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.
Shear Thinning Rheology
Meaning
Fluid behavior in which viscosity decreases under escalating shear strain governs the flow of molten polymers through narrow runners and gates. Injection moulders exploit shear thinning rheology to fill thin-walled cavities quickly without requiring excessive injection pressures. This non-Newtonian behavior occurs because the long polymer chains untangle and align themselves in the direction of the melt flow.
Process Advantage
Rapid movement of the polymer melt through the gate dramatically reduces the flow resistance. This viscosity reduction allows the moulding machine to fill complex, multi-cavity moulds at manageable pressures. Once the injection phase ends and the flow stops, the polymer viscosity returns to its high resting state.
This transition helps to pack the cavity and prevents the material from flashing out of the parting line.
Tool Design
Mold designers use the flow characteristics of the resin to calculate the optimum sizes of gates and runners. If the gate is too small, the local shear rate becomes too high, which can thermally degrade the polymer chains. This degradation leads to weak weld lines and brittle spots in the finished plastic part.
Accurate flow simulation software predicts these shear heating effects to prevent cosmetic defects and structural failures before the steel mold is cut.
Material Behavior
Different polymer structures show distinct responses to shear strain. Linear polymers like high-density polyethylene thin more readily than branched polymers like low-density polyethylene. Understanding these differences allows the processing engineer to adjust screw speed and injection velocity to match the specific resin being processed.