This procedure determines the rate of extrusion of molten resin or ground samples through a die of specified length and diameter under prescribed temperature, load, and piston position as a function of time according to ASTM D1238.
A resin sample can be analyzed directly, but preforms or other samples must first be ground to a particle size that can be loaded into the melt indexer. All samples are dried overnight to remove water. The test is run by quickly packing the melt indexer chamber with the dried sample. After a preset heat soak time, a piston with a defined dead weight load is applied to the melted material, forcing it through the capillary orifice. The amount of material pushed through the orifice within a defined time interval is collected and the length of time interval is recorded. This process is repeated a total of three times and an average weight is determined. The ASTM D1238 procedure defines the barrel temperature, piston load and standard designations for standard materials that should be used.
This test is routinely used for polyethylene and polypropylene; it is not recommended for PET.