What is Melt Flow Index (MFI) and Why Does It Matter in Polymer Processing?
1. Standard Testing Methodology (ASTM D1238 & ISO 1133)
MFI measures the mass of polymer (in grams) extruded through a standardized capillary die (2.095 mm diameter) in 10 minutes under a calibrated load and temperature.
Standard conditions vary by polymer: Polypropylene is tested at 230°C / 2.16 kg, whereas Polyethylene is typically tested at 190°C / 2.16 kg (or 5.0 kg / 21.6 kg for high-load HDPE pipe grades).
2. Low MFI vs High MFI: The Trade-off Rule
An inverse relationship exists between MFI and polymer molecular weight. Lower MFI means longer polymer chains, resulting in higher impact strength, superior Environmental Stress Crack Resistance (ESCR), but higher viscosity.
High MFI resins (e.g. PP MFI 11–25) flow readily into thin-wall cavity molds with low injection pressure, lowering energy consumption and cycle duration.
| Target Processing Method | Recommended Polymer | Ideal MFI Range (g/10min) | Key Processing Benefit |
|---|---|---|---|
| Thin-wall Injection Molding | PP Homopolymer H110MA | 11.0 – 25.0 | Fast cavity fill, zero flash |
| Blown Film Extrusion | LLDPE Film 218W | 1.0 – 2.0 | Stable bubble, high dart drop |
| Extrusion Blow Molding | HDPE 5502BN | 0.25 – 0.40 | High parison melt strength, zero sag |
| High-Pressure Pipe Extrusion | HDPE PE100 | 0.20 – 0.30 (190°C/5kg) | MRS 10.0 MPa, long-term crack resistance |
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