Hot runner or cold runner? A decision guide

The runner system is the channels that carry the plastic to the gate. A cold runner solidifies together with the part every cycle and is either discarded or reground. A hot runner uses a heated manifold to keep the melt hot all the way to the gate; no runner piece is formed.

Comparison

When is a hot runner the right choice?

• High-volume production: runner waste and cycle gains pay off the mold cost quickly.
• Expensive material (engineering plastics, filled grades): runner waste is a significant cost item.
• Thin-walled or multi-cavity molds: short flow path, lower pressure, balanced filling.
• Automation: Since you don't have to separate the runner, robot pick-off becomes easier.

When is a cold runner the right choice?

• Low-volume or prototype molds.
• Jobs with frequent color or material changes.
• Very heat-sensitive materials (e.g. PVC): a hot runner carries a degradation risk due to residence time.
• When budget and maintenance capacity are limited.

The cold-runner trap

If the cold runner is thick, it cools slower than the part, and that's what determines the cycle. Ejection has to wait for the runner to cool, not the part. So don't make the runner diameter unnecessarily large; the runner should be the smallest diameter that can still deliver the fill pressure to the cavities without needlessly wasting it. Otherwise both pressure drop and cooling time increase.

In between: hybrid solutions

There are systems that combine a hot-runner tip nozzle with cold-runner distribution, or valve-gated hot runners that leave no mark on the part surface; choosing these should be done together with the material and mold designer.

In the calculation tool you can select the runner type and see its effect on pressure distribution and cycle; you'll get a warning if the cold runner is cooling slower than the part.