Determination of melt mass flow rate (MFR) and melt volume flow rate (MVR)

ISO 1133-1, -2 and ASTM D1238

Extrusion plastometers provide standard values for the melt index (MFR) and volume index (MVR) of filled and unfilled thermoplastics to ISO 1133-1and -2, ASTM D1238. and other similar standards.
Schmelzindex-Bestimmung ISO 1133-1 und -2, ASTM D 1238

Methods for Melt Flow Determination

Method A, MFR

With this method, the extrudate is cut at constant intervals and its mass is determined by an analytical scale. The test result is the extruded mass per unit of time, which is stated in g/10 min. An operator must be present for the entire test sequence and therefore, it can be automated only to a limited extent.

Method B, MVR

This method determines the extruded volume of the polymer melt at regular intervals as opposed to the mass of the extrudate. The extrusion plastometer must be equipped with a piston displacement transducer. The MVR result is the extruded material volume per unit of time. It is specified in cm³/10 min and is calculated from the distance the piston travels per unit of time.

When meltingwith a homogeneous density distribution, the melt density can be used to convert the MVR value to an MFR value. If the plastic is unfilled, this is often not possible to do with a high level of accuracy due to the inhomogeneous distribution of the filler.

A significant advantage of this method is the elimination of mechanical cutting. The entire test sequence can be performed without any additional operator influence.

Method C to ASTM D1238, halved die dimensions

The area of application is polyolefins, which demonstrate an MRF value greater than 75 g/10 min. There are two alternatives for testing these materials: setting the test weight to a correspondingly low value or using a die with half the height and half the diameter. These options are also available for tests to ISO 1133-1. However, a direct comparison of results that are determined using the standard die is not possible.

Method D to ASTM D 1238, multi-step test

With many polyolefins it is common to state the MVR value for different load levels and determine the flow rate ratio (FRR). This requires measurements from several fillings when simple extrusion plastometers are used. Extrusion plastometers equipped with an automatic load change unit can measure measurement series across multiple load levels from a single filling of the extrusion barrel.

Zwick produces straightforward extrusion plastometers for Method A, together with instruments which feature automatic displacement measurement and can be used for measurements to Methods A and B.

Melt flow index-extrusion plastometers

The APC function – automatic optimization of measurement intervals

When measuring flow rates, measurement intervals must be set so that the measurement times are as great as possible, and in the case of MVR measurement, the measurement travel is as great as possible. This makes the method highly accurate. When outside of the optimal range, the number of measurement errors increases rapidly.

The Mflow and Aflow extrusion plastometers are equipped with the APC function. This function measures the speed of the piston shortly before actual measurement begins. Using this information, the best possible type of control, meaning travel or time control, is selected and set for the best suited measurement interval for the expected MVR value. Time-consuming pretests are no longer necessary and the process required for programming tests is reduced to setting a few test parameters that apply to all materials to be tested.

Relative_measurement error_MVR_value_en

ISO 1133-2 Testing Moisture-Sensitive and Quick Degrading Plastics

Special precautions must be taken for tests performed on these materials (for example, PBT, PET, or PA). First, these materials must be sufficiently dried and must be dry when poured into the extrusion barrel. An optional nitrogen blanket at the extrusion barrel prevents direct contact by the material with the ambient air. The test is run in defined time sequences and they are recorded by the software. The extrusion plastometers must meet specific conditions for this purpose in terms of spatial and temporal temperature distribution in the extrusion barrel.

Correlation of IV measurements for the MFR value for linear PET

The molecular weight of polyethylene terephthalate (PET) is usually described in terms of intrinsic viscosity. This is the IV value in dl/g. The longer the polymer chains, the higher this characteristic value. This makes it possible to demonstrate molecular chains and how they can occur when moisture that is too high during the melting process. The disadvantages of this method are that recyclers of PET in particular, are often not equipped to handle corrosive or toxic solvents. Furthermore, the fact that the test lasts a long time poses a practical problem. Therefore, measurement of the melt mass flow rate (MFR) has been used since the early 1990s in this area as well. Using testXpert II to control the Mflow and Aflow extrusion plastometers makes it possible to determine the correlation between IV values and the MFR by means of premeasurements, which can then be applied to other measurements.
Kraft-Weg Diagramm
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