While a number of major advances have changed several parts of the plastic injection molding world over the last few decades, one theme remains that’s been present for the duration: Most injection molds fall into two categories, either hot runner or cold runner systems. Each of these systems has benefits and drawbacks depending on the project need and the specific application, and understanding how these methods differ is often important for helping decide between them for your project needs.

At EnviroTech Custom Injection Molders, we’re happy to offer a huge range of custom injection molding services, with solutions ranging from numerous molding capabilities and machining techniques through laboratory testing and other forms of product quality control. What are the important differences between hot runner and cold runner systems for plastic injection molding, and why might you consider one or the other for your needs? Here’s a primer.

Cold Runner Mold Basics and Methods

In most cases, cold runner molds for plastic injection molding will come with either two or three plates, held together within the mold base. Molten thermoplastic will be injected into the mold via a nozzle in the sprue, filling the runner network that feeds into the mold cavities. The runners themselves, as the “cold” in the name suggests, are not heated, and serve as a delivery system that’s meant to evenly distribute molten plastic into the cavities – after which the system cools all the components involved, along with the actual molded part.

As we noted above, there are both two- or three-plate molds available within this method. Here’s how they differ in operation:

  • Two-plate molds: The sprue and runner system remain attached to the final product, with a special ejection system used to separate the molded component from the core mold.
  • Three-plate molds: A stripper plate is what contains the sprue, allowing delivery of plastic to runners connected in a separate cavity plate. The final molded part is formed and then cooled on the core half of the plate, with no need for any attachment of runners or gates. Through this system, the runner is automatically separated from the part as the mold is opened up.

Luckily, both of these systems allow for the runner to be reground and recycled, which limits waste concerns.

Pros and Cons of Cold Runner Molds

When it comes to cost-effectiveness within plastic injection molding, cold runner molds are usually the ideal choice in a vacuum. They come with lower total costs, plus lower maintenance expenses for the final part itself, making them attractive for projects with a limited budget. In addition, they are applicable to a variety of different thermoplastics, and are more flexible than hot runners since their locations can easily be changed.

On the flip side, cold runner molds – as we noted above – need to be manually separated from the finished part after each run in many circumstances. This means that if the runner is not reground and recycled after each run, the material will be wasted. This removal and recycling process takes time, naturally, and adds to the overall cycle time while slowing production.

Hot Runner Mold Basics and Types

Hot runner mold systems, on the other hand, are made up of two plates that will be heated within a manifold system. This manifold is in place to maintain a consistent temperature, and it does this by holding molten thermoplastic in the runners at the exact same temperature as the system’s heating cylinder. From here, heated runners deliver the plastic to nozzles that will then fill the core mold and begin forming the required part.

It’s important to note that during this format, the heated runner system sits in a separate plate that remains stationary during the molding process. When this is done, the molding plate opens to showcase the final part, one that has no runners attached to it whatsoever. In addition, there are two broad categories for hot runner molds:

  • Externally heated: Systems that are ideal for polymers sensitive to thermal changes.
  • Internally heated: Offering improved flow control for those who prioritize it.

As we touched on above, this process involves no attached runners to the mold once it’s complete. This, in turn, eliminates the need for recycling and grinding, and there will be no delays in cycle times due to these needs like there are in cold runner molds. There are also slight variations to this system known as insulated runners, which keep the plastic in a molten state – but this type is only used for specialized polymers and isn’t very common.

Pros and Cons of Hot Runner Molds

The primary benefit of hot runner molds, as we touched on above: Cycle time, which tends to be much faster than cold runner molds. This is because there are no runners present on the final mold, eliminating the need for recycling and regrinding of the runner and sprue – and this also limits the total amount of waste created, another major benefit of the hot runner mold format. Put together, these factors tend to mean total costs are relatively low, plus the mold automation process is highly efficient. On top of this, hot runner systems are capable of creating larger parts than cold runner parts.

On the downside, hot runner systems themselves are more expensive than cold runners up-front. They also require additional heating sources, as we detailed, and these come with some costs. Finally, even the tooling cost for hot runner molds tends to be higher than for cold runner systems, meaning the final cost of hot runner parts is usually a bit higher. In addition, these parts will usually require a bit more maintenance than cold runner molds do.

For more on the differences between hot runner and cold runner molds for plastic injection molding, or to learn about any of our custom injection molding services, speak to the staff at EnviroTech Custom Injection Molders today.