Melt Flow Index testing

By jnewmanazo1
Published: January 31st, 2013

Here is the scenario...I have a material cert from the manufacturer that shows a melt flow index of      11 g/10min with test conditions of 300°C/1.2Kg. We want to test the incoming material on our melt indexer but do not have a 1.2Kg weight. We only have a 4.2Kg weight. If I go ahead and run the test at 300°C/4.2Kg, is there some mathematical cartwheels I can do that would correlate the two indexes and tell me if they are close to equivalent?

ASTM D1238 (I believe) gives

ASTM D1238 (I believe) gives specifics for the test. All materials are tested at a specified temperature and weight. I would not suggest you simply change the weight and try to do some "mathematical cartwheels" (love that term) to determine what the MFI at the lower weight would be. Not sure you can even do that without having additional information. I guess you could do a benchmark study if you test the material that you are certain you have the actual ASTM data on with the correct weight and then use the higher weight and record the data to use that as your benchmark. Perhaps you can build an internal spec but I would test more that one lot of material in this manner because there probably won't be a linear relationship with changes in MFI at the lighter weight versus those at a heavier weight. PLUS the big problem is that I am pretty sure if there was a problem and you tried to reject the material your material supplier would not accept your findings since you did not follow the ASTM test procedures.

The weight drives the shear rate, which in turn drives the "viscosity" value in terms of grams/10 mins...if you can think of viscosity in those terms. Be careful of this test and how much you rely on it. It is at very low shear rates, done at one temperature, and the shear rates are not controlled in this test. Plus there are not flow corrections taken into account (entrance loss being one of them). If you are using the test for incoming lot to lot inspection, it should be fine provided you have the correct weight. Using the test to compare one grade of material to another to make a material switch may get you into some troubles...even if the two materials have the same MFI value.

Maybe it is just that I am

Maybe it is just that I am not familiar with how the Dynisco Indexer in question really works. I think that when you are initially setting up the test you have to enter temp and the load being used. Maybe the tester does the work and spits out the results no matter what weight you use therefore the MFR shown should be similar no matter which weight was selected? Or am I completely off base on this?

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