Unlocking the Power of Injection Molding Process: A Comprehensive Advanced Guide

Automatic production line for injection molding process

Plastic products have become ubiquitous in our daily lives. From the packaging that protects our food to the parts that make up our cars and electronics, plastics are all around us. But have you ever stopped to consider how these products are made? Perhaps, Injection molding is the most common manufacturing process used to create plastic products.

Plastic products produced by Firstmold through the injection molding process
Plastic products produced by Firstmold through the injection molding process

The utilization of injection molding has brought a groundbreaking change to plastic product manufacturing by enabling the fabrication of intricate shapes and designs with remarkable accuracy and consistency. With its ability to produce high volumes of identical parts, injection molding has become a critical component of modern manufacturing. This article explores different aspects of injection molding, including types, machinery, materials, mold design, quality control, and common defects and solutions. We will also look at the latest trends and advancements in injection molding technology and its impact on sustainability and the environment.

Introduction to the Injection Molding process

What is an injection molding process?

Injection molding produces plastic products by injecting molten plastic into a mold cavity to cool and solidify into the desired shape. Used to manufacture small to large and complex plastic parts. Injection molding is a popular choice for mass production due to its efficiency, speed, and ability to produce large quantities of identical parts with high accuracy and consistency.

Production of plastic products by injection molding process
Production of plastic products by injection molding process

In the injection molding process, raw plastic material is melted and injected into a mold under high pressure. Typically, the mold consists of two halves fastened to form a cavity corresponding to the shape of the desired product. As the plastic cools and solidifies, it takes on the shape of the mold cavity. The final product is then ejected from the mold and can be finished with additional processing steps such as trimming, polishing, or painting. Injection molding is used in various industries, including automotive, electronics, medical, and consumer goods.

The brief history of the injection molding process

The injection molding process was first developed in the 19th century when a billiard ball manufacturer named John Wesley Hyatt was looking for a way to replace ivory with a synthetic material. Hyatt experimented with different materials and processes, and in 1872 he patented the first injection molding machine. This machine used a plunger to push molten celluloid into a mold and then cooled the material with water.

In the years that followed, injection molding technology continued to advance. In the 1920s and 1930s, Bakelite, a type of thermosetting plastic, became a popular material for injection molding. In the 1950s, the first screw injection molding machine was developed, which allowed for more precise control over the injection process. This innovation paved the way for the development of new materials and the production of more complex parts.

Injection molding process in the past
Injection molding process in the past

Today, injection molding is a mature and widely used manufacturing process with many applications across many industries. Advances in materials, machinery and design software have made it possible to produce even more complex parts with greater efficiency and precision. As the demand for lightweight, durable, and cost-effective plastic parts continues to grow, injection molding technology is likely to play an even more important role in the future of manufacturing.

Workflow of the injection molding process

Material SelectionSelect the appropriate plastic material based on the desired properties and requirements of the finished part.
Mold DesignDesign the mold that will be used to shape the plastic material into the desired form. This includes specifying the part’s size, shape, and surface finish, as well as the placement and design of any features, such as channels or vents.
Mold FabricationCreate the physical mold using advanced machining techniques such as CNC milling, EDM, or laser cutting. The mold must be manufactured to tight tolerances and exacting specifications to ensure that the parts produced are of high quality.
Injection Molding Machine SetupInstall the mold onto the injection molding machine and configure the machine to the appropriate settings for the material and part being produced. This includes setting the injection pressure, temperature, speed, and other parameters such as clamping force or injection rate.
Injection Molding CyclePerform the actual injection molding process, which involves heating the plastic material to its melting point and injecting it into the mold. The material is then cooled and solidified to form the finished part.
Part FinishingTrim, polish, or otherwise finish the part as necessary to meet the required specifications and desired appearance. This may include secondary operations such as drilling, painting, or labeling.
Quality ControlInspect the finished part to ensure it meets the required quality, dimensions, and performance standards. This may involve visual inspection, dimensional checks, or testing for material properties such as strength or hardness.

The injection molding cycle is the process of molding plastic material into a desired shape using an injection molding machine. Here’s a detailed breakdown of each step in the injection molding cycle:


The first step in the injection molding cycle is to close the mold, or “clamp” it, so that the plastic material can be injected into it. The clamping force must be sufficient to hold the mold closed against the high pressures of the injection process.


