What Is Plastic Injection Molding and How Does It Work?

Plastic injection molding is essential across several industries. It works by creating a desired product from molten plastic material.

Table of Contents

Plastic injection molding is one of the processes of manufacturing plastic parts. The demand for intricate parts in high quantities is rising in the manufacturing industry. Thankfully, the plastic injection molding process has risen to this challenge.

But how does plastic injection molding work? This article explores the plastic injection molding process in detail as well as the advantages and limitations of the process.

What Is Plastic Injection Molding?

Mold for plastic manufacturing

Plastic injection molding is a popular process for manufacturing complex, intricate plastic parts and items. The manufacturing process involves injecting and filling molten plastic into custom-made molds according to specifications. After the mold cools and the plastic solidifies, you get your finished plastic component.

Plastic injection molding is essential in manufacturing because it helps create products of different sizes with intricate designs. This process manufactures plastic components across several industries, including medical, consumer goods, packaging, and automotive.

In addition, because the process manufactures products in large quantities, it helps control costs.

In 1872, John Kite invented the plastic molding machine for the production of plastic balls. The process was also used to make small items like combs and buttons. However, with the industry’s rapid growth, this process is now used to manufacture intricate and complex parts.

The Process of Plastic Injection Molding

Injection molding cycle

The plastic injection molding process involves a series of steps, including the following:

Step 1: Clamping

During the clamping step, a sufficient clamping force is applied to close the mold halves before injecting the molten plastic material. However, the clamping force still holds the mold in place, allowing the injected material to dwell in the cavities.

Step 2: Injection

Raw plastic pellets are melted and injected into the mold during this step. Factors affecting the injection process include the barrel and nozzle temperature, screw back pressure, and plastic flow rate. Barrel temperature affects the flowability of the molten plastic.

Set the barrel temperature between the melting point and the thermo-decomposition temperature of the plastic. If the temperature is too high, the plastic may overflow, resulting in flashes. Conversely, if the temperature is too low, it can slow the flow, resulting in melt lines and unfilled parts.

In addition, the nozzle temperature should be lower than the barrel temperature. Otherwise, the molten plastic will leak and decompose inside the nozzle. This then blocks the nozzle and results in bad finished products including blocked nozzles and poor shot quality.

Screw back pressure, also known as plastic pressure, is the pressure applied to the molten plastic by the screw. If the pressure is too high, it increases the flow of the plastic, which may lead to flash formation. Conversely, there is decreased flow with low pressure, which may lead to bubbles and voids.

The plastic flow rate is another factor affecting the injection process. The plastic’s viscosity depends on its flow rate. Therefore, the flow rate should be sufficient to inject molten plastic into the mold cavity without deformation.

Step 3: Cooling

After injecting the molten plastic into the mold and allowing it to dwell, it cools. During this step, the prevailing pressure is the holding pressure, ensuring that the molten plastic becomes compact during solidification. The mold has a coolant system that removes heat and facilitates cooling.

There is no set cooling rate and time. Factors affecting this rate and time include the mold design, the material used, and the final product expectations.

Step 4: Ejection

Ejection of finished product

After cooling the molded part, the mold is opened, and the finished product is ejected. The ejection system consists of ejection pins pushing the solidified product out of the mold plates. A mold release agent can also help to remove the molded part easily.

Step 5: Trimming

After the part is formed and ejected, the finishing process involves trimming the excess plastic. Trimming takes place in another machinery. After trimming and polishing the parts, they are packed and ready for distribution.

What Equipments Should be Used in Plastic Injection Molding?

Injection mold for plastic parts

Injection Molding Machine Components

The injection molding machine comprises a clamping unit, an injection unit, and an ejection system. The clamping unit is responsible for holding the mold plates in alignment, opening and closing the plates, and applying sufficient clamping force from the injection to the cooling step.

The injection unit comprises a feed hopper, barrel, heaters, and reciprocating screw. This unit supplies molten plastic, applies heat to the plastic, and homogenizes it before pushing it into the mold cavities. The ejection system facilitates the removal of molded plastic parts from the mold cavities.

Types of Injection Molding Machines

Cold runner vs hot runner injection mold

There are three popular injection molding machines, depending on their operation mechanism. These machines include hydraulic, electric, and hybrid.

  • Hydraulic injection molding machine

The hydraulic injection molding machine is relatively simple to operate and maintain. The machine injects molten plastic into mold cavities at high pressures.

This type of plastic injection molding machine is an excellent choice for businesses new to injection molding because it produces large parts in a short time. However, the downside to the hydraulic injection machine is its noise pollution.

  • Electric injection molding machine

As the name suggests, an electric injection molding machine uses electricity. Due to its energy efficiency, the machine is not noisy and does not lag when pressure changes occur.

