An Introduction To Hot Runner Systems In Injection Molding

What is a hot runner injection molding system?

A hot runner system delivers liquid plastic to a number of mold cavities in order to create a plastic product. It is an injection molding assembly that uses a heated manifold to keep plastic in a molten state. This heated plastic travels from the molding machine nozzle through internal channels called runners and can be delivered directly into multiple cavities at the same time.


Hot runner vs cold runner injection molding systems

Today, most plastic injection molding is done in one of two ways; hot runner systems and cold runner systems.


While the two systems differ in a number of ways, one of the key differences between the two systems is that hot runners are able to inject plastic directly into the part cavity. Thanks to this direct injection, the sprue and runners remain hot throughout the injection process so the only component removed each cycle is the part itself. Because of this, the mold can be cycled again very quickly. This feature makes for better-looking products, allowing for high detail and smooth surfaces due to a much smaller injection point. The increased cycling time and reduced plastic waste make hot runner systems ideal for high volume production.


These benefits do come with some drawbacks. In general, hot runners are more expensive than cold runners. The hot runner system itself is larger and more complex, resulting in a longer set up time and increased installation costs. It is important to know the strengths and weaknesses inherent to both systems, understanding that a different system will lend itself to a number of different products.


In general, hot runners are ideal for projects like:


-Cell phone cases

-Bottle caps

-Projects that require a high volume of inexpensive parts.


Cold runners are, and have long been, the most commonly used runner system for plastic injection molding. Cold runners inject plastic into a sprue, a central channel through which plastic flows into the runners. As the name implies, the plastic cools in a cold runner system. It solidifies in the cavities as well as in the sprue and runners. The excess plastic must be cooled and removed each cycle, which slows down the cycle time. In addition, plastic products formed using cold runners systems generally show more blemishes due to larger plastic entry points called gates.


Cold runners are a good choice for projects producing:


-Electronic housings and coverings where blemishes can be hidden


-Lower volume production