Once the mold is clamped, the plastic material is heated to its melting point and injected into the mold at high pressure. The material is injected into the mold through a gate, which is the opening in the mold through which the material enters.


After the plastic material is injected into the mold, it must be held under pressure for a while, known as the dwelling stage. This allows the material to fill all areas of the mold and ensures that the part is fully formed.


Once the dwelling stage is complete, the mold is cooled to solidify the plastic material. The cooling stage must be carefully controlled to ensure that the part is cooled evenly and to prevent warping or other defects.

Mold Opening

After the plastic material has cooled and solidified, the mold is opened, and the part is ejected from the mold. This is accomplished using ejector pins, which push the part out of the mold.


Once the part is ejected from the mold, it must be removed from the machine and any excess material must be trimmed off. This is known as the ejection stage.


The injection molding cycle is then repeated to produce additional parts, with the mold being clamped, injected, dwelled, cooled, opened, and ejected in a continuous process.

Each of these steps is critical to the success of the injection molding process, and careful attention must be paid to each stage to ensure that the parts produced are of high quality and meet the required specifications.

Applications of Injection Molding

IndustryApplications of Injection Molding
AutomotiveDashboard components, door handles, bumpers, interior trim, air ducts, battery cases, engine covers, and some structural components
Consumer GoodsBottles, containers, packaging, toys, razors, toothbrushes, and phone cases
MedicalSyringes, IV connectors, surgical instruments, hearing aids, inhalers, and prosthetic limbs
AerospaceLightweight, high-strength components for use in aircraft and spacecraft, turbine blades, fuel nozzles, and interior components
ElectronicsHousings, connectors, buttons, switches, and casings for a variety of electronic devices
ConstructionFittings, pipe systems, and other construction components
AppliancesParts for various home appliances, such as refrigerator components, dishwasher parts, control panels, knobs, and buttons
FurnitureChair components, table legs, and other furniture parts
Sporting GoodsHelmets, pads, and other sports equipment, as well as parts for equipment such as tennis rackets and golf clubs
PackagingCaps, closures, trays, blister packs, and clamshells

Types of injection molding

Types of injection molding machines

Type of Injection Molding ProcessDescription
Cold Runner Injection MoldingThe unheated channel in the mold is used for injecting plastic, while any plastic residue in the channel is usually discarded or recycled.
Hot Runner Injection MoldingPlastic injection into the mold is carried out through a heated channel that ensures temperature consistency and minimizes waste.
Insert Injection MoldingPre-formed parts or inserts are placed into the mold before the plastic is injected, creating a single, integrated part.
OvermoldingTwo or more materials are used to create a single part, with the second material injected over the first to create a final part that consists of multiple materials.
Multi-Shot Injection MoldingMultiple materials are injected into the mold in one cycle, allowing for the creation of complex parts with multiple colors, textures, or other features.
Gas-Assisted Injection MoldingNitrogen or another gas is injected into the mold after the plastic, which creates a hollow center in part, reducing weight and material usage.
Co-injection moldingTwo or more materials are injected into the mold simultaneously to create a part with multiple layers or properties.
Micro injection moldingExtremely small and precise parts are created using a specialized injection molding machine and tooling.
Liquid injection moldingThe injection of liquid silicone rubber into the mold, followed by curing, results in a highly flexible and heat-resistant part.
Powder injection moldingMetal or ceramic powders are mixed with a binder and injected into the mold, which is then cured and sintered to create a final part.
Reaction injection molding (RIM)Two liquid reactants are mixed and injected into the mold, where they cure to create a rigid or flexible part.
Structural foam moldingA chemical blowing agent is injected into the mold with the plastic, creating a foam core that provides strength and reduces material usage.

Main constituents of injection molding machine

Injection unit

The injection unit is responsible for melting and injecting the plastic material into the mold. It consists of a hopper that stores the plastic material, a screw or ram that pushes the material forward, and a heating element that melts it.

Mold or tooling

The mold is the component that shapes the plastic material into the desired form. It typically consists of two halves, a cavity and a core, that fit together to form the final shape. The mold is mounted onto the platens of the injection molding machine.

Clamping unit

The clamping unit is responsible for holding the mold in place during the injection process. It consists of a stationary platen and a movable platen that are connected by a hydraulic or mechanical system.

