Seven Types of Injection Molding Processes: What You Should Know

In manufacturing, injection molding is the core technology for producing plastic components. But did you know that injection molding encompasses multiple distinct processes? These processes provide tailored solutions for specific materials, complex structures, and functional requirements. Whether you're a product engineer, designer, or simply curious about plastic products, understanding these concepts will empower you to make better decisions and solutions.

Table of Contents

1. What is Plastic Injection Molding?

2. How Does Injection Molding Work?

3. Understanding Seven Injection Molding Processes

3.1 Traditional Injection Molding

3.2 Overmolding

3.3 Insert Molding

3.4 Two-Color Injection Molding

3.5 Thin-Wall Injection Molding

3.6 Gas-Assisted Injection Molding

3.7 Liquid Silicone Rubber (LSR) Injection Molding

4. Conclusion

5. Frequently Asked Questions

1. What is Plastic Injection Molding?

Injection molding is a forming manufacturing process. Whether you need small batches of parts for testing or large-scale production, injection molding is a viable option. More importantly, the unique process by which injection molding machines produce parts enables the creation of complex designs with intricate details—something not achievable by all manufacturing methods.

2. How Does Injection Molding Work?

The process can be summarized as follows: First, a mold is manufactured based on the product design. Then, plastic pellets are melted and injected under high pressure into the sealed mold cavity. After cooling and solidifying, the part is ejected from the mold, resulting in the final plastic product.

Step 1: Mold Manufacturing

Based on the product design drawings and requirements, the mold design is finalized. The mold cavity is formed through CNC machining and EDM (Electrical Discharge Machining). After assembly, debugging, trial molding verification, and adjustments, the final mold is obtained.

Step 2: Injection Molding

Injection molding involves forcing heated, molten resin into the mold cavity under high pressure. After cooling and solidification, the molded plastic part is formed. The process consists of six main stages:

1. Mold Clamping

Driven by the machine, the two mold halves rapidly approach, then smoothly close together. They are locked tightly with immense force. This forms the foundation for all subsequent actions, preparing a sealed “steel chamber” for plastic molding.

2. Injection

The screw rapidly advances the heated, molten plastic—now in a “syrup-like” state—from the barrel. Under high pressure, this molten plastic is injected through the mold's channels, rapidly filling the newly sealed cavity.

3. Holding Pressure

After filling the cavity, the screw does not immediately retract but maintains a certain pressure. Like “compacting,” it continuously injects a small amount of extra plastic into the cavity as it begins to cool and shrink. This ensures the product is dense, full, and free of sink marks.

4. Cooling

This is the most time-consuming stage of molding. The high-temperature plastic within the cavity gradually cools and solidifies through heat transfer via the mold's internal cooling channels. It transitions from a liquid state to a solid, ultimately achieving the desired shape and hardness.

5. Mold Opening

Once the product is cooled and set, the mold clamping force releases. The moving mold plate smoothly retracts, gradually separating from the fixed mold plate until fully opened. The formed product now rests on the moving mold side.

6. Product Ejection

Following mold opening, a pre-engineered ejector pin or ejector plate assembly activates, smoothly pushing the cooled, hardened product out of the mold. It is typically removed by a robotic arm or via automatic drop-out. This concludes a complete cycle, after which the mold prepares for the next clamping operation, repeating the process continuously.

3. Understanding Seven Injection Molding Processes

3.1 Traditional Injection Molding

Traditional injection molding, commonly referred to as injection molding, is a manufacturing process where plastic material is heated to a molten state, injected into a closed mold cavity, and cooled to solidify into the desired plastic product shape. As the most widely applied and highest-volume forming technology in the plastics processing industry, injection molding offers significant advantages including high production efficiency, high product precision, the ability to form complex-shaped parts, and ease of automation.

The core of the injection molding process lies in utilizing the high pressure provided by the injection molding machine to rapidly fill the mold with molten plastic, thereby replicating the shape of the mold cavity. The production process typically involves five sequential stages: mold closing, injection, holding pressure, cooling, and mold opening. Technological advancements have spawned various derivative processes such as overmolding, insert molding, two-shot molding, and gas-assisted molding, which are extensively applied across automotive, electronics, home appliances, medical devices, and packaging industries.

3.2 Overmolding

Overmolding is a versatile injection molding process that excels at producing complex plastic parts with multiple materials to enhance product performance and aesthetics.

