ASTM D1238 Standard Update: What You Should Know
The latest version of the ASTM D1238 standard, released on February 17, 2023, supports the use of force-controlled extrusion plastometers for the determination of MFR and MVR values. With this ASTM D1238 update, the standard opens up new control possibilities, in particular when it comes to the preheating phase of the test sequence. The test sequence can be automated over a wide MVR range, which to a great extent relieves personnel from the task of entering the correct test parameters. This makes work in the lab more efficient and ensures reproducibility of your tests when using different instruments at different locations, operated by different users. The Aflow extrusion plastometer is the first instrument that allows you to take full advantage of the new ASTM D1238 possibilities.
If volume or mass flow rates are to be determined on different polymers or different grades of polymers in the testing lab, the testing process can get complicated for laboratory managers: on the one hand, they have to know at what weight and temperature the polymer must be measured to remain in compliance with ASTM D1238. On the other, they have to control the volume of polymer and the sequence in the preheat phase of the test in such a way that the measurement of the melt flow rates starts precisely in the specified window, after 7 ± 0.5 minutes of preheating and at a piston height of 46 ± 2 millimeters. In addition, corrections are only permitted at the beginning of the preheating phase by means of purging, that is, by using a larger weight than the test weight. This significantly limits the control options in the later phases of the test. As soon as the start window has been correctly reached at 7 minutes and 46 mm under test load, the measurement according to ASTM D1238 is relatively easy. In Method B, flow rates up to 10 g/10 min are measured over a piston travel of 6.35 mm (1/4”), and flow rates starting at 10 g/10 min are measured over a piston travel of 25.4 mm (1”).
The Aflow detects the fill level in the barrel and measures early in the preheat phase
Force-controlled extrusion plastometers, such as the Aflow model from ZwickRoell, function similarly to a capillary rheometer and have significantly expanded control capabilities. Laboratory personnel may agree to use a test sample weight of 5 g for all MFR values above 1 g/10 min. In many cases, the material volume can be easily taken from a container, using the right size measuring scoop. Right at the start of the test, the instrument detects the fill level in the barrel and can measure early on in the preheat phase in which order of magnitude the MVR of the polymer is to be expected. Based on this ongoing available information, the instrument continuously calculates the ideal, ASTM standard-compliant, trigger position from which the piston must move freely under test load to reach the target window at the start of the test. With the flow rate that’s in place just before the target window is reached, the Aflow decides whether to measure over a ¼“ or 1” piston travel.
Laboratory tasks made simple with standard-compliant testing of every polymer
For the user this means that they simply add five grams of material into the extrusion barrel, start the test and rely on the fact that the test will run in compliance with the standard and with optimal test parameters. Lab managers no longer need lists with corresponding parameters for every different type of polymer and can be assured that technicians are running tests correctly. The benefits also extend to auditors: they no longer have to question how the lab ensures that every test on every polymer is run according to standard.
The ISO 1133-1 and ISO 1133-2 counterparts to ASTM D1238, have permitted the use of force-controlled extrusion plastometers for years. Therefore, many laboratories worldwide among molding compound manufacturers and compounders, already rely on the ZwickRoell Aflow for its robust construction, ease of operation, and reliable test results.
