What Is a Cold Runner Mould?

A cold runner mould is an injection molding technique that uses a nozzle to inject molten plastic resin into the mold. This flows and fills the runners that lead to the part cavity, causing it to cool as it does so.

The melt temperature delivered to each cavity during the molding cycle influences the shot-to-shot weight consistency and dimensional stability of parts molded with cold runner systems. Variation in the melt temperature increases shot-to-shot part weight variation, which leads to unacceptable parts and higher scrap rates.

Melt delivery management is an important aspect of molding, as it affects the final quality of a part and the success of the production line. The injection mold must be designed to provide the optimal melt flow, pressure, and temperature for each molded part.

To optimize the melt delivery process, it's important to understand how different cold runner shapes and sizes impact the polymer shearing, packing and cooling behavior, and gate sealing time of a mold. Understanding these processes helps reduce cycle times and make injection molding a more efficient, cost-effective process overall.

Two-plate and three-plate cold runner molds are the most common types of runner molds. These are generally less expensive than hot runner systems and easier to maintain.

The sprue and runners on cold runner molds can be removed manually or by machine. The sprue typically requires more inches of open stroke to remove than a hot runner sprue, adding time to the cycle. Also, the runner is usually thicker than the part and takes longer to cool enough for safe handling.