What is Injection Molding?

“Injection molding offers a transformative approach to producing a vast array of parts with precision and efficiency. It leverages sophisticated machinery to shape diverse parts & products, from everyday items to complex components for different industries.” 

What is Injection molding? It is one of the popular manufacturing approaches to produce precise parts or products. It encompasses both plastic and metal, catering to various applications. The process involves melting raw materials and injecting them into a mold, which cools and solidifies into the final part. Once the mold is created, it enables mass production of parts with consistent quality and precision. 

This article will explore the intricate details of injection molding, its processes, types, materials, injection molding cost, considerations, applications, and more.

The Basics of Injection Molding

Injection molding is a manufacturing process widely recognized for producing a vast array of plastic and metal components with precision and efficiency. It’s a cornerstone in the mass production of identical parts, utilized across industries from automotive to consumer electronics.

Working Principle of Injection Molding 

The working principle of injection molding revolves around melting raw material (typically plastic or metal) injected into a mold under high pressure. This mold is precisely crafted to represent the desired shape and size of the final product. The material is injected through a heated barrel, melting, and then forced into the mold cavity. This process is facilitated by a screw mechanism within the barrel that not only softens the material but also ensures it is evenly injected into the mold.

The real magic happens in the mold. Molds can be complex, with multiple cavities and intricate designs, allowing for the production of detailed and precise parts. Once the material is injected, it begins to cool and solidify, taking the shape of the mold. After cooling, the mold opens, and the part is ejected, usually with the help of ejector pins.

Four Stages of Injection Molding

There are four major stages in the working of injection molding, and each stage – Clamping, Injection, Cooling, and Ejection – works in harmony to transform raw material into complex and detailed parts. Let’s explore these stages to understand better how injection molding works to create intricate and consistent products.

1. Clamping

The first stage, clamping, involves preparing the injection molding machine for operation. The mold is securely clamped together by the machine’s clamping unit, ensuring it is tightly closed. This step is critical to prevent material from escaping during injection and maintain the final product’s desired shape and integrity.

2. Injection

In the injection stage, the raw plastic material, typically in pellet form, is fed into the machine, melted, and injected into the clamped mold. This phase requires precise temperature and pressure control to ensure consistent quality and detail in the molded part.

3. Cooling

Once injected, the material inside the mold begins to cool and solidify, taking the shape of the mold. Cooling is a crucial phase, as it significantly affects the quality of the final product. The cooling time varies depending on the material used and the thickness of the part.

4. Ejection

After cooling, the mold opens, and the part is ejected. Ejection must be done carefully to avoid damaging the newly formed part. The finished part is removed from the mold, and any excess material, like runners or flash, is trimmed off, completing the injection molding cycle. 

Related: 9 Best Injection Molding Materials

Structure and Components of Injection Molding Machine

Injection molding machines are complex devices designed to transform raw materials into molded parts through a precise, controlled process. Their structure typically involves two main sections: the injection unit and the clamping unit. The injection unit is responsible for melting and injecting the material into the mold, while the clamping unit holds and secures the mold during the injection and cooling phases. 

Injection Unit

The injection unit handles the melting and injection of the material. It consists of a hopper where raw materials are fed, a barrel heated to melt the material, and a screw mechanism that pushes the molten material into the mold. This unit is critical for precise control of temperature and injection pressure.

• Hopper: The hopper is where the raw material (either plastic pellets or metal powder) is stored before being fed into the barrel. It’s designed to allow a steady flow of material into the machine without clogging or spilling, ensuring a consistent supply for the molding process.
• Barrel and Screw: The barrel, equipped with heaters, melts the material fed from the hopper. Inside the barrel, a rotating screw not only aids in melting the material but also acts as a ram during the injection phase, pushing the molten material into the mold.
• Control System: The control system is another part of the injection unit. These systems regulate the process parameters like temperature, pressure, injection speed, and clamping force. They ensure consistent quality and precision in the molding process and allow for fine-tuning to meet the specific requirements of different materials and molds.

Clamping Unit

The clamping unit opens and closes the mold and ejects the finished part. It includes the mold itself, clamping mechanism, and ejection system. The clamping force must be sufficiently strong to resist the pressure of the injected material, so the mold remains closed and the part is accurately formed.

• Mold: The mold is a critical component and is custom-made for each specific product. Made from metal, it consists of two halves – the cavity and the core – which close tightly during the injection process and open for part ejection.
• Ejection System: Once the part is cooled and solidified, the ejection system is activated. This system often includes pins or plates that push the finished part out of the mold, ensuring it is ejected without damage.
• Hydraulic and Electrical Systems: Hydraulic systems are used to generate the force needed for the clamping and injection processes. Electrical systems, increasingly common in newer machines, offer benefits like energy efficiency and precision control.

