Injection Molds: Hot Runner versus Cold Runner Molds

July 8, 2021
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Plastic injection molding was introduced in the late 19th century, revolutionizing the way plastic products are created. While technology has significantly evolved over the years, a lot of injection molds still fall in one of the following categories:

1. Hot-runner mold systems
2. Cold-runner mold systems
Every system has its own pros and cons, meaning that one of them might be better suited for a specific application. Let’s see how they differ:
Hot-runner mold systems
Hot-runner mold systems are further classified into two types, both of which use two plates that a manifold system heat. Internally heated molds generate better flow control, allowing you to use more heat-sensitive polymers. Depending on your applications and materials, you may need (or prefer) one over the other.
Compared to cold runners, hot runners are costlier (in upfront costs and maintenance). However, they don’t produce any waste because they can handle larger parts and higher volumes.
Overall, hot-runner molds produce parts with faster cycle times and consistent quality. However, changing colors isn’t easy. Plus, hot runners can’t accommodate certain heat-sensitive polymers.
Pros
  • Perfect for higher volumes and larger parts
  • Little to no waste
  • Consistent quality
  • Fast cycle time

Cons

  • Making color changes is difficult
  • Not suitable for certain heat-sensitive polymers
  • Costlier to buy and maintain

Cold-runner mold systems

Cold-runner mold systems have the same temperature as the molds. They’re also classified into two types: a two-plate system and a three-plate system.

Two plate systems can handle most molds. Three-plate systems don’t require an ejection system to enable the part to be ejected from the runner. However, they’re more complex than a two-plate system—a system that requires an ejection system to remove the running and part from the mold. But using a three-plate system will be better if you’re dealing with complex designs.

In cold-running mold systems, the part should always be smaller than the runner. Otherwise, you may underfill the mold. Unlike hot-runner systems, cold-runner systems allow you to use more types of polymers without worrying about heat sensitivity.

Plus, cold runners are easy to maintain and aren’t as expensive as hot runners are. However, if you don’t melt the extra material or recycle, they may create waste.

Pros

  • Colors can be easily changed
  • Maintenance isn’t difficult
  • Fewer restrictions on types of polymers
  • Cheaper

Cons

  • Generates waste
  • Takes longer than hot-runner systems
  • Runners have to be removed

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