This is a answer to a question from a community member asking about plastic quality control. The question was: “Can we rely solely on MI (Melt Index) values to monitor the quality of plastic resins? And does an increase in MI indicate plastic property degradation?” Many people believe there is no direct correlation between MI values and cracking in plastic products.
When discussing whether using re-grind resin in plastics leads to degradation, Workingbear sought advice from industry experts. Most professionals agreed that it does cause degradation. However, opinions on whether MFI values can reliably indicate the presence of re-grind resin are divided within the industry. The conclusion? It’s uncertain but worth considering. Below is a summary of the information compiled by Workingbear.
It’s an open secret in the plastic injection molding industry that re-grind resin is often added during injection. Some manufacturers even claim that, given the current competitive environment and cost-cutting pressures, it’s impossible to make a profit without using some re-grind resin. Plus, they argue it promotes recycling and aligns with today’s emphasis on environmental sustainability. However, from a quality perspective, adding re-grind resin introduces risks of reduced strength and increased brittleness in the structure of plastic products.
The use of re-grind resin and recycled material in plastic injection molding is a persistent challenge for engineers. The more re-grind resin or recycled material is added, the more severe the degradation of the plastic becomes. This degradation weakens the material’s structural strength and increases the risk of brittle fractures, especially in stress-bearing areas such as hooks and screw holes.
However, incorporating re-grind and recycled materials in plastic parts is also an environmental issue. To support sustainability and reduce waste, adding an appropriate amount of re-grind or recycled material during the injection molding process can help balance material performance and environmental protection.
Workingbear’s company recently encountered a quality issue with a plastic product. The problem was, “Why is it that products made from the same type of plastic resin sometimes show normal MFI (Melt Flow Index) values, while others have unusually high values? When we asked the plastic injection supplier, they insisted they hadn’t added any regrind material.”
