Melt Flow index instruments are used particular in quality control of plastic manufacturers as well as processors.
It is a quick test which is standardized by a displacement/time measurement according to ISO and ASTM (DIN 53735, ISO 1133, ASTM D 1238).
During the measurement process, a constant shear stress is applied to the already melted plastic sample by applying a weight to a test punch.
Due to the constant shear stress, the melt index measurement is a one-point measurement, with a multi-load measurement different weights are applied resulting in different shear stresses.
However, the application of a test weight of a maximum of 20kg, only shearing rates that are far below those, as they are in the processing of plastics (extrusion, injection moulding, etc.) apply. As the MFR value is related to the average molecular weight of polymers, it provides a good indication of viscosity at low shear rates.
Due to the low shear rates, however, it is only a meaningful quick test and not a process-oriented characterization of the flow properties of plastics. Therefore a (high pressure) capillary rheometer is required.
A capillary rheometer, is generating the shear stress not by using weight furthermore by using a force- or speed-controlled motor (20-120 kN - depending on the application).
Thus, a capillary rheometer is not a standardized displacement-time measurement, but rather a dynamic viscosity measurement which is closed to polymer processing.