Methods

The test is performed by charging a measured quantity of sample into the preheated barrel of the plastometer, inserting the piston, allowing a specified preheat period (commonly around 4 to 5 minutes with occasional packing for powders), applying the standard load, and collecting the extruded strand either by timed cutting (Procedure A) or by monitoring piston displacement (Procedure B).

Governing Standards

• ISO 1133-1: Determination of the melt mass-flow rate (MFR) and melt volume-flow rate (MVR) of thermoplastics, Part 1: Standard method.
• ASTM D1238: Standard Test Method for Melt Flow Rates of Thermoplastics by Extrusion Plastometer.
• GB/T 3682.1: Chinese national standard, technically aligned with ISO 1133-1.
• JIS K 7210: Japanese industrial standard for melt flow rate of thermoplastics.

Procedure A — Mass Determination

The operator cuts extrudate segments at precisely timed intervals after the prescribed purge period, then weighs the cuttings. MFR is calculated from the extruded mass and collection time. This gravimetric procedure is the classical reference method and is suitable for all materials.

Procedure B — Volume Determination

An automatic device measures the distance the piston travels over a defined time or the time required for a defined distance. The result is reported as MVR and, if melt density is known or measured, can be converted to MFR. Procedure B reduces operator influence and is well suited to automated, high-throughput testing.

Standard Test Conditions

Temperature and load combinations are specified per material. Widely used conditions include 190 °C / 2.16 kg for PE, 190 °C / 21.6 kg for high-load HDPE flow assessment, 230 °C / 2.16 kg for PP, 200 °C / 5.0 kg for PS and ABS, 300 °C / 1.2 kg for PC, and 275 °C or 300 °C / 2.16 kg for PET depending on grade. Selecting the correct condition is critical, as results obtained under different conditions are not interchangeable.

Quality Controls and Best Practices

• Verify barrel temperature stability within the tolerance band specified by the standard before charging the sample.
• Purge and clean the barrel, piston, and die thoroughly between tests to eliminate cross-contamination and residue-induced errors.
• Pre-dry hygroscopic polymers (PA, PET, PC, PBT) using appropriate drying conditions to avoid moisture-induced degradation.
• Perform at least three valid determinations per sample and confirm that the scatter between readings falls within the repeatability limits of the standard.
• Ensure the testing is performed by qualified personnel in accredited third-party laboratories or properly calibrated in-house facilities with traceable weights, certified dies, and regularly calibrated temperature sensors.

Applications

Melt flow index data support decision-making across the entire polymer value chain:

• Incoming material inspection: verifying that purchased resin lots match the melt index grade declared on the certificate of analysis before production release.
• Resin grade selection: distinguishing between injection, extrusion, blow molding, and film grades when specifying materials for a given processing method.
• Process troubleshooting: investigating flow defects such as short shots, flash, sink marks, or poor extrusion output by confirming whether the material flow properties have shifted.
• Quality surveillance: monitoring batch-to-batch consistency and detecting polymer degradation caused by excessive regrind content, improper drying, or thermal abuse during processing.
• Recycling qualification: characterizing recyclates to confirm their suitability for defined applications and blend ratios.
• Research and development: comparing experimental polymerization results, peroxide-controlled rheology modifications, and additive effects on melt behavior.
• Supplier audits and commercial dispute resolution: providing objective, standardized evidence of material conformity.

FAQ

What is the difference between MFI, MFR, and MVR?

MFI (melt flow index) and MFR (melt mass-flow rate) refer to the same gravimetric quantity expressed in g/10 min, with MFR being the preferred modern term in ISO and ASTM standards. MVR (melt volume-flow rate) is the volumetric equivalent expressed in cm³/10 min, measured by piston displacement and converted to mass flow when melt density is known.

How does melt index relate to molecular weight and mechanical properties?

Melt index is inversely related to weight-average molecular weight and melt viscosity. In general, lower melt index materials exhibit higher toughness, better environmental stress crack resistance, and greater melt strength, while higher melt index materials fill thin-walled molds more easily but may sacrifice mechanical performance.

How much sample is required for a test?

A single determination typically consumes 3 to 8 grams of material depending on the expected flow rate, and a complete report with replicate determinations generally requires approximately 20 to 50 grams of pellets or granules.

Are ISO 1133 and ASTM D1238 results directly comparable?

Both standards use similar apparatus, but differences in die dimensions, tolerances, piston mass definitions, and procedural details can cause small systematic deviations, particularly at low flow rates. Results should always be reported together with the standard, temperature, and load used, and comparisons across standards should be made cautiously.

Why can two materials with the same melt index behave differently during processing?

Melt index is a single low-shear point measurement. Processing often occurs at much higher shear rates, so materials with different molecular weight distributions or additive packages may show identical melt indices yet very different actual processing behavior. Measuring the flow rate ratio under two loads or supplementing melt index data with capillary rheometry provides a more complete picture.

What does an out-of-specification melt index result indicate?

An elevated melt index usually signals molecular weight reduction from thermal oxidation, hydrolysis, over-drying damage, excessive regrind, or mislabeled material. An unusually low melt index may indicate contamination with a higher molecular weight resin, crosslinking, or a grade mix-up. Investigation should include verification of test conditions and material identification by complementary methods such as FTIR spectroscopy and DSC.