The “Melt Flow Index” (MFI) of plastic refers to the weight (in grams) of molten plastic that flows through a standardized tube with a specific diameter under a defined load (in kilograms) and temperature (in °C) within a set period of time (typically 10 minutes). A higher MI value indicates better plastic flowability, while a lower MI value suggests poorer flowability. This concept is similar to comparing clear broth and thick soup in the same type of container: when you open the valve, the amount of liquid flowing out within the same period will differ, as clear broth flows more easily than thick soup. Essentially, the thicker the liquid, the less it will flow out.

MFI: Melt Flow Index
MI: Melt Index
MFR: Melt Flow Rate

The MI is generally measured according to the ASTM D1238 standard. The process involves placing plastic or resin into a fixed-diameter metal tube, heating it to a specified temperature to melt the material, and then applying a defined load on a piston. The weight of the molten plastic extruded through a small hole over a 10-minute period is then calculated. The same method defines consistent parameters—such as the inner diameter of the tube, load, and temperature—depending on the material type (e.g., PC, ABS, PA6, PA66). In addition to testing MI, this method can also measure properties like melt density, viscosity, shear rate, and shear stress.

In practical operations, we rarely conduct a full 10-minute test to measure the melt flow because it’s time-consuming and wastes a significant amount of material. Over time, this can result in high costs, particularly for injection molding factories. Instead, we typically run tests for 10 or 20 seconds and use this data to estimate the 10-minute melt flow.

However, shortening the test time introduces measurement errors, especially with older equipment that lacked timers or automatic material-cutting features, which further increased inaccuracies. The test duration needs to be adjusted based on the material’s flow rate: if the flow is too slow, the test time should be extended; if the flow is too fast, the sample size should be increased to improve accuracy.

The primary reason we shorten the MI test duration isn’t just to reduce costs but to ensure the consistency of incoming plastic material quality. This is particularly critical for plastic pellets that have undergone third-party re-pelletizing and dyeing processes. It helps prevent unethical practices, such as mixing recycled material or substituting raw materials. For this reason, it’s essential to document the exact test conditions for each MI measurement. Once the testing conditions are defined, all future tests must follow the same parameters to minimize discrepancies and ensure consistent, reliable results.

The diagram below shows the measuring instrument and structure used in ASTM D1238 Procedure A.

In general, a higher MI (Melt Index) value means that the plastic has lower viscosity and shorter molecular chain structures. In practical applications, the MI value is often used for resin (plastic pellet) incoming quality checks and to monitor product quality during the injection molding process. Additionally, the MI value can serve as a reference to detect whether re-grind materials have been mixed into the plastic and can also be an important indicator to observe material degradation before and after injection molding.

You can think of plastic as a structure made of many intertwined long molecular chains. The longer the molecular chains, the more tangled they become, much like coiled springs. When longer springs are stacked together, they tangle more tightly, making the structure stronger and harder to separate. This also causes the plastic to flow more slowly when molten. On the other hand, plastics with shorter molecular chains have better flowability in their molten state.

When plastic is subjected to high temperatures and the extrusion force of the leadscrew in an injection molding machine, the original long molecular chains may break or shorten. This results in the molded plastic having an MI value approximately 20% to 30% higher than before molding (the exact increase depends on molding conditions and material properties). This explains why re-dyed plastic pellets often have a higher MI value than the original, undyed material. However, when the resin is dyed directly by the manufacturer during production, this issue usually does not occur since the material does not go through re-melting.

If plastic contains re-grind material, the post-molding MI value is typically 50% or more higher than that of the original material (depending on the molding conditions and material type). It’s also worth noting that MI testing can be directly affected by moisture levels, so plastic pellets must be dried and dehumidified according to their specifications before testing.

The duration of the test can also impact the accuracy of the results. If the sampling time is too short, it can lead to larger errors. Longer testing times require more samples, but the results will be more objective and reliable.

Below is an excerpt from ASTM D1238, detailing the materials, temperatures, and load conditions used for MI testing:

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