Thermoplastic Injection Molding: What is it?

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

The thermoplastic injection molding industry churns out literally thousands of products and supports hundreds of industries. Since the 1990s, nearly 20,000 different types of thermoset and thermoplastic materials have been used for injection molding. Industrialists commonly use dyes and other agents to alter the properties of the molten plastic resin, such as color, hardness and elasticity. The molds used to shape the molten plastic generally have to 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 usually designed to allow air bubbles to escape. Otherwise, during compression and heating, air bubbles could deform the plastic and even create internal burns of the finished components.

Pre-hardened steel dies 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 jobs, makers may use aluminum molds, which may be more cost effective for smaller operations. For tooling molds for industrial plastic injection work, manufacturers typically use one of two time-tested processes: electric discharge or standard machining. With the electric discharge process, a robot applies a voltage from a tool to alter 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.

Manufacturers can experiment with literally dozens if not hundreds of variations in 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 alterations can include the cooling time of the plastic, the pressure in the cavity, the composition of the colorant and the variety of plastic resin additives.

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