How to Design a Runner System for Multi-Cavity Molds

Step 1: Select Runner Layout Pattern

The runner layout determines how evenly resin distributes across all cavities. Three standard patterns cover most applications:

The H-pattern is the industry default for power-of-2 cavity counts because it provides natural balance — every cavity receives resin through an identical flow path length. This eliminates the need for flow adjustment during process setup. According to ScienceDirect's runner system design reference, naturally balanced runners achieve cavity-to-cavity weight variation below 1%, while artificially balanced runners typically achieve 2–5%.

Step 2: Size the Primary Runner

Runner diameter affects three competing requirements:

• Too small — High pressure drop, premature freeze-off, incomplete packing
• Too large — Excessive runner volume, longer cooling time, more material waste
• Optimal — Delivers adequate flow and packing pressure while minimizing cooling time

The empirical sizing progression:

Step 3: Design Cross-Section Geometry

Runner cross-section shape affects flow efficiency and ease of ejection:

• Full round — Most efficient flow (lowest surface-to-volume ratio). Requires machining in both mold halves — each half carries a semicircle. This is the preferred cross-section for three-plate molds.
• Trapezoidal — Machined in one plate only. Slightly less efficient than full round but simpler to manufacture. Common in two-plate molds where the runner is on the parting line.
• Half round — Semicircle machined in one plate. Not recommended — poor flow efficiency and difficult runner ejection.

Step 4: Balance Cavity Filling

Even with a geometrically balanced runner layout, real-world molds often show fill imbalance due to thermal variations across the mold and shear-induced flow effects in the runner bends. Two methods address this:

• Runner diameter adjustment — Increase runner diameter to cavities that fill last; decrease diameter to cavities that fill first. This is a permanent change to the mold steel.
• Flow rate adjustment pins — Adjustable pins or bolts inserted into the runner channel that restrict flow to over-filling cavities. This is adjustable in the field without modifying the mold steel — the preferred method for initial mold trials.

According to ISO 20457, cavity-to-cavity weight variation should be specified on the part drawing and verified during mold qualification. Typical specifications range from ±1% for precision parts to ±3% for general purpose molding.

Step 5: Select Runner System Components

For three-plate molds, the runner system requires these components beyond the runner channels themselves:

• Runner lock pins — Install 1 pin per 50–80 mm of runner length to hold the runner during first-stage opening
• Runner ejector sets — Install 1 set per 60–100 mm of runner length for reliable ejection
• Runner change pins — Install at every runner direction change between plates
• Sprue bushing — The entry point must match the injection machine nozzle SR