Injection molding machines in a large factory

A Guide to Hot Runner Systems

The plastic injection molding process utilizes all types of equipment, among them what’s known as a runner system. Runners are responsible for dispersing molten plastic resin from the nozzle of the injection unit into the mold/cavity. Typically, there are two types of runners that are used in the plastic injection molding process: cold runner and hot runners. Each type has their own advantages and disadvantages, but many mold makers today are looking towards hot runner systems due to their speed, potential ability to reduce waste, and overall cost savings. 

Components of a Hot Runner System

The components of a hot runner system are key to controlling the temperature and balancing the flow of resins in the injection molding process. The main components of a hot runner system include:

  • Heated Nozzle: The nozzle is linked either directly or through a sub-runner which will lead into the mold cavity. The heated nozzle can either be a needle valve or open-type nozzle. 
  • Heating Rods: Different types of heaters may be used to maintain the temperature of the runners and nozzles. 
  • Manifold: The manifold enables the flow of resin into different nozzles and injection points or gates.
  • Temperature Control Box: The control box enables setpoints to preheat the molding material and maintain the desired temperature.

It’s also important to note that sometimes hot runner systems are classified as either being “open-type” or “needle valve.” This is referring to the type of heated nozzle that is being used in the runner system itself. 

How Does a Hot Runner System Work?

The components, settings, and operation of a hot runner system are quite precise to ensure problems do not develop during production. The system works by using internally or externally heated components to ensure that the plastic resin remains in a molten state as it passes through gates and runners (the channels that run throughout the mold to allow resin to flow from the nozzle to the mold cavities).

Heating elements are typically placed near the center of the runner system, at the gates, and within the heated nozzle to maintain the high temperature requirement for molten plastic.


There are many advantages that a hot runner system can bring to the plastic injection molding process. This system can produce greater capacities and provide faster speeds. Additionally, hot runner systems reduce waste and cycle time because they don’t rely on cold runners in the molds themselves, which must be scrapped after every cycle. 

Simply put, hot runners are easier to work with because there are no solidified products or runners that need to be removed after the injection molding process. Instead, the mold is ready to use once the system has reached its operating temperature. Also, the lower injection pressures associated with hot runners will reduce the amount of deflection on the platen and mold, and effectively reduce the flash that can occur when a mold component moves.


A hot runner system is best for producing a high volume of inexpensive parts, such as bottle caps and cell phone cases, as well as in certain circumstances when a final component requires little or no finish or trimming. This system will also consistently produce a higher quality of injection molded parts in multi-cavity molds.

Learn More From Silver Basis Engineering

Our team at Silver Basis Engineering is happy to answer any questions you have about hot runner systems or the plastic injection molding process. We have over 20 years of experience using hot runner systems in plastic injection molding operations, and have a longstanding reputation for quality, service, and innovation. Contact us today to get in touch with one of our experts.