How Does Plastic Injection Molding Work?

The plastic molding injection or injection of thermoplastic is a flourishing sector in the plastics production. It is a manufacturing technique that allows parts to be made from plastics. A really fast-process to produce large numbers of the same plastic-product in a very short span of time.

Our today’s pick is How Plastic Injection Molding Works. We will understand this concept by watching a fabulous video produced by a popular Youtuber called Engineer Guy,

The Principle Of Plastic Injection Molding

The plastic Injection molding is normally used for a wide variety of parts, smaller components for vehicle body parts. Designing a plastic product is usually the job of an industrial engineer.

A qualified designer or a specialized engineer designs the injection molds carefully from metal to give accurate outcomes of  plastic part productions.

During the injection molding process, once plastic reaches the right temperature, the melting of plastic granules (in a sheath equipped with heating resistors) is injected under pressure into the cavity of a mold of an injection press, where it cools and takes the shape of the final product.

The Plastic Materials Used In Injection Molding

For injection molding, thermoplastic polymers are used, which can be frequently melted or softened by heating as well as solidified by cooling. It is necessary to define which type of thermoplastic material should be used, which depends on the type of product.

Plastic materials such as polycarbonate, polyethylene, nylon, polypropylene, and high-impact polystyrene are popular in use in the production industry. Besides, selecting the right content for the exact reinforcement can enhance production and reduce expenses.

How Does The Plastic Injection Mold Work?

In this video, we can see, a machine injects the molten polymer into a mold at very high-pressure with a cavity corresponding to the design of the part.

Then after the plastic-polymer moves into the cavity through a feed system of an injection molding machine. Later filling the cavity, the machine proceeds to compact the melted-plastic material that solidifies after cooling. And ultimately, the mechanism ejects the solid part.

The cycle time required to produce a single part is usually just a few seconds or minutes. A short cycle time allows you to significantly increase production volumes.

These injection molding mechanisms have many components, and they are accessible in different configurations. Such as a vertical or horizontal arrangement..

However, all injection molding machines use a power supply, an injection unit, mold parts, and a clamping assembly to execute the steps in the above-explained process cycle.

The Machinery For Plastic Injection Molding

Nowadays, many different types of plastic injection molding machines are in use; like, Motorized machines, Hydraulic machines, Hybrid machines, which are driven by servo motors and hydraulic motors.

Although, there is a nozzle that is a common feature in most machines. That injects the plastic material into the mold that has been previously heated. Moreover, the different plastic materials have different melting temperatures and diverse characteristics, but the principle of these machines is the same.

Furthermore, there is a hopper which is attached on the nozzle side, that serves as a container for the material to be heated. And the cylinder, which is where the material is heated and then inserted into the mold by the nozzle.

The Two sides Of The Mold: Side A And Side B

The injection-molded parts have two sides: side A, which stands the fixed-part of the mold (front half of the mold). And side B, which faces the movable section of the mold, which is the rear half of the mold. These two sides usually have different goals:

  • The A-side generally has a better visual appearance. And it is usually related to the aesthetic side. Faces on the side A will be smooth or textured as per the design term
  • The B-side usually contains hidden structural elements (but very important) of the piece (the bosses, ribs, ratchets, etc.) functional. Side B will often have a rougher finish, and we can observe some visible marks from the eject pins.

Interesting Fact About Plastic Injection

Approximately 50-percent of the typical cycle in the plastic injection is applied for cooling and solidifying the plastic parts. Minimization of product-design thickness is the core of speeding up production and reducing costs.

Advantages And Limitations Of Plastic Injection

Plastic injection is a long-established manufacturing technology, but it is continually getting fuller and better with new technological advances. Below is a brief overview of the pros and cons of plastic injection to help you determine the right solution for your application.


  • Manufacture of plastic parts in large quantities;
  • Varied material palette;
  • Very high productivity;
  • High reproducibility and stable tolerances;
  • Excellent visual appearance.


  • High starting costs for tools;
  • Design changes are expensive;
  • Longer delivery times than other technologies.

The EngineerGuy: Bill Hammack

Bill Hammack is a professor of engineering at the University of Illinois at Chicago. He explains various segments using real-life illustrations and logical reasoning, he provides in-depth, practical knowledge on just about anything from electronic engineering to chemical engineering. His way of teaching is so popular on YouTube and even in real life.  Visit his channel to know about different engineering science topics.

The YouTube channel: EngineerGuy

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