26 Aug 2019

Last Updated on



Since its inception in the 1960s, to its acceptance in the 1990s, the hot runner system has challenged tradition across the plastic injection molding business. With their increased cycle speed and decreased waste, hot runners are an attractive solution to many looking to diversify their injection molding process.


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


How does a hot runner injection molding system work?

A sprue and runner system used with a cold runner for plastic injection molding

A sprue and runner system used with a cold runner for plastic injection molding

Hot runners inject plastic directly into the cavities, as opposed to the sprue and runner system used in a cold runner system. One of the biggest advantages to a hot runner system is that the plastic in the runners will never solidify. This decreases the cycle time, allowing for faster processing. Another benefit of this system is a reduction in plastic waste, as the material does not harden until the mold is filled. The lack of a sprue system substantially cuts down on the amount of trimming required to smooth the final product. Hot runners operate through an additional manifold which is bolted to the mold assembly.


Major hot runner systems manufacturers

When considering the use of a hot runner system, it is important to understand the quality of its manufacturer. Internationally, Husky is one of the more well-known companies. The world’s largest brand name supplier of manufacturing solutions, Husky has a proven track record of delivering high-quality injection molding systems. The machines they produce can handle high volume and frequent use, while still maintaining a great deal of precision. 


Other hot runner manufacturers include companies like Maenner and Mold-Masters. Research is imperative when choosing a manufacturer. Will they provide training and support? Are the hot runners assembled on delivery? Will the system come with the controller? Knowing the answers to these questions will help when choosing a supplier.


Building a mold for plastic injection requires a considerable amount of communication with the manufacturer. The molds and the hot runner system must be perfectly compatible, and this requires that both parties work closely during the design and manufacturing process.


Do I need to find my own hot runner system supplier?

In most cases, the answer is no. Your mold maker will already have a system supplier that they work with. The hot runner system is so highly integrated into the mold that mold makers will have a preferred hot runner system supplier that they work with. It is important to talk to your service provider to learn which hot runner system manufacturer they use, and why. These systems work best when both companies have built a strong working relationship and have a good history of successful collaboration.


The hot runner injection mold system

A hot runner mold system is an assembly of many different parts:


-Manifold, generally heated by electricity

-A number of controllers, determined by the number of injection points

-Thermostat to control the heat of the manifold

-Plate, where the runners sit. This component is also heated


Hot runner price

Hot runner systems certainly have their advantages, but as we have discussed, they also come with a higher price tag. In general, you can expect to pay between $20,000 and $30,000 for a hot runner manifold, and another $5,000 for the controller. Beyond this, the price for an entire system begins to vary depending on the number of zones included in the manifolds. These zones, also known as drops, are the injection points for the molten plastic. The more drops a system has, the more cavities can be filled at once, and the more the system will ultimately cost. It is important to understand these costs when considering a hot runner system and to understand that these costs constitute an investment.


Types of hot runner systems

When we talk about hot runner systems, we are actually talking about two distinct versions of that system. This difference primarily involves the style of the injection point. These two different injection point types are known as hot tip and valve gate.



Hot tip hot runner system

Hot tip is the standard, general-purpose version of the hot runner system that we have discussed thus far. It is more economical than the valve gate system and is the most common hot runner system in use. Hot tip is ideal for mass production of simple parts and products. 


Valve gate hot runner system

A valve gate is a style of injection point featuring a mechanical valve that automatically closes when the machine injects plastic into the mold. This results in a smooth closure, leaving no visible nubs or abrasions on the surface of the product. A valve gate system is ideal for products that need to look great; highly aesthetic projects that will be noticed by the customer. This system is even more efficient than the general-purpose hot runner, and as such, is more expensive.  


The future of injection molding

While there will always be a need for cold runner systems, it is clear that hot runners are the technology of the future, and more importantly, the present. The reduction in cycle time and the removal of runner waste solidify the hot runner as an indispensable component of modern injection molding. Understanding that, it is important to know when to use one system or the other. With a greater awareness of hot runner systems and what they can achieve, it becomes possible to make more informed decisions in the years ahead.

If you are considering an injection molding system and are not sure which is right for you, please contact Rex Plastics with any questions. If you already know what you’re looking for, request a quote and get started today!


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