Hydraulic system

The hydraulic system provides the power to open and close the mold and operate the injection unit. It consists of a hydraulic pump, a motor, valves, and hoses.

Control system

The control system manages the operation of the injection molding machine. It consists of a control panel, which allows the operator to input settings and control the machine and sensors and switches that monitor the process.

Ejection system

The ejection system removes the finished part from the mold. It typically consists of ejector pins, which push the part out of the mold, and a plate that catches the part as it is ejected.

Materials for Injection Molding


These materials are polymers that become pliable when heated, allowing them to be molded into various shapes. Some common thermoplastics used in injection molding include:

Polyethylene (PE)Lightweight, durable, flexible, good chemical resistancePackaging, bottles, automotive parts
Polypropylene (PP)Versatile, lightweight, good chemical resistance, high melting pointPackaging, automotive parts, appliances
Polystyrene (PS)Lightweight, rigid, good insulation propertiesPackaging, disposable utensils, CD cases
Acrylonitrile Butadiene Styrene (ABS)Tough, rigid, good impact resistanceAutomotive parts, toys, electronics
Polycarbonate (PC)Clear, strong, good impact resistance, high-temperature resistanceSafety glasses, electronic components


These materials are polymers that undergo a chemical reaction during molding, forming a network of cross-linked molecules that give them rigidity and strength. Some common thermosets used in injection molding include:

EpoxyVersatile, good adhesion, excellent chemical resistanceAdhesives, composites, coatings
PhenolicRigid, heat-resistant, good electrical propertiesElectrical components, automotive parts
Polyurethane (PU)Durable, flexible, good impact resistance, good insulation propertiesFurniture, insulation, footwear


Elastomers, such as rubber and silicone, are highly flexible and elastic materials used in various applications.


Injection molding can also produce composites, which combine two or more materials to create a material with specific properties. For example, glass fiber can be combined with a thermoplastic to create a stronger and more rigid material than the base plastic.

The choice of material depends on the requirements of the final product, including its strength, flexibility, heat resistance, and other properties. Other factors, such as the cost of the material and the ease of processing, also play a role in selecting the material for a particular project.

Mold Design and Fabrication

The mold is a critical component of the process, as it determines the shape, quality, and consistency of the finished product.

The mold design and fabrication process typically involves several stages, including:


The first step is to design the mold using CAD (computer-aided design) software. The mold designer must consider factors such as the material being used, the part geometry, and the injection molding process parameters.


Before the mold is fabricated, a prototype is usually created to test the design and ensure that it meets the requirements. This may involve 3D printing or other rapid prototyping methods.


Once the design has been finalized and tested, the mold is fabricated using CNC (computer numerical control) machining or other methods. The mold must be precise and durable enough to endure the high pressures and temperatures of the injection molding process.

Testing and tweaking

After the mold is fabricated, it must be tested and tweaked to ensure that it produces high-quality parts. This may involve adjusting the mold design, the injection molding process parameters, or both.

Mold Filling Simulation

Mold filling simulation is an important tool used in the injection molding process. It is a computer-based simulation that is used to predict how the molten plastic will flow into the mold during the injection molding process. By simulating the filling process, engineers can identify potential issues such as air traps, weld lines, or sink marks that can affect the quality of the finished product.

Mold filling simulation typically involves the following steps:

Model creation

The first step is to create a digital 3D model of the part to be molded and the mold cavity using CAD software. The model must be accurate and take into account the properties of the material being used.

Material selection

The material properties must be input into the simulation software, including the viscosity, temperature, and shear rate.

Boundary conditions

The filling process is simulated by setting the injection molding process parameters, such as injection speed, pressure, and holding time.


The simulation software predicts the filling process, including the flow front, pressure distribution, and cooling behavior. Engineers can use this information to optimize the mold design and the injection molding process parameters.


The simulation results can be used to make design changes to the mold or to the process to improve the quality of the final product.

Mold filling simulation is a valuable tool in the injection molding process, as it allows engineers to optimize the design of the mold and the process parameters before production begins. This can save time and money by reducing the number of design iterations and improving the overall quality of the finished product.

In-Mold Labeling

In-mold labeling (IML) is a technique used in the injection molding process to apply labels or graphics directly to the surface of a molded plastic part. This is done by placing a pre-printed label or film into the mold cavity before injecting the plastic material. The label or film is fused with molten plastic during the molding process, becoming an integral part of the final product.

