Meaning
Rheological changes occur when the flow characteristics of a polymer melt vary over time or across different batches. While viscosity drift can result from moisture content in hygroscopic resins, it also stems from the thermal degradation of the polymer chains during prolonged residence times in the barrel. Moulders observe this phenomenon as changes in the pressure required to fill the cavity at a constant speed.
Rheology Change
Variations in the molecular weight of the resin directly influence the resistance to flow. When a batch contains a high percentage of regrind, the viscosity drift usually moves toward a lower resistance as the polymer chains are shortened by multiple heat cycles. This shift causes the material to flow more easily into thin sections but increases the risk of flash.
Process Shift
Maintaining consistent part dimensions requires a stable relationship between speed and pressure. If viscosity drift occurs, the machine must adjust its hydraulic force to maintain the same injection velocity. Modern controllers can compensate for minor shifts but extreme variations lead to part weight instability.
The injection curve shows a clear deviation when the material flow properties change.
Batch Instability
Storing resin in humid conditions without proper drying is a common cause of flow inconsistency. Hydrolysis breaks the bonds in polymers like nylon or polyester, creating a noticeable viscosity drift that ruins the mechanical properties of the part.
