Plastic Injection molding process

What is injection molding process

Molding Process or moulding process? Which one is what? Typically, the meaning for both of them is the same. But the difference is in their local spelling. Usually, on the internet, we will find both Injection molding and injection moulding. Continue reading this article “Plastic Injection molding Process,” we hope you will understand everything to make your research easier.

However, Let’s come to the definition of the Injection Molding process. An Injection molding is a process where the machine manufactures plastic products through the injection of molten plastic. Usually, the Plastic injection process of injecting happens inside a metallic mould box. And then, the mould cools down the injected molten material. After that, The frame opens, and the product is ready for extraction. Typically, the shape and size depending on the internal structure of the mold.

The injection molding process can make shapes of many things. For example, bottles, packaging, toys, automotive parts, musical instruments, and most of the plastic products are available today.

In this article, we will discuss the Plastic injection molding process steps, types of Plastic injection molding process. Furthermore, we will discuss the structure and feature of the Injection metallic moulds. Additionally, we will review the details of Injection molding materials and their types. At the end of this technical writing, we will have a glance at a few books on the Injection molding process.

The Basic injection molding process steps

The injection moulding process is very famous in the plastic industry. There are lots of real Plastic Injection machine videos on YouTube. In this section, we will point out the necessary steps of the process. Anyway, Due to its number of steps, we call this as six steps Injection Molding process.

Before we go for the process, we would like to remind one thing. The material used in the injection process can be various. Through this, not only the plastic but also the machine can extract the thermoplastic of other raw materials too.

Here, we have categorized the steps into the necessary six steps. We will start with the input of the raw materials. And then we finish at the ejection point of the machine.

Feeding Hopper

A conical shaped container placed at the top of the machine is known as the Feeding hopper. It can be various shapes. But most commonly, it is a conical shaped.

Considering the color and the raw materials of the thermoplastic product, the number of feeding hopper depends. Feeding the conical-shaped hopper and mixing the raw materials is known as the blending process. So, we position our different feeding hopper according to the raw materials. We very often blend two or more elements. There’s a point where all of them meet. The blending process is the very primary step to be careful because the design of thermoplastic depends on it.

However, there are a significant number of ways for plastic blending. But among them, the most common methods are; Manual blending, Proportional loaded blending, Volumetric blending, Gravimetric blending. 

Usually, we blend the raw materials in powdered form. After we feed the hopper, the elements reach the mixing chamber. There’s a rotating mixture inside the mixing chamber.

After then, the raw material directly enters the barrel. We will find a motor attached to the barrel. It rotates the screw inside the barrel. The ram or the screw pushes the raw material to the heating stage. The temperature before this stage can be room temperature.


The blended raw materials are now inside the heating chamber. The heating chamber generally consists of a few thermosetting systems. A homogenous heating system is the priority to get an ideal product. The fluctuations of the heating system may cause the instability of the product structure. Typically the range of the heating system is between 160˚ C to 200˚ C for plastic. For other materials, we should change the heating medium. Why? Because their melting point is not as convenient as the plastic. The system usually uses different heating mediums, such as hot water, steam, hot oil, induction heating, etc. 

Hot water is a very cost-effective medium to melt our plastic materials. It can reach up to 160˚ C. For the higher melting point material, and we can use hot oil because it can reach 320˚ C. On the other side, we can also use induction heating because it is easier to demonstrate than the others. But, it costs higher than the different medium settings. It can reach more than 200˚ C. Lastly, and the stem is the most widely used heating medium in the industry because it has energy 18 times than the oil and six times the water.

After all, when the heating is over, the molten material is ready for the injection.

Closing the Mold

The cavity of the mould tool gives the desired size of the product. The engineers design the mould tool before we construct it on the machine. They adjust the machine in such a way that the mould tool can open and close when it is necessary. So, when our molten plastic or the raw material is ready for the injection, the mould tool closes.