Another characteristic of the electric plastic injection molding machine is that it is a high-volume production process, producing plastic components in a short time. The downside to the electric molding machine is its high upfront cost.

  • Hybrid injection molding machine

The hybrid injection machine is the best of both worlds. The machine is energy-efficient and does not produce as much vibration. It is also precise and accurate, making it ideal for manufacturing plastic parts with tight tolerances.

Injection Mold Components

Injection molded products

All types of injection molding machines use a mold. The mold consists of two plates, and they are attached to the clamping unit. The plates have a cavity which is a shaped section that gives the molten plastic its final form.

The injection mold also has a parting line that may be straight or curved. This line usually indicates the separation between the two mold halves.

What Are Mold Design and Construction?

Injection molded plastic parts


Mold design and construction are integral to the success and functionality of the desired plastic part. Custom mold design provides limitless possibilities. Depending on the application, you can create molds for manufacturing anything from mechanical components to medical devices. Mold design also ensures product consistency, reducing manufacturing errors significantly.

Skilled designers and programmers create new molds using the latest CAD-CAM software during the design process. Before manufacturing the mold, the 3D design is tested and optimized till it is ready for CNC machine. Afterward, a rough mold shape is carved out of aluminum or steel.

Aluminum and steel are the most common materials for carving molds. Although steel molds are durable and allow more shots per mold, they are slower and more expensive to make. Aluminum molds, on the other hand, are faster and more affordable to make. Nevertheless, they have a short lifespan and only allow a few shots per mold.

After choosing a suitable material for the mold and carving out the rough mold shape, precision machining is used to finalize the ultimate shape of the mold. The mold is then assembled. Consequently, these molds have a high degree of accuracy.

Factors Considered in Mold Design

Intricate and small plastic components

Factors to consider when designing a mold for a plastic part include the material options, critical tolerances, and sink marks. You should also consider the ejection way, parting line, mold size, and cavity layout.

Types of Mold

When designing and constructing molds, it is essential to note that there are different types. The types of molds include hot-runner, cold-runner, two/three plate, and family mold.

Hot runner mold has a nozzle with heat. After molding, the plastic part does not have a sprue, and the gate mark is very small. On the other hand, cold runner mold has a sprue bush, and it is necessary to cut off the sprue after molding. This type of mold also leaves a very big gate mark.

Two and three-plate molds are both cold runner molds. The mold leaves a big gate mark in two-plate molds, and you need to cut the sprue gate after molding. Three plate mold has an additional runner plate compared to the two-plate mold. This mold leaves very small gate marks, and the sprue is cut off automatically.

What are the Different Types of Plastic Injection Molding?

injection molded plastic

Conventional Injection Molding

Conventional injection molding involves injecting molten plastic into molds to form plastic parts in several shapes. This injection molding method has several advantages, including precision in producing complex designs and fast mass production.

In addition, with conventional injection molding, your plastic parts can have many detailing and remarkable features.

Insert Molding

Insert injection molding process molds plastic parts around inserts. These inserts may be non-plastic, and they are often simple objects. Insert molding combines plastic and other materials into a single unit, reinforcing the plastic part and improving its tensile strength and wear resistance. During the insert molding process, the insert is placed in the mold cavity before the molten plastic is injected.

Insert molding is often used in the electronics industry. It is used to embed electronics into plastics and to manufacture wiring and insulated pipes.


Overmolding is similar to yet different from insert molding. In the over-molding process, the base layer is first molded. Afterward, the molten plastic is molded over the base part. Overmolding creates a single product with high tensile strength and extended shelf life.

Overmolding is often used to manufacture ergonomic grips on power tools, car tail lights with colorful parts, and hardware tools, including screwdrivers, pliers, and hammers.

Micro Molding

Micro molding

Micro molding is used to produce microplastic parts. Micro parts usually weigh less than a gram. The micro molding injection process is highly specialized and high precision, producing plastic components in their desired shape in thousands and millions.

Micro molding is often used to manufacture lenses and sensors for optical devices, microelectronic devices, grommets, and receiver protection for hearing aids.

Liquid Injection Molding

Liquid injection molding produces rubber products from liquid silicone rubber and similar elastomers. This injection molding method is versatile and precise. This thermoset process mixes platinum-cured liquid silicone rubber before injecting it into a heated, sealed mold. Next, the mold heats the liquid mixture to initiate curing. Once it hardens, the molding machine ejects the product.

Liquid injection molding is used in several industries, including medical, automotive, and food and beverage. In addition, it is used to manufacture gaskets, sensors, seals, and vibration dampeners.

Injection molding machine

What Are the Advantages of Plastic Injection Molding?

