Hot runner systems are feed systems for injection molds which imbue molten plastic from a machine nozzle into a cavity. Hot runner systems consist of various components, each with specific functions.
How a Hot Runner System Works
Hot runners are complex systems because they must keep the plastic moldable at a high and uniform temperature while keeping the rest of the system cool so the plastic can be subsequently solidified. Main components of the system include the nozzles, manifolds, flow control, and connections.
- Nozzles are the link to the cavity. They either directly connect into the part or into a sub runner which leads into the cavity.
- Manifolds are the devices used to hold the nozzles and guide the plastic melt from the distributor bushing to the respective nozzle gates. They come in a variety of standard shapes and can also be built to specification.
- Flow control uses valve technology to govern the flow of molten plastic through the nozzle.
- Connections are made throughout the system to provide functionality. They include electrical, thermocouple, hydraulic, pneumatic, and coolant connections.
Hot Runner System Types
There are two types of hot runner systems, insulated runners and heated runners.
- Insulated runners have oversized passages formed in the mold plate. They are a more simple design and are less costly to build than heated runners. They suffer from freeze-ups at the gate, problems in uniform mold filling, and long start-up periods for stabilization. They also require a fast cycle to maintain the melt state of the plastic.
- Heated runners include externally and internally heated designs. Internally heated systems have largely become obsolete because externally heated runners tend to distribute and control heat better. Heated runners boast better heat distribution than insulated runners, but have the trade-off of a higher cost and more complicated design. The thermal expansion of the mold products also needs to be factored in these systems.
Advantages of Hot Runner Systems
Reasons a buyer should consider selecting a hot runner system for plastic molding include fast and efficient processing, consistency, little material waste, and high quality molds. These advantages are due to the effects of molding and forming using a melt of uniform temperature.
Deterring factors from the use of hot running systems include high component cost, strict requirements for compatible process equipment, and complex operation and maintenance. These disadvantages stem from the complexity and intricacy of the hot running components and system.
There are a number of factors that should be taken into account when considering the right hot runner system.
Consider the Design
Hot runner systems generally have unique requirements, even with standardized systems. The hot runner manufacturer should be included in the process of designing the system to ensure the right fit.
Consider the Application
Hot runner system configurations vary greatly with application. For instance, the design will vary depending on whether the system should gate directly to the part or use a sub-runner attached to the part. Configurations will also change with high-temperature needs, high-volume projects, or other application specific requirements.
The type of material being molded and the color requirements influence the type of hot runner system needed. Because of the expense of hot running, cost is usually also a main factor. However, the benefits of these systems can many-times outweigh initial costs.