As its name, injection is the core process of injection molding process. While looking at the machine, we will find a barrel-shaped hollow tube in the middle of the injection machine. But, if we look inside of this, we will find a ram which is attached to a motor. With the rotation of the piston, the materials inside the barrel move forward. Here, The mechanical part is the size of the barrel and the size of the mold cavity. It comes back and forth, depending on the opening and closing of the old tool.

The force comes from the screw surfaces on the ram. We can call it as a continuous process. At the end of the barrel, we will find a nozzle. It closes the pore until the mold tool is not ready for the injection. While the orifice opens at the injector, the piston pushes the mold forward. Hence, the molded plastic goes inside the mold cavity. Due to the excessive force, they occupy each of the space they get.

The injection is an essential part of injection molding process. Because of these characteristics, we call it as an injection molding machine. After the molded plastic goes inside the cavity properly, the orifice closes instantly. Afterward, The mold tool is ready for the cooling operation.


The next step of injection molding process is the cooling part. It is a critical part of all the steps because it has an impact on the quality of the product. It’s cycle time is a significant part here to consider. Here, the time required to cool a piece has a vital role in finding the cycle time. Again, a non-uniform cooling may cause residual stresses, shrinkage, and warping. Therefore, we must focus on the uniform cooling system.

It can last in the two-third of the injection molding process. The medium we use here is water or oil. Usually, The temperature is cooled in an external heat exchanger before it enters the cooling channel. An optimized cooling system requires less time to cool the molded plastic. The hot, molded plastic then turns into room temperature. It turns into a solid material subsequently. Afterward, it turns into the shape contained inside the mold cavity. Finally, it is ready to go outside the mold tool.


After the cooling process is done, the mold tool opens. Then the cooled hard plastic comes out of the molding machine. The ejected plastic will go for checking. And then, after the successful verification, the plastic product goes for the packaging. In the same way, the defect products get separated from the other. At this stage, the injection molding process is over.

Because of this six continuous process, the injection molding process optimized the difficulty of manufacturing.

Different Types of injection moulding

Based on the different working principles and development of the tool industry, the moulding process has also developed in various categories. We know, the similar Injection molding process can not make all the different products. Hence the different molding process produces different product designs. For example, can we make the plastic handled knife using the traditional injection molding process? No, right? Because we will need different kinds of molding process. Usually, Using the insert molding process, we can put the metallic knife inside the plastic. Then, we can attach the plastic with the metals. Therefore, based on various commodities, injection molding can be divided into three main categories. They are two-shot injection molding process, Insert molding process, over-molding process. Let’s describe them one by one.

Two-shot molding process

Two-shot Injection Molding process

The two-shot molding process is a kind of injection process where two or more barrels inject the molded plastic into the mold tool. It has other names such as multi-shot molding process, double shot molding process,  dual-shot moulding process, etc. The working principle of this process is almost similar to the traditional injection process. Just the difference is in the number of feeding hopper. Multiple feeding hopper attached to the barrel blends. The attracting benefits of this molding process are:

  1. Part consolidation.
  2. Improved efficiency.
  3. Improved quality.
  4. Complex molding.

Insert molding process

Insert Molding Process. The metallic screws are attached to the plunger and the injector injects the plastic beside the screws.

The process of molding that allows the plastics to bond to insert placed within a mold cavity is called the insert molding process. Have you seen toys with a metal screw hole? How does the injection molding add the molded elements there? Through the insert process, we can do this efficiently. First, we can determine the position of the screw, and then we can insert the moulded plastic. Similar to this, there are other examples we can easily manufacture them through this process. Such as stainless steel insert, Nylon Mesh, Stainless Steel, Threaded Brass, Stainless Steel Pin Insert. The benefits of this process are:

  1. Improves component reliability
  2. Improved strength and structure
  3. Reduces assembly and labor costs
  4. Reduces the size and weight of the part
  5. Enhanced design flexibility.

