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. A hot runner keeps that plastic molten instead.

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 runner	Hot runner
How it works	Unheated channels; plastic freezes each shot	Heated manifold and nozzles; plastic stays molten between shots
Runner scrap	A solid runner every shot, scrapped or reground	Little or none
Tooling cost	Lower	Higher, with heaters and temperature controls to fit and maintain
Cycle time	The runner must cool too, which can lengthen the cycle	Often shorter, since only the part has to cool
Color and material changes	Quick; the old runner comes out with the parts	Slower; the heated channels must be purged
Heat-sensitive plastics	Shorter time at temperature	Longer heat exposure can degrade some resins
Best fit	Prototype tools, low to mid volumes, frequent color changes	High volumes where material savings pay back the system

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 reground 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.

Related Services at NICE Rapid

• Plastic Injection Molding — single- and multi-cavity tooling, from aluminum molds for short runs to P20/H13 steel for production volumes.
• LSR Molding — self-sealing silicone molds, with a cold runner system specified where material saving is required.
• Request a Quote — share your design files and annual volume; our team confirms tooling lead time and unit cost.

Sources

1. US Patent 9,610,721, Background — cold-runner definition (runners solidify each cycle and are "removed from the mold as a solid mass of plastic"); hot-runner definition (polymer "remains molten"); cold runner "typically discarded as scrap or reground"; large cold-runner volume "can extend the cycle times"; hot-runner material must be purged on color or material changes and is exposed to heat longer; runner nearest the cavity "typically ranges from about 1.5 to about 3 times the nominal wall thickness" — patents.google.com
2. US Patent Application 2003/0185931, Background — hot runners are heated above the resin's solidification temperature while cold-runner temperature "generally approximates the temperature of the mold"; hot runners "tend to be more costly to fabricate and maintain and typically take up more space"; added thermal history can degrade the resin; "cold runner systems are oftentimes the systems of choice"; balanced runners for multi-cavity molds — patents.google.com
3. MIT OpenCourseWare, 2.008 Design and Manufacturing II, Lecture 03: Injection Molding II (Spring 2025) — runner definition; runner too big ("longer cooling time, more material, cost"), too small ("short shot, sink mark"), too long ("pressure drop, waste"); runner balancing; three-plate mold with two parting lines — ocw.mit.edu