The Cold Runner Trap: Why Saving 10% on Scrap Plastic Will Cost You the Entire Mold

You are reviewing a new mold design for a multi-cavity plastic part. To save on material costs and reduce regrind, someone on the team suggests reducing the cold runner diameter from 6.0 mm to 4.5 mm.

It seems like a smart, lean manufacturing move. But when the mold goes to the press, the technicians can't fill the cavities, the parts are full of sink marks, and trying to fix it results in flashing the tool.

Why? Because you ignored the fluid dynamics of polymer melt.

Melt flow in a circular runner is governed by a variation of the Hagen-Poiseuille equation for pressure drop (ΔP). While plastics are non-Newtonian, the fundamental mathematical relationship remains brutally unforgiving:

ΔP = (8 · Q · η · L) / (π · r⁴)

Where:

• ΔP = Pressure drop through the runner
• Q = Volumetric flow rate
• η = Melt viscosity
• L = Length of the runner
• r = Radius of the runner

The most critical variable in that entire equation is r⁴ (the radius to the fourth power) sitting in the denominator.

💡 The Example

Let’s look at the math of the proposed "cost-saving" change.

Your original runner has a radius of 3.0 mm.
r⁴ = 3.0⁴ = 81.0

You reduce the radius to 2.25 mm (a 4.5 mm diameter).
r⁴ = 2.25⁴ = 25.6

The Result: By reducing the runner diameter by just 25%, the denominator in your pressure drop equation shrinks from 81.0 to 25.6.

You didn't just increase the pressure drop by 25%. You more than tripled it (81.0 / 25.6 = 3.16).

An injection molding press only has a finite amount of available injection pressure (typically 20,000 to 30,000 psi). If you burn all your available pressure just forcing the plastic through a starved, restrictive feed system, you will reach a "pressure limited" state. You will have zero pack-and-hold pressure left when the plastic actually reaches the cavity, guaranteeing dimensional instability, voids, and high internal stress.

🛠️ The Solution

1. Budget Your Pressure: Never guess runner sizes. The feed system (sprue, runner, and gate) should be mathematically sized so that it consumes no more than 20% to 30% of the machine's maximum available injection pressure.
2. Use Full-Round Runners: If you must optimize, optimize the shape, not just the size. Never use half-round runners. They have a terrible volume-to-surface-area ratio, meaning the plastic freezes prematurely. Full-round runners provide the lowest pressure drop and best flow dynamics.
3. Cut Steel Safe: It is incredibly easy to machine a runner larger if you need less pressure drop. It is very expensive to weld a runner shut and re-machine it smaller. Always start with a conservatively sized feed system and open it up only if the data demands it.

Stop trying to save pennies in the runner system if it means sacrificing the pressure you need to mold a profitable part.