The melt-flow index (MFI), also known as the melt-flow rate (MFR), is one of the most important specification values reported on any polypropylene datasheet. Yet it is also one of the most misunderstood figures by buyers and processors alike. In this article we explain what MFI measures, why it matters, and how to match it to your application.
What Is the Melt-Flow Index?
The MFI quantifies how easily a molten polymer flows under a standard load. It is measured by extruding the polymer through a calibrated capillary at a fixed temperature and pressure, and expressing the result in grams per 10 minutes. For polypropylene, the standard conditions are 230 °C with a 2.16 kg load.
A higher MFI means the material flows more easily — it is "thinner" when molten. A lower MFI means the material is more viscous and "thicker".
Why MFI Matters for Processing
MFI directly influences how a polymer behaves inside a mold or die. Selecting a grade with the wrong MFI for your process can lead to short shots, flash, sink marks, or extended cycle times.
High MFI (Easy Flow)
- Fills thin walls and complex geometries quickly
- Reduces injection pressure and clamping force
- Shorter cycle times — ideal for high-volume consumer goods
- Generally lower impact resistance due to lower molecular weight
Low MFI (High Viscosity)
- Better mechanical properties and impact strength
- Suited to thick-wall parts and structural components
- Requires higher processing pressure and temperature
- Preferred for pipes, sheets, and heavy-duty applications
Matching MFI to Your Application
For general-purpose injection molding of caps, closures, and housewares, an MFI around 3.0 g/10min — like DOT RAD's Bl3 grade — offers an excellent balance of flow and mechanical performance. For thin-wall packaging, higher-flow grades (10–30 g/10min) are preferred, while extrusion and pipe applications demand low-flow grades below 1.0 g/10min.
Always cross-reference the MFI with your tooling design, wall thickness, and expected cycle time. When in doubt, our technical team can recommend the optimal grade for your specific application.
