Closed-Loop Valve Gate Control Mechanics in High-Cavitation Tools
Closed-loop servo valve gate control uses cavity pressure and needle position feedback to eliminate weight variance and cut cycle times in multi-cavity tools.
Velocity Profiling
Meaning
Control strategies divide the injection stroke of a screw into distinct zones with different forward speeds. The use of velocity profiling allows a moulder to adjust the flow rate of the melt front as it moves through different geometries of the mould cavity. This control is critical for maintaining a uniform filling pattern in parts with varying wall thicknesses.
Injection Control
Programmed speed steps reduce the velocity as the melt passes through the restrictive gate to prevent high shear rates. With velocity profiling, the controller ramps up the speed as the flow front enters wider areas of the cavity and slows it down again as it reaches the end of the part. This variable speed schedule minimizes polymer degradation while ensuring the cavity fills completely.
Defect Elimination
Uncontrolled flow rates lead to surface defects such as jetting, gate blush, and burn marks caused by trapped air. Applying velocity profiling helps prevent jetting by ensuring that the plastic emerges from the gate at a speed that allows it to form a stable, spreading melt front. Slower speeds at the end of the stroke also allow the cavity air to vent safely, preventing combustion of the gas.
Process Repeatability
Consistent screw speed execution is necessary to hold the same cushion volume and transition point from cycle to cycle. Because resin viscosity varies with shear rate, velocity profiling provides a stable shear history for the polymer melt, which leads to predictable shrinkage and part dimensions. This predictability allows the moulder to maintain tight tolerances during long production runs, even when using resins with minor batch-to-batch variations.