Melt Index

Melt index, also called melt flow index or melt flow rate, is measured using ASTM D1238. A small sample of resin is melted inside a heated cylinder and pushed through a fixed-diameter die under a specific weighted load, typically 2.16 kilograms for polyethylene, at a controlled temperature, usually 190 degrees Celsius for PE resins. The weight of the polymer that flows out in ten minutes is the melt index, reported in grams per 10 minutes.

A low melt index, such as 0.5 to 1.0, means the resin is highly viscous and resists flow, which generally translates to higher molecular weight and better mechanical properties like toughness and tear strength, but it also means higher extrusion pressures and more motor load on the line. A higher melt index, such as 2.0 or above, flows more easily, processes faster, and is often used where flowability into a die or onto a chill roll matters more than maximum strength. Most film-grade polyethylene resins used in blown film fall somewhere between roughly 0.3 and 2.0 melt index, since resins much above that range generally lack the melt strength needed to hold a stable bubble.

Melt index is one of the first specifications an extrusion engineer checks on a resin's data sheet, alongside density, because together those two numbers narrow down what kind of film properties and processing behavior to expect before a single pound of resin ever goes through the extruder.

A line that's struggling to maintain bubble stability on a new resin lot might trace the problem back to a melt index that's higher than the previous lot, since a more free-flowing melt can lower melt strength and make the bubble harder to control.

Melt index is often confused with viscosity in a general sense, but the two move in opposite directions: a higher melt index actually means lower viscosity (the resin flows more easily), while a lower melt index means higher viscosity. It's a common mix-up worth double-checking when comparing resin grades.