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Types of Molds in Injection Molding

Injection molds may be classified by multiple different categories based on their features, such as runner system, plate amount, and part capacity. These classifications are not exclusive, but rather every mold will fit a label within each category. For example, you may have a mold that consists of a cold runner system, three plates, and has multiple cavities. Understanding what each of these categories means for your molding process will help prepare you to select the right type of mold for your part. 

The Importance of the Right Mold

Choosing the correct mold type for your production is crucial because the type of mold will directly impact product quality, production speed, and, as a result, overall costs. While the type of mold you choose will also influence upfront cost of the mold, it is important to consider that the cost will change if more complexity or stronger tool material is required, such as side actions or pick-outs, steel in place of aluminum, or the mold is to be used for overmolding or insert molding. 

There are multiple different ways to classify a mold based on different attributes of the mold. Below, we’ll discuss the primary three categories: 

Type by Runner

The first category of types of injection molds classifies a mold by its type of runner system. A runner system is a series of channels, including a gate, sprue, and runners, that direct the molten plastic material from the machine’s nozzle to the desired areas of the mold’s cavity. 

Hot Runner

A hot runner system involves runners that are either internally or externally heated by rods, coils, or other heating elements. Because it must be heated while the mold is cooled, this type of runner system is encased within a stationary manifold plate. Since the runners are held permanently within this plate, they will not eject with the part, so the molded part comes out clean and devoid of extra plastic waste attached. This reduces waste, and there are no added production steps to remove the runners and recycle them as there are with cold runner systems, meaning production is allowed to move much more quickly. This type of mold is typically used for large volume productions with multiple mold cavities because it is expensive to heat and requires more maintenance that its cold runner sibling. Additionally, because the runners are hidden, it can be difficult to ensure the runners are clean and clear of leftover plastic from previous productions.