What is a hot runner?

Hot runner technology, introduced to the plastics industry over 50 years ago, revolutionized injection molding processing capabilities by improving molded part quality, enhancing operational efficiencies, reducing scrap and saving money.

A hot runner system is a molten plastic conveying unit used within an injection mold. In other words, a hot runner system consists of heated components (generally via electricity) used inside the plastic injection molds, which brings the molten plastic from the barrel of an injection molding machine into the cavities of the mold. The sizing of hot runner melt channels depends on many factors such as the type of resin, the injection speed, fill rate, and the molded part. A temperature controller (standalone controller or controls from the injection molding machine) heats the hot runner system within the injection mold and the resin inside the machine barrel to processing temperature and injects the resin into the mold. The resin travels through the inlet, down into the manifold which then splits to the various nozzles and through injection points (or gates) into the final mold cavity where the final part is formed. Today’s molds can have anywhere from 1 to over 192 nozzles depending on the plastic parts being manufactured.

Prior to hot runner technology, cold runners were widely used on injection molds. Cold runner molds faced many challenges in conveying the resin from machine barrel to cavities without affecting the flow and thermal characteristics of the resin. With the advancement of resin types and the complexity in mold and part designs, it became more and more difficult to control the molding process via cold runner molds to produce molded parts of acceptable quality.

However, with the introduction of hot runner technology with advanced thermal controls, processing of wider ranges of resin became more practical and convenient to injection molders. Unlike a cold runner mold, the hot runner components are individually heated to ensure the resin maintains the temperature continuously through the mold. The temperature of each hot runner heated component can also be precisely controlled to ensure the process is optimized to the requirements of each type of resin delivering the highest possible part quality. Today, hot runners are capable of producing highly complex parts in a wide range of sizes which are utilized in every industry.

Elements of a Hot Runner

§  Locating Ring – The locating ring aligns the injection mold with the platen of the molding machine. It ensures there is proper alignment of the mold with the machine.

§  Inlet – When resin is injected into the mold, this is the entry port where the resin enters from the injection machine nozzle. Depending on the type of resin and the design of the hot runner the inlet component may be heated in order to optimize the molding process.

§  Manifold – The manifold enables the flow of resin into different nozzles and injection points (gates). Manifolds are normally used where multiple cavities are injected or where more than one nozzle/gate per part is needed. Manifolds are available in a wide range of materials, designs and shapes and are often optimized to improve the molding process using CAE analysis. There are 2 main types of manufacturing techniques, gun drilled and 2 piece brazed. Gun drilled are often ideal for simpler, more economical systems while 2 piece brazed is often favoured when tighter performance criteria is required (balance, faster color change). 2 piece manifolds are also ideal for multi material or multi color molding applications.

§  Nozzles –Nozzles are components where the resin is injected into the cavity through a gate. Depending on the design, nozzles are typically installed into the mold plate with or without a manifold. A wide range of nozzle designs are available, using different materials, in order to achieve the processing characteristics of various resins that best suit the application.

§  Heater Technology – Heater technology is the basis of all hot runner systems and significantly affects molding process and part quality. There are several heating options each with their own pros and cons. Selecting the right hot runner depends on the requirements such as molding process, part performance, reliability and cost. Most common hot runner technologies have heaters with heater bands/plates, paste-in/flex heaters or brazed-in heaters.