MFR and MVR in Plastics: What Melt Flow Values Really Mean

The Short Answer

• MFR measures the mass of polymer extruded through a standard die in a specified time.
• MVR measures the volume extruded under the same type of controlled conditions.
• Temperature and applied load are an essential part of every result.
• Values obtained under different test conditions should not be directly compared.
• A higher result indicates greater flow through the test die under the stated conditions—not universally better processability.
• Within comparable grades of the same polymer, higher flow often corresponds to lower melt viscosity and may be associated with lower molecular weight.
• MFR and MVR do not independently predict impact strength, tensile performance, heat resistance or finished-part quality.

What Are MFR and MVR?

MFR stands for melt mass-flow rate. It is the mass of molten thermoplastic that passes through a standardised die during a defined period under a specified temperature and load. It is normally expressed in grams per 10 minutes:

g/10 min

MVR stands for melt volume-flow rate. It describes the volume of material passing through the die and is normally expressed as:

cm³/10 min

The standard method is defined in ISO 1133-1. The test uses an extrusion plastometer equipped with a heated cylinder, piston and standard die.

ISO 1133-1 describes two principal procedures:

• Procedure A determines MFR by measuring the mass of the extruded material.
• Procedure B determines MVR from piston displacement over time.

MFR can be calculated from MVR, or MVR from MFR, when the melt density at the test temperature is known.

MFR and MVR Are Not Interchangeable Numbers

MFR is based on mass, while MVR is based on volume. The distinction becomes particularly relevant when comparing materials with different densities.

This may apply, for example, when comparing:

• an unfilled polymer with a mineral-filled compound;
• an unreinforced grade with a glass-fibre-reinforced grade;
• materials containing different levels or types of fillers.

ISO 1133-1 identifies MVR as particularly useful when comparing thermoplastics with different filler contents. However, even MVR should not be treated as a complete description of rheological behaviour.

Both measurements represent one defined test point—not the complete viscosity profile of the material.

Why Test Conditions Matter

An MFR or MVR value is incomplete without its test conditions.

A technically meaningful datasheet entry should include:

• the test temperature;
• the applied load;
• the unit;
• the applicable test method.

For example, reporting only:

MFR: 12 g/10 min

does not provide enough information for a reliable comparison.

The same material can produce different flow-rate results when tested at a different temperature or under a different load. Therefore, two values should only be directly compared when the measurement method and conditions are equivalent.

This is especially important when comparing datasheets from different suppliers or different polymer families. A numerically similar MFR does not necessarily mean that two materials will behave in the same way during injection moulding or extrusion.

Does a Higher MFR Mean Easier Processing?

Under identical test conditions, a higher MFR means that a greater mass of material passed through the standard die during the test. A higher MVR means that a greater volume passed through it.

This generally indicates higher flow under those particular test conditions.

It can be advantageous in applications involving:

• thin walls;
• long flow paths;
• complex geometries;
• multiple cavities;
• lower available injection pressure;
• short filling times.

Commercial high-flow grades are therefore often developed for thin-wall and high-speed injection-moulding applications.

However, higher MFR does not automatically guarantee easier processing.

According to ISO 1133-1, the shear rates generated during the standard test are considerably lower than those normally encountered during industrial processing. Consequently, two materials with similar MFR or MVR values may respond differently to the much higher shear rates present in an injection-moulding machine.

Actual mould filling also depends on factors including:

• melt temperature;
• mould temperature;
• injection speed and pressure;
• gate dimensions and location;
• runner design;
• wall thickness;
• flow length;
• venting;
• fibre or filler content;
• moisture and material preparation.

MFR or MVR can support material screening, but neither value replaces processing trials or a full rheological analysis.

Does a Higher MFR Mean Lower Molecular Weight?

For comparable grades within the same polymer family, tested under the same conditions, higher flow often correlates with lower melt viscosity and may indicate lower average molecular weight.

Longer polymer chains generally create more molecular entanglements and greater resistance to flow. Shorter chains will often flow more readily under the same conditions.

However, this relationship is not universal.

Research on commercial thermoplastics has shown that useful correlations can be established for linear polymers with comparable molecular-weight distributions. The relationship becomes more complex for branched polymers and materials with different molecular architectures.

MFR is influenced not only by average molecular weight but also by:

• molecular-weight distribution;
• chain branching;
• copolymer structure;
• fillers and reinforcements;
• modifiers and processing additives;
• moisture;
• contamination;
• thermal and processing history.

MFR should therefore not be presented as a direct measurement of molecular weight. Molecular weight is determined using dedicated analytical methods, such as gel permeation chromatography, where appropriate.

The technically correct conclusion is:

Within a comparable material family and formulation, a higher MFR may be associated with lower molecular weight. It is not a universal rule for comparing different thermoplastics or compounds.

Can MFR Predict Mechanical Properties?

MFR and MVR cannot independently predict the mechanical performance of a finished component.

ASTM D1238 states that melt-flow data are useful for assessing the uniformity of polymer flow rate, but should not be used to infer the uniformity of other properties without a validated correlation supported by additional testing.

A higher-flow grade may differ from a lower-flow grade in:

• tensile strength;
• elongation;
• impact resistance;
• fatigue behaviour;
• creep resistance;
• environmental stress-cracking resistance;
• weld-line performance;
• long-term thermal performance.

The direction and magnitude of these differences depend on the polymer, formulation and grade design. It is therefore incorrect to assume that a higher MFR always means lower strength—or that two grades with similar MFR will provide equivalent mechanical performance.

