Different Types of Injection Moulding Processes – An Overview

Injection moulding is a highly efficient and versatile process for manufacturing plastic parts. It enables the production of complex products using a variety of materials within a very short timeframe. Consequently, the process has become increasingly advanced over the years. New variants are introduced annually that enhance its capabilities, sustainability, and cost-effectiveness. The entire spectrum of injection moulding processes encompasses a range of materials, mechanisms, delivery methods, and more.

Types of Injection Moulding

Injection moulding processes can be categorised in various ways, depending on the criteria used for classification. These criteria may include the working principle of the process, the type of plastic used in the injection moulding process, the types of runner systems, end-product specifications, and other relevant factors.

To provide a comprehensive overview, we will first examine a classification based on the type of material used, followed by a classification that highlights the unique features of different injection moulding processes.

Categorisation Based on Material

• Thermoplastic injection moulding

• Thermoset injection moulding

• Metal injection moulding (MIM)

Thermoplastic Injection Moulding

Thermoplastic injection moulding is one of the most popular and versatile methods for creating lightweight and durable plastic products. Thermoplastic polymers are recyclable materials that can be remolded once a product is no longer needed. They soften when heated and solidify upon cooling, allowing this process to be repeated to produce new items from old ones. Additionally, this method is generally more cost-effective than other manufacturing processes.

Thermoplastic injection moulding is utilised to manufacture a variety of consumer, industrial, and medical products. Some examples include:

• Consumer products: Chairs, clothing (synthetic fibres), toys, appliances, storage bins, cleaning products and more.

• Industrial products: Pumps, gears, ropes, containers and more.

• Medical products: Medical implants, diagnostic tools, respiratory devices, anesthesia equipment, tubing, medical packaging, dental aligners, syringe seals, wound dressings, oxygen masks, ventilator bags and splints for fractures, sprains and strains.

Thermoset Injection Moulding

Unlike thermoplastic polymers, thermosetting polymers are designed for single use and cannot be recycled. This process is often used to manufacture metal replacement parts from plastic in industries such as aerospace, automotive, industrial and medical.

Thermoset injection moulding is similar to thermoplastic molding. In this process, a thermosetting material is heated and injected into a mould where it is permanently set and cured. A common example of a thermosetting material is epoxy. Once epoxy is poured into a mould, it undergoes an irreversible chemical reaction and hardens permanently. It cannot be remelted or reshaped to create new products.

Thermosetting injection moulding can be quite cost-effective for large production runs compared to traditional manufacturing processes. It is also fast, repeatable and offers a wide range of material options to meet the specific requirements of various products. Some examples of products made using thermosetting injection moulding include:

• Aerospace products: Aircraft structure, engine and interior components, protective coatings, adhesives, sealants and more.

• Automotive products: Dashboards, bumpers, fenders, A-pillars, engine components, brake pads, ignition parts, electrical switches, body panels, heat shields, seats and more.

• Industrial products: Pump housings, pipes, valves, gears, insulators, circuit breakers, equipment panels, containment systems and more.

Metal Injection Moulding

Metal injection moulding is a variant of plastic injection moulding in which fine metal powder mixed with a polymer binder is injected into a mould to create metal parts. After moulding, the binder is removed (debinding) and the part is sintered. As-sintered MIM parts typically reach ~96–99% of the alloy’s theoretical density (near-wrought), and optional hot isostatic pressing (HIP) can raise density further (often >99.5%) for demanding applications.

The process excels at small, complex components (often ≤100 g), enabling thin walls and net-shape features. Note that while injection is quick, overall lead time is driven by debinding and sintering, with throughput achieved by processing many parts per furnace batch.

Categorisation Based on Special Features

Some injection moulding processes are customised for specific products or design features. In this section, we will examine the following unique injection moulding processes:

• Cube moulding

• Gas-assisted injection moulding

• Liquid silicone rubber injection moulding

• Thin-wall injection moulding

• Structural foam moulding

• Micro injection moulding

• Reaction injection moulding

• Fusible core injection moulding

• Overmoulding and insert moulding

Cube Moulding

Cube Moulding

Cube moulding is a specialised form of injection moulding process that uses a cube-shaped mould to produce circular plastic components. The cube can rotate along the vertical axis, allowing for the use of its multiple sides to mould various parts. Although this technology is relatively new, it boasts high efficiency and can reduce the production time per component to less than 0.25 seconds, offering about a 40% reduction in cycle times.