It involves molding two or more plastics together to form a single, integrated product. Overmolding can refer to two distinct molding processes:

1. Using a softer material, typically a rubber-like substance called a thermoplastic elastomer (TPE), as an outer layer covering a harder core material known as the substrate. This form is also called secondary material injection.

2. One material forms the outer skin of the molded part, while another material forms the internal core. This form is also known as co-injection molding.

Primary application areas for overmolding:

1. Consumer goods: e.g., tools, home appliances, and electronics

2. Automotive industry: e.g., handles, grips, and trim components

3. Medical industry: e.g., handles and grips for medical devices

4. Electrical industry: e.g., connectors and cables

3.3 Insert Molding

Insert molding is a process that integrates preformed inserts with plastic material. The fundamental principle involves placing metal or other material inserts into the mold cavity, followed by injecting molten plastic. This process ensures the plastic and insert fuse tightly during molding, forming an integrated composite structure. This technique is widely used in automotive, electronics, and home appliance industries to manufacture products with specialized functionalities or structural requirements.

During insert injection molding, there are two methods for positioning inserts within the mold:

1. Manual placement: This significantly increases the injection molding cycle time and is suitable for small-batch production or products with complex structures.

2. Automated placement: This is the preferred choice for high-volume production, as it reduces human error, improves efficiency, shortens the injection molding cycle, and enhances product reliability.

Primary application areas for insert molding:

1. Electronics and Electrical Industry: USB/Type-C connectors, circuit modules with metal pins, switch buttons.

2. Automotive industry: Steering wheel buttons/knobs with threaded inserts, engine sensor housings, high-voltage connectors for new energy vehicles.

3. Medical devices: Handles for surgical knives/scissors, disposable endoscope components, diagnostic equipment probes.

4. Industrial equipment and tools: Power tool handles/gears, instrument housings with embedded nuts, industrial gears.

3.4 Two-Color Injection Molding

Two-color injection molding refers to the process of forming two plastics (or two plastics of different colors) into a single integrated part during a single injection molding operation. In this document, the first injected material is termed the base material, and the second injected material is termed the overlay material.

The most common configuration for two-color injection molds involves two identical moving dies corresponding to two different fixed die cavities. After the first injection of the base material, the mold opens. The moving mold then rotates 180° using the injection machine's rotary mechanism. The mold closes again, and a second injection occurs using a different color or different material (the overlay material). After the second mold opening, the ejector action removes the part that has undergone both injections. The first injection of the base material and the second injection of the overlay material occur simultaneously, requiring the injection machine to have two injection nozzles—one for each material.

The most typical example of dual-color injection molding in daily production is the toothbrush. A toothbrush is a product combining hard and soft plastics. The main body uses hard plastic to provide sufficient strength, while the hand-contact area employs soft plastic to ensure tactile comfort. Dual-color injection molding also enables toothbrushes to feature vibrant, colorful appearances.

Dual-color injection molding is primarily applied in:

1) Power switches, mobile phone buttons, automotive switches, etc., to meet partial light-guiding or translucency requirements.

2) Handheld products like walkie-talkie housings, toothbrush handles, power tool grips, wrenches, and thermos cups to enhance tactile feel.

3) Products requiring multi-color aesthetics, such as keyboards. Dual-color injection ensures visual appeal without worrying about color wear during use.

4) Products requiring localized plating, such as keys on feature phones.

5) Products requiring waterproofing to achieve water resistance.

3.5 Thin-Wall Injection Molding

Thin-wall injection molding is a specialized technique for manufacturing plastic parts with wall thicknesses below conventional standards. Parts typically under 1mm thick are considered thin-wall; for larger components, wall thicknesses may range between 2–4mm. Compared to conventional injection molding, thin-wall injection molding produces lighter, thinner, structurally sound parts while enhancing production efficiency.

Key Advantages of Thin-Wall Injection Molding：

Advantage	Description
Weight Reduction and Cost Savings	Thin-walled designs use less material, reducing raw material and transportation costs. Ideal for electronics, automotive, and packaging industries.
High Production Efficiency	Thin walls cool faster, shortening the cycle time by 30-50% and increasing production capacity.
High Design Flexibility	Enables the manufacturing of micro-fine and complex structural parts, balancing functionality and aesthetics.
Environmentally Friendly	Reduced material usage and shorter production cycles lower energy consumption and decrease carbon emissions.

