Runner Diameter Calculator
Determine the optimal runner diameter based on part weight and runner length. The calculator accounts for runner cross-section type and number of cavities, computing pressure drop and material waste.
Input Parameters
Results
Fill in the data and click Calculate
ARGUS automatically optimizes the runner system based on mold geometry
Runner diameter affects pressure drop, runner cooling time, and material waste — ARGUS balances these parameters automatically.
How do we calculate runner diameter?
Runner diameter is a compromise between pressure drop, material waste, and cooling time. A runner that is too small causes excessive pressure drop and cavity fill problems; one that is too large extends cycle time (the runner must cool) and increases material waste in cold-runner systems.
The calculator applies an empirical diameter selection formula based on part weight, runner length, and number of cavities. The formula includes correction factors for different runner cross-section types.
d — runner diameter [mm]
m — part weight [g]
n — number of cavities
flength — runner length factor
ftype — cross-section type factor
The full round runner is the most hydraulically efficient — it has the lowest perimeter-to-cross-section-area ratio, minimizing freeze-off and pressure drop. The trapezoidal runner is a compromise used when the mold splits in a single plane. The half-round runner is least efficient but simplest to machine.
Typical runner diameters
The following indicative values apply to a round runner up to 100 mm long:
Medium parts (10–100 g) — 5–8 mm
Large parts (100–500 g) — 8–12 mm
Very large parts (>500 g) — 10–16 mm
For long runners (>150 mm) the diameter should be increased by 10–20% to compensate for the additional pressure drop. In multi-cavity molds with a symmetrical runner layout, flow balancing is critical — all cavities should fill simultaneously.
When to consider a hot runner?
If material waste on the cold runner exceeds 15–20% of shot weight, or runner cooling time is limiting cycle time, it is worth considering a hot runner system. The investment typically pays back at volumes above 50,000 parts. The hot runner expansion calculator will help select the system parameters.
ARGUS automatically optimizes the runner system based on the full project context
See it for yourself — book a presentation and discover how ARGUS balances runner parameters against process requirements.