The process uses multiple parting lines that are sequentially injected with material to create multi-material or multi-component parts. The cube mould is rotated either 90° or 180° between successive injections. While the second injection occurs on one side, the first injection is repeated on a different side of the cube. This allows for injection, cooling, and part ejection to be conducted simultaneously on multiple sides, effectively doubling productivity.

Cube moulding offers several advantages, including a reduced space requirement (higher output per footprint), the capability to produce highly complex parts using multiple materials and colours, and quick cycle times (up to 10,000 parts per hour). It also supports diverse applications, allowing for the easy creation of one-, two-, or three-component parts. Additionally, there is a high potential for automation, as operations such as inserting, unscrewing, assembling and testing can be automated to achieve consistent quality without human intervention. Other benefits include a lower clamping force and compatibility with high-volume production.

Gas-Assisted Injection Moulding

Gas Assisted Injection Moulding

Gas-assisted injection moulding uses pressurised gas to create hollow spaces or cavities within the moulded part. Gases exert equal pressure in all directions when contained within a closed environment. This property is leveraged in this injection moulding process to ensure uniform wall thickness throughout the moulded component.

The final products from this moulding process exhibit a smooth, high-quality, glossy finish. Additionally, the process requires lower clamping pressures, which in turn reduces both costs and wear on the moulding machine.

As the gas fills the thicker sections, the likelihood of sink marks appearing on the part also decreases.

Sink marks are shallow depressions on the surface of injection-moulded parts that occur due to uneven cooling. The surface cools more quickly than the core material, causing the core material to pull the surface material inward and leaving small crater-like depressions on the surface.

Liquid Silicone Rubber Injection Moulding

This liquid silicone injection moulding process uses silicone, a thermosetting polymer known for its unique properties, including a smooth surface finish, high-quality appearance, durability, biocompatibility, thermal stability, and excellent electrical and chemical resistance.

Unlike typical injection moulding materials, silicone remains in a liquid state at room temperature and can be poured directly into the moulding machine without the need for heating. However, it does require vulcanisation—a process in which rubber is hardened using heat and sulphur.

Thin-Wall Injection Moulding

Creating thin walls presents a significant challenge in injection moulding. For specialised mass production applications, using the thin-wall injection moulding process is considerably more effective. This method employs specialised equipment designed to manufacture thin-walled products for various applications.

The thickness of a product largely depends on its size. For small components, thin walls may have a thickness of less than 0.5 mm. In contrast, larger parts can have significantly greater thicknesses while still necessitating thin-wall injection moulding equipment. The key differentiator in this process is the flow length-to-thickness ratio, with some components exhibiting a ratio greater than 200.

Thin-wall injection moulding offers several advantages, including high material efficiency, rapid cycle times and cost savings. This technique is commonly employed to produce containers, enclosures and equipment housings. 

Structural Foam Injection Moulding

Structural Foam Moulding

Structural foam moulding uses gases mixed with the plastics to force the plastic material against the mould walls. This process is a form of low-pressure injection moulding.

In this process, thermoplastic and thermosetting polymers are mixed with nitrogen gas during the melting phase and injected into the mold. The incorporation of nitrogen induces foaming in the material. The gas dilutes the core while pushing the material outward toward the mould walls. When the material contacts the cold walls of the mould, the foam bubbles collapse, and the material solidifies, resulting in increased density at the walls. Consequently, a solid outer layer with a lighter core is achieved.

Structural foam injection moulding does not require steel moulds due to the use of lower pressures. Instead, aluminium or other lighter metals are used, making it more cost-effective. The finished parts can be larger compared to those produced by other injection moulding processes, making it suitable for manufacturing bigger components such as car roofs, housings, plastic pallets, trim panels, large equipment covers, kiosk enclosures and more.

The process is highly cost-efficient, and the increased porosity provides the components with exceptional thermal and acoustic insulation. However, it has several drawbacks, including lower production speeds, the requirement for thick walls (at least 1/4 inch or 6 mm), increased post-processing, and rougher surface finishes.

