Meaning
Cumulative shear stress and thermal exposure experienced by a polymer melt during processing affect its final molecular structure and flow behavior. The accumulation of melt shear history can alter the entanglement density of the polymer chains and reduce the melt viscosity. This change influences how the resin fills the mould cavity during subsequent injection steps.
Moulders must monitor this variable to ensure part consistency.
Viscosity Reduction
High shear rates in the runner system of an injection mould heat the polymer through viscous dissipation. Due to melt shear history, the resin flows faster than predicted.
Part Performance
The physical properties of a molded plastic part are directly linked to the alignment and length of its polymer chains. When melt shear history includes excessive shear stress, the polymer chains are broken or highly oriented. This orientation increases the strength of the part in the flow direction but makes it weak in the transverse direction.
Such anisotropy can lead to warping and premature failure under load. Moulders should balance the injection speed to minimize this orientation.
Extruder Tuning
Extruder screws must be designed to melt the resin without causing excessive mechanical stress. To optimize the melt shear history, engineers adjust the screw speed and the temperature profile of the extruder barrels. A lower screw speed reduces the shear rate in the mixing zones.
This adjustment prevents the resin from overheating and degrading. It ensures that the physical properties of the incoming virgin resin are preserved during the compounding step.
