Melt Flow Index (MFI, also called Melt Flow Rate or MFR) measures how easily a molten resin flows under a standardized temperature and pressure, expressed in grams per 10 minutes. It reads like a niche spec sheet number, but it directly determines how a resin fills a mold and how consistent the final bottle wall thickness will be.
A resin with a high MFI flows more easily, useful for filling thin-walled or complex geometries quickly, but prone to flash and less structurally rigid. A low-MFI resin flows more slowly and resists deformation better, but can struggle to fill thin sections or fine detail, contributing to short shots or incomplete fills.
In the injection stretch blow molding process covered previously, the preform's resin needs a melt flow characteristic that supports even stretching during the blow stage. A resin with inconsistent or mismatched MFI can produce bottles with uneven wall thickness (thin in some zones, thick in others), which affects both structural integrity and how the bottle looks on shelf under light.
Resin suppliers can shift MFI batch to batch within nominal spec tolerances. Consistent bottle quality therefore depends on a molder who tracks incoming resin lots and adjusts process parameters accordingly. Incoming resin QC covers this at Jadewell.
Grade selection and MFI control are the same decision made twice. Compare properties across PET, HDPE, and PP before locking a spec, then request a quote on the grade and wall tolerance required.