A Comprehensive Guide of Plastic Injection Molding Process

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Plastic injection molding plays a very important role in today’s manufacturing industry. The process involves the injection of molten material into a mold cavity under high pressure to form parts. 

Injection molding offers several advantages such as high production efficiency, high quality parts, and production of complex geometry parts. This has seen the process gain various applications in manufacturing of medical equipment, electronics, aerospace, and automotive parts. 

That said, we will look at everything you need to know about injection molding in this article. But first, let’s take a quick glance into the history of plastic injection molding!

Plastic Injection Molding Process 1

Operation principles of injection molding (Image Source: Pinterest)

The Origin of Injection Molding

The origin of injection molding dates back to the mid-19th century, when inventors began carrying out tests with the use of heated material to create mold products. 

Modern injection molding was developed during world war II when James Watson invented the first injection molding machine. The machine featured a screw injection system. The 1960s and 1970s saw improvements in materials and machinery thus expanding its application.

The 1980s saw the introduction of hot runner systems for production efficiency while the 1990s dated the introduction of computer Aided design (CAD) and Computer aided manufacturing (CAM) softwares. These softwares came on board to streamline part design and production. 

In recent decades the advancement in automation and robotics has seen injection molding develop into a multi-billion dollar industry. 

Understanding the Basics

Plastic injection molding is a manufacturing process that is used to produce mold parts through injecting molten plastic material through a gate and into a mold cavity. The molten plastic materials cools and solidifies taking the shape of the mold cavity which is then ejected as the final product. This process is widely used for its efficiency and production of parts with complex geometries that need extreme precision and accuracy. 

Some of these material used in injection molding include:

  • Acrylonitrile Butadiene Styrene (ABS)
  • Polyvinyl Chloride (PVC)
  • Polystyrene (ps)
  • Polycarbonate (PC)

The Step-by-Step Process 

Injection molding goes through several processes that play an important role in the production of parts that are up to the required specification. These processes include:

1. Material Selection

Choosing the right plastic material is important in achieving the parts application requirements. Different materials vary in terms of properties such as varying flexibility, heat resistance, chemical resistance and strength. Choosing a material that is suited for a certain product application ensures optimal performance. 

The durability of the product highly depends on the material used. Through selecting materials that have high mechanical properties, durability and resistance to wear and tear products can withstand the harshness of its application.

The aesthetic appearance of the finished product is very important in the brand perception. Materials vary in textures, surface finishes and color and choosing a material that promotes product appeal manufactures can attract customers.

2. Mold Design

Mold design is another component that plays a very crucial role in  injection molding. The mold design is influenced by several factors such as part complexity, surface finishing requirements and also  tolerances. 

Parts that are complex and require accurate and precise geometries can use the Computer Aided Design (CAD) software in order to achieve these desired features. To achieve the desired tolerances of the parts mold designers must take into account factors such as condensation, cooling rates and material flow into the cavity. 

Use of Computer Aided Design (CAD) software can help in producing, analyzing and optimizing the mold design.The CAD software can be used to generate a 3D model to get a visual representation of the parts and address any problem that arises.

3. Mold Creation

Mold making techniques have advanced over the years from traditional machining to 3D printing. The traditional machining techniques involve processes such as turning, drilling, shaping and milling. 

This technique relies mostly on tangible cutting tools and direct interacting with the material.It offers high precision but it is time consuming. 3D printing is the creation of a 3D object achieved using additive processes. 

In the additive process the object is created by laying down successive layers of material until the object is created. It allows for production of parts with complex geometries.

4. Plastic Melting

During injection molding, the plastic material that is injected into the mold must be molten to be of uniform and required density. This process happens in the molding barrels with the help of heater bands and the action of the screw which contribute to the quality of the melt. 

The sources of the heat to melt the plastic material  comes from the heater bands and the shear heat from the rotating screw. The barrel has different zones with the front part being the melting zone.

The pellets are subjected to high temperatures and pressure forcing them to melt. Once the melting is done the molten material is injected into the mold cavity.

5. Injection 

The process of injection molded plastic into the mold cavity is crucial. Before injection the two halves of the mold are closed and clamped together by the injection molding machine.this ensures the mold remains sealed during the process. 

The molten plastic is injected into the mold cavity under high pressure in the injection unit that is positioned above the mold. The high pressure forces the molten plastic material to flow into the cavity filling the cavity to form the shape of the mold.

6. Cooling and Solidification 

Cooling and solidification has a great  influence in part quality, cycle time and efficiency in the long run. The mold is equipped with cooling features consisting of channels through which a coolant circulates. 

