Meaning
Viscosity variation occurring when a recycled polymer is subjected to repeated mechanical work during processing.
Molecular Degradation
Successive passes through an extruder break the long polymer chains into shorter segments through a process called scission. Regrind shear sensitivity describes how quickly the material loses its structural integrity under these conditions. High temperatures and high screw speeds accelerate this breakdown.
A material with high sensitivity will show a dramatic increase in melt flow rate after only one recycle loop.
Process Control
Moulding shops must limit the percentage of regrind mixed with virgin resin to maintain part consistency. Because regrind shear sensitivity affects how the melt flows into the cavity, it can cause shifts in part weight and dimensions. Sophisticated control systems monitor the injection pressure to compensate for these variations.
If the sensitivity is too high, the material is unsuitable for structural applications.
Economic Impact
The ability to reuse scrap directly affects the profitability of a moulding operation. Understanding regrind shear sensitivity allows for the optimization of the regrind ratio without sacrificing quality. Testing involves comparing the properties of the material after multiple extrusion cycles.
This data helps in selecting resins that are stable enough for closed-loop manufacturing.
