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Injection molding: hot vs cold runner systems

In injection molding, there are two types of systems used to create molds: hot runner and cold runner systems. There are unique advantages and drawbacks to each, and knowing a little more about these two systems will help you to make the right decision for your manufacturing processes.

Cold runner injection molding systems

In a cold runner system, there are usually two or three plates in the base of the mold. When the plastic is injected into the mold, is has to be done through the sprue. Then, it moves into the runners and lastly, it enters the mold cavity parts through the gating system.

In a two-plate mold, the structures of the runner and cavity components are attached. An ejection system is used to separate these from the mold. In a three-plate mold, the runner is on a separate plate, which means the cavity components can be ejected on their own.

In a cold runner system, the runner can be recycled and reground. The advantage is that wastage is curbed; the drawback is that cycle time may be increased.

Advantages and disadvantages of cold runner injection molding

Advantages

  • Is cheaper than hot runner systems.
  • Colours can be changed fast.
  • If the process is automated, it can be very quick to remove runners and therefore cycle time is reduced.
  • It can be used for almost all polymers, engineered plastics and commodity plastics.

Disadvantages

  • The system is slower than hot runner injection molding.
  • Wastage may occur where sprue bushes cannot be reground.

Hot runner injection molding systems

In this kind of injection molding, two plates are heated together with a manifold system, which sends the molten plastic into specialised nozzles. From here, the plastic arrives at the cavity components.

The system is heated by dedicated manifold heaters and probes. When heated internally, the flow of plastic is controlled by specialized, external temperature controllers.

Unlike cold runner systems, in the most common hot runner systems, there are in fact no runners, which means that regrounding and recycling doesn’t impact on cycle time.

Advantages and disadvantages of hot runner injection molding

Advantages

  • Incredibly fast cycle times.
  • No plastic waste due to lack of runners.
  • No capital investment required for robotics systems to remove runners.

Disadvantages

  • More expensive than cold runner systems, with higher maintenance costs.
  • More difficult to change colours.
  • Higher possibility of down time.
  • Not ideally suited to heat-sensitive polymers and resins.

All your injection molding needs catered for

Choosing the right type of injection molding system is critical to ensuring a high-quality end result that is also cost-effective. If you aren’t sure which to choose, or you’d prefer to eradicate the expense involved in setting up your own injection molding system, talk to us.

With over 40 years in the industry, we offer a complete end-to-end plastics solution, including injection molding, rotational molding, extrusion and full assembly lines. If you’re looking for quick turnaround times and superior quality components, we can help.

Learn more about our custom plastic injection molding products or get in touch with us to chat about your specific requirements.

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