Step-by-step Injection Molding Process

The injection molding process is a series of careful steps, starting with selecting appropriate materials and designing the mold. It involves melting the material, injecting it into the mold, cooling, and solidifying, followed by ejection and post-processing. Each step is critical to achieving the desired quality and precision of the final product.

Let’s take a look at each step in detail.

Step 1: Material Selection

Material selection is pivotal in injection molding, as it directly influences the final properties. This step requires a deep understanding of material science to ensure compatibility with the design and intended use.

You need to choose based on required characteristics like durability, flexibility, and resistance to environmental factors.The decision hinges on the intended application of the final product, considering factors such as strength, weight, heat tolerance, and chemical resistance. 

Step 2: Designing the Mold

Injection molds are the accurate mirror of the final shape and dimensions, typically crafted from metals like steel or aluminum, and contain two primary parts: the cavity and the core. Designing mold can be achieved with Advanced CAD software. 

CAD design of Injection Mold 

The quality and design intricacy of the mold determines the precision and quality of the molded parts. Furthermore, this stage often involves significant investment, as the mold’s durability and accuracy are paramount for successful production.

Step 4: Mold Manufacturing

Advanced manufacturing techniques, including CNC machining and electrical discharge machining (EDM), are employed to create molds that can withstand the pressures and temperatures of the injection process. 

Step 4: Melting the Material

Temperature control is crucial here, as it affects the viscosity of liquified material and the overall quality of the molded product. For plastics, pellets are fed into a heated barrel where they melt before injection. In metal injection molding, metal powders are mixed with a binding agent and heated until they reach a molten state. 

Step 4: Injection of molten material

Injecting the molten material 

The injection is a high-precision stage where the molten material is injected into the mold under significant pressure, commonly ranging between 70 and 112 MPa (10–16 Kpsi). This ensures the complete filling of the mold cavity, capturing the intricate details of the design. In this stage, it is crucial to maintain the correct pressure and volume of material. Any inaccuracies during injection can lead to defects in the final product, such as a warping or incomplete filling. 

Step 5: Cooling and Solidification

Post-injection, the material within the mold begins to cool and solidify, a phase crucial for the product’s structural integrity. Material transforms from molten to solid form in this cooling period. The duration of this cooling process varies based on the material’s properties and the size of the part.

Cooling Time (Tc) = (K * V * ΔT) / Q . Source 

Where Tc is Cooling time in seconds), V is the volume of the molded part, ΔT is the temperature difference between the melt temperature and the ejection temperature, Q is the cooling rate, and K is the constant depends on the material and mold properties. 

Step 6: Ejection

Parts ejection from the injection mold

After solidification, the part is ejected from the mold, typically pins, are activated to carefully release the part without causing any damage. In metal injection molding, additional steps like de-binding and sintering are involved. Debinding removes the binding agent and sintering strengthens the part by heating it without melting.

Step 7: Post-Processing

The post-processing phase involves trimming excess material and applying finishing touches such as polishing or painting. For metal parts, this stage might include additional machining or heat treatments to enhance mechanical properties or to achieve finer tolerances and surface finishes.

Step 8: Quality Control

Quality control is a critical step to ensure it meets predetermined standards and specifications. This inspection might involve dimensional checks, material testing, and surface examination. Parts that fail to meet quality criteria are either reworked or discarded, maintaining the integrity of the production batch.

Step 9: Assembly and Packaging

In the final step, parts are assembled if they form a component of a larger product. Subsequently, they are packaged, often with protective materials to prevent damage during transportation. This stage is tailored to fit the product’s specific distribution and end-user requirements. 

What Are the Different Types of Injection Molding?

Injection molding, a versatile manufacturing process, adapts to various applications through its different types. Each type caters to specific requirements, ranging from the material used to the complexity of the product design. The diversity in injection molding types allows for diverse products, from simple household items to intricate automotive components.

Let’s discuss different types of injection molding methods in brief.

Thin-Wall Injection Molding

Thin-wall injection molding specializes in producing ultra-thin plastic parts while maintaining strength and durability. It’s widely used in creating lightweight, fast-production items like packaging materials and containers. The main challenge is ensuring these thin components retain structural integrity.

Thin-wall injection molding 

This molding type demands precise molding process control to achieve the desired thinness without compromising quality. It’s ideal for applications where weight reduction is essential but strength cannot be sacrificed, such as in certain electronic device casings.

Liquid Silicone Rubber Injection Molding

Liquid Silicone Rubber (LSR) injection molding creates flexible, durable, and thermally stable parts. It’s particularly suited for products that must withstand extreme temperatures and stress, such as medical devices and heat-resistant cookware.

