What Is Your Resin's Melt Flow Index and Why Can It Ruin Mold Filling?
The melt flow index (MFI) measures how fast your resin moves through the equipment. When it isn't right for your process, no masterbatch adjustment can fully fix an incomplete mold fill.
The mold has delivered parts with an incompletely filled corner for three runs in a row. The operator raises injection pressure, then barrel temperature, and when neither fixes the defect, suspicion falls on the recently purchased masterbatch. The color gets checked, the dosage gets checked, the supplier gets checked. No one has checked the piece of data that was on the resin's technical sheet from the start: its melt flow index.
The melt flow index (MFI) measures how fast your resin moves through the equipment. When it isn't right for your process, no masterbatch adjustment can fully fix an incomplete mold fill.
The incomplete fill no one connects to the base resin
When a part comes out with an unfilled area, the first suspect is usually color: insufficient dosage, poor dispersion, or a low-quality masterbatch. It's a reasonable hypothesis, but it leaves out the variable that makes up most of the part's mass: the resin itself.
What the melt flow index actually measures
MFI indicates how fast or slow the molten material moves through the equipment under standard pressure and temperature conditions. A resin with a lower MFI than your mold requires flows with more resistance, and that resistance translates directly into incomplete fill zones, especially in geometries with thin walls or long flow paths.
Why masterbatch can't compensate for an unsuitable MFI in your process
Resin makes up between 95% and 98% of the final product's mass. Masterbatch contributes color and some functional properties, but its volume within the mix is too small to fully change the flow behavior that the base resin defines. If the melt flow index isn't right for the process, productivity and mold filling are affected no matter how much you adjust the color dosage.
The mistake of treating each variable separately
When the plant team reviews masterbatch, temperature, and pressure as isolated problems, weeks can go by adjusting parameters that don't address the real cause. The correct diagnosis starts by confirming whether the MFI of the resin in use matches what the process and the mold's geometry require.
What your quality team should require before approving a resin or a lot change
Every resin lot's technical sheet should include the confirmed melt flow index, not just a generic reference value from the supplier. Before running a full lot, it's worth validating a small sample and comparing fill behavior against the mold's history. At Pigmentos Químicos we review the MFI declared by the client as part of the color development process, because that figure shapes much of the final result on the plant floor.
Frequently asked questions
Can masterbatch correct an unsuitable MFI?
Not completely. Masterbatch contributes color and some functional properties, but it can't change the flow behavior the base resin defines on its own.
How do I know if the incomplete fill comes from the MFI or the color?
Compare the defect against previous runs with the same resin and the same masterbatch. If the incomplete fill appears right after a resin lot change, the MFI is the first thing to validate.
Do all resins of the same grade have the same MFI?
Not necessarily. There can be variation between lots of the same commercial grade, which is why it's worth confirming the value on each lot's technical sheet instead of assuming the general reference figure.
