What is the Melt Flow Index?

Dynisco Subject Matter Experts

09.14.2023

What is Melt Flow Index?

Polymer science is a complex field. It covers a breadth of quantitative and qualitative approaches to materials analysis and is replete with intricate jargon. One central concept that may be prone to misconception is the melt flow index (MFI). MFI is simply a measure of material output, and not an important value that can be used in production or quality control. This is a measure of how easily a thermoplastic melt will flow. But it neither gives a direct expression of molecular weight nor viscosity. Rather, the melt flow index is an empirical value that ultimately depends on test conditions. So, what makes it important?

This article delves into the intricacies of MFI, elucidating its significance, measurement techniques, and relevance in the polymer industry.

Defining the Melt Flow Index

The melt flow index is often abbreviated as MFI or the melt flow rate (MFR).The quantifiable measurements are expressed as grams per 10 minutes (MFR), grams per cubic centimeter (MVR) and melt density. While this test method does not compute viscosity, MFR and MVR can be correlated to viscosity.  In simpler terms, it gauges the materials ability to flow under specific temperature and force conditions. It defines the mass of a polymer, which is used to determine the material output in weight over time. The force used in the melt flow test is exerted through predetermined weights at specific temperatures listed in the relevant test standards.

The Importance of MFI in Polymer Science

MFI serves as a critical indicator for those involved in the processing of thermoplastics. It offers insights into many key factors, including:

• Processability: MFI offers an indication of how easily polymers can be processed using different methods, enabling manufacturers to select appropriate systems (i.e. extrusion vs injection molding).
• Quality control: MFI values can offer insights into polymer batch consistency and uniformity, helping manufacturers detect and eliminate variations.
• Material selection: MFI may be used as a material selection criteria.
• Addressing recyclability: Understand the impact of using recycled materials in a process. Key information that helps drive the percentage of recycled material that can be used at any given time in the process to maintain quality of the product. 

The melt flow index offers invaluable insights for plastics processors and recyclers, ensuring that the quality of the end product remains consistent. Moreover, understanding the MFI is instrumental in optimizing the processing conditions, thereby ensuring efficiency and quality in production.

Measuring MFI: The Role of the Melt Flow Indexer

MFI is precisely measured using a specialized laboratory instrument called a melt flow indexer. This instrument meticulously maintains a designated temperature range. Concurrently, it employs a consistent weight load to extrude the material through a capillary. An example of such sophisticated equipment is the Dynisco LMI5500 Melt Flow Indexer.

Test Methods and ASTM D1238

The determination of MFI is not a one-size-fits-all approach. Various test methods exist, with ASTM D1238 being a prominent standard. This method is widely recognized and adopted  for its accuracy and reliability in measuring thermoplastic melt volume flow rate.

Molecular Weight and Its Relation to MFI

An interesting facet of MFI is its indirect correlation with the molecular weight of the polymer. Typically, a higher MFI indicates a lower molecular weight and vice versa. This relationship is crucial as the molecular weight can influence various properties of the polymer, from its mechanical strength to its processing behavior.

In the ever-evolving domain of polymer science, understanding and accurately measuring the melt flow index remains a cornerstone. Dynisco manufactures state-of-the-art laboratory melt flow indexers for those who seek precision, reliability, and industry-leading technology when performing critical testing of polymer behavior. Explore the legacy and innovation of Dynisco's Melt Flow Indexers and elevate your polymer processing endeavors.