House Blends Shipped under a Branded Resin Grade Name
House blends sold under branded grade names alter molecular weight distribution and melt rheology, requiring lot-level shear, thermal, and impact testing.
Melt Shear Sensitivity
Meaning
The degree to which the viscosity of a molten polymer decreases in response to an increase in the rate of shear flow defines the processability of the resin. This characteristic, known as melt shear sensitivity, governs the behavior of polymers as they travel through narrow gates and thin part walls. The property applies to pseudo-plastic materials like polystyrene and polycarbonate but does not apply to Newtonian fluids that maintain constant viscosity regardless of flow speed.
Rheological Behavior
Polymers with high shear sensitivity experience rapid thinning as flow velocity increases. This flow property relies on the untangling of long polymer chains when they are forced through high-velocity areas. Linear polymers generally show different sensitivity than branched molecular structures.
Melt Flow
In injection molding, high melt shear sensitivity helps fill thin-walled sections without requiring extremely high melt temperatures. Viscosity drops dramatically as the polymer passes through the gate, which facilitates rapid mold filling. This behavior allows molders to reduce injection pressures during the filling phase.
Process Adjustment
Processing technicians use shear heating to lower the flow resistance of the material without degrading it in the barrel. However, if the shear rates are too high, the polymer can degrade and cause cosmetic defects such as silver streaks or splay. Injection speed profile must be tuned to stay within the shear limits of the resin.