What are Injection Molders?

Salvatore Salamone

Injection molders are machines that shape plastic into commonly used objects. Items manufactured using injection molders include automotive body panels, cell phone shells, plastic packaging, plastic toys, medical devices, and even toothbrushes. Injection molders create these items by melting plastic resin and injecting it into a metal mold. Today, injection molders are widely used because of their flexibility allowing the creation of many different types of products. They are also highly desirable because they make very economical use of raw plastic resin. As such, they are ideal for the mass production of numerous commodity products.

Plastic auto-rickshaw made with multiple injection-molded parts.
Plastic auto-rickshaw made with multiple injection-molded parts.

Injection molders can be quite large, often filling an entire room. The machines vary greatly depending on the type of product being produced. However, all injection molders share a common set of elements. In general, they have a core part that is shaped like a long tube. At one end of the tube, a hopper is used to feed plastic resin pellets into the machine. The pellets are heated and forced down the tube in the direction of the mold.

Colored pellets used for plastic injection molding.
Colored pellets used for plastic injection molding.

As the pellets are melting, injection molders frequently mix additives into the resin to give the plastic desired characteristics. For example, additives are commonly used to provide color, increase strength, or improve the flexibility of the final product. Once the additives and the resin are combined and melted, the mixture is injected into a mold. The mold sits at the end of the tube, in the shape of the product to be created.

The injection of the melted plastic into the mold is done using one of two techniques. One method relies on a large screw called a reciprocating screw. The screw grinds the resin pellets and helps mix any additives. As the screw turns, it forces the melted resin into the mold. A second method uses what is called a ram injector. A ram injector forces the melted resin into a mold using a hydraulically-powered ram or plunger.

The mold that receives the melted plastic is constantly cooled. This cooling solidifies the plastic, forming it into the shape of the desired product. When the formed item is completely cooled, the mold is opened, and the created product is then ejected from the injection molder.

These basic steps in creating plastic products and the design of injection molders have not changed much for decades. Going forward, the only big change on the horizon is for the machines to become more efficient. To that end, future generations of injection molders are expected to use less electricity.

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