Determination of Melt Flow Rate of Plastics: MFR/MVR Testing Methods and Applications

Melt flow rate (MFR) determination is one of the most widely performed tests for characterizing the flow properties of thermoplastic materials, providing essential data for quality control, material grading, and process optimization. This technical article explains the principles, standards, sample requirements, testing methods, and industrial applications of plastic melt flow rate measurement in accordance with ISO 1133 and ASTM D1238.

Technical Overview

Melt flow rate (MFR) determination, also known as melt index testing, measures the ease with which a thermoplastic polymer melt flows under a specified temperature and load. The test is performed using an extrusion plastometer (melt flow indexer), in which a measured quantity of molten polymer is extruded through a standardized die by the weight of a piston and a specified load. The result is expressed either as the melt mass-flow rate (MFR) in grams per 10 minutes or the melt volume-flow rate (MVR) in cubic centimeters per 10 minutes.

MFR values are inversely related to melt viscosity and therefore provide an indication of molecular weight: a higher melt flow rate generally corresponds to a lower molecular weight and easier processability, while a lower melt flow rate indicates a higher molecular weight and greater melt strength. Because the test is rapid, relatively inexpensive, and highly reproducible when performed under standardized conditions, it has become the primary acceptance criterion in resin specifications, incoming material inspection, and production quality control for commodity and engineering thermoplastics.

The principal standards governing this test include ISO 1133-1 and ISO 1133-2 (Conditions for measuring melt flow rate and melt volume-flow rate of thermoplastics), ASTM D1238 (Standard Test Method for Melt Flow Rates of Thermoplastics by Extrusion Plastometer), and GB/T 3682.1 and GB/T 3682.2 for the Chinese national equivalent. These standards define the apparatus geometry, temperature and load conditions, measurement procedures, and reporting requirements.

Testing Samples

Pelletized Resins

The most common sample form is virgin resin pellets supplied directly from polymer producers or compounders. Pellets should be representative of the lot and free from contamination. Typically, 3 to 8 grams of material are required for a single determination, with additional material recommended for repeat measurements.

Powder and Granular Materials

Powdered polymers, such as certain grades of polyethylene produced by slurry processes, PVC powders, and rotomolding grades, can be tested directly. Care must be taken to ensure uniform packing of the powder in the barrel to avoid voids that would distort the measured flow rate.

Finished Parts and Regrind

Material cut or milled from finished products, runners, sprues, or regrind streams may be tested to verify material identity, assess degradation after processing, or monitor recyclate quality. Samples should be reduced to a size that fits freely into the barrel, and any non-polymeric attachments, coatings, or inserts must be removed beforehand.

Hygroscopic Materials

Moisture-sensitive polymers such as polycarbonate (PC), polyamide (PA), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), and acrylonitrile-butadiene-styrene (ABS) must be pre-dried before testing in accordance with the relevant material standard or supplier recommendation. Residual moisture causes hydrolytic degradation and bubble formation, leading to falsely elevated and erratic melt flow values.

Testing Items

• Melt Mass-Flow Rate (MFR): the mass of extrudate in grams extruded in 10 minutes under specified conditions of temperature and load, determined by Method A.
• Melt Volume-Flow Rate (MVR): the volume of extrudate in cubic centimeters per 10 minutes, determined by measuring piston travel (Method B) or by weighing combined with melt density.
• Melt Density: the density of the polymer melt at the test temperature, calculated when both MFR and MVR are measured.
• Flow Rate Ratio (FRR): the ratio of flow rates measured at two different loads at the same temperature, used as an indicator of molecular weight distribution and shear sensitivity.
• Material Identification and Lot Consistency Evaluation: comparison of measured values against resin datasheet specifications.

Testing Methods

Apparatus and Test Conditions

The extrusion plastometer consists of a heated steel barrel with an internal diameter of approximately 9.55 mm, a removable standardized die with a bore of 2.095 mm diameter and 8.0 mm length, and a weighted piston that applies the specified load. The barrel temperature is controlled to within ±0.5 °C of the specified condition. Standard condition pairs of temperature and load (including total loads of 1.20 kg, 2.16 kg, 3.80 kg, 5.00 kg, 10.00 kg, and 21.60 kg) are assigned to each polymer family; for example, polyethylene is typically tested at 190 °C with a 2.16 kg or 21.6 kg load, polypropylene at 230 °C with a 2.16 kg load, and polycarbonate at 300 °C with a 1.20 kg load.

