Hot runner molding

Hot runner injection molding

What is a Hot Runner System?

Hot runner injection molding is a system used within injection molds to maintain the molten state of plastic from the injection molding machine all the way to the mold cavity, without the need of a runner.

Traditional “cold runner” systems involve the plastic being injected into a primary runner, which then fills secondary runners, distributing the molten plastic to multiple part cavities.

The plastic in these runners solidifies and is typically ejected with the part, leading to material waste and additional processing steps.

1. Process

This is a block that is heated and distributes the molten plastic to various nozzles. The manifold ensures even heating and consistent flow of the molten plastic.

These are the components that directly feed the molten plastic into the mold cavities. They are heated and maintain the plastic’s molten state right up to the point of injection.

These are critical to the hot runner system as they control the temperature of both the manifold and the nozzles. This ensures consistent material flow and quality.

2. Advantages

Since there’s no cold runner, there’s no wasted plastic from the runners. This can lead to significant material savings.

Since there’s no cold runner, there’s no wasted plastic from the runners. This can lead to significant material savings.

Hot runner systems can provide more consistent plastic temperature and flow, leading to more consistent part quality.

With no runner to remove, secondary operations like trimming or degating can be minimized or eliminated.

3. Disadvantages

Hot runner systems are more expensive than cold runner molds, increasing the initial investment.

The system’s complexity means more potential points of failure and may require specialized maintenance.

If not properly managed, the continuous heating can degrade some sensitive materials.

4. Applications

Due to the advantages in cycle time and material efficiency, hot runner systems are often used for large production runs.

For small parts, the weight of the runner in a cold runner system can sometimes be more than the part itself, making hot runner systems more efficient.

Hot runner systems can provide better flow and fill for complex or multi-cavity molds.

For materials sensitive to shear or prolonged heat, the direct flow from the machine through the hot runner to the part can reduce the chance of material degradation.


In conclusion, hot runner injection molding is a sophisticated technique that offers material savings, faster cycle times, and more consistent quality compared to traditional cold runner systems. The decision to use a hot runner system is often based on production volume, part design, material used, and overall project budget.

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