How Does Injection Molding Work?

Injection molding is a manufacturing process widely used in the production of a vast array of plastic parts, ranging from small components to entire products. It offers high production rates, precision, and reproducibility, making it an essential technology in many industries. This article delves into the working principles and processes involved in injection molding.

What is injection molding?

How does injection molding work

The so-called injection molding refers to the method of injecting heated and melted materials into a mold, cooling and curing them, and obtaining molded products. The specific process is to feed granular or powdered plastic from the hopper of the injection molding machine into a heated barrel, which is heated and plasticized into a molten state. It is then pressurized by a screw and injected into a low-temperature, closed mold cavity through a nozzle at the end of the barrel, which is cooled and hardened to maintain the shape given by the cavity. After opening the mold and removing the colloid, a work cycle is completed.

How does injection molding work?

The basic steps of injection molding process:

1. Machine Setup

The first step involves setting up the injection molding machine, which consists of a clamp unit, injection unit, and hydraulic or electric drive system. The clamp unit holds the mold halves securely in place, while the injection unit houses the screw-type plunger that forces the molten plastic into the mold cavity.

2. Mold Preparation

The mold, typically made of steel or aluminum, is designed to produce the desired part shape. It consists of two halves, the stationary half (fixed to the injection molding machine) and the movable half (which opens and closes). The mold cavity, which will impart the final shape to the plastic part, is created by the precise mating of these two halves.

3. Material Preparation

The plastic material, in the form of pellets or granules, is fed into the injection unit. The plastic is then heated to a molten state, ready for injection into the mold. The temperature required depends on the type of plastic being used.

4. Injection Phase

With the mold clamped securely, the molten plastic is forced into the mold cavity by the plunger. The high pressure ensures that the plastic fills the cavity completely, replicating the shape of the mold cavity. This process is known as the filling stage.

5. Packing and Cooling

Once the cavity is filled, the plunger continues to apply pressure to the plastic, compacting it and ensuring there are no voids or shrinkage. This phase is called packing. Simultaneously, cooling channels within the mold circulate water or oil to rapidly cool the plastic, solidifying it into the desired shape.

6. Ejection

After sufficient cooling, the clamp unit opens, and the movable half of the mold retracts, exposing the solidified plastic part. The part is then ejected from the mold cavity, either manually or automatically, and is ready for further processing or assembly.

7. Post-Processing

Depending on the part and its intended use, post-processing steps may include trimming excess plastic, deburring, sanding, painting, or other finishing operations.


Injection molding is a highly technical and precise process that requires careful attention to detail and a deep understanding of materials, machine operation, and mold design. With the right setup and expertise, it can produce high-quality plastic parts at scale, efficiently meeting the demands of modern manufacturing.

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