What Is a Cold Runner?

A cold runner is an unheated channel system in an injection mold that carries molten plastic from the sprue to the gates. The plastic in it cools and solidifies along with the parts on every shot, then is ejected as a solid runner that is cut off and scrapped or reground. In short, a cold runner is the simple, low-cost way to feed a mold, and the price is a little plastic scrap on every cycle.

What Is a Runner in Injection Molding?

The runner is the network of channels that distributes plastic from the sprue to each gate and cavity. In a cold-runner mold those channels are cut into the mold plates and sit at mold temperature, below the plastic's melting point, so the plastic in them freezes with the part. When the mold opens, the sprue, runners and parts come out together as one "tree", and the parts are separated from it.

There are two common cold-runner layouts. In a two-plate mold the runner sits on the main parting line and each part is gated from the side. A three-plate mold adds a second parting line that separates the runner from the parts, which lets the runner sit above the parts and feed them from the top rather than the edge.

In a multi-cavity mold the runner is usually balanced: it branches symmetrically, so every cavity sees the same flow length and fills at the same time.

Hot Runner vs Cold Runner

Cold runners are often the default choice because they are cheaper to build and maintain, take up less space in the mold and are more forgiving of heat-sensitive materials. Hot runners earn their cost when volumes are high enough that scrap and cycle time dominate the part price.

How Is a Cold Runner Designed?

• Size the runner to the part. One common convention sets the runner feeding each cavity at about 1.5–3 times the part's nominal wall thickness, so it stays open long enough to pack the part.
• Keep it short. A long runner means more pressure drop, more scrap and more cooling time.
• Don't oversize it. A runner that is too large wastes material and adds cooling time; one that is too small causes short shots and sink marks.
• Balance multi-cavity layouts, so no cavity overfills while another is still filling.
• Plan for regrind. Check whether the part's material and specification allow regrounded runners to be blended back in, and at what ratio.

Feed-system design is set during mold design for every plastic injection molding project. In liquid silicone rubber molding the logic is reversed: the mold is hot and the runner is kept cool so the silicone does not cure in it.

Frequently Asked Questions

What is a cold runner mold? It is an injection mold whose runners are not heated. The plastic in the sprue and runners solidifies with the parts on each shot and is ejected with them, then cut off and scrapped or reground.

What is the difference between a hot runner and a cold runner? A hot runner keeps the plastic in its channels molten with a heated manifold, so there is little or no runner scrap and cycles are often shorter. A cold runner is unheated and freezes each shot, which creates scrap but costs less to build and maintain.

What is a runner in injection molding? The runner is the channel system that carries molten plastic from the sprue to the gates of each cavity. It can be cold, freezing with the part each cycle, or hot, kept molten by heaters inside the mold.