The heat from the molten plastic is absorbed by the coolant facilitating lowering the temperature of the material. Proper Cooling is important in achieving uniform part cooling. 

With the molten plastic cooling it starts to change its state from liquid to solid state. The solidification begins at the mold wall and progresses inward. Solidification must be controlled to achieve desired part and mechanical properties.

Plastic Injection Molding Process 2

Plastic Injection Mold (Image Source: Pinterest)

7. Ejection 

Ejecting the finished part from the mold is accomplished using ejector pins. An ejector pin is a system that separates the molded part from the mold cavity. When the mold opens after cooling and solidification of the plastic the ejector pin pushes the finished part out of the mold.

Pin ejectors allow effective part removal from the mold. It is important to choose a suitable type of ejector pins that do not affect the aesthetical properties of the finished part.

The ejector pins have several advantages which include: efficient part removal, cycle time optimization, prevention of part distortion and automation of the ejection process.

8. Post Processing 

Post processing takes place after the completion of injection molding.This enhances the properties of the molded parts for their application and involves corrective measures to eliminate surface defect and processes for decorative purposes. 

Post processing services in injection molding include:

  • Trimming and deflashing:  This involves the removal of the excess material and trimming to get the desired final product.
  • Surface finishing: This is where the surface of the molded part is polished to achieve a specific texture or smoothness.
  • Painting and coating: This is the application of paint ,coating to add color to improve part resistance to environmental factors.
  • Assembly: when the final product is composed of multiple components post processing may include an assembly step to combine them.

Challenges and Solutions 

Manufactures are faces by several challenges in injection molding process which include:

1. Sink Marks 

Sink marks are depressions or indentations on the surface of an injection molded part that is caused by variations in the cooling rate of the plastic material. These variations can result in uneven shrinkage and warpage of the part. 

They occur mainly in thick sections of the part where the cooling rate is lower. High melting temperature can prolong the cooling rate leading to shrinkage variation leading to sink marks. Furthermore improper gate placement can result in uneven material flow and cooling leading to sink marks. 

Sink marks can be reduced through several  design and optimization processes. These include,through designing parts with uniform thickness to ensure uniform cooling, proper gate placement and design and choosing a mate.

2. Warpage

This is a common challenge faced by manufacturers. Warpage is the distortion of a molded part, often caused during the cooling process in the injection molding process. This can cause parts to bend, twist, fold leading to potential complications in the final product assembly.

Warpage is caused by several varying factors: cooling rate, cavity pressure and the fill rate,gate size and location. Warpage can be minimized through: cooling optimization, proper mold design, proper gate design.

3. Incomplete filling 

Incomplete filing in injection molding occurs when molten plastic material fails to completely fill the mold cavity. This results in production of partially formed parts. This greatly impacts negatively the quality of the part. 

It can be as a result of inadequate pressure during injection molding, poor venting which results in trapped air in the cavity, constricted flow paths. To minimize incomplete filling you need to: optimize injection components such as injection pressure and temperature, improve on venting  and proper gate design.

The injection molding industry is a crucial sector of modern manufacturing. It is used in the production of a wide range of products. Just like any other industry the injection molding industry is evolving driven by changing consumer demands, technological advancement. 

Technological Trends

The injection molding industry is evolving seeing a range of technological advancement that are transforming how products are being manufactured. 

The following are some of the technology trends: 

  • 3D printing – It helps manufacturers create complex parts that require accurate and precise geometry and features. 
  • Automation and robotics – This innovation brings about increasing product efficiency and improved product quality and also reduces labor cost.
Plastic Injection Molding Process 3

3D Injection Molding Machine (Image Source: Pinterest)

Material Trends

Material trends are brought about by the ever changing needs and environment concern. This includes the use of sustainable materials to produce eco-friendly parts. 

Some of these sustainable materials used include compostable and biodegradable materials. Others are the use of lightweight and high performance materials.


Plastic injection molding has greatly revolutionized modern manufacturing. From the use of technology in production of high quality products, complex parts, fast production and cost efficiency to the use of eco-friendly materials. 

Injection molding helps cut across all industries in producing parts that are used in automotives, electronics, aerospace and even consumer goods. It is crucial to consider keenly all the steps in the injection molding process in order to achieve the desired finished parts requirements. 

Embracing innovation and best practices should not be about you just being ahead of the competition but also a driving force towards continuous production improvement. This will also ensure that your customers are satisfied and help you unlock new market opportunities.


Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

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