Over molding process

Over molding Process

It is a process where the first molded substantial part is further molded inside another mold tool. Therefore, it is quite similar to the insert molding. But the difference between over-molding and insert molding is in the repetitiveness.

Over molding can perform two or more repetitive cycles of injection. For example, we are trying to make a plastic car that has three layers of protecting shield. How to manufacture it? We will need three repetitive processes to finish manufacturing. That is the principle of an over-molding injection process. The benefits of this process are:

  1. Textures that make handheld items more accessible and safer to use
  2. A greater variety of styles and colors for parts and finished products
  3. Enhanced opportunity for customization
  4. Vibration and noise reduction
  5. Protection from sharp corners or edges
  6. Defense against water, dust, and other contaminants, resulting in longer product life
  7. Greater electrical resistance
  8. Increased shock absorbency

Plastic injection molding machine

A plastic injection molding machine has another name that is the plastic injection press. The plastic injection molding machine produces the thermoplastic product through the injection process.

Structure and Unit of the machine

The basic structure of this machine has two units. They are:

  1. Clamping Unit: The clamping unit consists of the mold, motor drive for clamping, and tie bars. The primary mechanism of the clamping group is to ensure the opening and closing of the mold. At the same time, it provides the ejection of the substantial part.
  2. Injection Unit: The injection unit consists of a motor screw drive, reciprocating screw, and barrel, heaters, thermocouple, and ring plunger. The injection unit has a good number of works to do. First, it ensures the blending of the raw materials. Second, it provides homogenous heating on the blended materials. Third, it also ensures a smooth injection into the mold. Tremendous pressure is created here while injecting into the mold.

Types of injection molding machine

We have described the operation of this machine in the Injection molding process part. We knew from above that following the six steps mechanism of the injection molding, we could easily manufacture a plastic product. However, An injection molding machine can be of three kinds in nature.

  1. Hydraulic: Most of the machining system in today’s world runs off the hydraulic system because it doesn’t pollute the environment. At the same time, the ram provides a lot of pressure on the mold. It is very much effective and reliable.
  2. Mechanical: From the beginning of technology, humankind has practiced using mechanical based machines. Mechanical machines are more stable. It needs manual control. Because of the mechanical systems, the device seems more significant. It needs more labor to operate the machine. Besides, it is not faster as the hydraulic and electric systems.
  3. Electric: Electric machines are faster than the other two machines. The operator can observe the practical steps through the digital management system. It can be controlled using a computer or other operating systems. Electric machines are expensive but reliable. It saves time, and the production is much faster.

Plastic Injection Moulds

A Plastic Injection Mould is a customized machine tool that is necessary for the plastic injection molding to mold molten plastic into solid plastic. Engineers design the specific injection mold for particular design. The most common injection mould is the two-plate injection mold. Moreover, there are other types of injection moulds. They are:

  1. Cavitation: This mold has a single cavity that’s produces one part per cycle.
  2. Family Mold: It has a single mold base but two or more different cavities. It can produce two or more separate parts per cycle.
  3. Unscrewing Molds: This mold is prevalent to produce the threaded holes within a plastic part. An automatic system with a small driver extracts the undercut features. It can be both internal or external. The extraction is continuous into the press cycle.
  4. Hot and cold runner molds: Hot runner uses a temperature-controlled manifold to reduce runner scrap from the cycle significantly. But the cold runner is the most common method for the traditional tooling where they utilize sprues and runners to gate into the part.
  5. Insulated runner molds: These tools resemble more traditional cold runner. But it cartridge heaters or other methods of heating to form a surrounding layer of molten plastic.

Injection molding Materials

Another thing you should consider about injection molding process is materials. Thermoplastics are polymeric, which can be molten by heating, reshaping, and solidified by cooling. It is the physical properties of the polymers. However, While manufacturing, it is essential to distinguish the types of plastic materials for different purposes of the production.  Depending on that, we have categorized them into six classes. We will demonstrate some examples of them that is used to manufacture in the industry.  

