Meaning
Rheological discrepancy occurs when two polymer components used in a blend or co injection process have significantly different viscosities under processing temperatures. A melt flow rate mismatch can result in poor dispersion, flow instability, and severe physical defects in the molded part. It represents a critical variable that processors must manage when compounding recycled resins or mixing masterbatches with virgin materials.
Viscosity Discrepancy
Flow differences lead to uneven shear heating and non uniform melt fronts in the mold. When a melt flow rate mismatch exists between the masterbatch carrier resin and the base resin, the two materials do not melt at the same rate. This causes the lower viscosity component to flow ahead of the main melt stream during injection.
It can lead to jetting or uneven filling of the mold cavities.
Dispersion Failure
Effective blending requires the two polymers to co deform under shear force. If a melt flow rate mismatch is too high, the low viscosity resin behaves like a lubricant, slipping past the high viscosity resin without transferring the shear necessary to disperse additives. This failure results in localized clusters of pigment or additive concentration.
The molded part exhibits poor color consistency and erratic physical properties, which often leads to the rejection of entire production batches.
Mechanical Defect
Uneven flow during molding leads to internal stress and delamination. Under a severe melt flow rate mismatch, the finished part can suffer from weak weld lines and peeling surfaces. These weaknesses reduce the impact strength and long term durability of the plastic component.
Moulders can mitigate this by adjusting zone temperatures or choosing carrier resins that match the flow of the base material.
