Melt Flow Index (MFI) Guide

Melt Flow Index (MFI), also known as Melt Flow Rate (MFR), is the most commonly used indicator to evaluate the flowability of thermoplastics in the molten state.

1. Core Definition

MFI is defined as the mass of polymer melt that flows through a standard die (capillary) within 10 minutes under specified temperature and load conditions.

Unit: g/10 min

Physical Meaning:

Higher MFI → Lower melt viscosity → Better flowability → Lower molecular weight

Lower MFI → Higher melt viscosity → Poor flowability → Higher molecular weight

2. Test Standards and Conditions

Testing must strictly follow international standards. Different materials require different temperature and load conditions.

Main Standards:

ISO 1133 (International)

ASTM D1238 (USA)

Typical Test Conditions:

Material	Temperature	Load
PP (Polypropylene)	230°C	2.16 kg
PE (Polyethylene)	190°C	2.16 kg
ABS	220°C	10 kg
PC (Polycarbonate)	300°C	1.2 kg

3. Test Method

(1) Preparation

Add plastic granules into a preheated barrel and allow them to melt.

(2) Loading

Apply standard weight on the piston to generate pressure.

(3) Measurement

After stable flow, cut the extrudate at fixed time intervals.

(4) Calculation

Weigh the sample and convert it into mass per 10 minutes (g/10 min).

4. Key Influencing Factors

(1) Molecular Weight (Most Critical)

Higher molecular weight → longer chains → more entanglement → lower MFI

(2) Molecular Structure

More testing and flexible chains → higher MFI

(3) Additives

Plasticizers / oils → increase MFI

Fillers / glass fiber → decrease MFI

(4) Test Conditions

Higher temperature or load → higher MFI value

5. Industrial Applications

(1) Material Selection

Injection molding (thin-wall / complex parts):

Use high MFI (e.g., 30–50) for better filling

Extrusion / blow molding (pipes / bottles):

Use low MFI (e.g., 0.5–5) for better melt strength

(2) Quality Control

Monitor batch consistency

Detect degradation of recycled materials (MFI usually increases)

(3) Process Optimization

Guide temperature and pressure settings

6. MFI vs. MFR vs. MVR

MFI / MI / MFR: Mass flow rate (g/10 min), most commonly used

MVR: Volume flow rate (cm³/10 min), suitable for materials with density variation

7. Melt Flow Index Testers

MFI is measured using a Melt Flow Index Tester (MFI Tester / Melt Flow Rate Tester).

1. Classification by Function

(1) Manual Type

Manual cutting, weighing, and calculation

Low cost, basic testing

Measures MFR only

(2) Automatic Cutting Type

Automatic timing and cutting, manual weighing

Improved consistency

Suitable for factory QC

(3) Fully Automatic Type

Automatic cutting, weighing, calculation, and printing

Displays MFR, MVR, density

High precision temperature control

(4) Multi-load Type

Supports multiple weights (2.16 kg, 5 kg, 10 kg)

Used for FRR (Flow Rate Ratio)

(5) Dual Measurement Type (MFR + MVR)

Measures both mass and volume flow

Suitable for filled/modified plastics

2. Classification by Method

Mass Method (Mainstream, ISO 1133 / ASTM D1238)

Volume Method (MVR)

Advanced models with auto cleaning and feeding

3. Accuracy Comparison

Type	Temperature Accuracy	Timing Accuracy	Weighing	Repeatability
Manual	±1–3°C	Large error	Manual	5%–15%
Auto Cut + Manual Weigh	±0.5–1°C	<0.1 s	Manual	2%–5%
Fully Automatic	±0.2–0.5°C	<0.05 s	Automatic	1%–2%
High-End (MFR+MVR)	±0.1°C	<0.01 s	Auto + displacement	<0.5%–1%

4. Key Factors Affecting Accuracy

Temperature stability (most critical)

→ 1°C variation may cause 3%–10% error

Manual cutting errors

Dirty or degraded die

Non-standard weights or piston misalignment

Summary

Manual tester: Suitable for rough evaluation only

Semi-automatic: Best cost-performance for factory QC

Fully automatic: Required for R&D and certification

High-end dual method: Used for advanced material characterization

2026-04-22 14:36