Hot-Runner vs.
Cold-Runner Systems

Key Takeaways

• Hot-Runner Systems improve efficiency and reduce waste by keeping plastic molten, eliminating excess material, and speeding up production cycles.

• Cold-Runner Systems offer a cost-effective and simple solution for low-volume production, though they generate more material waste and have longer cycle times.

• Vital Plastics manufactures with both runner systems and offers expert engineering guidance to help customers choose the optimal solution.

Understanding the Differences

The difference between hot-runner and cold-runner systems in injection molding is the handling of the plastic material during the molding process. In a hot-runner system, molten plastic is injected into the mold cavities through heated runners, which keeps the plastic in a molten state throughout the process. This eliminates the need for cooling and ejection of solidified material, reducing waste and improving cycle times. Hot-runner systems are typically used for high-volume, high-precision applications where material savings and efficiency are crucial.

A cold-runner system involves plastic flowing through unheated runners.  Once the parts have been filled, the material in the runner system will cool and solidify.  The runner system is ejected with the parts after the cycle and may be used as regrind back into the process or may be discarded, dependent upon customer requirements.  Cold-runner systems are generally less expensive to set up but may lead to higher material waste and longer cycle times compared to hot-runner systems. Cold-runners are often used in lower-volume or less complex molding applications. 

Benefits of Hot-Runner Systems

1. Reduced Scrap and Material Waste: Hot-runner systems leave the plastic in a molten state, eliminating the solidified waste typically generated in cold-runner systems. This approach significantly reduces material costs, waste, and ultimately the part price.
2. Faster Cycle Times: Since there is no need to cool and eject solidified runners, hot-runner systems streamline the injection process, leading to quicker production cycles and eliminates the need to remove and dispose of a solidified runner.  This method also reduces part price.
3. Improved Part Consistency and Precision: Hot-runner systems provide uniform material flow and temperature control, ensuring better consistency and quality in complex or thin-walled parts.
4. Proven and Effective:  The hot-runner systems Vital specifies for tooling are extremely reliable, and allows Vital to improve cycle times and efficiencies. Multiple customers of Vital Plastics utilize hot-runner systems and Vital Plastics has been building them for several years.

Downside to Hot-Runner Systems:

1. Higher Initial Cost: The equipment required for hot-runner systems is more expensive compared to cold-runner systems, leading to a higher upfront investment.
2. Increased Maintenance Complexity: Due to the intricate nature of hot-runner systems and the need for precise temperature control, maintenance and repairs can be more complex and costly.
3. Difficult Color or Material Changes: Purging hot-runner systems when transition between colors or different resins can be more difficult given the design of the tool. Hot-runner systems may be less desirable if parts will have frequent changes in either color or resin.

Benefits of Cold-Runner Systems

1. Lower Initial Cost: Cold-runner systems are less expensive to build into the tool and maintain compared to hot-runner systems, making them more cost-effective for smaller production runs.
2. Simplicity and Ease of Maintenance: Cold-runner systems are simpler in design and easier to maintain due to the lack of complex temperature control mechanisms, making repairs less expensive and less time-consuming.
3. Suitable for Low-Volume Production: Cold-runner systems are ideal for low-volume or prototype runs, where the cost of a hot-runner system may not be justified due to smaller quantities and less precise requirements.

Downside to Cold-Runner Systems

1. Increased Material Waste: Since the plastic solidifies in the runner system, excess material is discarded after each cycle, leading to higher scrap rates and increased material costs (subject to allowable regrind percentage).  This process will ultimately increase the part price.
2. Longer Cycle Times: Cold-runners require additional time for the plastic to cool and solidify before ejection, which can slow down the overall cycle time and reduce production efficiency, increasing part price and decreasing tooling capacity.
3. Need for Secondary Operations: The scrap from cold-runner systems must be removed from finished parts. This process can be manual or automated through robotics, but it is an added step to the manufacturing process.

It is important to carefully consider the differences between hot-runner and cold-runner systems before investing in a production mold, which is why Vital Plastics evaluates all customer requirements to recommend the best fit for each project.

Contact us to learn more about the differences between runner systems and explore our full range of injection molding capabilities.

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