Shrink rate
Dimensions after fill and pack.
Shrink rateGlossary
What Is Melt Flow Index? Injection Molding Explained
Melt flow index is a lab measure of how easily a resin flows under standard test conditions.
Glossary
Melt flow index measures how easily a resin flows under standard lab conditions
Buyers use MFI to compare grades when fill difficulty is a concern.
Higher MFI grades generally fill thin walls more easily in many families.
Impact and strength can trade off when MFI rises within the same resin family.
Shop floor shear rates differ from lab tests, yet MFI still helps shortlist grades.
Selection
When MFI belongs in material conversations
Pair MFI with part geometry, not in isolation.
- Long flow lengths with thin nominal wall
- Large parts where last fill region shorts on first trials
- Comparing two approved grades within one family on the same tool
Go deeper
Where this term connects
Each link is a different angle on the same program, not a duplicate definition.
Gate
Restriction affects shear at entry.
GateHygroscopic resin
Moisture changes process window.
Hygroscopic resinPlastic molding
Production on hardened steel tools.
Plastic moldingVisual reference
Photos from molding and tooling programs
Images are selected from the NetProto Forge media library for this topic.
Injection molded production parts on a tray
Injection molded production parts on a tray
Engineer reviewing a molded part drawing
Engineer reviewing a molded part drawing
Plastic housing components from a steel tool
Plastic housing components from a steel tool
FAQ
Melt flow index questions
Is MFI the same as viscosity at the press?
No, but it correlates for comparing grades in RFQ review.
Does higher MFI always mean lower strength?
Not always. Read datasheets for the properties you need.
Should MFI appear on the mold drawing?
Material spec belongs on part and mold docs, not orphan MFI alone.
Next step
State resin family on RFQ
Upload CAD and material targets for fill review.