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.
Trapezoidal Runner Section
Meaning
Feed channel geometries cut exclusively into a single mold plate feature tapered sidewalls and a flat base to facilitate ejection while providing fluid delivery. Machining a trapezoidal runner section offers a cost-effective compromise between full round profiles and flat modified-trapezoid shapes in two-plate tooling. Boundary limits restrict this geometry to materials that release easily from single-sided plate cuts without sticking in un-drafted corners.
Machining Efficiency
Single-plate cutter paths cut tooling fabrication costs in half compared to matching dual-plate circular channels. Utilizing a trapezoidal runner section eliminates runner mismatch misalignment across parting lines.
Ejection Behavior
Draft angle along the runner sidewalls governs how easily solidified feed channels release from plate cavities upon mold opening. Incorporating a trapezoidal runner section with a five to ten degree side taper prevents runner hanging on the stationary mold half during ejector stroke execution. Semi-crystalline polymers with high shrinkage rates release readily, whereas sticky amorphous polymers like thermoplastic polyurethane require higher draft angles or polished surface finishes.
Regrind resin content increases thermal contraction variance, which can cause sticking if draft angles are insufficient. Molders specify cold runner surface finishes of N5 or better along the tapered walls to ensure consistent ejection during high-speed production cycles.
Flow Profile
Corner sharp radii in flat-bottomed channels increase pressure drop and promote skin growth during filling. Operating with a trapezoidal runner section creates higher shear stress near channel base corners than circular runners, leading to minor pressure losses. Moulders compensate for this pressure loss by slightly increasing channel depth or boosting injection velocity settings.