ASTM D1238 - Testing High Fluidity Samples with a Piston Holding Device
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. So, we performed the tests on a semi-automatic CEAST MF50 tester, equipped with a piston holding device. This feature consists of a retractable piston support that removes any load from the sample and prevents the piston from unwanted travelling during pre-heat period. The device is based on a pneumatic cylinder with a fork-shaped support that is automatically activated according to the test method settings. The operator is able to manually adjust the piston resting position before starting the test. Using the MF50, we tested the samples and easily obtained the desired starting position for measurements, together with a predictable starting time in the range specified within the standard.
The Instron® CEAST MF50 multi-weight Melt Flow Tester performs melt flow rate (melt index) tests according to MFR and MVR methods that fully comply with ASTM D1238 procedures A, B, C, D, and ISO 1133 A and B, for single weight and multiple weight tests . Multi-weight tests can include up to 5 masses in increasing, decreasing or free sequence, with fully customizable preheating and measurement steps. All masses are pre-installed and managed by the machine without operator intervention, eliminating all safety concerns. A high-resolution direct-contact encoder measures piston displacement. The temperature profile in the barrel is extremely stable and accurate. Additional advanced features include a large touch-screen display for full test programming, management and diagnostic functions, with an embedded PC based on Windows CE, able to store test methods and test results. Easy data export through USB and Ethernet ports. Compacting and purging functions can be supported by the unique load cell feature. Automatic melt cutting and barell cleaning accessories are also available.
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