In the second phase of this process, the focus is on maintaining the unique properties of silicone, like flexibility and heat resistance, making it a preferred choice for intricate designs in healthcare and culinary applications.

Cube Molding

Cube molding is an innovative injection molding technique that uses a rotating cube-shaped mold. This unique approach allows for simultaneous multi-material injection and more complex part designs. It’s particularly effective for high-volume production where efficiency and precision are paramount.

In cube molding, different sides of the cube can be used for various stages of the molding process. This means while one side is being injected, another can be cooling, and a third can be ejected. This simultaneous operation reduces cycle time and increases production efficiency, making it ideal for intricate parts that require multiple materials or inserts.

Gas-Assisted Injection Molding

Gas-assisted injection molding is a technique where gas (usually nitrogen) is used to create hollow sections in plastic parts. This method is beneficial for reducing material use, minimizing warping, and enhancing the strength-to-weight ratio, especially in larger, complex parts.

The process is particularly valuable in manufacturing handles, panels, and other large parts where internal stability and surface finish are critical. It combines material efficiency with structural integrity, making it a sought-after method in various industries.

Micro Injection Molding

 micro-injection molding machine

Micro injection molding caters to the production of extremely small, high-precision parts, commonly used in medical and electronic applications. This method requires meticulous precision and control, as slight deviations can significantly impact the component’s functionality.

The challenge lies in consistently producing such tiny parts with intricate details, demanding advanced technology and skilled operators. It’s a key technique in miniaturized components, where accuracy is paramount.

Overmolding

Overmolding is a process where a layer of material is molded over an existing part, enhancing its features. This technique is commonly used in consumer electronics to add soft-touch surfaces or aesthetic layers to a rigid base.

Overmolding 

The second stage of over-molding often involves adding functional characteristics, such as improved grip or shock absorption, to an item. This method is instrumental in producing multi-textured or multi-material products, combining functionality with aesthetic appeal.

Read More:

• Transfer Molding: Definition, Process, and Applications
• Compression Molding: Type, Pros, Cons, and Uses

Plastic Injection Molding

 Plastic parts created with injection molding 

Plastic injection molding is a widely used process for manufacturing high-volume plastic parts. It involves melting plastic granules and injecting them into a mold to form a specific shape.

This method is highly efficient for mass production, allowing for the creation of intricate and consistent designs. It’s essential for industries ranging from automotive to consumer goods. The following are the common Plastic Injection Molding Materials 

• Polyethylene (PE)
• Polypropylene (PP)
• Polystyrene (PS)
• Polyvinyl Chloride (PVC)
• Polyethylene Terephthalate (PET)
• Acrylonitrile Butadiene Styrene (ABS)
• Polycarbonate (PC)
• Polyamide (Nylon)
• Polybutylene Terephthalate (PBT)
• Acetal (POM)
• Polymethyl Methacrylate (PMMA)
• Thermoplastic Rubber (TPR)
• Thermoplastic Polyurethane (TPU)
• Polyphenylene Oxide (PPO)
• High Impact Polystyrene (HIPS)

Particular Considerations

Considering the melting temperature and flow properties of different plastics is crucial in plastic injection molding. It ensures the smooth filling of the mold and the quality of the final product.

Another consideration is the shrinkage rate of each plastic type. Adequate allowance for shrinkage must be factored into mold design to achieve accurate dimensions in the finished part.

Applications

It’s vital in automotive manufacturing, producing components like dashboards and bumpers. Plastic injection molding is extensively used for creating various consumer goods like toys and household items. 

The medical industry relies on it for making sterile, disposable items such as syringes. In electronics, it’s used for encasing devices like smartphones and remote controls.

Related: ABS Injection Molding: A Guide to Achieving Plastic Products

Metal Injection Molding

Metal injection molding (MIM) is a process used to manufacture high-volume metal parts with complex geometries. It is known for its ability to produce strong & intricate parts used for small, complex components requiring high strength. It’s a cost-effective alternative to traditional metalworking techniques like machining and casting. The following are the Metal Injection Molding Materials; 

• Stainless Steel
• Titanium
• Tungsten
• Nickel Alloys
• Cobalt Alloys
• Copper Alloys
• Iron-Nickel Alloys

Particular Considerations

The choice of metal powder affects the part’s final properties, such as strength and corrosion resistance. Binder removal and sintering conditions must be precisely controlled to achieve optimal part density and mechanical properties.

Injection molded metal parts 

Applications

• Widely used in the medical sector for surgical tools and implants due to biocompatibility.
• Ideal for manufacturing complex parts in the aerospace industry, such as components for engines.
• In the automotive sector, it’s used for producing small, intricate components like gears and connectors.
• MIM is also prevalent in consumer electronics for manufacturing precise metal parts in gadgets.

