The Importance of Melt Flow Index in Polymer Processing

In polymer processing, MFI is perhaps the most important variable that eventually dictates the quality and performance of plastic materials. From plastic product manufacturing to polymer testing and even choosing a material fit for any production process, holding on to the Melt Flow Index Tester MFI can truly make or break a product. What exactly is MFI, its importance, and its effects on polymer processing? Let us discuss everything related to the melt flow index in this blog.

What Is Melt Flow Index? 

The melt flow index is the flow rate of a polymer in a molten form. It is determined by measuring the amount of polymer. Flowing in grams through a die of standard dimensions within a time interval at a predetermined temperature and load.

Generally, it is expressed in grams per 10 minutes (g/10min); a high MFI value shows the polymer flows easily (low viscosity), whereas a low MFI value describes harder flow (high viscosity).

In other words, MFI indicates the degree of "runniness" or "thickness" of a polymer after it is heated to its melting temperature. So the easier it flows, the higher the MFI will be, and vice versa. 

The MFI in Polymer Processing: Why Is It Important? 

The Melt Flow Index is important in polymer processing due to the following reasons:

Material Selection and Consistency

The MFI helps manufacturers in the selection of an ideal polymer for their application. Different products and processes require polymers with different flow properties. For example:

1. Injection molding is done with polymers that flow with ease to fill the molds with precision and volume.
2. Extrusion uses polymers with various flow characteristics to form products such as films, pipes, or fibers with a consistent shape and size.

If the ASTM MFI stays unchanged, then the batch of polymers will behave in the required manner during the manufacturing process, which reduces the variability in the product and hence increases its reliability and quality.

Determination of Processing Conditions

While processing the polymer, one should also consider the behaviour of polymers at high heat and pressure. Processing temperature and pressure are usually set to the values given by the MFI to make the polymer that is going to be processed render good properties. For example, polymers having higher melt flow index values are usually required to be processed at lower temperatures or under less pressure, while polymers with lower melt flow index values generally are processed at comparatively higher temperatures or pressures.

Predicting Product Properties

The MFI value can give some indication of the physical properties of the final product, such as:

1. Strength and durability: A lower MFI generally means stronger materials that are more resistant to tough conditions, as polymer chains are tightly packed and less mobile.
2. Flexibility and elongation: A polymer with a higher MFI would behave more flexibly and would exhibit more elongation properties.
3. Molecular weight distribution: The MFI can give information about the molecular weight and weight distribution of the polymer. A polymer with a broader molecular weight distribution will be better balanced between processing ease and product strength.

Quality Control and Compliance

The MFI is extensively used for control during quality assurance. Based on the MFI of polymers being tested from various batches, a manufacturer will ascertain if the material complies with the specified specs. This is especially pertinent for industries wherein strict compliance to standards is required-of automotive, medical, and packaging, to mention a few. Deviations outside an acceptable MFI range signify potential for contaminants in the polymer, bad compounding, or improper storage.

Cost-Benefit Efficiency

The MFI serves as a parameter for cost and efficiency determination. A polymer with a high MFI may be a costly material, but it also could afford shorter cycle times and thus efficiency in production. Conversely, polymers with lower MFI may need greater processing times but produce products that are more durable and less costly in the end. Knowing MFI will make material selection and process parameter choices easier for the manufacturer, and the resulting condition should provide the highest cost-performance ratio.

How Is a Melt Flow Index Measured?

At an international level, the melt flow index is measured per ASTM D1238 and ISO 1133. In general, from a small start, the following cables are followed in bulk:

• Sample preparation: A small amount of polymer sample is weighed into the barrel sample, which is usually in tiny pellet or powder form.
• Heating: The barrel is heated to a pre-selected temperature, usually the melting point of the polymer, to melt the polymer.
• Flowing: The piston, under a known pressure, forces the molten polymer to flow through the die for a given period (usually 10 minutes). The amount of polymer flowing through the die within the set time is measured and recorded as the Melt Flow Index.

The resulting value, called MFI, is given as the corrected mass of the polymer that flows within 10 minutes under standard conditions.

Types of MFI Testing

There are various types of MFI tests depending on the material and the performance required. For example:

• Standard MFI Test: This is the ordinary test to determine the rate of flow under normal conditions.
• High-Temperature MFI: Used for testing materials that are processed at very high temperatures.
• Low-Temperature MFI: Suitable for polymers that are heat-sensitive or need testing at low temperatures.

Applications of MFI in Polymer Processing

The Melt Flow Index Tester value controls and modifies various stages of polymer processing and is responsible for the flow characterization of the final product.

• Injection Molding: Injection molding uses the MFI to select polymers that flow adequately to fill intricate molds. Adequate MFI will allow the resin to completely fill the mold and maintain the shape until cooling; poor MFI allows mold filling inconsistently, leaving defects like voids or incomplete filling.
• Extrusion: Film, pipe, or sheet extrusion is controlled depending on the MFI. The higher the MFI, the more the polymer can get extruded with low pressure and heat, making extruding itself an easy process.
• Blow Molding: A polymer is supposed to flow in blow molding evenly with thin stretch layers. The polymer is assessed using MFI to produce an even, thin layer with the required shape.
• Film Fabrication: Plastic films for packaging, etc., require MFI measurement for the determination of thickness and uniformity of the film. Thus, choosing the most suitable MFI will result in producing excellent films.

Limitations of MFI

While the MFI is an important parameter, it does have its limitations:

• Not a Complete Measure of Quality: MFI does not show mechanical properties of polymers such as tensile strength, impact resistance, or heat deflection temperature. 
• Variability in Results: Test parameters such as temperature, pressure, and sample preparation vary, and these increase variability in the MFI results. 
• Representation of Long-Term Performance: MFI mostly evaluates flow behaviour during processing but does not indicate how the polymer will perform over time in the real field.

Conclusion

A crucial tool of polymer processing is the Melt Flow Index (MFI), which manufacturers and material suppliers use to quantify a polymer’s flow characteristics. Material selection, processing conditions, quality control, and prediction of final product properties depend on it. However, although MFI provides important information, a polymer user comprises many more parameters while considering the ultimate suitability of a polymer for a particular application.

Thus, by understanding and applying the Melt Flow Index, manufacturers can better tailor their processing methods, establish product uniformity, and, lastly, assure that they receive optimum results in terms of performance and cost.

For more in-depth information on polymer testing and processing, head over to Test Your Product and get advanced testing options for your materials, satisfying the highest quality standards for your products.