Introduction
The Melt Flow Index (MFI) is a crucial property of plastic materials, especially in manufacturing processes like injection molding and extrusion. It helps determine how easily a plastic will flow when melted, which directly impacts processing and the quality of the final product.
What is Melt Index of Plastic?
The “Melt Index” (MI) of plastic, also known as the “Melt Flow Index” (MFI) or “Melt Flow Rate”(MFR), measures how much melted plastic flows through a specific diameter tube in 10 minutes under set weight and temperature conditions.
The melt flow index test is a crucial method for assessing the flow properties of thermoplastic materials, providing essential metrics such as melt index and melt flow index, which are vital for ensuring the quality and processability of plastic products.
A higher MI value means the plastic flows more easily, while a lower MI value means it flows less easily. Melt flow rate values are key indicators of a material’s ability to flow under pressure, relating to molecular weight and viscosity, and are important for selecting appropriate grades of polymer for various processing methods.
Why Melt Flow Rate is Important for Injection Molding?
The Melt Index (MI) value is important for injection molding because it affects several key aspects of the molding process and the quality of the final product:
Flow Characteristics: MI measures how easily the molten plastic flows. A higher MI, or high melt flow rate, indicates better flowability, which helps the plastic fill intricate mold designs and thin-walled parts more effectively.
Processing Conditions: MI affects the temperature and pressure settings during molding. Plastics with a high MI generally require lower temperatures and pressures, which can reduce energy use and extend the life of machinery.
Cycle Time: Higher MI materials typically fill molds faster and cool down more quickly, leading to shorter cycle times and increased production efficiency.
Material Properties: The MI value impacts the mechanical properties of the finished product. Plastics with high MI may have shorter molecular chains and can be less strong or impact-resistant compared to those with lower MI. This is because a high melt flow rate is associated with low molecular weight, indicating shorter molecular chains and lower strength.
Surface Quality: Materials with appropriate MI values can provide better surface finishes and reduce defects like flow marks or voids.
Consistency: Maintaining the right MI ensures consistent processing and product quality, minimizing variations that can affect the final product’s performance and appearance.
Molecular Weight Distribution: The broadness of the molecular weight distribution can be assessed by comparing ratios of melt flow rates for a given material under different conditions. This is important for selecting appropriate materials for various processing applications.
Method of Testing Melt Flow Index
The Melt Flow Index (MFI) test measures how easily a thermoplastic polymer melts and flows. The test involves the following steps:
Step 1: Preparation:
The Melt Flow Index (MFI) test measures how easily a thermoplastic polymer melts and flows. The test involves the following steps:
Step 2: Loading:
A specific weight (usually in kilograms) is applied to a piston inside the heated barrel. This piston applies pressure to the molten plastic, forcing it through a die with a standardized diameter.
Step 3: Extrusion:
The sample is allowed to melt completely for a few minutes. The tester is then activated, and the piston begins to move downward at a constant speed, pushing the melted plastic through the die’s orifice.
Step 4: Measurement:
The amount of plastic that flows through the die in 10 minutes is collected and weighed. This weight, in grams, represents the Melt Flow Index.
Step 5: Calculation:
The Melt Flow Index is calculated based on the rate of extrusion, which is determined by the time it takes for a specific amount (e.g., 10 grams) of polymer to flow through the die.
Metal Index Measurement Standard
The MI (Melt Index) is usually measured using the ASTM D1238 method. In this process, plastic or resin is placed in a metal tube with a fixed diameter and heated until it melts. A set weight is then applied to a piston, pushing the melted plastic through a small hole. The flow rate ratio (FRR) is determined using Method D, which is essential for understanding the flow behavior of materials during testing.
The weight of the plastic that comes out is measured. Since the tube diameter, load, and temperature are constant for the same material, this method can also be used to measure other properties like melt mass flow rate, melt volume flow rate, melt density, viscosity, shear rate, and shear stress.
A higher MI value usually means the plastic has lower viscosity and shorter molecular chains. In practice, MI is used to check the quality of resin when it’s received, monitor product quality during injection molding, and detect if recycled material has been mixed in. It can also help explain any degradation that happens before or after injection molding.
Common Plastics Melt Index
The following is an excerpt from ASTM D1238 regarding material, temperature, and load conditions for the MI test:
| MATERIAL | MFI (°C/Kg) |
| Acrylonitrile-butadiene-styrene (ABS) | 220/10 |
| Acetals (copolymer and homopolymer) | 230/1.2 |
| Acrylic | 230/1.2 |
| ABS+PC | 230/3.8 |
| Ethylene-chlorotrifluoroethylene copolymer | 271.5/2.16 |
| Ethylene-tetrafluoroethylene copolymer | 297/5.0 |
| Nylon PA66 | 275/0.325 |
| Nylon PA6 | 235/2.16 |
| Perfluoro (ethylene-propylene) copolymer | 372/2.16 |
| Perfluoroalkoxyalkane | 372/5.0 |
| Polycaprolactone | 125/2.16 |
| Polychlorotrifluorethylene (PCTFE) | 265/12.5 |
| Polyether sulfone (PES) | 380/2.16 |
| Polyethylene (PE) | 125/0.325 |
| Polycarbonate (PC) | 300/1.2 |
| Polymonochlorotrifluoroethylene | 265/21.6 |
| Polypropylene | 230/2.16 |
| Polystyrene (PS or HIPS) | 200/5.0 |
| Polysulfone (PSU) | 343/2.16 |
| PBT | 250/2.16 |
| PET | 285/2.16 |
| Poly (phenylene sulfide) (PPS) | 315/5.0 |
Effect of Recycled Materials on Molecular Weight Distribution
We can think of plastic as being made of long, tangled molecular chains. The longer these chains are, the more they get tangled, like coiled springs. When they are more tangled, they stick together more tightly, making the melted plastic flow more slowly. When plastic is heated and goes through an injection molding machine, the machine’s screw can break these chains, affecting the flow properties of the material.
If you test the MI of the plastic after molding, you’ll usually find it’s 20-30% higher than before, depending on the molding conditions and material. This also explains why dyed plastic resin (unless originally colored by the manufacturer) usually has a higher MI than undyed resin. When the manufacturer dyes the resin during production, the MI stays lower because there’s no need to re-melt it. The mass flow rate of the material can also be influenced by these factors.
The melt flow test is a standard method for evaluating the flow properties of thermoplastic materials during the extrusion molding process. It helps determine material processability through standardized procedures.
For plastic parts made from recycled material, the MI value is often more than 50% higher after molding compared to before (depending on conditions and materials). Also, moisture can affect the MI test results, so it’s important to dry the resin properly before testing.
The length of the test also matters. Shorter tests can be less accurate, while longer tests are more reliable but need more samples. Melt flow testing is essential for polymer processing, as it determines the flow capability of polymers and helps in selecting appropriate resins and optimizing production efficiency.