Hot runners are heated nozzles and manifold assemblies used in injection molding to keep plastic fluid between cycles, reducing waste and cycle times. They are more expensive to install but save money in high-volume production. The traditional cold molding process creates solidified corridors that waste material and increase costs. Hot runners use electric heating elements to keep the plastic fluid, and there are two types of heating elements: internal and external. External heating is more efficient and has largely replaced internal heating.
A hot runner is a heated nozzle and manifold assembly installed on injection molding equipment. This assembly allows the plastic filler left in the feed mechanism to remain fluid after injection while the part itself cools and solidifies. Injection molds equipped with hot runners are cheaper, characterized by faster cycle times and less material waste. These savings are possible because the manifold and nozzles keep the plastic inside them fluid between injection cycles, eliminating the time and material waste associated with solidified “corridors” in conventional cold molds. Installing a set of heated runners greatly increases the cost of any mold, limiting the viable use of the devices to high-volume production processes.
Injection molding is a manufacturing process in which granulated materials, typically various grades of plastic, are melted and injected into a mold under pressure. Once the injection process is complete, the mold and parts are allowed to cool and solidify, allowing the product to be ejected from the mold cavity. The path within the mold taken by the molten plastic before reaching the actual cavity is known as the manifold or channel. Typically, these consist of one or more narrow channels. each terminating in a nozzle which forms the entrance to the cavity. In conventional cold forming processes, the plastic left in these channels and nozzles cools and solidifies together with the molded part.
These solidified sprues or “corridors” are then rejected before the next injection cycle begins. This not only represents a waste of material, particularly in cases where the skates cannot be recycled, but also adds a manufacturing step to the process. This increases cycle times, reduces productivity and increases the unit cost of the parts produced. Using a hot runner system almost completely eliminates these problems by keeping the plastic in the manifold runners and the nozzle fluid between injection cycles. This is accomplished by including electric heating elements in the manifold and nozzles, which keep these parts at a constant temperature of approximately 550-590°F (290-310°C).
Hot runner heating elements are divided into two categories: internal and external types. The external hot runner heaters are located inside the manifold body immediately adjacent to the runners and around the outside of the nozzles. The internal heating elements are located inside the channels and nozzles. While the internal heating method is used in some specialist applications and older machines, the more efficient external heating method has largely replaced it. While the use of hot runner systems represents significant savings, the associated high installation costs limit their use.
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