ASTM D1238: How to Use a Melt Flow Index Tester to Determine the Melt Flow Rate of Polymers

The ASTM D1238 test method is used to determine the Melt Flow Index (MFI) of thermoplastic polymers. The ASTM D1238 MFI is a basic measurement for quality control and acceptance of incoming products. It is part of the ‘ID’ of a polymer and used to verify if its grade is within the required fluidity range. The ASTM D1238 method is commonly used for polyolefins (polyethylene such as HDPE, LDPE, LLDPE, and polypropylene).

ASTM D1238 Test Method

ASTM D1238 covers the measurement of the rate of extrusion of molten resins through a standard die (2.095 x 8 mm). The test is performed under prescribed conditions of temperature, load, and piston position in the barrel as the timed measurement is being made. The standard includes four different test procedures:

A, B, C, and D. The main difference between ISO 1133 and ASTM D1238 is that, in the latter,  procedure C is related to polyolefins with a high flow rate and prescribes the use of a die with half dimensions (length and diameter), which reduces the flow rate. High fluidity samples can also be tested by using a die plug, inserted before the material charge and removed prior to removing the piston/weight support.

ASTM D1238 Procedure D is related to the application of different weights to the same sample charge to obtain the melt mass-flow rate (MFR) data under different test conditions. 

This allows the automatic calculation of the Flow Rate Ratio (FRR), which is the ratio of the average flow rate at the higher load to the average flow rate at the lower load. FRR is commonly used as an indication of the way in which rheological behavior is influenced by the molecular mass distribution of the material.

Plastic Melt Flow Index Testers: Key Recommendations

The Instron MFi Series is ideal to test the melt flow properties of a High Density PE sample at 190° C using different weights.

The Instron MFi7 is equipped with a high-resolution encoder and a motorized weight lifter, as well as an automatic cutting system that sections the extrudate at precise, repeatable intervals. In addition, its unique load cell-controlled material compacting system, capable of applying a controlled force of up to 750 N, accurately defines and monitors the material softening level prior to testing. This controlled pre-heating and compaction phase is critical, as it has a strong influence on measurement repeatability.

At the end of the test, we recommend cleaning the instrument thoroughly with our barrel cleaning device to reduce the testing time and to optimize the quality control process.

For chemically aggressive materials, such as PVC or Fluoropolymers, we developed a special corrosion-resistant kit (barrel, die, piston) that can be provided with all our instruments.