I receive this question very often, and feel it would be great for the blog…
Can you briefly explain melt flow index, and how processors use it?
Melt flow indexing is the most popular, and yet least accurate way to determine material viscosity. The melt flow index (MFI) is the measure of how many grams of polymer pass through a standardized capillary under a standard load over 10 minutes. The value obtained through the melt flow index test is a single data point. The melt flow index only tests the material at one shear stress, and temperature.
In general, a higher melt flow index indicates a lower material viscosity. This means that a material with a melt flow index of 20 flows easier than a material with a melt flow index of 5. Melt flow index information from different materials and material grades may be used for a rough comparison of flow characteristics for different materials.
Many processors use this data to qualify incoming materials and to help anticipate changes in the process. For example, if the lot of material you are processing has a MFI of 10, and a new lot has an MFI of 15… you can anticipate issues such as flash, over packing, or overweight product and make the appropriate adjustments.
To obtain more accurate and relevant viscosity data… it is better to perform rheology tests using a capillary rheometer or a parallel plate rheometer. Many companies will also perform in-mold rheology tests using actual production molds.