Plastic Injection Process Explained

What is plastic injection?

Plastic injection is a long-developed practice of manufacturing and is able to effectively and repetitively create parts and components with identical geometric characteristics. Injection molding is not like machining. The latter cuts a solid piece of material to attain the desired form. Plastic injection, on the other hand, fills a liquified material into a mold to carry out the geometric design. Aside from the fact that this method is simple yet effective, its versatility and flexibility also show in the material it can process.

The most frequently used materials in injection molding are metals, plastic, glass, and elastomers. The molding process that produces metal components is called die casting, and injection molding used to form plastic parts is called plastic injection molding. In this article, we will talk about the main components of an injection molding machine in general, and break down the process step by step. We hope you can walk away with the basic ideas of what plastic injection is all about and you can surely contact us to learn more about the details. Let's read on.

Process Explained

The plastic injection molding process is so straightforward that it can pin down to only four (or three) steps: raw material preparation (feeding and injection), molding, and ejection. In the following section, we will put focus on each step of the process and how the machine part works as well.


The first two steps of the process are important because they prepare the plastic material for molding and the preparation profoundly affects the precision of the end result. At the very beginning of the process, the raw material(s) comes in the form of thermoplastic pellets. The pellets are fed into the hopper of the injection molding machine to enter the injection unit. The injection unit is essentially a massive syringe, with a barrel to contain the thermoplastic and a tiny outlet (a nozzle) to inject the material into the mold. An injection molding machine has the injection unit in either a horizontal or vertical configuration.


Entering the injection unit, there is a screw that advances the plastic material. As the screw drives the plastic pellets to the nozzle, the heater on the barrel melts the pellets at the same time, turning the pellets into a plastic slurry. Heating is vital in a precision plastic injection molding process because it controls the viscosity and velocity of the material, determining the effectiveness of the injection and the wall thickness and evenness of the product. In addition to heating, the amount of plastic material injected into the mold is also under strict control.

The amount in each injection cycle is called a shot. Too much/less injected material leads to too thick/thin walls of the product. If the wall is too thick, it takes more time to cool down and solidify, or it just doesn’t solidify enough. On the other hand, the thin wall makes the product brittle and vulnerable. It could also happen that the shot is not enough to form a complete product. Either way, the product is NG and will be rejected.


The molding process requires three primary parts, the two mold halves, and the clamping unit. The two mold halves grant the final form of the product. The geometric design of the product is created on the mold halves and together they form the molding cavity, which is filled with molten material. The physical strength of the part (structurally speaking) partially comes from the mold design. A good mold design can successfully present intricate geometric details on the part, ensure great physical strength, allow the material to efficiently spread across every corner of the cavity, and render easy ejection of the part from the mold halves. A molding cycle usually takes only several seconds. In some high-production settings, the mold opens instantly right after the injection.

The mold halves are supported by a clamp (typically a hydraulic clamp). The clamp controls the opening and closing of the mold and the clamping force is an essential factor in heavy-duty manufacturing processes. Depending on the force output of the clamp, the products the injection molding machine can manufacture vary. For example, when processing products that are heavier, larger, or require a higher injection pressure, the clamp with a higher tonnage force is needed. The general-purpose injection molding machine usually has a tonnage ranging from 4 to 20 tons, while the machine for the heavy industry may require a clamping force of up to several thousand tons.


Molded part ejection is the last step of the entire process. It takes place when the molded part thoroughly cools down. In a precision plastic injection process, the time it takes to inject the material, mold, and cool down the part is calculated to guarantee the quality of the result and to achieve the best production rate possible. It is common for the mold to feature holes or channels for the coolant to pass through, aiming to reduce the overall cooling cycle. There are also components such as ejection pins to assist this process. Some polymers are prone to clinging when solidifying. It is detrimental to ejection and will deform or distort the part. The pins, therefore, play a crucial role to help remove the part from the mold.

The injection process is complex to maneuver also because of the shrinkage of the material. Most thermoplastics have the tendency to shrinkage when cooling down. Each type of polymer or collective use of polymers shows a different level of shrinkage. In the home appliance industry and general manufacturing industry, a tight tolerance of shrinkage is not always necessary. However, in the automotive, medical, electronic, aerospace industries, etc., tolerance matters because parts are likely to fail or underperform, and such circumstances are unacceptable. Most (if not all) leading plastic injection molding firms have their own know-how or patented formulas to ensure the utmost quality of their products.

Why is Plastic Injection Prevalent across Industries?

The major reason why the plastic injection process is prevalent is its cost-effectiveness. Some may think that it costs quite a lot to invest in mold design and mold making. However, once the mold is made, it virtually needs zero maintenance if the manufacturing process is done correctly and most of the cost lies solely on the thermoplastic resins. Also, owing to the nature of this method, it is ideal for mass production for it can go fully automated easily. The production rate can be significantly higher than most manufacturing processes today.

Apart from the cost-effectiveness, plastic injection is also flexible and versatile. First of all, this method is able to realize complex dimensional and geometric characteristics. The same process that makes toothbrushes and food containers can also make valves, electronic housings, automotive parts, etc. Moreover, the possibility of using multiple polymer resins to achieve desired physical characteristics also shows why it is preferable. Contact us to know the materials we use for our products.


As stated, plastic injection molding holds a broad breadth of applications. You can find plastic injection molding products in industries such as medical, general appliance, healthcare, automotive, engineering, military/defense, aerospace, construction, agriculture, food processing, and electronics, to name a few. Leave us a message here right away if you want to know more about the process or our service. Thanks for your time. We hope you find this article helpful. There will be more articles coming up.