Mechanical and thermal requirements must be checked using the relevant values from the technical datasheet and, where necessary, verified in the final component.

MFR and MVR in Quality Control

MFR and MVR are widely used in quality control because they provide a relatively fast way to monitor whether the flow behaviour of a material remains within an established range.

They can be used to:

• compare production batches;
• monitor consistency of incoming material;
• identify a change in flow behaviour;
• support process troubleshooting;
• monitor recycled or reprocessed material against an approved baseline.

An unexpected change in melt-flow value may indicate that something has changed. It does not, by itself, identify the cause.

Possible influences include changes in formulation, molecular structure, moisture, contamination, degradation, filler content or test procedure. Additional investigation is required before drawing a conclusion.

Moisture- and Temperature-Sensitive Materials

Some thermoplastics are particularly sensitive to moisture or to the time spent at elevated temperature during testing.

For these materials, inadequate drying or excessive residence time can change the polymer during the measurement and affect the result. Hydrolysis, chain scission, condensation or cross-linking may alter flow behaviour.

ISO 1133-2 specifies a more tightly controlled method for materials whose rheological behaviour is sensitive to moisture and/or time-temperature history. It includes stricter control of temperature, test duration and sample preparation.

For sensitive engineering thermoplastics, repeatable results require:

• preparation according to the material supplier’s instructions;
• controlled drying where required;
• consistent sample handling;
• controlled residence time;
• an appropriate test method;
• documented test conditions.

A result obtained from improperly prepared material may describe the condition created during the test rather than the condition of the original granulate.

How Should Technologists Use MFR and MVR?

MFR and MVR are most useful as part of a wider material-selection and process-validation process.

Before comparing values, check:

1. Polymer family
   Are the materials based on the same polymer or a genuinely comparable formulation?
2. Test method
   Were both results obtained using the same applicable standard?
3. Temperature and load
   Are the test conditions identical?
4. Unit
   Is the value reported as MFR in g/10 min or MVR in cm³/10 min?
5. Material condition
   Were the samples dried and prepared in a comparable way?
6. Reinforcement and additives
   Are the grades unfilled, reinforced, flame-retardant, impact-modified or otherwise formulated differently?
7. Mechanical and thermal requirements
   Does the grade meet the required impact, stiffness, strength, heat-resistance, flammability and environmental conditions?
8. Component and tool design
   Are the flow length, wall thickness, gate system and expected weld lines compatible with the material?
9. Processing validation
   Has the material been tested in the intended mould, process window and final application?

Common Interpretation Errors

Comparing values measured under different conditions

An MFR measured at one temperature and load cannot be reliably ranked against a result obtained under different conditions.

Treating MFR as a universal viscosity value

MFR is an empirical single-point flow measurement. It is not a complete viscosity curve and does not describe the material’s response across the range of shear rates present during processing.

Assuming that the highest value is the best choice

A very high-flow grade may support filling, but the application may require a different balance of impact strength, creep resistance, chemical resistance or long-term durability.

Using MFR as proof of degradation

A change in MFR may be consistent with degradation, but it is not proof. Moisture, contamination, formulation differences and measurement variation can also affect the result.

Comparing unrelated materials by MFR alone

Identical numerical values do not make two different polymers technically equivalent. Their viscosity response, shrinkage, crystallisation, thermal properties and mechanical behaviour can remain completely different.

Frequently Asked Questions

What is the difference between MFR and MFI?

MFI, or melt flow index, is a commonly used industry term. Current ISO terminology distinguishes between MFR—melt mass-flow rate—and MVR—melt volume-flow rate. A technical datasheet should state which value is being reported, together with the unit and test conditions.

Is a high MFR better for injection moulding?

Not automatically. A higher MFR may help fill thin walls or long flow paths, but successful moulding also depends on component geometry, tooling, processing conditions and the material’s behaviour at injection-moulding shear rates.

Can MFR values from different polymers be compared?

A numerical comparison can be made, but it may have little technical meaning. Direct ranking is most reliable for comparable materials tested under identical conditions. MFR alone should not be used to determine whether one polymer can replace another.

Does higher MFR always mean lower molecular weight?

No. This relationship may apply within comparable grades of the same polymer family, but MFR is also influenced by molecular-weight distribution, branching, additives, fillers, moisture and material history.

Can MFR determine whether a recycled material is suitable?

MFR can support quality control and indicate changes in flow behaviour, but it cannot independently confirm composition, contamination level, degradation, mechanical performance or suitability for a specific application.

Can two materials with the same MFR be processed using the same settings?

Not necessarily. Similar MFR values do not guarantee identical viscosity at processing shear rates or equivalent thermal, mechanical and shrinkage behaviour. Processing parameters should be established for the exact material and application.

Is MVR better than MFR for filled compounds?

MVR can be particularly useful when comparing materials with different filler contents because it is based on volume rather than mass. Density, test conditions and the complete formulation must still be considered.

MFR Is a Selection Tool—not a Final Decision

MFR and MVR provide useful information about polymer flow under standardised conditions. They support grade comparison, quality control and the initial assessment of processing behaviour.

They should not be interpreted as universal indicators of quality, processability, molecular weight or final-part performance.

The correct material decision combines melt-flow data with:

• the complete technical datasheet;
• component geometry;
• tool design;
• actual processing conditions;
• mechanical and thermal requirements;
• regulatory requirements;
• application testing.

At Danje-Polymer, we support customers in interpreting material data, selecting suitable grades and validating materials in the intended application.