  1. ABS or Acrylonitrile Butadiene Styrene: ABS is the most popular material for injection molding process. ABS is always used to manufacture drain pipe systems, plastic clarinets, golf club heads, automotive parts, common appliances in a kitchen, LEGO bricks, and most of the other products.
  2. Polyethylene: Polyethelene is used to manufacture plastic bags, plastic films, containers including bottles, etc.
  3. PC or Polycarbonate: PC is used to make DVDs like DVDs, eyewear lenses, medical devices, automotive components, and cellular phones.
  4. Polyamide: Nylon is very famous in the medical industry. Besides, it is used to manufacture automotive products, sports equipment, apparel and footwear, industrial components.
  5. Polypropylene or PP: PP is very famous for flexibility. It is used to manufacture utensils, athletic apparel, area rugs, and automotive parts such as car batteries.

Related books that can help you learning more about injection molding process.

We knew that an injection molding consists of an injection unit and a clamping unit. The following five are the name of a few books. Few well-known engineers wrote the books. Each of the books contains different parts of the injection molding machine.

Handbook of Troubleshooting Plastics Processes: A Practical Guide

John R. Wagner, Jr edit this book. It provides a framework of how to overcome the troubleshoot of injection molding process. We will find the polymer processing technologies from the book. All the 21 chapters in the book are full of complete knowledge of injection molding process. From the preface of the book, we will find the authors detailed the summary of the book. After that, Every chapter describes the process, along with the troubleshoot problems. The most common issues are discussed with the root causes and potential technical solutions. Lastly, this book provides numerous case studies along with the illustration of the troubleshooting process.

How to Make Injection Molds

Georg Menges, Walter Michaeli, Paul Mohren are the authors of this book. It focuses on the making of the injection moulds. From the preface, they said, “Injection molds are high-precision tools whose smooth-operation on a day-to-day basis is vital to the economic success of many plastics processors. Consequently, errors in the design and construction of molds can have grave consequences. That is where this book comes in.“ Building on the earlier editions, it draws extensively both on the literature and on the development work carried out at the Institute for Plastics Processing in the Technical University of Aachen with the aid of substantial private and public research funding.

Injection Mold Design Engineering 2E

Injection mold is the core of injection molding process. It is essential to master the know how to make the injection molds. David O. Kazmer is the author of the book focused on the cost-effective ways how to make the injection mold. From the preface of the book, the author said that the mould design had been more technical trade than an engineering process. So, the cost is the most crucial factor here. Therefore, considering the price, he implanted many ways of making the mold. This book will be very much helpful for learning plastic injection process. There are lots of practical information to know about the requirements and necessary steps.

Plastic Part Design for Injection Molding 2nd Edition

Hanser Publisher, Munich publish this book. The author of the book is Robert A. Malloy. This book focuses on the Plastic part design technology. The prototyping and experimental stress analysis is its priority. There are lots of practical knowledge to learn from this book. Besides, it shows the various innovative reviews on how to make the plastic parts. This book will be very much helpful for thermoplastic product design.

Injection Molding Advanced Troubleshooting Guide: The 4M Approach

Hanser Publisher, Munich also publish this book. The author of the book is Randy Kerkstra, Steve Brammer. This giant book is a bank of information on injection molding technology. There are 47 chapters where each of the parts on injection molding technology is there. Besides, it contains the overall process of injection molding. This excellent book is a complete lesson on every knowledge. This book also includes a summary of every part of the lessons from the previous four books. However, This book can be a fantastic choice for engineers.


Starting from the definition of the Injection Plastic molding process, we have observed their procedures above. Following step by step, we tried to show the necessary injection process on the Injection molding process section.

However, there are other categories of plastic molding processes in plastic product manufacturing. If you are interested, please visit here to have a summary of them. The definition and a short process explanation are well explained there. We hope it will make your research easier. 

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