Meaning
Frictional forces occur between the moving layers of a molten polymer as it flows through a nozzle or a mould cavity. This shear rate stress describes the resistance of the fluid to deformation at a specific velocity gradient. In the world of non-Newtonian plastics, the viscosity of the melt decreases as the speed of the flow increases.
Flow Dynamics
High velocities near the gate can generate measurable shear rate stress, leading to localized heating and potential degradation of the resin. This phenomenon is known as shear thinning and is a main factor in the design of injection moulds. Understanding the relationship between pressure drop and flow rate helps in sizing the runners and gates correctly.
Defect Correlation
Excessive stress in the melt can cause cosmetic defects like jetting, splay, blush or structural weak spots in the finished part. Monitoring the shear rate stress helps engineers avoid over-shearing the material, which can break the long polymer chains or the fibers in a reinforced grade. A balanced flow prevents these issues and ensures a uniform fill.
Rheological Profile
Capillary rheometers are used to measure these properties across a range of temperatures and pressures. This data allows for the accurate simulation of the injection cycle.
