Melt flow index (MFI)

Definition (what it is)

  • Melt flow index (MFI), also called melt flow rate (MFR), is a standardized measure of the mass of a thermoplastic polymer that flows through a specified capillary die under a prescribed temperature and load over a fixed time (typically 10 minutes). It is reported in grams per 10 minutes (g/10 min).
  • The related measure melt volume-flow rate (MVR) reports the extruded volume (cm³/10 min). For a given test condition, MFR ≈ MVR × melt density.
  • MFI/MFR and MVR are determined using an extrusion plastometer (melt indexer) per standards such as ASTM D1238 and ISO 1133.

What it indicates (key characteristics)

  • Flowability and viscosity: Under the stated test condition, higher MFI indicates lower apparent melt viscosity and easier flow; lower MFI indicates higher viscosity and more resistant flow.
  • Molecular weight link: For a given polymer family, lower MFI generally corresponds to higher average molecular weight and, often, higher melt strength and better mechanical/thermal/chemical resistance; higher MFI typically reflects lower molecular weight and easier processing.
  • Quality control and specification: Commonly used as a quick screening metric for grade selection, process setup, and lot-to-lot consistency. Shifts in MFI can indicate changes in molecular weight due to degradation, branching, or compounding effects.
  • Sensitivity to test conditions: MFI strongly depends on the test temperature, applied load, and die geometry. Values are only comparable when measured under identical conditions and methods.
  • Molecular weight distribution (MWD): Multi-load testing (e.g., flow rate ratio, FRR) compares MFI at two loads to provide an indirect indication of MWD breadth; broader MWD can improve processability but may affect properties.

Test method and reporting

  • Key elements: Method (ASTM/ISO and procedure), test temperature, applied load (e.g., 1.2, 2.16, 5.0, 10 kg, etc.), die dimensions, and any conditioning (e.g., pre-drying) should be reported.
  • Typical test conditions by polymer family (examples; always confirm the relevant standard):
    • Polyethylene (PE): 190 °C/2.16 kg
    • Polypropylene (PP): 230 °C/2.16 kg
    • Polycarbonate (PC): 300 °C/1.2 kg
    • Polyesters (PET, PBT), polyamides (PA): polymer-specific temperatures and loads; many require pre-drying
    • High-performance polymers (e.g., PEEK, PPS): 360–400+ °C at specified loads
  • Reporting example: “MFR = 12 g/10 min (230 °C/2.16 kg, ASTM D1238)”
  • Moisture and thermal history: Moisture-sensitive polymers (polyesters, polyamides) must be dried; hydrolytic or thermal degradation can artificially raise MFI.

Limitations and pitfalls

  • Single-point, low-shear measure: MFI provides one data point at a relatively low and material-dependent shear rate; it does not capture full non-Newtonian behavior, temperature dependence, or melt elasticity.
  • Not a substitute for full rheology: It cannot predict high-shear viscosity, die swell, melt strength, sag resistance, or time-dependent behavior. Capillary and rotational rheometry are needed for detailed design and simulation.
  • Condition dependence: Results from different temperatures, loads, or die dimensions are not directly comparable. Instrument setup, pre-shear, dwell time, and cutting method can affect outcomes.
  • Compounding and reprocessing effects: Fillers and additives usually lower MFI (increase viscosity). Thermal/oxidative/hydrolytic degradation tends to increase MFI; chain extenders or branching agents generally decrease MFI by increasing molecular weight.

Relevance and applications

  • General manufacturing:
    • Injection molding: MFI guides grade selection and process parameters to achieve complete filling, manage injection pressure, and balance flow with mechanical performance and weld-line strength.
    • Extrusion (profiles, sheet, cable, film): Lower MFI (higher viscosity) supports melt strength and dimensional stability in film blowing and thermoforming; moderate MFI supports stable throughput in profile and sheet extrusion.
    • Blow molding: Typically favors lower MFI for controlled parison behavior and uniform wall thickness.
    • Fiber spinning: Requires controlled MFI for stable throughput and desired filament properties.
    • Additive manufacturing (filament/pellet extrusion): MFI (or analogous flow metrics) informs printability and extrusion consistency.
  • Electric vehicle (EV) relevance (examples):
    • Connectors and high-voltage components: Selecting MFI helps ensure fill of thin sections, dimensional accuracy, and insulation integrity for housings, terminals, and busbar overmolding.
    • Battery systems and thermal management: For cell spacers, potting/encapsulation, and cooling components, MFI informs impregnation and void-filling capability around complex features.
    • Structural and semi-structural polymer/composite parts: In short- and long-fiber thermoplastic compounds, MFI influences fiber wet-out, fiber-length retention, mold filling, and cycle time.
    • Recyclate integration: Monitoring MFI supports qualification and control of recycled content, flagging molecular-weight shifts that impact processing and properties.

Synonyms and related terms

  • Synonyms: Melt flow rate (MFR; melt mass-flow rate), melt index (MI).
  • Related: Melt volume-flow rate (MVR), melt density, capillary rheometry, zero-shear viscosity, shear rate, shear thinning, melt elasticity, molecular weight (Mw), molecular weight distribution (MWD), polydispersity index (PDI), flow rate ratio (FRR).

Practical notes

  • Choose MFI ranges appropriate to the process: very low MFI for high melt strength (e.g., blow molding, film blowing), moderate for extrusion/profile, higher for injection molding of thin-wall or complex parts. Optimal values are polymer- and part-specific.
  • Use MFI as a screening and QC metric; for rigorous design, correlate MFI to full viscosity curves at processing-relevant shear rates and temperatures.