Micro Injection Moulding

Micro injection moulding is a specialised injection moulding process that produces miniature plastic components typically weighing less than one gram. These parts find use as micro gears, medical syringes and needles, micro implants, connectors, and in electronic circuit boards. This process is characterised by its high precision, as the parts must adhere to tolerance ranges measured in microns. Additionally, they may incorporate intricate features such as thin walls and micro holes.

The production process is similar to standard injection moulding, but it operates on a microscopic scale. The injection moulding machine is equipped with a micro injection unit to accommodate the small components. Material quantities weighing mere fractions of a gram are precisely injected into the mould. Otherwise, insignificant features like parting lines can make or break a part in micro injection moulding.

Micro-injection moulded parts are increasingly being used in the medical industry due to their size advantages. These components enable the safe performance of minimally invasive surgeries, including neurosurgery and aortic procedures.

Reaction Injection Moulding

Reaction Injection Moulding

Reaction injection moulding (RIM) uses two or more reactive liquid polymers to produce strong and durable components. The two monomers, typically a resin and a hardener, are combined in a specialised mixing chamber to form a homogeneous mixture. Once prepared, this mixture is injected into the mould at relatively low pressures (up to 100 bar) until the mould cavity is completely filled.

The mixture undergoes an exothermic reaction within the mould, which may be accompanied by gas emissions or foaming, followed by solidification. The solid components are then ejected from the mould and sent for post-processing as necessary.

Some processes may involve the incorporation of reinforcing materials, such as glass fibres or mica, to enhance the strength and stiffness of the final part. These processes fall into two categories: structural reaction injection moulding and reinforced reaction injection moulding.

In structural RIM, reinforcing agents, such as carbon fibre meshes, are positioned in the mould cavity prior to the injection of the liquid mixture. As the mixture solidifies around the fibres, the fibre structure enhances the strength of the component.

In reinforced RIM, reinforcing agents such as glass fibre and silica are combined with the liquid mixture before injection.

Fusible Core Injection Moulding

Fusible core injection moulding, also known as lost core injection moulding, is a specialised variant of the injection moulding process used to create internal cavities or undercuts that cannot be achieved with demouldable cores. Demouldable cores are those that can be removed from the parts after the injection process.

In such cases, we use fusible cores that either dissolve on their own or can be melted later to separate them from the finished part. This process is referred to as soluble core injection moulding when the core is composed of plastic.

Fusible core injection moulding consists of three main stages: core preparation, core insertion into the mould and shooting the mould, and removal of finished parts and the melting of the core.

The core may consist of a low-melting-point metal, such as a tin-bismuth alloy, or a soluble polymer. These materials typically have a melting temperature around 150 °C. It is crucial to ensure that the cores are non-porous to prevent defects in the final product. Additionally, polymer cores offer the advantage of being manufactured in-house using conventional injection machines.

The cores are then inserted into the mould. This process can be as straightforward as placing the core and closing the mould. However, for more complex parts, automation provides superior results due to its enhanced accuracy and speed. Once the core is securely positioned and the mould is closed, molten plastic is injected into the mould. Upon solidification, the core is removed from the moulding using a hot bath or through induction heating.

While the first fusible core injection moulding process was patented in 1968, it did not gain widespread adoption until the automotive sector turned to it to manufacture parts such as intake manifolds and brake housings.

Overmoulding and Insert Moulding

Overmoulding

Overmoulding is a specialised type of injection moulding process that involves the sequential moulding of two or more plastic parts one over the other, across multiple stations to create a multi-material part. This process uses multiple injection units that supply various cavities. The base, referred to as the substrate, is moulded first, followed by the higher layers being moulded as the part progresses through the different stations. When two materials are used in the moulding process, it is commonly known as two-shot injection moulding.

Overmoulding enables the production of multi-material components, such as plastic parts with rubber handles. This technique is commonly employed in the manufacturing of grips, toothbrushes, knobs, perfume bottles and more.

A variant of overmoulding is insert moulding, which involves using a prefabricated substrate that is coated with plastic material. For example, a metal screwdriver can be encased in a plastic handle. The substrate is secured inside the mould, and molten plastic is injected into it. Insert moulding products are ubiquitous, appearing in various forms such as cables, pacemakers, electrical sensors, fasteners, and more.

The key difference between the two processes is that overmoulding involves moulding a rubber-like plastic around another plastic material, while insert moulding refers to the moulding of plastic around a non-plastic object.