Melt Index (MFI/MFR) Testing for New Materials: Standards, Methods, and Applications

Melt index determination is one of the most fundamental rheological tests for characterizing the flow behavior of thermoplastic and advanced polymer materials. Performed to internationally recognized standards such as ISO 1133 and ASTM D1238, melt flow rate (MFR/MVR) testing provides a rapid, cost-effective indicator of molecular weight, processability, and batch-to-batch consistency. This article explains the principle, sample requirements, test procedures, and practical applications of melt index measurement for new materials.

Technical Overview

Melt index determination—also known as melt flow index (MFI), melt mass-flow rate (MFR), or melt volume-flow rate (MVR) testing—is a single-point rheological measurement that quantifies the ease of flow of a molten thermoplastic polymer. The test is conducted using an extrusion plastometer, which consists of a temperature-controlled heated barrel, a weighted piston, and a calibrated capillary die with a bore of 2.0955 mm and a length of 8.000 mm. Under a specified combination of temperature and load, the molten material is extruded through the die, and the mass or volume of extrudate collected over a defined time interval is used to calculate the flow rate.

MFR is expressed in grams per 10 minutes (g/10 min), while MVR is expressed in cubic centimeters per 10 minutes (cm³/10 min). Because melt viscosity is inversely related to molecular weight, the melt index serves as an indirect but highly practical indicator of average molecular chain length: a higher melt index generally corresponds to a lower molecular weight and easier flow, whereas a lower melt index indicates a higher molecular weight and greater melt strength. For new materials—including biodegradable polymers, high-performance engineering plastics, recycled compounds, and modified formulations—melt index measurement is a critical parameter for research, development, quality control, and material certification.

The principal standards governing this test include ISO 1133-1 and ISO 1133-2, ASTM D1238, GB/T 3682.1 and GB/T 3682.2, and JIS K 7210. Accredited third-party laboratories routinely perform melt index testing to these standards to support supplier qualification, incoming inspection, product release, and regulatory compliance.

Testing Samples

Polyolefins and General-Purpose Thermoplastics

• Polyethylene (LDPE, LLDPE, HDPE)
• Polypropylene (homopolymer, random and impact copolymers)
• Polystyrene, high-impact polystyrene (HIPS)
• ABS and SAN resins
• Polyvinyl chloride and thermoplastic polyurethane (TPU)

Engineering and High-Performance Polymers

• Polycarbonate (PC)
• Polyamide (PA6, PA66) and glass-filled nylon compounds
• Polyoxymethylene (POM), PBT, PET
• Polyphenylene sulfide (PPS), polyetheretherketone (PEEK), and fluoropolymers
• Polymethyl methacrylate (PMMA)

New and Advanced Materials

• Biodegradable and bio-based polymers such as PLA, PBAT, PBS, and PHA
• Recycled and regranulated thermoplastics
• Filled, reinforced, and modified compounds, including masterbatches and nanocomposites
• Blends and alloys developed for specialty applications

Sample Forms and Conditioning

Acceptable sample forms include pellets, granules, powder, film or sheet cuttings, and ground material from finished parts. Hygroscopic materials such as PA, PC, PET, PBT, TPU, and PLA must be dried prior to testing in accordance with the relevant material specification or ISO 1133-2, because residual moisture causes hydrolytic degradation that artificially inflates the melt index and may create unsafe sputtering in the barrel. Typical sample consumption is approximately 3 to 8 grams per determination, depending on the expected flow rate.

Testing Items

• Melt Mass-Flow Rate (MFR): the mass of extrudate in grams expelled in 10 minutes, reported in g/10 min
• Melt Volume-Flow Rate (MVR): the volume of extrudate in cm³ expelled in 10 minutes, reported in cm³/10 min
• Melt Density: the density of the molten polymer at the test temperature, required for MFR–MVR conversion and Procedure B calculations
• Flow Rate Ratio (FRR): the ratio of flow rates measured at two different loads, providing insight into shear sensitivity and molecular weight distribution

Testing Methods

Procedure A: Cut-and-Weigh (Mass-Based) Method

In Procedure A, the operator charges a pre-weighed quantity of sample into the barrel, allows a defined preheating period of approximately five minutes at the set temperature, applies the specified load, and then cuts and collects timed segments of extrudate. The segments are weighed, and the MFR is calculated directly from the collected mass and measured time interval. Procedure A does not require knowledge of melt density and remains the reference approach for many quality control applications.

