Direct Pin Sensor Placement and Dynamic Viscosity Control in Micro-Moulds
Direct pin pressure sensing in micro-moulds compensates for melt viscosity drift by dynamically adjusting injection speed to hold precise shear rates.
Micro Runner Shear
Meaning
Rheological behavior describes the intense frictional heating and molecular orientation that occurs when molten polymer flows through narrow delivery channels in a mould. High micro runner shear increases the melt temperature and reduces the viscosity of the flowing resin just before it enters the cavity. This localized heating can assist in filling thin-walled parts but also increases the risk of polymer degradation if the gate design is incorrect.
Viscosity Reduction
Non-Newtonian fluid dynamics cause the viscosity of the polymer melt to drop sharply under high deformation rates. As the molten plastic passes through micro runners, the shear rate climbs, aligning the molecular chains in the flow direction. This alignment reduces flow resistance, allowing the melt to travel further into complex mould details.
The sudden reduction in viscosity is temporary and reverses once the flow stops and the material begins to cool.
Degradation Consequence
Excessive shear forces generate intense localized heat that can exceed the thermal limits of the resin. This thermal stress breaks polymer chains, resulting in reduced mechanical strength and severe cosmetic defects like silver streaks on the part surface. To prevent these defects, tool designers must optimize the runner diameter and gate transition angles.
Process engineers can also adjust injection velocity to keep the shear heating within safe limits. In extreme cases, the polymer can release acidic gases that corrode the steel surfaces of the mould cavity.
Cavity Filling
Thin-wall moulding requires a precise balance between flowability and structural integrity. Controlled micro runner shear allows the use of higher molecular weight resins that would otherwise freeze before filling the cavity. This processing strategy helps achieve thin-walled geometries while maintaining the impact strength of the finished product.