Primary Applications of Thin-Wall Injection Molding:

1. Medical Devices: Syringes, catheters, surgical instruments, etc.

2. Consumer Electronics: Mobile phones, laptops, camera housings, etc.

3. Automotive Components: Engine hoods, interior trim parts, etc.

4. Food packaging: lightweight containers, bottle caps, medical packaging, etc.

3.6 Gas-Assisted Injection Molding

Gas-Assisted Injection Molding (GAIM) is an advanced plastic injection technology. It involves injecting high-pressure gas (typically nitrogen) into molten plastic. The gas propels the plastic to fill the mold cavity and forms hollow structures during cooling. This technology combines features of traditional injection molding and blow molding, finding extensive application in automotive, home appliances, furniture, and other sectors for producing lightweight, high-rigidity, low-warpage plastic components.

Compared to conventional injection molding, gas-assisted injection molding offers significant advantages:

Indicator	Traditional Injection Molding	Gas-assisted Injection Molding
Part Weight	Solid, heavier.	Hollow structure, 20%-40% weight reduction.
Shrinkage/Warpage	Prone to shrinkage and warping in thick-walled parts.	Gas holding pressure eliminates shrinkage; deformation rate reduced by over 50%.
Mold Pressure	High (requires high clamping force).	Low (only 1/3 to 1/2 of traditional molding).
Material Utilization	Low (more scrap from runners/gates).	High (simplified runners, scrap reduced by about 30%).
Applicable Scenarios	Thick-walled or uniform wall thickness parts.	Thick or uneven wall thickness parts (e.g., handles, instrument panels).

Gas-assisted injection molding is primarily applied in:

1. Automotive sector: Door handles, roof grab handles, bumpers, instrument panels, steering wheels, etc.

2. Home appliances and daily goods: Chair armrests, bases, large toys, office furniture components, panels, etc.

3.7 Liquid Silicone Rubber (LSR) Injection Molding

LSR is a non-toxic, heat-resistant, highly resilient flexible thermoset material offering excellent transparency, tear strength, rebound elasticity, yellowing resistance, thermal stability, breathability, heat aging resistance, and weatherability. Its rheological behavior is characterized by low viscosity, rapid curing, shear thinning, and a relatively high thermal expansion coefficient. LSR is a platinum-catalyzed, two-component rapid-curing material that enables high-volume, rapid, repeatable mechanical production via injection molding.

Liquid silicone rubber injection molding is an innovative silicone rubber processing technology that combines high-performance liquid silicone rubber with equipment capable of precise and stable injection molding. It requires only two components (which may include auxiliary components like colorants) to be loaded into the equipment, with the entire process—from feeding, metering, and mixing to molding—completed automatically. This processing technology achieves simplified processes, reduced processing time, material savings, and enhanced efficiency. Furthermore, the production process generates virtually no scrap, contributing to environmental sustainability.

Liquid silicone rubber is an indispensable material in the production and design of health products, automotive components, baby products, medical supplies, diving equipment, kitchenware, daily necessities, electrical insulation materials, and seals.

4. Conclusion

This article mainly introduces seven common injection molding processes, covering their concepts, procedures, advantages, and application areas. Understanding these seven injection molding processes can provide more suitable technical options for product design and manufacturing. If you have needs for injection molding, overmolding, insert molding, or two-color molding, please feel free to contact Alpine Mold. We have over twenty years of extensive experience in these areas.

5. Frequently Asked Questions

1. What are the different types of injection molding?

Primary injection molding types include single-cavity molds, multi-cavity molds, family molds, two-color molds, overmolding molds, insert molds, pneumatic molds, hot runner molds, and cold runner molds.

2. What are the different types of molding processes?

Common molding process types include injection molding, extrusion molding, compression molding, rotational molding, blow molding, hot pressing, die casting, precision casting, powder metallurgy forming, and fiber-reinforced composite forming.

3. What is DFM in injection molding?

DFM refers to considering manufacturing feasibility during the product design phase to ensure designs can be produced efficiently and economically via injection molding. DFM analysis evaluates mold structure, material selection, injection molding processes, product dimensional tolerances, wall thickness, venting design, and other factors. Its purpose is to optimize designs, reduce production costs, enhance part quality, and ensure products are easy to manufacture and assemble.