The relationship between MFI and plastic secondary/recycled materials
Regarding the use of re-grind resin in plastics and whether it causes degradation, most experts agree that it does. However, using MFI (Melt Flow Index) as a definitive way to determine if recycled material has been added is debated in the industry. The general consensus is that MFI can be a useful reference, but it’s not a guaranteed indicator. Here’s a summary of the key points:
According to Wikipedia, MFI measures the flow rate of molten plastic, which depends on the size and entanglement of molecular chains. Theoretically, for thermoplastic materials, repeated heating and cooling cycles should not change the molecular chain size, so MFI should remain consistent.
However, for MFI to remain unchanged after adding re-grind resin, certain conditions must be met:
The re-grind must come from the same type and brand of resin, ideally from the same production batch, usually as sprues from the original virgin resin.
The re-grind must be free of added substances or contaminants.
The re-grind must be stored properly and kept dry.
The injection molding machine’s screw should not damage the polymer chains excessively.
Many believe MFI can indicate the presence of re-grind based on the following:
The screw in the injection molding machine can shorten the polymer chains with each re-melt, causing MFI to increase.
Re-grind is often stored in poor conditions, exposed to the elements, which can degrade its quality.
Re-grind might be added during modification or coloring, especially in black resins, where different brands or types of plastics might be mixed. This makes it difficult to predict whether the MFI will increase or decrease, but it generally tends to rise.
Interestingly, the MFI of a resin typically increases by 20%-50% after being re-ground from a molded product. The exact increase depends on the practices of the injection molding factory.