Designing Balanced Multi Cavity Runner Systems for Polymer Injection Tooling
Balanced multi cavity runners match flow length, shear history, and pressure drop across all branches to achieve simultaneous mold cavity filling.
Pressure Drop Runner Branching
Meaning
Flow channel cross-section step-downs at runner junctions offset cumulative friction losses as molten polymer travels through branching networks. Applying pressure drop runner branching equalizes pressure delivery at each gate location by progressively reducing runner diameters along downstream paths. Application stops at micro-moulding scales where surface tension and wall slip effects dominate flow behavior over shear fluid mechanics.
Hydraulic Resistance
Friction along feed channel walls absorbs hydraulic energy during fill cycles. Implementing pressure drop runner branching maintains constant pressure gradients across all active flow paths.
Branch Sizing
Calculating diameter transitions across primary, secondary and tertiary channels requires applying power-law fluid models. Incorporating pressure drop runner branching ensures that flow resistance per unit length increases in downstream channels to balance shortened remaining flow distances. Sizing formulas scale sub-runner diameters to roughly seventy percent of main runner diameters at each binary split junction.
When processing glass-filled resins, precise diameter steps prevent localized fiber orientation changes that lead to part warpage. Regrind addition increases melt elasticity, which alters entrance pressure loss at branch splits, forcing process engineers to adjust injection speed to maintain design pressure distributions.
Fill Balance
Symmetrical hydraulic head across gates ensures simultaneous cavity volumetric filling. Utilizing pressure drop runner branching prevents early cavity packing and localized flash in multi-cavity tools. Balanced filling lowers part mass variability and stabilizes part dimensions across multi-cavity production runs.