What is the Injection Molding Process?

The injection molding process is a technique used to create a wide range of objects such as plumbing parts, plastic toiletry items, toothbrush handles, toys, and even the dashboards in cars. Materials used in the injection molding process vary, but they frequently include plastics or synthetic polymers that can be easily molded. Low marginal cost per molded item, variability of material, general efficiency, and nearly complete products are all advantages of the injection molding process. The initial investment in the machinery can be expensive, however, and the cost of powering the machines can be high as well, presenting a few disadvantages to injection molding.

To begin the injection molding process, plastic granules are dispensed from a hopper into a heated container with a plunger. The plunger is usually moved by a hydraulic pump that presses the heated plastic horizontally towards the mold. As the plastic moves through the chamber it passes a series of heaters that eventually melt it, and then it reaches a nozzle that directs it past a gate and into the mold. Once inside the mold, the plastic cools and hardens. The mold itself is kept at a cool temperature to facilitate efficient cooling of the plastic, which begins almost immediately.

The mold must maintain a set amount of pressure to make up for the natural shrinkage of the plastic material as it cools. Often times, clamping units are included in the system to create this pressure. Once the plastic resin has cooled sufficiently, the mold is opened and the plastic part is removed. The empty mold is then ready to close and receive the next shot, or dose, of melted plastic for the next part.

While the injection molding process is quite streamlined, there are several problems that can potentially occur if the equipment malfunctions. Many of these problems occur in the beginning of the process, and can be fixed by adjusting the machinery. Common complications include burning and general imperfections in the plastic parts, warping of the parts, and incomplete filling of the mold.

Burning and general imperfections are most frequently caused by overheating. If the plastic is left in the heating chamber too long, or if the heaters are powered too high, the plastic could burn or develop gas bubbles that create an uneven surface. Warping is also a result of a temperature error, but it usually involves an uneven mold surface temperature. This causes the plastic inside the mold to cool at different rates, shrinking unevenly and warping the plastic part. Finally, if the nozzle malfunctions, then the mold might not be fully filled, causing another error in the shape of the plastic parts.