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What Are the Steps to Injection Molding Plastic?

Read on to learn about injection molding plastic, which involves fabricating plastic components of varying complexity, sizes, and application. 

What Are the Steps to Injection Molding Plastic - KB Delta

One of the popular methods for manufacturing plastic components is injection molding. This process, which is not overly complicated, is favored chiefly due to its tendency to facilitate extensive production runs of identical components. Moreover, it is the least expensive method of producing plastic parts in medium to high volumes.

The entire plastic-making process requires a horizontal or vertical injection mold press, raw plastic resin, and a specially-shaped mold. First, the plastic resin material is melted in the injection molding machine. Then it is injected into the mold, where it cools down, solidifies, and takes the shape of the final component or components.

The price-per-unit cost after creating the initial mold is comparatively low. In addition, since injection molding generally creates perfectly molded and identical products each time, it cuts down significantly on waste material. This is why injection molding is preferred over other manufacturing processes, such as CNG machining.

These are the primary reasons plastic injection molding continues to proliferate. The industry was valued at $258 billion in 2019 and is estimated to reach $372.9 billion in less than ten years.


The 6 Steps of Injection Molding Plastic

The entire injection molding process, from start to finish, generally consists of 6 easily-repeatable steps:


  1. Clamping
  2. Injection
  3. Dwelling
  4. Cooling
  5. Mold opening
  6. Ejection


Let’s look at these steps, assuming that the mold has been created already.


1. Clamping

This is the first stage in which each half of the injection mold gets attached to the injection molding machine through the powered clamping unit. Remember that injection molds are generally made up of 2 clamshell-style pieces.

The clamping unit pushes these two clamshell-style pieces until they close. It exerts a sufficient force that keeps the mold securely closed while the resin gets injected into the mold cavities.

The size of the molding press determines the time it will take to clamp the mold and hold it closed. Therefore, clamping is an essential factor to consider as it determines the overall size of the injection molding press required to create the plastic component.

Knowing the size press your component, you will immediately know at least one element the injection mold requires to create your plastic component.


2. Injection

The next phase in this process involves injecting the resin raw material into the injection molding press. The resin is then advanced toward the mold via the injection unit.

During this procedure, the raw resin material gets melted as it gets exposed to severe heat and pressure. The injection unit guides the molten plastic by using the build-up of pressure packs and holding the material within the mold’s cavities.

Several combinations of settings must be applied for accurate melt temperatures. This includes:


  • The nozzle temperature
  • The barrel temperature
  • The injection mold


The nozzle and barrel temperatures considerably impact the flowability of the resin from the start to the point where it gets solidified. The temperature of the mold also affects the flow of the resin through the mold while cooling down the molded component.

The nozzle and barrel temperatures need to be set carefully. If the temperature is too low, the final product will have several unfilled parts. On the other hand, if the temperature is too high, the result is a flash and an overflow. Therefore, the temperature must be set perfectly and appropriately for optimum results.

Plastic pressure is another crucial factor in injection molding. This refers to the physical force applied to the melted or molten plastic by the head of the screw when the latter moves forward. This process is controlled by an automated system of the injection molding press.

Pressure and temperature remain the two most crucial elements that must be adjusted when establishing a stable and perfect injection molding process.


3. Dwelling

During the dwelling phase, the melted plastic fills the mold. Pressure is then carefully applied to the mold to ensure the molten plastic fills up every cavity.

This action ensures that the product or component created will be perfectly identical to the mold.


4. Cooling

At this stage, the molten plastic or resin inside the mold cavities solidifies and takes up the shape of the component. Cooling starts right inside the mold immediately after the resin makes contact with the interior surfaces of the mold.

The manufacturer of the resin material would have provided specifications that ensure the resin ends up as the perfect component required. The mold designs, the component to be constructed, and the raw resin material used significantly determine the cooling period or how long it will take for the resin to cool.

The design of the injection mold incorporates internal cooling for extensive periods.


5. Mold Opening

As soon as the component cools, a clamping motor opens up the two parts of the mold to make a simple and safe removal of the final element.


6. Ejection

After opening the mold, an ejection mechanism pushes out the cooled, molded component from the mold. Since the molten plastic fills up every cavity of the mold and takes up its shape, it will require some force to eject the molded component after sufficient cooling.

After ejecting and removing the components, the mold closes so that the next shot is injected, and the next cycle begins.


The Tooling Process 

Injection molding has several advantages that manufacturing companies love. But the entire process has a few drawbacks. For instance, manufacturers must deal with upfront tooling costs and considerable lead time.

Tooling may be the first phase or stage of the entire injection molding process. It is a heavily-involved undertaking that must be completed once for every product. First, product teams will need to design and prototype a component – usually via 3D printing – and create a mold that can be utilized reliably to make the component.

This calls for comprehensive testing and optimization. But as soon as the mold is complete, the same injection molding process can be performed thousands of times to create identical plastic components.



The cost and time it takes for tooling make injection molding a daunting prospect. However, as soon as the first mold is perfectly created, injection molding becomes the easiest or most seamless way of creating numerous identical components at a meager cost.

This makes injection molding one of the fastest-growing and most popular practices within the manufacturing industry today.



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