The flowability of materials or plastics can be tested both with a melt flow tester, also known as a plastometer, and with a capillary rheometer. However, the measuring systems are optimized for different applications.
Melt index testers are very easy to operate and quickly provide a single key figure. In contrast, a capillary rheometer makes it possible to determine viscosity at all shear rates relevant to processing technology. Capillary rheometer tests are generally more complex and take longer. In comparison, melt flow index testers are generally more affordable.
The following application example clearly shows the different capabilities of the two measurement systems.
The diagram shows the results of a comparison of melt index measurement and capillary rheometer on two batches of the same material. Material 1 causes problems in processing by injection molding, while material 2 works without problems.
In the incoming goods inspection by means of MVR tests, both batches are inconspicuous. Both are within specification and differ only slightly. In a comparison test on a capillary rheometer, the viscosity is measured over a very wide shear-rate range. Especially the higher shear rates relevant for processing show a clear difference between both batches. Material 1, which is problematic during processing, has a significantly higher viscosity at high shear rates.
This difference in viscosity at higher shear rates explains the different behaviour in the process. This material characteristic cannot be determined with the basic MVR-test with only one weight. By means of multi-weight-measurements, such as those possible with our melt flow index tester mi40, this characteristic can also be detected by determining the flow rate ratio (FRR). With the add-on "viscosity measurement" the mi40 visco is also directly able to measure viscosity at higher shear rates.