What is thermoplastic injection molding?

Toy with injection molded thermoplastic parts.

Thermoplastic injection molding is a process used in manufacturing to create a variety of parts and components for industries ranging from aerospace, automotive, and construction. Thermoplastics, such as phenolics and epoxy, are heated in a molten resin and then injected into a mold that is usually made of aluminum, steel, or metal alloy. The molten plastic is then squeezed into the mold and allowed to cool. Machines remove the plastic component or part from the mold, and this hardened part can then be used to build a larger product, such as a child’s toy or a car door.

An old phone made by thermoplastic injection molding.

The thermoplastic injection molding industry literally produces thousands of products and supports hundreds of industries. In the 1990s, nearly 20,000 different types of thermosetting and thermoplastic materials were used for injection molding. Manufacturers often use dyes and other agents to change the properties of molten plastic resin, such as its color, hardness and elasticity. The molds used to shape molten plastic usually must be pre-cut in a separate process, using sophisticated tooling procedures to properly prepare them. A device called a sprue allows molten resin to enter the mold and fill the cavity. Molds are also typically designed to allow air bubbles to escape. Otherwise, during compression and heating, air bubbles can deform the plastic and even create internal burns on the finished components.

LEGOsĀ® are made with acrylonitrile butadiene styrene, a type of thermoplastic.

Pre-hardened steel molds tend to be more expensive but may last longer; therefore, manufacturers often use these higher quality, harder steel molds for high volume thermoplastic injection molding jobs. For more boutique industrial work, manufacturers can use aluminum molds, which can be more economical for small-scale operations. For tool molds for industrial plastic injection work, manufacturers typically employ one of two proven processes: electrical discharge or pattern machining. With the electrical discharge process, a robot applies a voltage from a tool to change the shape of the base metal. With standard machining, a more conventional process, a machine or tool physically deforms the mold into its final shape.

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Manufacturers can experiment with literally dozens if not hundreds of variations on the thermoplastic injection molding process to optimize their processes. Changes can be made to the pressure applied to the mold, the injection speed of the plastic resin, or the geometry and structure of the molds. Other changes may include plastic cooling time, cavity pressure, dye composition, and variety of plastic resin additives.

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