The practice of plastic injection molding dates back to the late 19th century, and since that time, the process has evolved together with technology to become the fundamental manufacturing process it is today.

But despite the evolution and transformation of injection molding over time, its foundation remains mostly the same, with two primary categories of injection mold runner systems – hot runner and cold runner. Each of these runner types offers specific advantages, as well as drawbacks, and thus they are ideal for different applications and products.

In this post, we will examine hot runner and cold runner systems – how each works, as well as their benefits and limitations. By understanding these systems, manufacturers can make more informed decisions and determine which process is best suited for their part.

What are cold runner and hot runner molds?

Cold Runner Molds

As the name implies, a cooling process is a primary differentiator in the cold runner system. Cold runner molds are typically made up of two to three plates. A nozzle is used to inject molten thermoplastic into the mold, filling the runners that lead to the mold cavities. The runners carry the molten material to the mold cavities, and the cold runner cools multiple components of the mold system; including the runner, sprue, and gate, as well as the molded part itself.

While the final product varies based on whether a two-plate mold or three-plate mold was utilized, cold runner molds typically produce a final part with at least the sprue attached, if not the sprue and the runner system. In both systems, the runner is often reground and recycled to reduce waste.

Hot Runner Molds

In hot runner molds, a manifold system is used to heat a mold made up of two plates. The manifold is used to regulate the temperature, which it achieves by ensuring the molten thermoplastic in the mold’s runners maintains the same temperature as the heating cylinder. Heated runners distribute the molten material to nozzles, which form the part by filling the mold cavity.

Located in a separate plate, the heated runner system is fixed throughout the molding process. When the process is complete, the mold opens containing the molded part. The final product in hot runner molds does not have attached runners.

While there are a variety of types of hot runner systems, the majority fit into two main categories: internally heated, which are known for providing superior flow control; and externally heated, which are ideal for polymers and other sensitive materials.

Benefits and Drawbacks

Cold Runner Molds: Benefits
Cold runner molds are generally less expensive as the runner system is less complex, as well as lower maintenance costs overall.

Cold Runner Molds: Drawbacks
The main drawbacks of a cold runner system relate to cost, waste, and ability to hold tolerances. A cold runner increases the amount of material being consumed each cycle. This translates to a higher part cost. Runners, while often able to be recycled and the material reused, are inherently waste and require additional steps to manage. Also, cold runner systems aren’t able to hold tolerances as tightly as a hot runner system.

Hot Runner Molds: Benefits
Generally, the cycle times for hot runner systems are significantly faster than those for their cold runner counterparts. The fact that the runner is eliminated entirely is a significant factor in reducing cycle and production times, as well as minimizing waste.
Hot runner systems are also better suited for applications that require tight tolerances.

Hot Runner Molds: Drawbacks
The main drawback of a hot runner system is cost. A hot runner system will increase the cost of your initial tool investment. The costs of maintenance for a tool containing a hot runner system are also higher.

Conclusion

Both cold runner and hot runner injection molding have limitations and benefits. By understanding these systems, manufacturers can make informed decisions on which to utilize for which parts, based on compatibility with various materials, budget, and other important factors.

Hot or cold – we’re here to help

At New Berlin Plastics, we know plastic injection molding. We understand the strategic advantages and limitations of both systems, and our knowledgeable team is experienced working in both hot and cold runner processes.

We offer custom solutions to help make your manufacturing processes more efficient and improve your bottom line. Contact us today at 262-784-3120, or visit us online at nbplastics.com to learn more.