High efficiency and accuracy

The plastic injection molding process is highly efficient and accurate. The process uses a mold to shape molten plastic. Consequently, all manufactured parts are identical, even with multiple batches.

Wide range of materials

The injection molding machine can handle and mold a wide range of materials, including chemical-resistant plastics and thermoplastic materials, into desired shapes and sizes.


The injection molding process is a high-volume production process that saves costs across multiple manufacturing cycles. However, your raw material and design complexity increase the cost of the process. Nevertheless, compared to other manufacturing methods, injection molding is more affordable.

Design flexibility

The plastic injection molding process is flexible – it can manufacture simple and complex plastic parts. In addition, it is easy to customize the molds and print a brand name or logo on plastic parts during the manufacturing process.

Consistency and repeatability

Although the plastic injection molding process molds multiple plastic parts in a single production cycle, the parts have a consistent quality. However, even if there are parts that don’t meet the standard, the rejection rate is minimal and often negligible.

injection molding

Applications of Plastic Injection Molding

  • Automotive industry Plastic injection molding: It is widely applicable in the automotive industry. It is used to produce plastic parts like door panels and dashboards.
  • Medical industry plastic injection molding: It is precise and accurate, making it ideal for producing stethoscopes, heart pumps, and blood pressure cuffs.
  • Electronics industry: Electronic components like plugs, switches, and computer components are manufactured using plastic injection molding.
  • Consumer goods industry: Plastic injection molding process is used in the consumer goods industry to mold non-toxic plastic materials. These materials are safe for handling food.
  • Packaging industry: Plastic injection molding is used to mold molten plastic into suitable packaging products. Because of the versatility of the packaging industry, the molding process is used to produce plates, plastic bags, and bubble wrap according to specifications.

What are the Challenges and Limitations of Plastic Injection Molding?

Although plastic injection molding offers numerous benefits for the manufacturing industry, this process still has its limitations. Below are the following challenges and limitations of plastic injection molding.

Environmental concerns

Although the injection molding process is low-waste, there are still concerns about plastic waste. However, plastic waste is bad for the environment. A thoughtful product design and an efficient production process should limit waste. It should also account for recycling whenever possible for a more eco-friendly process.

Material selection

Plastic injection molding machines can mold several materials, including polyethylene, nylon, and polypropylene. However, you cannot just use any material for your product. Therefore, it is essential to consider the tensile strength of the desired product before choosing a material.

Design limitations

Plastic molding processes like 3D printing allows you to upload a file and make a design change whenever necessary. But you can’t just make a design change with plastic injection molding. Instead, you may have to create a new mold from scratch.

injection molding

Tooling and equipment costs

Initial startup costs of injection molding are high. Creating molds for each plastic part you want to mold is necessary. Tooling for complex molds is even more expensive than simple designs. Nevertheless, because the molds are reusable, you can save on equipment costs in the long run.


Plastic injection molding is useful in the medical, automotive, and consumer goods industries. The process produces minimal waste, and although it has a high upfront cost, it is cost-effective in the long run.

Furthermore, because the process involves using molds, it efficiently produces consistent and high-quality products. You can also check out rubber moldings like compression molding and die cutting.

Contact Hongju For High-Quality Products!

Hongju is a leading rubber products manufacturer with processing capabilities, including injection molding. With over two decades of experience, we offer top-of-the-line solutions. You can contact our team for a prototype regardless of the product. We customize plastic and rubber products according to your specifications.

Get Instant Quote Now!

Share The Post Now:


Hey there, I’m David!

I’m the founder of Hongju Silicone. I have been in this field for more than two decades. If you are looking for custom-made silicone rubber products, feel free to ask me any questions.

You may also find these topics interesting

viton rubber

Viton Rubber: The Definitive Guide

The rapidly evolving demands of the modern world mean materials must operate in harsher environments for longer. In the case of elastomers, even materials such as nitrile rubbers can fall short of expectations. This is where Viton rubber comes into play.

Read More »
4 types of silicone

4 Types of Silicone and Their Distinctions

Silicone rubber, an elastomeric material known for its exceptional properties, plays a pivotal role in various industries, from medical devices to automotive applications. Understanding the 4 types of silicone and their distinctions—ranging from liquid silicone rubber (LSR) to high-consistency rubber (HCR)—is crucial for selecting the right material for your needs.

Read More »
epdm rubber

EPDM Rubber: The Definitive Guide

EPDM, or Ethylene Propylene Diene Monomer rubber, is a versatile material known for its durability and weather-resistant properties. It is used in a wide range of industries, including roofing, automotive, and construction.

Read More »

Request for Quote

*All inquiries will be responded to within 24 hours.

Send Your Inquiry

*We respect your confidentiality and all information is protected.

Download Catalog

Fill in the form below, and we will send you our entire catalog immediately!