In-mold labeling offers several advantages over traditional labeling methods. These include:

Durability: Because the label or graphic is fused with plastic material, it is highly durable and resistant to scratching, fading, and peeling.

Cost-effectiveness: In-mold labeling eliminates the need for a separate labeling process, reducing costs and production time.

Aesthetics: In-mold labeling allows for high-quality graphics and intricate designs to be applied directly to the surface of the product, enhancing its visual appeal.

Efficiency: In-mold labeling is an automated process that can be integrated with the injection molding process, allowing for high-speed production.

In-mold labeling is commonly used in the packaging industry, particularly for food and beverage containers, as well as in the production of consumer goods such as automotive parts, appliances, and electronics. It is also popular in the medical industry for the production of disposable products such as syringes and test tubes.

Injection Molding Automation

Injection molding automation refers to the use of robotics and other automated systems to perform various tasks in the injection molding process. Automation can be applied to various stages of the injection molding process, including material handling, mold loading and unloading, part removal, and quality inspection. The use of automation in injection molding offers several benefits, including increased efficiency, improved quality, and reduced costs.

One of the main advantages of injection molding automation is increased efficiency. Automated systems can perform tasks quickly and accurately, reducing the cycle time of the injection molding process. This not only improves productivity but also allows for the production of higher volumes of parts in a shorter amount of time.

Another benefit of injection molding automation is improved quality. Automated systems can perform tasks with greater precision and consistency than manual labor, reducing the likelihood of defects and errors in the final product. Automation can also improve quality control by allowing for real-time monitoring and inspection of the production process.

In addition to efficiency and quality, injection molding automation can also reduce costs by eliminating the need for manual labor and reducing the likelihood of production errors and defects. It can also reduce material waste by ensuring precise material handling and minimizing scrap.

Quality Control of injection molded parts

Quality control is crucial in injection molding, utilizing different techniques to ensure that the final product meets the necessary specifications and standards. Quality control starts with selecting the right materials, continues through the molding process, and ends with the final inspection of the finished parts.

During the injection molding process, quality control measures are employed at different stages to ensure that the produced parts are high quality. One key stage is during the mold’s design and construction. The mold design must be carefully evaluated to ensure that it can meet the required specifications for the final product. This can be done through mold-filling simulation and other computer-aided design tools.

Another important quality control measure is monitoring the process parameters during the molding process. This involves monitoring factors such as temperature, pressure, and injection speed to ensure that they are within the recommended range for the material being used. This can be done using sensors and other monitoring equipment.

Quality control also involves the inspection of the finished parts to ensure that they meet the required specifications. This can be done using various methods, such as visual inspection, dimensional inspection, and functional testing. Visual inspection involves examining the parts for any defects such as warpage, sink marks, or flash. The dimensional inspection involves measuring the parts to ensure that they meet the required tolerances. Functional testing involves testing the parts for their intended use to ensure that they meet the required performance criteria.

3D Printing and Injection Molding

3D printing and injection molding are two different manufacturing processes, each with advantages and disadvantages. The choice between the two depends on various factors, such as the part’s complexity, the required quantity, the desired material properties, and the available resources.

3D printing is ideal for producing small quantities of complex parts, especially those with intricate geometries that traditional manufacturing processes cannot easily produce. It also offers the advantage of producing parts on demand and with minimal lead times. Additionally, 3D printing allows for rapid prototyping and design iteration, which is essential in product development.

Injection molding, on the other hand, is more suitable for the high-volume production of parts with consistent quality and performance. It enables efficient and rapid production of large quantities of parts with minimal waste. Injection molding also allows for the use of a wide range of materials with varying properties, including high-performance thermoplastics, which are not typically used in 3D printing.

In summary, Both 3D printing and injection molding have their advantages and disadvantages, and the selection between them depends on the specific product requirements.

Coatings and Finishes

Coatings and finishes in injection molding refer to the final layer of protection or decoration applied to the molded plastic parts. These coatings can help improve the aesthetic appeal of the parts, provide additional protection against wear and tear, or offer specific functional benefits such as UV resistance, anti-static properties, or enhanced grip.

