A single number is tempting. An MFR of 25, an RV of 2.4 — it looks like enough to judge a material. For polyamides especially, it is one of the quickest ways to buy the wrong grade.
The problem
A buyer sees one flow or viscosity value on a datasheet or COA and decides from it whether a material suits injection moulding or extrusion — or whether two lots are equivalent. MFR/MFI and RV are both 'flow-related', so they get treated as interchangeable. They are not.
Why it happens
MFR (melt flow rate, sometimes MFI) and RV (relative/solution viscosity) describe different things. MFR is a melt-flow measurement under a defined temperature and load — a processing-oriented number. RV is a solution measurement that tracks molecular weight — a material-structure number. They correlate loosely within one polymer, but the relationship shifts with the polymer, the additive package and, critically, moisture. For PA6 and PA66, moisture alone can move an MFR reading enough to make a good lot look wrong — or a wrong lot look fine.
As rough orientation only: higher MFR / lower RV means easier flow and lower molecular weight — often suited to thin-wall, fast-fill injection moulding. Lower MFR / higher RV means higher melt strength and molecular weight — typically what extrusion, blow moulding or high-mechanical parts need. The direction, never the decision on its own.
How to read it properly
Never judge from one value in isolation. Put the full picture together before you decide:
- Test method — know whether you are looking at MFR or RV, and the standard behind it — they are not comparable across methods.
- Temperature & load — an MFR only means something with its test conditions (e.g. 275 °C / 0.325 kg); compare like with like.
- Moisture state — was the sample dried before testing? For PA, an undried sample reads differently.
- RV / molecular weight — for polyamides, RV is often the more honest indicator of whether a grade matches your process.
- The application — injection vs extrusion vs blow moulding decides which end of the range you actually need.
Key takeaway
One flow number is an orientation, not a specification. For PA6/PA66, read MFR and RV together, with their test conditions and moisture state — a single value compared across different methods or moisture states will mislead you.
Match the viscosity window to the process, not the headline number to the habit.
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