Method A: Mass Measurement (Manual Cutting)

In Method A, the operator charges the preheated barrel with the sample, inserts the piston, and allows a specified preheat period (commonly around 5 minutes, with optional packing) for the material to melt. After the designated conditioning time, timed extrudate cuts are collected at defined intervals within the specified measurement window. The collected extrudate segments are weighed, and the MFR is calculated from the extruded mass and the cutting interval. Method A directly yields the melt mass-flow rate and is suitable for materials without an established melt density value.

Method B: Displacement Measurement (Automatic)

In Method B, the MVR is determined from the distance traveled by the piston over a measured time interval, using an automatic displacement transducer, without collecting or weighing the extrudate. MFR can then be calculated from the MVR if the melt density at the test temperature is known. Method B eliminates operator cutting error, provides shorter test cycles, and offers improved precision, particularly for high-flow materials with short extrusion intervals. ISO 1133-2 additionally specifies a semi-automatic procedure for materials sensitive to timed preheat, requiring measurement to begin immediately after loading with minimal thermal contact time.

Procedure Summary

• Clean and preheat the instrument to the specified temperature and stabilize temperature control.
• Charge the barrel with the conditioned sample, insert the piston, and apply the specified load after the required preheat or as dictated by the selected method.
• Collect timed extrudate cuts (Method A) or record piston displacement (Method B) within the valid measurement range, discarding material extruded outside the specified piston travel band.
• Calculate MFR or MVR, average the valid determinations, and report the material, condition code, temperature, load, method used, and individual and mean results.
• Perform at least two, and preferably three, determinations and verify that the scatter between values meets the repeatability limits of the standard.

Applications

Melt flow rate determination is applied across the entire thermoplastics value chain. Polymer producers use it for in-process control of polymerization and compounding, ensuring that production lots fall within the specified melt index window for each grade. Injection molders, extruders, and blow molders rely on incoming MFR inspection to confirm that purchased resin matches the certified specification, because flow rate directly affects cavity filling, cycle time, die pressure, and dimensional stability of finished parts.

Specific applications include grade differentiation of polyolefins such as PP, PE (HDPE, LDPE, LLDPE), and their copolymers; quality assessment of engineering resins including PC, PA, PBT, PET, POM, and ABS; evaluation of degradation in recycled materials and regrind; comparison of flow behavior before and after processing or reprocessing; calculation of flow rate ratio to characterize molecular weight distribution for film blowing, pipe extrusion, and fiber spinning grades; and support of failure analysis when process variations or material mix-ups are suspected. Accredited third-party laboratories routinely perform MFR testing for supplier qualification, lot release, certification to ISO 1133 and ASTM D1238, and dispute resolution between trading parties.

FAQ

What is the difference between MFR and MVR?

MFR is a mass-based result expressed in grams per 10 minutes, obtained by weighing timed extrudate cuts, while MVR is a volume-based result expressed in cubic centimeters per 10 minutes, obtained from piston displacement. The two values are related through the melt density at the test temperature: MFR equals MVR multiplied by melt density.

How much sample is required for a melt flow rate test?

A single determination typically requires approximately 3 to 8 grams of material depending on the expected flow rate, and about 20 grams in total is recommended to allow multiple determinations for repeatability in accordance with the standard.

Why must hygroscopic resins be dried before testing?

Moisture in polymers such as PC, PA, PET, PBT, and ABS causes hydrolytic chain scission and generates bubbles in the melt during heating, which inflates and destabilizes the measured flow rate. Pre-drying ensures that the result reflects the true flow properties of the material rather than moisture-induced degradation.

What does a higher melt flow rate indicate about a polymer?

A higher melt flow rate indicates lower melt viscosity, which is generally associated with lower average molecular weight and easier flow during processing. A lower melt flow rate corresponds to higher molecular weight, higher melt strength, and greater resistance to flow, which is preferred for applications such as blown film, pipes, and blow-molded containers.

Which standard should be selected, ISO 1133 or ASTM D1238?

Both standards use similar apparatus and principles but differ in procedural details, tolerance requirements, and condition designations. The choice is typically dictated by the material specification, customer requirement, or regional practice; ISO 1133 is common in Europe and international trade, while ASTM D1238 is widely used in North America. Results from the two methods are generally comparable but should not be assumed to be interchangeable without verification.