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Melt Flow Index (MFI) Testing

Over many years' experience with melt flow index (MFI) testing with many operators we have concluded that for our purposes as a custom injection molder, we do not sweat getting exact numbers compared to the numbers published by the resin supplier. We have built up our own database of acceptable ranges of melt flow index based upon our testing alone.

If there comes a time where a dispute occurs, i.e. we have a reading unacceptable outside of the range of our database of readings and we have revivified by repeated tests. We will use an A2LA registered outside lab service to perform a more sophisticated characterization of the resin. This may be via FTIR, GPC, TGA looking for the presence of contaminants or hybrid molecules of the same resin, and assuming we detect something abnormal, take this to resin maker. This generally ends the dispute. Fortunately this doesn't happen very often with the major resin makers and is more likely to occur with compounders or even bulk distributors who have been known to make packaging/labelling errors when they repackage the material in bags or barrels out of a bulk train car load or similar shipment that they have received.

Also, we keep a small 50-100 gram sample of the resins upon receipt for the purpose of being able to compare to past receipts or troubleshooting our melt flow index test. If we have not used the retained sample in a year we discard it.

For the benefit of those who may not be familiar with the material tests listed above:
FTIR= Fourier Transform Infrared Spectroscopy
GPC= Gel Permeation Chromatography
TGA= Thermo-Gravitation Analysis
Not all of these tests are suitable to all resins and some are more expensive than others. Contact your A2LA registered lab for guidance. Tell them what your concerns are and ask which tests are most appropriate, and how much that they will cost.

Our experience has been that the FTIR is probably the test that comes closest to a "universal" test applicable to almost any resin or fluid (those of you who are evaluating the quality of your lubricating and hydraulic fluids).

The biggest drawback to FTIR is that the laboratory must have a standard FTIR fingerprint on file and with some specialty resins and compounded resins, this is not always the case.

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