ISO 1133 covers the test methods used to determine the Melt Flow Index (MFI) of thermoplastic polymers.

According to ISO 1133 and ASTM D1238, the MFI is the weight of polymer melt flowing out from a standard die (2.095 x 8 mm) at a given temperature and with a standard weight applied to the piston.

The ISO 1133 MFI is used to verify if a plastic material's grade is within the required fluidity range. This is commonly used for polyolefins (polyethylene HDPE, LDPE, LLDPE, and polypropylene PP).