Cold runner systems in injection molding refer to the configuration where the molten plastic material used to form the parts is conveyed through channels (runners) that are not heated and thus cool alongside the parts themselves. These systems consist mainly of two plates: the sprue plate, which receives the molten plastic from the injection machine, and the runner plate, which channels the plastic to each mold cavity.

In cold runner molds, the runners are cooled and solidified with the molded parts. After ejection, the solidified runners must be separated from the finished parts and are often recycled or discarded, leading to potential material waste. However, cold runner systems are popular due to their simplicity, lower initial cost, and flexibility, as they can be used with a wide variety of materials and colors.

Components of Cold Runner Systems

Cold runner systems in injection molding consist of several key components, each contributing to the process of forming plastic parts. Here’s an overview:

  • Sprue: This is the channel through which molten plastic flows from the injection machine’s nozzle into the mold. It marks the beginning of the material’s path in the mold structure.
  • Runner: The runner is a network of channels branching from the sprue, guiding the plastic to different parts of the mold. In cold runner systems, this plastic solidifies and must be separated from the final parts.
  • Gate: The gate is the opening between the runner and the mold cavity. It controls the plastic’s entry into the cavity, affecting the part’s appearance and integrity.
  • Mold Cavities: These are the spaces in the mold designed to shape the molten plastic into the desired final parts. Each cavity represents the negative space of the intended part.
  • Ejection System: Once the plastic has cooled and solidified, the ejection system is activated to remove the parts and the solidified runners from the mold. This system typically includes pins or plates that push the finished parts out of the mold cavities.

The Process Flow in Cold Runner Systems

The process flow in cold runner systems for injection molding follows several key steps, ensuring the efficient creation of plastic parts. Here’s a detailed overview:

  1. Molten Plastic Injection: The process starts with the injection molding machine heating plastic pellets until they melt. The molten plastic is then injected under high pressure into the mold through the injection nozzle.
  2. Flow Through Sprue: The molten plastic first enters the mold through the sprue, a primary channel that directs the plastic from the injection nozzle to the runner system.
  3. Distribution via Runners: From the sprue, the molten plastic flows into the runners. The runners are a network of channels that distribute the plastic evenly to different parts of the mold. In cold runner systems, these channels are not heated, and the plastic within them will eventually cool and solidify.
  4. Entry Through Gates: The molten plastic moves from the runners into the mold cavities through gates. The gates control the flow of plastic into each cavity, influencing the quality and consistency of the final parts.
  5. Cooling and Solidification: Once the plastic fills the mold cavities, it begins to cool and solidify, taking the shape of the cavities to form the finished parts. Meanwhile, the plastic in the runners also cools and solidifies.
  6. Mold Opening and Part Ejection: After the plastic has fully solidified, the mold opens, and the solidified parts, along with the attached cold runner system (sprue, runners, and gates), are ejected from the mold.
  7. Runner Removal: The solidified runners, which are not part of the final product, are then manually or automatically removed from the finished parts. In many cases, these runners can be recycled by grinding them into pellets and reusing them in the molding process.


Cold runner systems in injection molding offer a range of advantages that make them particularly suitable for certain types of production scenarios. Firstly, they come with lower initial costs compared to hot runner systems, which makes them an attractive option for companies operating within tight budget constraints or those engaged in small-scale productions. The flexibility offered by cold runner systems in terms of material and color changes is also significant, as it allows for quick and easy switches between different types of plastics and colors. This is especially beneficial for manufacturers who need to produce parts in small batches or wish to frequently change colors.

Furthermore, the simplicity in design and operation of cold runner systems translates into ease of maintenance and reduced downtime, an advantage for manufacturers seeking straightforward solutions with fewer technical complexities. They are particularly favored for prototyping and short production runs, given their capacity for quick and cost-effective mold modifications.

Another notable advantage is the ability to recycle the runners produced in cold runner systems. Unlike hot runner systems, where the material remains molten, the solidified runners from cold systems can be ground up and reused, contributing to material savings and environmental sustainability. Additionally, cold runner systems present a reduced risk of thermal degradation of the material since the plastic cools down more uniformly.


Cold runner systems in injection molding, while beneficial in certain scenarios, have several disadvantages that may impact their effectiveness for particular applications. This issue is compounded by longer cycle times, as both product and runners must cool and solidify before ejection, reducing overall production efficiency.

While cold runner systems might save on the energy required for heating, the extended cycle times and the energy needed for regrinding and recycling runners may lead to higher overall energy consumption.


Cold runner systems offer a practical solution in injection molding, particularly for projects with limited budgets or requiring frequent changes in materials or colors. Despite their limitations, they remain a valuable option in many scenarios due to their cost-effectiveness and flexibility.

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