Electrical hot runner technology and applications

Hot runner basics

How melt delivery technology can help reduce material waste and increase productivity

What is a hot runner system? For those who are not familiar with the injection molding process, this may be the first question. Hot runner systems are an important part of injection molding processing. Since its inception, hot runners have continued to play a key role in plastics processing. While hot runners increase the initial cost of tooling, they bring many improvements to production in the long run, making them a more economical option.

What is a Hot Runner?

Hot runners increase productivity and system performance, produce better-looking injection molded parts, increase flexibility, and improve process monitoring. In addition, they improve energy efficiency and avoid scrap, which in turn speeds up production cycles and reduces component costs. The hot runner also delivers the melt to the mold, resulting in high-quality gates, excellent cavity-to-cavity balance, fast color changes, unlimited design freedom, and a more customized product. Hot runner solutions enable processors to achieve low cost, high quality and high throughput. Hot runner systems excel in both hot spot and valve gate configurations and are optimized for processor-specific applications.

The benefits of using hot runner technology are most significant for applications with high throughput and long product life, but it can also be particularly beneficial for low-cavity molds where speed and consistency are critical for continuous improvement. This article will provide an overview of how hot runner melt delivery systems work and how they can improve the injection molding process.

Limitations of cold runners

In the injection molding process, one of the most important functions of the mold is to transport the melt from the barrel to the cavity. Melt delivery management has a significant impact on the quality of the end components and the successful operation of the production line. Precise control of resin flow, pressure, and temperature is essential. The success of the molding process is directly related to the design of the melt delivery system, so a detailed comparison of hot and cold runner schemes is beneficial.

Cold runners are solidified plastic fluid channels that exist within the mold and must be removed from the mold and injection molded parts after the molding process is complete. Channels machined on the parting surface of the mold result in the creation of cold runners that direct the flow of molten resin from the gate into the cavity. After the molten resin cools and hardens, it is ejected along with the injection molded part, and the next production cycle begins. In general, cold runner systems require less initial cost or initial capital investment in the tool. They tend to be used in low-cavity molds with shorter run times or lower production frequencies.

However, demand for some injection molded parts has increased and there has been a shift to higher cavity counts. When this happens, the cold runner has a significant impact on processing and associated costs. A larger runner system means more resin is injected, cooled, and processed. The layout of cold runners can be complex, and large spacing between cavities must be maintained in order to accommodate the runners and facilitate exhaust.

Since the melt still flows into the cavity during the filling phase, the cold runner also extends the production cycle. Cold runners are typically much thicker than injection molded parts, and as a result, the cooling time is also long. Obviously, one of the most important considerations is that the cold runner wastes resin, resulting in high costs. Processors have to discard the runners, regrind them for recycling and other processing, or sell them as bulk scrap at a very low price.

The advantages of the double-plate cold runner mold include a compact design, low investment costs, and short residence times. It also features simple maintenance, quick color change, and easy processing of gates. The disadvantages are reflected in the generation of end-of-life plastics, secondary operations that increase processing costs by running to remove excess gate material, long production cycles, and quality problems such as poor surface finish and poor dimensional stability.

Although the scrap from the three-platen cold runner system can be re-grinded, this process can create other problems. Processors have to face a number of issues around the use of regrind cold runners in new applications in the future. Does the part need virgin materials to be manufactured, is it feasible to use only a small percentage of regrind material, is there unwanted material in the scrap, can foreign debris clog the gate, and there are also many problems in the gate material removal process, such as whether the runner is broken or has to be removed. There may also be a need for manual labor and associated fixed capital expenditures for the purchase of automatic gate material removal equipment. The investment can be quite high and the payback period (ROI) is long.

Hot  runner resistance winding machine.