Melt Flow Index (MFI / MFR): Testing, Significance & Standards (ISO 1133 / ASTM D1238 / IS 2530)

Melt Flow Index (MFI), or Melt Mass-Flow Rate (MFR), measures the ease of flow of molten thermoplastics through a standardized orifice under specified temperature and load. Expressed in grams per 10 minutes ($\text{g/10 min}$), MFI is the universal quality control parameter used by resin producers, compounders, and processors to verify raw material lot consistency, estimate molecular weight ($M_w \propto 1/\text{MFI}$), and select grades for injection molding (high MFI) vs extrusion (low MFI).

By completing this lesson, you will be able to:

• Execute MFI tests under ISO 1133 Method A (mass) and Method B (volume / MVR) standards.
• Calculate Melt Mass-Flow Rate (MFR) and Melt Volume-Flow Rate (MVR) from extrudate cut-off weights.
• Compare standard test conditions ($190^\circ\text{C}/2.16\text{ kg}$ for PE; $230^\circ\text{C}/2.16\text{ kg}$ for PP).
• Diagnose thermal degradation and moisture-induced hydrolytic degradation (PET/Nylon) via MFI shift.

Standard Extrusion Plastometer Setup

Resin charge is heated inside a vertical barrel and extruded through a standard capillary die (diameter $D = 2.095\text{ mm}$, length $L = 8.000\text{ mm}$) by a weighted piston.

graph TD
    A["Pre-heated Resin Barrel (Standard Temp e.g. 190°C / 230°C)"] --> B["Piston & Standard Load Application (2.16 kg / 5.0 kg / 21.6 kg)"]
    B --> C["Extrusion through Capillary Die (D = 2.095 mm, L = 8.000 mm)"]
    C --> D["Timed Extrudate Cut-Offs (Interval t = 30s to 240s)"]
    D --> E["Weighing Cut-offs & Calculation of MFR (g/10 min)"]

Melt Mass-Flow Rate (MFR) Equation

$$\text{MFR} = \frac{600 \times m}{t}$$

Where:

• $m$ = Average mass of extrudate cut-offs in grams ($ext{g}$)
• $t$ = Cut-off time interval in seconds ($ext{s}$)
• $600$ = Conversion factor to standard $10\text{ minute}$ ($600\text{ s}$) basis

Worked Numerical Example:

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Problem: A Polyethylene (PE) resin sample is tested at $190^\circ\text{C}$ under a $2.16\text{ kg}$ load (ISO 1133 Condition D). The timed cut-off interval is set to $t = 30.0\text{ seconds}$. Five consecutive extrudate cut-offs yield an average mass $m = 0.115\text{ grams}$. Calculate the Melt Mass-Flow Rate (MFR) in $\text{g/10 min}$.

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Solution:

$$\text{MFR} = \frac{600 \times 0.115}{30.0} = \frac{69.0}{30.0} = 2.30\ \text{g/10 min}$$

Grade Assessment: An MFR of $2.30\text{ g/10 min}$ ($190^\circ\text{C}/2.16\text{ kg}$) qualifies this HDPE resin as a blow molding or film extrusion grade.

• Resin Quality Control: Verification of raw polypropylene MFI ($230^\circ\text{C}/2.16\text{ kg} = 12.0\text{ g/10 min}$) for injection moulding. (Illustrative Indian industry scenario based on CIPET lab testing protocols).
• Recycled Plastic Grading: MFI testing to detect polymer degradation in recycled post-consumer flakes.

• Inverse Relationship: High MFI indicates low molecular weight and low melt viscosity.
• Standard Orifice: Die diameter $= 2.095 \pm 0.005\text{ mm}$, length $= 8.000 \pm 0.025\text{ mm}$.
• MVR: Melt Volume-Flow Rate expressed in $\text{cm}^3/10\text{ min}$ (ISO 1133 Method B).

1. ISO 1133-1:2022 — Plastics — Determination of the melt mass-flow rate (MFR) and melt volume-flow rate (MVR) of thermoplastics.
2. ASTM D1238-20 — Standard Test Method for Melt Flow Rates of Thermoplastics by Extrusion Plastometer.
3. IS 2530:1963 — Methods of Test for Polyethylene Materials.