Types of Plastic Injection Molds

Plastic injection molds create objects when melted plastic is forced into mold cavities. They often are used in mass-manufacturing operations to create products such as toys, kitchen utensils, bottle caps and cell phone stands. You can spot items constructed using an injection mold by the "flash lines" of extra plastic around the seams. Here are four major types of injection molds commonly used.

Cold Runner Molds

Cold runner molds are the most common type of plastic injection molds. These molds use a sprue and runner to inject plastic into each cavity. However, the sprue and runner harden along with the part once it solidifies. Workers then cut these pieces off and recycle them. This can create a problem with waste, which may make them less economical to use.

Two-Plate Cold Runner Molds

Two-plate cold runner molds are so named because they feature two plates that fit together. There is only one parting plane, and that is where the runner must be located. This is the simplest type of cold runner mold.

Three-Plate Cold Runner Molds

Three-plate cold runner molds have three pieces, which means there are a pair of parting planes. The three pieces are the runner plate, the moveable plate and the floating plate. The runner plate contains the sprue and half the runner. The moveable plate contains the part's ejector system and half the part's geometry. The floating plate contains the other halves of the runner and geometry. The system can be located at either parting plane, and the piece can be gated anywhere on the surface to provide flexibility.

Hot Runner Molds

Hot runner molds deliver plastic in a different way. While cold runner plastic injectors cool along with the part, the runners in hot runner molds are situated inside the mold and kept at a temperature higher than the plastic's melting point. This means the sprue and runner never have the chance to harden. This also reduces the amount of wasted plastic. However, hot runner plastic injection molds require more skill and maintenance and have a higher baseline cost. However, they often pay for themselves based upon the material savings.

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