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Zwick Roell melt flow index testers – extrusion plastometers to suit every test-throughput level

Melt flow tests are a simple method of characterizing the flow properties of a plastic melted mass. This test method is widely used, especially in quality assurance and goods inwards testing. The demands placed on the extrusion plastometer vary according to which processing stage in the plastics industry is involved.

A simple explanation of the basic principles of melt flow testing

ISO 1133 - Determination of the melt mass flow rate of thermoplastics

The essential elements of a melt flow index tester, together with the accuracy requirements and test methodology, are specified in national and international standards. The standards involved are ISO 1133, JIS K 7210, ASTM D1238 as a general method and ASTM D3364 for tests on PVC, plus stipulations contained in specification standards.

Determination of melt flow index as per Method A: MFR - melt mass-flow rate
In this method the extrudates are cut off at uniform time-intervals and their mass is then determined on analytical scales. The test result is the extruded mass per unit of time, usually stated in g/10 min The quality of the MFR result obtained is dependent on the accuracy of the cutting times and the weighed mass. When low MFR values are involved, manual cutting is only recommended if long cutting times are used.

Determination of melt flow index as per Method B: MVR - melt volume flow rate
This method requires the extrusion plastometer / melt flow index tester to be equipped with a piston transducer. The MVR result is the extruded material volume per unit of time and is expressed in cm³/10 min. It is calculated from the distance that the piston travels per unit of time. A significant advantage of this method is the elimination of mechanical cutting. With synchronized travel-time values high accuracy can be achieved, even with short measuring times and short piston-travel. This enables up to 40 individual measurements from one barrel filling, depending on the material involved, accuracy requirements and MVR result.

Determination of melt flow index to Method C - ASTM D1238
Most commonly used are dies with a height of 8.00 mm and a bore diameter of 2.095 mm. For measurement of MFR values greater than 75 g/10 min the standards recommend a ‘half-height / half-bore-diameter’ die with dimensions 4.00 / 1.050 mm. This is designated as Method C in ASTM D1238.

Determination of melt flow index as per Method D - ASTM D1238: multi-stage tests
With many polyolefins it is usual 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 multiple load-levels from a single filling.