It is common for fiber lengths to be reduced by 50% during molding. As the fiber content increases, the contributions from this ingredient become more important. Whenever MFR tests are performed on glass-filled materials, the measurement is a composite of the response from the polymer and the fiber. But high-shear processes like injection molding also cause fiber length to be reduced, and this reduction also contributes to the change in the MFR measurement. We have established that processing can reduce the length of the polymer chain, and this is the primary area of interest when we perform MFR tests on molded parts and the related raw materials. This is especially true of reinforcements such as glass fiber. The problem with comparing the MFR results of pellets and parts when fillers are present pertains to the influence that the process has on the integrity of the filler. This increase is detectable at any shear rate, but it is more noticeable at low shear rates where the lack of orientation makes the filler particles or fibers much more resistant to flow. Because fillers do not melt at processing temperatures, they tend to increase the melt viscosity of a polymer. One excellent example of this principle is the influence of fillers. In general, the more ingredients there are in the compound, the more difficult it becomes to make a straightforward interpretation of the MFR test results.

Other constituents such as flame retardants can further complicate the response of the material to processing.

Melt viscosity test