Meaning
Dynamic flow of molten polymer into a closed mold impression under controlled velocity and pressure establishes part geometry during injection molding. Molders monitor mold cavity filling to ensure uniform melt front progression and prevent defects like short shots or flash. The injection stage transforms hydraulic or electric actuator movement into volumetric melt flow through runners and gates into complex cavity geometries.
Flow Kinetics
Non-Newtonian polymer melts exhibit shear thinning behavior as flow velocity increases through narrow gate restrictions. High shear rates lower melt viscosity, reducing injection pressure requirements while raising localized frictional heating. Laminar flow fronts freeze upon contacting cold mold steel, forming a thin skin layer that constricts the core flow channel.
Defect Prevention
Imbalanced fill patterns across multi-cavity tools cause localized overpacking and uneven part shrinkage. Air traps occur when converging melt fronts seal off venting channels before air escapes from cavity pockets. Adjusting fill velocity profiles prevents jetting and reduces weld line severity in structural components.
Process Control
Cavity pressure sensors detect the exact moment transition from velocity-controlled filling to pressure-controlled packing should occur. Decoupled molding techniques isolate filling speed from packing pressure, holding part weight tolerances across raw material lot variations. Consistent fill times maintain thermal balance across long production runs.
