Calculate the Melt Flow Index of Plastics Using Correct Methods

Knowing the Melt Flow Index (MFI) tester becomes essential for professionals dealing with plastics, whether it be during product development, sourcing of materials, or quality control in the lab. But let's face it-MFI seems like just another technical jargon from the field of polymer science. 

True to its implications, in this piece we will use very simple language and eventually guide you through its calculation. You will know what MFI is and how to carry out its measurement through means accepted by the industry after reading the article in its entirety. 

Understanding Melt Flow Index (MFI)

Melt Flow Index, or sometimes Melt Flow Rate (MFR), is a measure of the ease of flow of a thermoplastic when melted; the "thinness" or "goopiness" of melted plastic, if you will. 

Higher the MFI, the easier the flow of that plastic in the molten state. This also usually indicates a lower viscosity (a more runny material). Conversely, the lower the MFI, the thicker the plastic is in the molten state and the slower it flows. 

The MFI understanding enables the manufacturers to

• Select appropriate plastic for injection molding, extrusion, or 3D printing.
• Control process consistency between batches produced.
• Comparison of materials from different suppliers.

Importance of MFI

Imagine trying to use honey in a soda fountain: not going to work, right? A similar fate will befall you if you select a plastic with an inappropriate flow rate for your application. 

For thin-wall, highly detailed parts, like electronic casings, use a plastic with a high MFI so that it fills the mold easily.

For thick-walled structural or load-bearing parts (think gears or automotive parts), a lower MFI is preferred. A lower MFI means thicker plastics that are stronger and tougher. 

Getting the right MFI = getting the performance and finish you need in your product.

Equipment Used to Measure MFI

Standard measurement of MFI uses the melt flow indexer or K extrusion plastometer. It consists of a preheated barrel with a small die at the bottom through which the molten plastic is extruded under a specified weight.

In general, the setup should include

• A Melt Flow Indexer
• A preheated barrel (190°C or 230°C, depending on the thermoplastic under investigation)
• A standard weight for testing (2.16 kg or 5 kg)
• A good stopwatch
• A cutter or blade for slicing the extruded filament
• An analytical balance to weigh the samples accurately

Calculation of MFI

The melt flow index is measured by the weight of the plastic that flows out from the die during a fixed period. The basic formula states that. 

MFI (g/10 min) = (Weight of extrudate in grams / Time in seconds) × 600

So this is a flow rate calculated in grams per 10 minutes, the unit standard of industries. 

Step-by-Step Procedure for MFI Testing 

The ASTM D1238 standard provides the MFI test method that is most widely accepted. The procedure is simplified as below: 

Step 1: Preheat the Machine

Heat the melt flow indexer barrel to the proper temperature for testing, which will vary according to the plastic being tested. For example, polypropylene would be tested at approximately 230°C.

Step 2: Load the Plastic Sample

Place approximately 4 to 5 grams of plastic pellets into the barrel. Slightly compact this sample with the piston so that there are no air pockets.

Step 3: Apply the Test Load

Add the appropriate test weight (usually 2.16kg for general use). This weight pushes the molten plastic through the die. The material is allowed to condition under load for a set preheat time of a standard 7 minutes.

Step 4: Extrusion

After this time, the material is extruded through the die. Cut the extruded strand with a cutter or blade after specified time intervals, mostly every 30 seconds.

Step 5: Weigh the Extruded Segments

Collect the plastic that has been extruded and weigh it on an analytical balance. Accuracy is critical in the weighing procedure, as any small difference will have drastic implications for the calculated MFI.

Step 6: Do the Calculation

In our example, 3.5 grams of plastic were collected in 30 seconds. Using 

MFI = (3.5 g / 30 s) × 600 = 70 g/10 min

This is the Melt Flow Index for this particular sample.

Some Tips for Reliable and Consistent Results

Use standard equipment- consistency in MFI measurements is of utmost importance.

• Regularly calibrate your melt flow indexer and use standard equipment. Accuracy depends on regular maintenance and calibration.
• In your testing, ensure the time intervals always remain the same. Use a stopwatch to ensure that extruded samples are sliced at the same intervals during every measuring procedure. 
• If possible, test as many samples as you can. Running multiple tests on the same samples and then averaging the results gives a better value for MFI. 
• Record everything, including material type, temperature, die dimensions, weight used, and sample batch. This will enable tracing back from any variation in test results. 

Real-World Example: 

Suppose your company makes polypropylene caps for medical syringes. To achieve a high-quality mold filling and precise definition of the part, the resin to be used must have an MFI of about 12 g/10 min. Should a batch from a new supplier be found to test at 6 g/10 min, the material could simply be too viscous; an impracticality would impede mold filling, extend cycle time, and may cause defects. 

Early testing of MFI could prevent many production failures from arising. It is a small investment in time and trouble that will gain an inspector and performance. 

Final Thoughts

The melt flow index is a useful and popular method for molten plastics to characterize their flow. Despite being technical at first sight, its basic intent is to estimate the behavior of a material during processing and ultimately in service. Whether in R&D, production, or quality control, knowing and measuring MFI will serve as a reliable guide to instruct in making any selection with certainty on materials and processes. 

At Test Your Product, we help companies determine essential attributes such as MFI with clarity and accuracy. Should you have any queries concerning your materials or need to test them against industry standards? Let us guide you through every step. 

Need help testing your plastic's MFI? Contact the lab crew at Test Your Product, and we will think through all the technical stuff for you so that you can concentrate on producing excellent products.