The Basics of Plastic Injection

Plastic injection is a popular manufacturing method for producing durable, functional parts. This is done by inserting a liquid resin into a hollow space of a mold, where it hardens to form the desired shape. There are many different types of plastics that can be injected into the mold, and different materials require specific techniques to achieve the best results. This process is also very fast, and it is a good choice for producing high-volume parts in a short amount of time. The key to success is a well-designed product and the correct molding techniques.

The first step in plastic injection is the clamping or closing of the two halves of the mold to secure them together before injection begins. This is accomplished by the powerful force applied by the machine’s clamping unit, which can run from less than 5 tons to over 4000 tons. Choosing the right clamping pressure is critical to quality; too much or too little pressure will cause leaks in the molded part and can damage the equipment over time. Cracked platen and mold plates, sheared hydraulic cylinders, crushed mold vents and other problems can be caused by improper clamping pressure.

Once the mold is closed, the heated plastic is injected into the cavity through the sprue and runner system. The sprue is a channel that connects the nozzle of the injection unit to the front half of the mold. The runner system is a series of channels that carry the molten plastic from the sprue to each of the part cavities. The sprue and runner systems also contain gates that direct the flow of molten plastic into each cavity. The gates can be either hot or cold, and some are equipped with ejector pins that help eject the finished part from the mold.

After the molten plastic is transferred to the cavities, holding pressure is applied until the cavity is 95% filled. This step helps improve the compactness of the molded part and compensates for thermal shrinkage during cooling. It also prevents flashes, which are voids in the molded part due to heat loss during the transfer and filling process.

Depending on the application, the injection speed is adjusted to achieve a uniform melt distribution and fill rate throughout the entire cavity. The injection process is repeated until the molded part meets the design specifications.

A wide variety of thermoplastics are used in injection molding, including commodity plastics like polycarbonate, nylon, polypropylene and ABS — as well as more advanced resins such as PEEK. Most thermoplastics can be used with standard injection tools, although some require specialty molds. For example, some thermoset resins such as phenolic and epoxy require hot molds to accelerate the chemical reactions and increase the speed at which they solidify. The hot molds also provide a thermally isolated environment for the injection system and chemical precursors. When the molded parts are ready to be removed, valves close to isolate the injection system and the chemical precursors, and the mold opens to eject the molded parts.

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