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Definition of Plastic Injection Molding and How it Functions

Plastic injection molding is a complex manufacturing process that uses molten material to create different parts. It involves injecting molten plastic material into a mold, where it cools and solidifies into a final product. Plastic, metal, and glass are commonly used materials. Plastic injection molding has existed since the 19th century and remains among the top ways to produce complex parts under minimal expenses. Injection molding is a popular option for countless industries because the process allows the creation of a wide range of plastic materials and finishes.

Plastic molding injection is the most popular plastic manufacturing method in the world. It is widely known for its various benefits including low labor cost, high tolerance precision, and durable products. The process involves mass production, creates complex parts, and is environmentally friendly. Aerospace, medical, and automotive industries are some of the most demanding manufacturing projects worldwide that use plastic injection molding.

How injection molding works

Before the actual procedure begins, you must consider your product design. Ensure you have all the materials for your final product to avoid making mistakes that can make your production process inefficient. Consider your product’s application, aesthetics, manufacturability, and assembling during product design. In most instances, computer software aids in product designing since it is more convenient and fast.

Mold design is another essential factor before starting your molding process. Mold design depends on the type of product you want. Usually, manufacturers make molds using metal materials to enhance strength and effectiveness during plastic molding. Aluminum, hardened steel, and beryllium copper alloy are the commonly used metals. Ensure you choose the best mold design to achieve quality results.
Plastic molding is complex, but various stages can help you understand how it functions. The stages include:


In the clamping phase, a hydraulically actuated clamp ensures the two parts are firmly closed together to withstand the extraordinary pressure produced in the injection stage. The clamping unit holds the mold together and ensures the molten material forms fully. Most plastic injection molding machines are classified by the clamping force they reach. After the part is correctly cooled, the clamp releases it through ejection, and the molding machines prepare for the following process.


Injection involves feeding color and resin pellets, shavings or granules to a barrel through a hopper. The feeding can be manual, automatic, or vacuum. A heating element encloses the barrel, and the raw materials are subjected to high temperatures and pressure until they melt and become malleable. The screw shear action also plays a role in this process.

The screw carries the molten polymer resin to the nozzle. There is a check ring that prevents the resin from flowing backward. The injection molding machine parts are well set to ensure the molten resin gets to the injection point in a homogenous state and at the correct pressure.


The molten polymer resin is conveyed effectively into the split mold assembly, which consists of a platen-mounted moving core and stationary cavity. There are gates at the cavity of varying sizes and different locations. The type, size, and location of these gates determine the entire quality of the end product.
Molds have internal channels that allow liquids to be pumped immediately after injection. The fluids help in cooling just before ejection. During the cooling phase, plastic parts form in the right shape and may shrink slightly. The molding haves does not open until the needed cooling time is achieved. The cooling period depends on the properties of the plastics used in the procedure.


After cooling, the mold opens, and the in-built injector pins in the mold eject the plastic part. If you look at some plastic materials in your house, you can find ejector pin marks. The mold then recloses to repeat the plastic molding process.

Post-ejection stage

Sometimes, your final product may need finishing, like polishing, dying or removing spurs. Other machines or human operators can do these processes. Once all the procedures are complete, the products will be packed and distributed to manufacturers.

Plastic injection molding is an efficient process of producing mass and complex products at low labor costs. It may seem like a simple procedure of melting plastic materials and cooling them to achieve a final product, but it entails critical elements. You must have the appropriate design, tools, manufacturing process, and efficient machine to meet your specific requirements.

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