Melt Flow Index General Information (FAQ)

General Information

We specify the HDPE reference material M80064 at the test conditions of 190°C usual for PE with a weight of 2.16kg. However, the material is very temperature stable and can still be measured with good repeatability up to approx. 250°C.

Basically, PP is not a moisture-sensitive material. However, under certain circumstances it is possible that moisture is present on the surface of the granulate. In these cases, bubbles can be seen in the strand due to the evaporation of the water. This can lead to higher measured values, as the density decreases.

Especially due to the higher stiffness of the glass fibre reinforced material, air inclusions can also occur due to too weak pre-compression, which then lead to higher measured values. If that is the case a higher, automatically applied pre-compression load might help.

The geometries of the piston and the barrel have very fine tolerances and the gap dimensions at the piston tip have a great influence on the measurement result. Changes in surface roughness can also lead to falsified measurement results. In any case, we recommend to repair or replace an apparently corroded test piston or inspection barrel.

The piston speed is determined by stopping the time the piston needs for a certain distance. For this length, called resolution by us, three pre-settings are stored on the machine. The normal resolution of 0.7 mm/test point was chosen to record 40 measuring points in the ISO 1133 measuring range with a length of 30 mm.

The number of test points can also be reduced, for example to shorten the measuring time or to limit the measuring range. This can make sense for thermally unstable or especially fibre-filled materials, where the measured value changes significantly over the barrel length, due to fibre orientation and fibre deposition. By limiting the measuring distance, the reproducibility can be increased in some cases.

Such special weights are basically possible. The integration of the special weight into a weight magazine such as the mi40 may also be possible, but must be checked by our design department in each individual case. Please contact us.

For particularly low viscosity polymers we generally use method B. Since these are mostly pure, unfilled polymers, the melt density is known from the literature. The test barrel is filled completely to approx. 120 mm above the capillary. A capillary plug is necessary to prevent material loss during filling and during the melting time. We have already performed MFR-measurements with values of up to 4000 g/10 min on our mi40.

Typically, a capillary plug is used to prevent the material from trickling out through the capillary during filling and pre-compression. The capillary plug is closed before filling. The material is usually filled and compacted in portions. The capillary plug is opened after the melting time.

In cases with extremely sticky and fine powder, an inlet can also be used. This is a test channel insert which is filled in cold condition on a press and is only inserted into the test barrel at the beginning of the melting time. This option is available for the mi40. Please contact us!

The maximum service life is generally not limited, but depends strongly on the materials tested. The standard test barrel is suitable for polyolefins or other unproblematic polymers and typically shows little wear even after years of heavy use. If material with a high fibre content is regularly measured, a particularly abrasion-resistant test barrel material should be selected to significantly increase the service life. Especially for fluoropolymers, a special corrosion resistant barrel has to be used. A standard barrel is destroyed by the acids released after only a few measurements. The test barrel of our plastometers MI-3, MI-4 and mi40 can easily be exchanged in a few minutes.

The test barrel should always be smooth like a mirror after cleaning. A matt finish indicates damage to the surface. The test barrel and piston are measured by our service staff during maintenance and compared with the tolerances permitted by the standards.

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