PaintingApplication of liquid paint to the surface of the partAutomotive, consumer goods, appliances, toys, packaging
Powder coatingElectrostatic application of dry powder to the surface of the part, followed by curing in an ovenAutomotive, appliances, outdoor furniture, sports equipment
AnodizingThe electrochemical process adds a layer of oxide to the surface of the part, increasing its resistance to wear and corrosion.Aerospace, electronics, automotive, medical devices
PlatingElectrochemical deposition of a thin layer of metal onto the surface of the partAutomotive, aerospace, electronics, medical devices
Laser etchingUse of a laser to remove material from the surface of the part, creating a permanent, high-contrast markElectronics, medical devices, automotive, aerospace
TexturingAddition of a pattern or texture to the surface of the part, using techniques such as sandblasting, acid etching, or mold texturingConsumer goods, appliances, automotive, toys
Hot stampingApplying a dry transfer foil to the surface of the part using heat and pressure creates a metallic or holographic effect.Automotive, consumer goods, cosmetics
Pad printingTransfer of ink from a silicone pad to the surface of the part, creating a detailed, high-resolution image or textElectronics, medical devices, automotive, consumer goods
Vacuum metalizingA thin layer of metal is deposited onto the part’s surface in a vacuum chamber, creating a reflective or decorative surface.Automotive, consumer goods, toys, packaging

Common Injection Molding Defects and Solutions

While injection molding is an effective manufacturing process, it is not without its flaws. There are a number of common defects that can occur during the injection molding process. These defects can result in parts that do not meet the required specifications and can impact the overall quality of the finished product. Early identification and resolution of defects are crucial in minimizing their impact on production and preventing the production of defective parts.

Some common injection molding defects include:

Sink marks: These are depressions in the part’s surface caused by shrinkage during cooling. Solutions to sink marks include increasing the packing pressure, the hold time, and the gate size.

Flash: Flash occurs when molten plastic escapes from the mold during the injection molding process, resulting in thin protrusions on the part. Solutions to flash include reducing the injection pressure, increasing the clamping force, and adjusting the temperature.

Warpage: Warpage occurs when the part cools unevenly, resulting in distortion or bending. Solutions to warpage include optimizing the gate location, increasing the cooling time, and using a higher mold temperature.

Short shots: Short shots occur when the mold is not completely filled with plastic. Solutions to short shots include increasing the injection speed, injection pressure, and shot size.

Burn marks: Burn marks occur when the plastic is overheated, resulting in discoloration or black marks on the part. Solutions to burn marks include reducing the melt temperature, increasing the injection speed, and reducing the hold time.

Industry 4.0 and Smart Manufacturing

Injection molding manufacturers are leveraging Industry 4.0 and smart manufacturing technologies like IoT, AI, and machine learning to streamline production processes and minimize downtime.

Additive Manufacturing

Additive manufacturing or 3D printing technology is increasingly being used to produce injection molds. By enabling the production of intricate shapes and designs while reducing waste and shortening production cycles, this method is a cost- and time-effective solution.

Automation and Robotics

Automation and robotics are used to improve efficiency and reduce labor costs in injection molding. Collaborative robots or cobots are increasingly used to work alongside human operators in injection molding cells.

Multi-Material Molding

Multi-material injection molding, where two or more materials are molded into a single part, is gaining popularity in the automotive, electronics, and medical industries. It enables the production of complex parts with different properties, such as hardness, flexibility, and color.

Sustainable Materials

Environmentally sustainable materials, such as bioplastics, are increasingly being used in injection molding. Bioplastics are biodegradable and have a lower carbon footprint compared to traditional plastics.

Advanced Control Systems

Advanced control systems are being incorporated into injection molding machines to improve the accuracy and consistency of the molding process. These systems enable tighter control of key parameters, such as temperature, pressure, and speed, resulting in more precise and repeatable parts.

High-Speed and High-Pressure Molding

High-speed and high-pressure molding are becoming more common in the injection molding industry. These methods enable the production of parts with thinner walls and more complex geometries, reducing material usage and cycle times.


That’s all of the advanced knowledge you need to know about the injection molding process. I am Ray Liu. I have been working in the injection molding industry for over 10 years. If you like this article, please feel free to forward it to your friends. If you still don’t understand something, welcome to contact me, and I will answer all your questions about the injection molding process.

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