Injection Moulding Definition: A Comprehensive Guide

Injection Moulding Definition: A Comprehensive Guide

Injection molding is a manufacturing process that produces parts from a variety of materials by injecting molten material into a mold. In this process, plastic pellets or powder are first heated to a molten state and then injected into a pre-designed mold through high pressure. As it cools and solidifies, the molten material gradually solidifies and forms the desired part shape.

The history of injection molding dates back to the late 19th century, when people began using this method to manufacture plastic products. Over time, injection molding technology has been continuously improved and developed, making it an integral part of modern manufacturing.

During the injection molding process, a range of equipment and tools are used. These include injection molding machines, molds, cooling systems, control systems, etc. Injection molding machine is the core equipment of injection molding, which can provide sufficient pressure and speed to inject molten material into the mold. The mold is designed according to the shape and size of the required part, and it can be heated to facilitate the flow and solidification of the molten material. The cooling system is used to control the temperature of the mold to ensure that the part is adequately cooled and maintains its shape. The control system is responsible for monitoring and adjusting parameters throughout the injection molding process to ensure part quality and consistency.

However, some problems and failures may occur during the injection molding process. For example, part defects, mold damage, machine failure, etc. may occur. To resolve these issues, some troubleshooting and maintenance work is required. This may include inspecting and repairing machine parts, replacing damaged molds, adjusting process parameters, etc.

Injection molding has a wide range of applications. It can be used to manufacture various plastic products, such as auto parts, home appliance casings, electronic product casings, medical equipment, etc. Injection molding has become one of the most commonly used manufacturing methods in modern manufacturing due to its efficiency, precision and repeatability. Whether for large-scale or low-volume production, injection molding delivers high-quality and consistent parts.

Table of Content

  • What is Injection Moulding?
  • History of Injection Moulding
  • Equipment for Injection Moulding
  • Injection Moulding Process
  • Injection Moulding Troubleshooting
  • Injection Moulding Applications

What is Injection Moulding?

Injection moulding (or molding in the US) is a manufacturing process that can produce parts from various materials by injecting molten material into a mould. The mould is a hollow cavity that has the shape and size of the desired part. The molten material can be a metal, glass, elastomer, confection, or most commonly a thermoplastic or thermosetting polymer. The material is fed into a heated barrel, mixed by a helical screw, and forced into the mould cavity under high pressure. The material then cools and hardens in the mould, forming the part. The part is then ejected from the mould and the cycle repeats.

Injection moulding is widely used for mass production of parts that have complex shapes and features. It can produce parts with high accuracy, consistency, and quality. It can also reduce waste and cost by using less material and minimizing finishing operations.

History of Injection Moulding

The concept of injection moulding dates back to the late 18th century, when the first injection moulding machine was invented by James Watt to produce metal buttons. However, it was not until the late 19th century that injection moulding became more practical and popular with the development of thermoplastics and thermosets. Thermoplastics are polymers that can be melted and reshaped multiple times, while thermosets are polymers that can only be cured once and cannot be remelted.

The first injection moulding machine for thermoplastics was patented by John Wesley Hyatt in 1872. He used a plunger to inject celluloid into a mould to make billiard balls. In 1903, Arthur Eichengrün developed the first injection mouldable thermoset called Bakelite, which was used for electrical insulators and telephone casings. In 1926, Eckert and Ziegler invented the first screw injection machine, which improved the mixing and heating of the material. In 1939, James Watson Hendry built the first gas-assisted injection moulding machine, which reduced warping and improved surface quality.

The injection moulding industry grew rapidly after World War II with the introduction of new materials such as nylon, polystyrene, polyethylene, polypropylene, and acrylic. These materials offered better properties and lower costs than traditional materials such as metals and wood. Injection moulding also enabled the mass production of consumer goods such as toys, appliances, furniture, and packaging.

In recent years, injection moulding has evolved with the advances in technology and innovation. Some of the modern developments include:

  • Computer-aided design (CAD) and computer-aided manufacturing (CAM) software that allow for faster and more accurate design and production of parts.
  • Computer numerical control (CNC) machines that automate the machining of moulds with high precision and efficiency.
  • Scientific moulding that applies scientific principles and data analysis to optimize the injection moulding process and quality.
  • Multi-shot or multi-component injection moulding that can produce parts with multiple materials or colours in one cycle.
  • Micro-injection moulding that can produce parts with micro-scale features and dimensions.
  • 3D printing or additive manufacturing that can create complex parts without using a mould.

Equipment for Injection Moulding

The main equipment for injection moulding consists of three parts: the injection unit, the mould, and the clamp.

Injection Unit

The injection unit is responsible for melting, mixing, and injecting the material into the mould. It consists of three components: the hopper, the barrel, and the nozzle.

  • The hopper is a container that holds the raw material in solid form such as pellets or granules. The material is fed from the hopper into the barrel by gravity or a mechanical device.
  • The barrel is a cylindrical chamber that has a helical screw inside. The screw rotates and moves forward to transport the material along the barrel. The barrel is heated by electric heaters or oil to melt the material. The screw also acts as a plunger to inject the molten material into the mould through the nozzle.
  • The nozzle is a tapered tip that connects the barrel to the mould. It has a valve that controls the flow of the material into the mould. The nozzle can have different shapes and sizes depending on the type and viscosity of the material.


