Melt Flow Index
A relatively simple method to provide a general and relative characterization of how a given polymer will flow when melted is a test called the melt flow index (MFI), which is also sometimes called the melt flow rate (MFR)
In this test, a small sample of plastic is heated and forced through an orifice by a weighted piston. The results are recorded as the weight of material “extruded through the orifice” in a given amount of time (usually grams per minute at a given temperature). Typical values could be anywhere from 0.1 grams/minute for a very stiff, hard to flow material, up to 100 grams/minute for a very easy flowing material. A material with an MFI value of <1.0 is called a fractional melt material. The test methods and conditions are defined in ASTM D1238-Standard Method for Flow Rates of Thermoplastics by an Extrusion Plastometer. Because of its simplicity and repeatability this is a very common test used in industry, both for comparing one material to another and for quality control testing of a given material. The
figure below shows a typical testing apparatus.
Extrusion plastometer (melt index tester).
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