ASTM D1238 defines the method to measure the rate of extrusion of molten thermoplastics using a Melt Flow Tester. The sample is extruded through a die with specified dimensions under prescribed conditions of temperature and load. For materials with melt flow rate (MFR) greater than 10 g/10 min, the standard specifies the use of a support for test masses – and if needed for the piston, too – in order to prevent the piston from travelling during the pre-heat period. The goal is to ensure that enough sample is left for the measuring phase and that measurements can start at the correct distance from the die at the right time.

We were asked to test high fluidity samples of PP (polypropylene) and PA (nylon) having an MFR higher than 50 g/10 min. This required significant care and adjustments of the test method in order to achieve reproducible results with a basic melt flow tester.