LimsPulse
Polymers, plastics & coatings · Thermoplastic granules, powder or regrind (example: polyethylene)

Melt flow index of plastics

A few grams of polymer are melted in the heated barrel of a melt flow indexer and extruded through a standard die under a fixed load. Timed cut-offs are weighed and the result is expressed as grams per 10 minutes.

  • Chemicals & materials
  • Extrusion plastometer (Procedure A, mass method)
  • MFI tester
  • Analytical balance
  • Vacuum oven

Reference: Typical method based on ISO 1133-1 and ASTM D1238 (Procedure A). Use the test condition from the material specification.

PRINCIPLE

Why this method works

At a fixed temperature and load, the rate at which polymer melt flows through the standard die depends on its viscosity. A higher MFR means an easier-flowing, usually lower-molecular-weight grade. The mean mass of cut-offs taken over a known time is scaled to 600 s (10 minutes).

Instruments used

SAMPLE PREPARATION

Preparing the sample

  1. 01

    Condition

    Dry the polymer if it is hygroscopic or if the specification says so.

  2. 02

    Weigh charge

    Weigh 4–8 g of granules, depending on the expected MFR.

  3. 03

    Load & preheat

    Charge the barrel, compact and preheat for 5 minutes.

  4. 04

    Extrude & cut

    Apply the load and take timed cut-offs in the measuring zone.

  5. 05

    Weigh

    Weigh the cooled cut-offs and calculate MFR.

Procedure

  1. 1Set the instrument to the test condition, for example 190 °C/2.16 kg for polyethylene or 230 °C/2.16 kg for polypropylene, and let it stabilise.
  2. 2Dry hygroscopic polymers (for example PA, PET, PC) as specified; polyolefins usually need no drying.
  3. 3Charge the barrel within about 1 minute and compact the granules with the packing rod.
  4. 4Insert the piston and preheat for 5 minutes.
  5. 5Add the test weight. Discard the extrudate until the piston's lower reference mark reaches the barrel top.
  6. 6Cut the extrudate at a fixed time interval (for example every 30 s for MFR 3–10 g/10 min) and collect at least three bubble-free cut-offs.
  7. 7Weigh each cut-off to 1 mg when cool and calculate the mean.

Standard preparation

  1. 1Run a reference polymer of known MFR periodically as a check on the instrument.

Notes for analysts

  • MVR (cm³/10 min) is measured by piston travel (Procedure B). MFR = MVR × melt density.
  • A rise in MFR of moulded parts or regrind compared with virgin resin indicates chain scission during processing.
  • Moisture in hygroscopic polymers causes hydrolysis and high, erratic MFR; dry first.
TEST CONDITIONS

Method parameters (typical)

Test conditionPolymer-specific, for example PE 190 °C/2.16 kg; PP 230 °C/2.16 kg; PS 200 °C/5 kg
Die2.095 mm bore, 8.000 mm length
Preheat time5 minutes
Charge massAbout 4–8 g, depending on expected MFR
Cut-off intervalChosen from the expected MFR (shorter for high MFR), as the standard's table suggests
ReplicatesAt least three cut-offs per test
BEFORE YOU REPORT

System suitability & sequence

ParameterLimit
Cut-off agreementMasses of individual cut-offs typically agree within about 10% (or the limit in the standard)
Reference polymerWithin its assigned range

Typical sequence

  1. 1Stabilise temperature
  2. 2Reference polymer check (periodically)
  3. 3Sample test
  4. 4Clean barrel and die
CALCULATION

Calculation with a worked example

Formula
MFR (g/10 min) = m × 600 ÷ t

Where

m
Mean mass of the cut-offs (g)
t
Cut-off time interval (s)
600
Seconds in 10 minutes

Given

Condition
Polyethylene, 190 °C / 2.16 kg
Cut-off interval (t)
30 s
Cut-off masses
0.312, 0.318, 0.315, 0.309, 0.316 g

Working

  1. 1
    Mean mass m = (0.312 + 0.318 + 0.315 + 0.309 + 0.316) ÷ 5 = 1.570 ÷ 5 = 0.314 g
  2. 2
    MFR = 0.314 × 600 ÷ 30 = 6.28 g/10 min

MFR (190 °C/2.16 kg) = 6.3 g/10 min

ACCEPTANCE CRITERIA

What result passes

  • Compare with the grade specification or supplier CoA, for example nominal MFR ± a stated tolerance (often about ±10–20% of nominal, depending on grade).
  • Always report the test condition with the result, for example 'MFR (190 °C/2.16 kg) = 6.3 g/10 min'.
METHOD VALIDATION

Validation parameters (ICH Q2(R2))

ParameterTypical requirement
PrecisionRepeatability on replicate charges; compare with precision data in ISO 1133 / ASTM D1238
AccuracyReference polymer of assigned MFR
In LimsPulse · Sample Management

Run this method in LimsPulse: specifications and STPs under version control, results calculated and checked against limits, and a signed Certificate of Analysis.

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