Melt Flow Index Interpretation Calculator
Interpret MFI test results to determine shear rate, apparent viscosity, molecular weight distribution breadth, and recommended processing method.
About this calculator
Melt Flow Index (MFI, per ASTM D1238 / ISO 1133) reports how many grams of polymer flow through a standard 2.095 mm die in 10 minutes under a fixed weight — a single, easy number, but not directly comparable across labs or materials without converting it into real rheological terms. This calculator does that conversion: it turns the mass flow rate into a volumetric flow rate (using melt density), then computes the wall shear rate at the die (gamma-dot = 4Q/(pi·r³), the standard capillary rheometry relation for flow through a cylindrical die) and the wall shear stress from the applied test load and the die/piston geometry of the plastometer itself. Dividing stress by shear rate gives the apparent viscosity at that specific shear condition — useful for comparing lots or for feeding into mold-flow estimates, but only valid at the shear rate implied by the standard test, not across the full range a polymer sees in real processing.
A second MFI reading at a higher load (Flow Rate Ratio, FRR = high-load MFI / standard MFI) indicates how broad the molecular weight distribution is: a narrow MWD resin flows almost the same regardless of load, while a broad one shows dramatically higher flow at higher loads because its long-chain fraction shears free more easily. The tool uses MFI ranges to suggest a likely process (very low MFI favors extrusion and blow molding, which need melt strength; high MFI favors injection molding and fiber spinning, which need fast fill). Keep in mind these are rule-of-thumb ranges — actual process selection also depends on part geometry, wall thickness, and required properties, not MFI alone.
Inputs
Results
Wall shear rate (1/s)
19.43
Apparent viscosity (Pa·s)
996.1
Figures current as of 2023. Source: ASTM International, ASTM D1238-23, Standard Test Method for Melt Flow Rates of Thermoplastics by Extrusion Plastometer
How to Use This Calculator
- Enter MFI at standard load (g/10 min), MFI at high load (g/10 min), and Melt density (g/cm³).
- Set Standard test load (kg) and Die diameter (mm).
- Review Wall shear rate (1/s) and Apparent viscosity (Pa·s).
- Use Wall shear stress (Pa) and Flow Rate Ratio (FRR) to inform your decision.
How the result changes with Die diameter (mm)
| Die diameter (mm) | Wall shear rate (1/s) | Apparent viscosity (Pa·s) |
|---|---|---|
| 1.05 | 154.37 | 62.85 |
| 1.57 | 46.18 | 314.17 |
| 3.14 | 5.77 | 5,026.72 |
| 5 | 1.43 | 32,318.13 |
What each input means
- MFI at standard load (g/10 min)
- Melt flow index measured at standard load (e.g., 2.16 kg for PE per ASTM D1238).
- MFI at high load (g/10 min)
- MFI measured at high load (typically 21.6 kg) for FRR calculation.
- Melt density (g/cm³)
- Polymer melt density at test temperature (PE ~0.76, PP ~0.73, PS ~0.96).
- Standard test load (kg)
- Dead weight load used for standard MFI measurement.
- Die diameter (mm)
- Extrusion plastometer die diameter. Standard is 2.095 mm.
What each result means
- Wall shear rate (1/s)
- Apparent shear rate at the die wall.
- Apparent viscosity (Pa·s)
- Ratio of wall shear stress to wall shear rate.
- Wall shear stress (Pa)
- Shear stress at the die wall from applied load.
- Flow Rate Ratio (FRR)
- Ratio of high-load MFI to standard MFI. Indicates MWD breadth.
- MWD breadth (1-4)
- 1 = narrow, 2 = moderate, 3 = broad, 4 = very broad molecular weight distribution.
- Processability score (1-10)
- Higher = easier flow. Based on MFI range.
- Process code
- 1 = extrusion, 2 = blow molding, 3 = injection molding, 4 = fiber/film.
- Vol. flow rate (cm³/s)
- Volumetric flow rate through the die.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersMFI at standard load (g/10 min) = 10, MFI at high load (g/10 min) = 100, Melt density (g/cm³) = 0.95, Standard test load (kg) = 2.16 = 5 input(s) provided
- Calculate Wall shear rateWall shear rate = (4 * volumeFlowMm3) / (π * pow(dieRadius, 3))19.43 = 19.43
- Calculate Apparent viscosity996.1 = 996.1
- Calculate Wall shear stressWall shear stress = (dieRadius / 1000) * pressureDrop / (2 * (dieLength / 1000))19358.67 = 19358.67
- Calculate Flow Rate RatioFlow Rate Ratio = mfiHighLoad / mfi10 = 10
Figures and sources
- ASTM D1238 — Standard Test Method for Melt Flow Rates of Thermoplastics by Extrusion Plastometer (2023) — ASTM International, ASTM D1238-23, Standard Test Method for Melt Flow Rates of Thermoplastics by Extrusion Plastometer
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
What does the Flow Rate Ratio (FRR) tell me about my resin that the standard MFI number doesn't?
FRR is the high-load MFI divided by the standard-load MFI, and it indicates how broad the molecular weight distribution (MWD) is. The calculator reads FRR under 20 as narrow MWD, 20-40 as moderate, 40-80 as broad, and above 80 as very broad — because a resin with a wide spread of chain lengths shows a much bigger jump in flow at higher load than one with uniform chain lengths.
Why does the calculator need die and piston dimensions, not just the two MFI readings?
Apparent viscosity requires actual shear stress and shear rate at the die wall, and those depend on the plastometer's physical geometry — die diameter, piston diameter, and die land length — not just the mass flow number. The tool converts MFI to a volumetric flow rate, then plugs the standard capillary geometry (die radius 1.0475 mm, piston diameter 9.5504 mm, 8 mm land length) into the shear-rate and shear-stress relations to get a real Pa·s viscosity.
How does a higher MFI translate into a recommended process?
The tool uses simple MFI bands: below 1 g/10min it recommends extrusion, 1-6 blow molding, 6-50 injection molding, and above 50 fiber or film spinning. That progression reflects that low-MFI (higher-viscosity) resins have the melt strength extrusion and blow molding need to hold their shape, while high-MFI resins flow fast enough for quick mold fill or fine fiber draw-down.
Is the apparent viscosity this calculator reports valid across my whole processing range?
No — it's only valid at the shear rate implied by the standard MFI test itself, which is a low shear rate compared to what a polymer experiences in an injection mold gate or extrusion die. It's useful for lot-to-lot comparison or as a rough input to mold-flow estimates, but not as a stand-in for a full shear-rate viscosity curve.
Related Calculators
The questions that sit next to this one — chosen by subject, including calculators filed under a different category.
Injection Molding Clamp Tonnage Calculator
Calculate required clamping force for injection molding from projected part area, cavity pressure, number of cavities, and runner system.
Injection MoldingPlastic Shrinkage Calculator
Calculate mold cavity dimensions from finished part size and material shrinkage rate to ensure correct part dimensions.
GemologyRefractive Index Calculator
Interpret RI measurement and birefringence for gemstone identification against a reference database.
PlasticsExtrusion Die Swell Calculator
Calculate die swell ratio from material elasticity, L/D ratio, shear rate, and melt temperature. Determine required die dimensions to achieve target extrudate size.
PlasticsThermoforming Draw Ratio Calculator
Calculate areal and linear draw ratios for thermoformed parts. Estimates wall thinning, minimum wall thickness, and forming feasibility.
More in Manufacturing, Industrial & Coatings.