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#027 Hot Runner Technology

Category : Molding Technology
January29, 2010

A hot runner has the feature that it is possible to carry out injection molding without generating runner parts such as scrap, etc., by incorporating an electrically heated runner part in the mold.

Since hot runners make it possible to make the amount of scrap generated during the large scale manufacturing of plastic injection molded products extremely small, hot runners are a technology that make it possible to achieve great reductions in the cost of scrap processing. In addition, since the filling pressure loss that occurs in the case of cold runners becomes small in the case of hot runners, it is possible to suppress a fall in the filling pressure. Further, it is also possible to aim at shortening the cooling cycle. Hot runners have been used in Japan in a large number of cases, such as for the production of food containers, medical implements, automotive parts, etc.

Hot runner systems are broadly classified into those in which the mechanism part is a manifold part and those in which the mechanism part is a nozzle part. In addition, a controller is also required for carrying out the temperature control of the hot runner part.

Hot runners can be designed and built in-house within the company, or can be purchased from outside as systems. In general, the method of purchasing and applying a system available in the market is used most often. About 20 Japanese and foreign companies are supplying such systems to the market, and all of them have their own technical features for the method of heating or for the heat transmission structure. For example, the external heating method or the internal heating method can be used as the nozzle heating method. Further, various valve gate structures have been developed, such as the one in which the gate is opened and closed mechanically by force.

Since there are different types of hot runners that are used depending on the material to be molded and since molding is difficult in the case of some hot runners, it is necessary to sufficiently study in advance the type of plastic and the presence or absence of glass fibers, etc. During the design of the mold, it is necessary to propose the structure for incorporating a hot runner or manifold, the countermeasures against thermal expansion, the cooling structure, the maintenance structure, etc. The design is different from that of an ordinary mold, and it is necessary to thoroughly investigate the thermal calculations and the strength calculations. In addition, since recovering the initial investment for hot runners is difficult unless the production quantity of the molded product is large, it is necessary to verify the plan in detail as to how much planned production is to be made using that mold. In recent times, a trend can be seen in using valve gates or new types of hot runners in the development of molded products of super engineering plastics whose material costs are high. Even in Japan this technology is being considered to be used more and more frequently in the future as a means for making the costs of molding materials a minimum and for realizing molding that is in harmony with the environment.

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