The mould is a metal tool that has one or more cavities that have the shape and size of the desired part. The mould can be made of steel or aluminium, and it can be machined, cast, or 3D printed. The mould can have different features and components such as:

  • Core and cavity: The core is the part of the mould that forms the internal features of the part, while the cavity is the part of the mould that forms the external features of the part. The core and cavity are usually separated by a parting line, which is where the two halves of the mould meet.
  • Sprue and runner: The sprue is the channel that connects the nozzle to the runner, while the runner is the network of channels that distributes the material to each cavity. The sprue and runner can be cold or hot, depending on whether they are cooled or heated to maintain the temperature of the material.
  • Gate: The gate is the opening that allows the material to enter each cavity from the runner. The gate can have different shapes and sizes depending on the type and flow of the material.
  • Vent: The vent is a small gap or hole that allows air and gas to escape from the cavity during injection. The vent prevents air traps, bubbles, and burns in the part.
  • Ejector: The ejector is a mechanism that pushes or pulls the part out of the mould after cooling. The ejector can be a pin, a blade, a sleeve, or a plate, depending on the shape and location of the part.


The clamp is a device that holds and closes the two halves of the mould together during injection. The clamp can be hydraulic, mechanical, or electric, depending on the force and speed required. The clamp has two components: the fixed platen and

Injection Moulding Process

Injection moulding is a process of producing parts by injecting molten material into a mould. The material can be metal, glass, elastomer, or polymer. The mould is a metal frame that has the shape of the desired part. The injection unit heats up the material and pushes it through a nozzle into the mould cavity. The material cools and hardens inside the mould, forming the part. The mould then opens and ejects the part. Injection moulding is widely used for making various products, such as buttons, combs, car panels, and plastic toys.

Injection Moulding Troubleshooting

Injection moulding troubleshooting is the process of identifying and solving problems that occur during the injection moulding process. Injection moulding is a widely used technique for producing plastic parts with complex shapes and high dimensional accuracy. However, injection moulding can also result in various defects that affect the quality and functionality of the parts. Some of the common injection moulding defects and their possible causes and solutions are:

  • Weld lines: These are lines or marks that appear on the surface of the part where two or more flow fronts meet. Weld lines can weaken the part and affect its appearance. Weld lines can be caused by poor venting, low melt temperature, low injection speed, or complex geometry. To prevent or reduce weld lines, improve the venting of the mould, increase the melt and/or mould temperature, increase the injection speed, or modify the part design to avoid converging flow fronts .
  • Delamination: This is when layers of material separate from each other within the part. Delamination can compromise the strength and integrity of the part and make it susceptible to cracking or breaking. Delamination can be caused by contamination of the material, moisture in the material, or excessive shear stress during injection. To prevent or reduce delamination, ensure that the material is clean and dry before injection, reduce the injection pressure and speed, or use a material with higher melt strength .
  • Burn marks: These are dark or black spots on the part surface that indicate thermal degradation of the material. Burn marks can affect the appearance and performance of the part and indicate poor process control. Burn marks can be caused by trapped air or gas in the mould cavity, excessive melt temperature, excessive injection speed, or insufficient cooling. To prevent or reduce burn marks, improve the venting of the mould, lower the melt and/or mould temperature, lower the injection speed, or increase the cooling time .
  • Sink marks: These are depressions or dimples on the part surface that occur due to shrinkage of the material in thick sections. Sink marks can affect the appearance and dimensional accuracy of the part and indicate uneven cooling. Sink marks can be caused by insufficient packing pressure, insufficient cooling time, high mould temperature, or thick wall sections. To prevent or reduce sink marks, increase the packing pressure and time, decrease the mould temperature, or reduce the wall thickness .
  • Flash: This is excess material that extends beyond the part edge along the parting line or ejector pins. Flash can affect the appearance and functionality of the part and indicate poor mould fit or alignment. Flash can be caused by too much injection pressure, too much material, low material viscosity, or worn or damaged mould components. To prevent or reduce flash, lower the injection pressure and speed, lower the shot size, increase the material viscosity, or repair or replace the mould components .

Injection Moulding Applications

Injection moulding is a manufacturing process that involves injecting molten material into a mould cavity. The material can be plastic, metal, ceramic, glass or rubber. The mould cavity is shaped like the desired product, such as a bottle cap, a toy, a medical device or a car part. Injection moulding can produce complex and intricate shapes with high precision and accuracy. Injection moulding has many applications in various industries, such as:

  • Packaging: Injection moulding can create durable and lightweight containers, lids, caps and closures for food, beverages, cosmetics, pharmaceuticals and other products. Injection moulding can also produce labels, tags and stickers that adhere to the containers.
  • Automotive: Injection moulding can create parts and components for cars, trucks, motorcycles and other vehicles. Injection moulding can produce bumpers, dashboards, door handles, steering wheels, airbags, seat belts and more. Injection moulding can also create decorative and functional accessories, such as mirrors, lights, knobs and buttons.
  • Medical: Injection moulding can create devices and equipment for medical and dental use. Injection moulding can produce syringes, needles, catheters, implants, prosthetics, braces and more. Injection moulding can also create surgical instruments, such as scalpels, forceps and clamps.
  • Electronics: Injection moulding can create housings and cases for electronic devices, such as phones, tablets, laptops and cameras. Injection moulding can also produce circuit boards, connectors, switches and sockets. Injection moulding can also create components for electrical appliances, such as fans, heaters and blenders.
  • Toys: Injection moulding can create toys and games for children and adults. Injection moulding can produce action figures, dolls, puzzles, Lego bricks and more. Injection moulding can also create educational and interactive toys, such as robots, musical instruments and puzzles.