The Impact of Melt Flow Index (MFI) on Injection Molding

What is Melt Flow Index (MFI)?

The Melt Flow Index (MFI), also known as Melt Flow Rate (MFR), is a measure of a plastic’s flowability in its molten state. Simply put, it indicates how many grams of molten plastic can be extruded in 10 minutes under high temperature and pressure.

How is Melt Flow Index (MFI) Tested?

Testing Method: Following international standards (e.g., ASTM D1238 or ISO 1133), plastic pellets are heated to a specified temperature (e.g., 190°C) and then forced through a standardized die under a defined load (e.g., 2.16 kg). The weight of extruded plastic in 10 minutes is measured (unit: g/10min).
Common Notation: For example, PP (230°C/2.16kg) = 25 g/10min means polypropylene extrudes 25 grams in 10 minutes at 230°C under 2.16 kg pressure.

What Does High/Low Melt Flow Index (MFI) Indicate?

High MFI (e.g., 50 g/10min): The plastic flows easily, making it easier to process, but it may have lower strength.
Low MFI (e.g., 2 g/10min): The plastic flows poorly, making processing more difficult, but it typically offers higher strength.

How Melt Flow Index (MFI) Affects Injection Molding

MFI directly impacts production efficiency, product quality, and cost in injection molding.

1.1 Flowability

High MFI Plastics (e.g., PP, PE): Flow like “thin syrup,” ideal for thin-walled, intricate parts (e.g., bottle caps, packaging films).
Low MFI Plastics (e.g., PC, PA): Flow like “thick honey,” better suited for thick-walled, high-strength products (e.g., gears, automotive parts).

1.2 Injection Pressure & Temperature

High MFI: Requires lower pressure and temperature, reducing energy consumption and machine wear.
Low MFI: Needs higher pressure and temperature; otherwise, incomplete filling (“short shot”) may occur.

1.3 Product Appearance & Quality

High MFI Pros: Fills molds easily, good for complex shapes.
Cons: Prone to flash (excess material), higher shrinkage, possible warping.
Low MFI Pros: Smoother surface, higher strength, lower shrinkage, better dimensional stability.
Cons: Poor flow may cause “weld lines” (weak seams where melt fronts meet).

1.4 Material Selection by MFI Range

MFI (g/10min)	Applications	Example Materials
1–5	High-strength, thick parts (e.g., automotive, engineering plastics)	PC, PA66, POM
5–20	General-purpose parts (e.g., household items, toys)	PP, HDPE
20–50	Thin-walled, fast-molding (e.g., packaging, caps)	LDPE, PP
>50	Ultra-thin products (e.g., fibers, films)	LLDPE, specialty PP

How to Choose the Right Melt Flow Index (MFI) Material?

Part Design: Thin/complex → High MFI; thick/strong → Low MFI.
Machine Capability: High MFI works with standard machines; low MFI may need high-pressure machines.
Cost: High MFI materials are often cheaper and faster to process but may sacrifice strength.

Common Issues & Solutions

Issue	Cause	Solution
Incomplete filling (short shot)	MFI too low	Increase temp/pressure or switch to higher MFI
Flash (burrs)	MFI too high	Reduce pressure or use lower MFI
Visible weld lines	Poor melt fusion (low MFI)	Raise mold temp, optimize gate design