Piezoelectric Cavity Pressure Sensor Setup in Micro Injection Moulding
Direct piezoelectric cavity pressure sensing in micro injection moulding requires sub-micron bore tolerances, rigid preloading, and active thermal drift compensation.
Melt Front Velocity
Meaning
Linear speed at which the molten polymer advances through the mould cavity during the filling stage determines the local shear rate and molecular orientation. Optimizing the melt front velocity prevents defects such as jetting, hesitation, and surface dullness in moulded parts. This variable depends on both the screw speed and the flow cross-section of the cavity.
Flow Dynamics
Molten resin behaves as a non-Newtonian fluid whose viscosity decreases as shear rates rise. As the melt front velocity increases, the shear-thinning behavior reduces the flow resistance. This effect helps in filling thin sections.
Process Control
Injection moulding machines control the screw position over time to maintain a constant melt front velocity as the flow path changes. If the cavity widens, the machine must accelerate the screw to prevent the flow front from slowing down and cooling prematurely. Modern controllers use multi-step injection profiles to adjust the volumetric flow rate based on the cavity geometry.
Defect Prevention
Slow flow fronts allow the resin to cool too quickly, causing premature solidification and surface blemishes near the gate. Conversely, an excessive melt front velocity causes frictional heating that can degrade the polymer and generate burn marks or flash. Maintaining a uniform velocity throughout the fill phase is essential for producing high-quality parts with uniform shrinkage.