---
term: "Melt Flow Index"
category: "mfi-testing"
shortDefinition: "MFI (MFR) is the extruded mass of polymer per unit time through a standard die under specified temperature and piston load, typically reported in g/10 min."
formula: "MFR = m / t"
url: "https://vectorbtc.com.tr/resources/glossary/melt-flow-index/"
---

The **melt flow index (MFI)**, also called **melt mass-flow rate (MFR)**, is a single-point **rheological QC test** for thermoplastics. Molten polymer is extruded through a **standard capillary die** (2.095 mm diameter for Procedure A/B) while a **defined piston load** produces a nominal shear stress regime.

Test temperature and load are chosen per material grade—for example polyethylene is often tested at **190 °C** with **2.16 kg** load, while other resins use 230 °C or higher loads (5 kg, 10 kg, 21.6 kg) to bring flow into measurable range.

MFI inversely correlates with **average molecular weight** only qualitatively; broad polydispersity or long-chain branching can produce similar MFI with different processability. It should complement **intrinsic viscosity**, capillary rheometry, or dynamic shear rheology for process design.

Moisture-sensitive polymers must be **dried** before testing; trapped moisture hydrolyzes chains or causes foaming, biasing MFI high. Barrel residence time and piston insertion depth must follow standards to avoid thermal degradation artifacts.

Processors use MFI for **lot-to-lot incoming inspection** and adjusting extrusion blow molding or injection molding parameters, but final part properties still depend on **shear history, cooling rate, and orientation**.