Why Does Injection Part Strength Degrade After Using Recycled Materials
Introduction
Using recycled materials (re-grind resin) in plastic parts is a challenge for engineers. The more recycled material is added, the more severe the plastic degradation becomes, leading to reduced structural strength and an increased risk of brittle fractures, especially in stress-bearing areas like hooks and screw holes. However, using recycled plastic is often seen as an environmental necessity.
But why does using recycled material weaken the plastic?
This issue can be explored from two angles:
Source of Recycled Materials: Plastic manufacturers often add recycled materials not just for environmental reasons, but to cut costs, as recycled material is cheaper. However, these recycled materials may not be of the same quality as the original plastic. For example, if the specified plastic is Sabic PC, the manufacturer might use recycled PC from various sources, or even mix it with ABS, leading to further degradation. Additionally, recycled materials are often stored improperly, exposed to the elements, which can further degrade their quality.
MFI Value Before and After Injection Molding: Even if the manufacturer uses high-quality recycled material, the Melt Flow Index (MFI) typically increases by 20-30% after injection molding, indicating reduced material strength. This occurs because the long molecular chains that give plastic its strength are broken during the injection process. As the plastic is pushed and sheared by the heated screw in the injection machine, these chains are shortened, weakening the material’s overall strength.
Summary
In summary, the degradation of plastic strength when using recycled materials is due to both the quality of the recycled material and the physical stresses introduced during the injection molding process.