Procedure B: Displacement (Volume-Based) Method

In Procedure B, an automatic transducer tracks the piston travel distance during the test. The MVR is calculated from the piston displacement, elapsed time, and known barrel geometry. MFR can subsequently be derived if the melt density is determined or assumed. Procedure B offers improved repeatability and reduced operator dependence and is emphasized in the current edition of ISO 1133-1.

Standard Test Conditions

The test temperature and load are selected according to the material specification or the standardized condition tables within ISO 1133 and ASTM D1238. Commonly used condition pairs include:

• Polyethylene: 190 °C with 2.16 kg or 21.6 kg (high-load condition for high-viscosity HDPE)
• Polypropylene: 230 °C with 2.16 kg
• Polystyrene and ABS: 200 °C with 5.0 kg, or 220 °C with 10 kg
• Polycarbonate: 300 °C with 1.2 kg
• PBT: 250 °C with 2.16 kg
• Biodegradable polymers such as PLA: 190 °C with 2.16 kg

Procedure for Materials Sensitive to Time-Temperature History or Moisture

For low-flow materials or polymers whose rheology is highly sensitive to thermal exposure and moisture, ISO 1133-2 specifies a half-dimension die and shortened preheating to minimize thermal and hydrolytic degradation during measurement.

Test Procedure Summary

• Clean and preheat the barrel and die to the specified temperature, controlled within ±0.5 °C
• Charge the conditioned sample, insert the piston, and observe the required preheat period
• Apply the specified load and allow the extrusion to stabilize, discarding the initial segment as required by the standard
• Collect timed extrudate segments (Procedure A) or record piston displacement (Procedure B)
• Calculate MFR or MVR, verify repeatability between determinations, and report the test conditions in full

Applications

Melt index data are used across the entire lifecycle of new material development and commercialization. Accredited third-party laboratories provide melt flow testing results that are widely accepted for supplier approval, material datasheets, and compliance documentation.

• Incoming resin inspection and supplier qualification for consistent feedstock quality
• Grade identification and differentiation between extrusion, injection molding, blown film, and fiber-spinning grades
• Batch-to-batch consistency control in compounding and masterbatch production
• Evaluation of degradation, recycling effects, and reprocessing history on molecular weight
• Research and development support for modified plastics, controlled-rheology polypropylene, biodegradable formulations, and reactive extrusion processes
• Verification of conformance to purchase specifications, industry standards, and regulatory requirements

FAQ

What is the difference between MFR and MVR?

MFR is a mass-based result expressed in g/10 min, obtained by weighing the extrudate, while MVR is a volume-based result expressed in cm³/10 min, derived from piston displacement. The two values are related through the melt density at the test temperature, and both describe the same flow behavior under identical test conditions.

Which standard should be used: ISO 1133 or ASTM D1238?

ISO 1133 (or the equivalent GB/T 3682 series) and ASTM D1238 describe similar instruments but differ in procedural details, tolerance requirements, and reporting rules. The choice depends on customer specification, end market, and regulatory requirements. Results from different standards should only be compared when the test temperature, load, die geometry, and procedure are identical.

How are the test temperature and load selected?

Temperature and load are specified in the material datasheet or determined from the standardized condition tables in ISO 1133 and ASTM D1238. High-viscosity materials may require high-load conditions (for example, 21.6 kg for HDPE) to obtain a measurable flow rate within the valid measurement window.

What does a higher melt index indicate?

A higher melt index indicates lower melt viscosity, which usually reflects a lower average molecular weight or a degree of degradation. Such materials fill thin-walled molds more easily but may exhibit lower mechanical strength and reduced melt strength for processes such as blown film extrusion.

Why must hygroscopic samples be dried before testing?

Moisture in polymers such as PA, PC, PET, and PLA causes hydrolytic chain scission at test temperatures, leading to an artificially elevated melt index and poor repeatability, as well as potential hazardous spattering. Proper conditioning in line with the material specification or ISO 1133-2 is essential for accurate results.

Can melt index testing fully predict processing behavior?

Melt index is a single-point, low-shear measurement and is excellent for quality control and grade comparison, but real processing often involves much higher shear rates. For comprehensive processability assessment, melt index data are commonly complemented by capillary rheometry in accordance with ISO 11443 or ASTM D3835.