Read More: CNC Milling for Metal Parts: Precision in Practice 

Surface Finishing of Injection Molded Parts

Surface finishing of injection molded parts enhances their appearance and functionality. These finishes can range from aesthetic improvements to functional coatings that alter the part’s properties.

Finish on injection molded plastic part

Different techniques are used depending on the desired outcome and the material of the part. These finishes can provide improved durability, aesthetic appeal, or special features like UV resistance.

• Painting: Adds color enhances visual appeal and offers a protective layer against environmental factors.
• Plating: Applies a metal layer for aesthetics and wear resistance and is ideal for parts needing a metallic look; more expensive.
• Polishing: This creates a glossy, high-quality finish and removes surface imperfections for smoothness.
• Sandblasting: Achieves a matte texture by blasting the surface and preparing parts for further finishing, like painting.
• Laser Engraving: Adds detailed designs or text precisely and is ideal for permanent, wear-resistant markings.

Injection Molding Cost: Compared with Other Approaches

Injection molding is often considered for its cost-efficiency in mass production. While the initial setup and mold costs are high, the per-unit cost significantly decreases with volume, making it more economical for large-scale manufacturing compared to other methods.

In contrast, 3D printing and CNC machining have different cost structures. 3D printing is more cost-effective for low-volume, complex designs due to minimal setup costs. CNC machining, while versatile, incurs higher costs in materials and labor, especially for complex parts or large quantities.

Table: Cost Comparison of Injection Molding, 3D Printing, and CNC Machining

Related: 3D Printing Vs. Injection Molding: Choose the Best Method for Your Project

What Are the Advantages of Injection Molding?

Injection molding is renowned for its efficiency in mass production. It enables the creation of large quantities of parts with consistent quality and precision, making it ideal for manufacturing on a grand scale.

This process also allows for high versatility in design and material selection. It’s capable of producing complex shapes that might be challenging or costly with other manufacturing methods.

Advantages

• High efficiency in mass production
• Consistent part quality
• Capability to produce complex shapes
• Wide range of material options
• Economical for large production runs
• Reduced waste compared to traditional manufacturing

What Are the Applications of Injection Molding?

Injection molding’s versatility makes it a staple in various industries. From automotive to consumer goods, its applications are vast due to its ability to produce diverse part designs and complexities. In the medical field, injection molding is used for creating sterile, disposable items, while in the automotive industry, it is essential for producing durable, high-precision parts.

Table: Injection Molding Applications

China Injection Molding Services

China’s injection molding services are renowned for their cost-effectiveness and large-scale production capabilities for diverse industries globally. Chinese injection molding services can handle rapid production turnaround and high-volume orders.

Prolean Injection Molding Services: from Mold Creation to Surface Finish

We offer comprehensive injection molding services, ensuring quality from mold creation to the final surface finish.

Our Services include:

• Custom mold design and manufacturing
• Precision injection molding for plastics and metals
• Advanced quality control measures
• Post-molding operations like painting and polishing
• Rapid prototyping for design verification
• Large-scale production capability for high-volume orders

Read more: 

• Why is CNC Machining Better than Injection Molding for Thermoplastic Prototyping? 
• Injection Molding Gate: Expert Insights on Design & Manufacturing

Summing Up

Injection molding is a transformative technique in modern manufacturing, offering precision and efficiency in producing diverse parts. Different metal and plastic molded parts are crucial for various industries and applications. Its stages of clamping, injection, cooling, and ejection, allow for the mass production of intricate designs with consistent quality. It’s not just limited to standard processes but also includes specialized forms like thin-wall, liquid silicone rubber, and gas-assisted molding. 

FAQs

What are the primary benefits of injection molding?

Injection molding offers high-volume production, consistent part quality, and the ability to create complex shapes with high precision.

How does the design impact the injection molding process?

Design dictates the mold’s complexity, material choice, and manufacturing feasibility, impacting overall production efficiency and part quality.

How are advancements in technology influencing injection molding?

Technological advancements lead to more precise control, reduced waste, and the ability to mold complex designs. Innovations like 3D printing for mold making also streamline the process.

What are the cost considerations for starting an injection molding project?

Initial costs include mold design and production. Material choice and part complexity also affect overall expenses. Economies of scale play a crucial role in long-term cost efficiency.

What is splay in injection molding?

Splay appears as silver streaks or splashes on a molded part’s surface, usually caused by moisture in the material or excessive injection speed.

What is 2k injection molding?

2K (two-component) injection molding combines two different materials or colors in a single molding process, enabling complex, multi-material parts.

Resources

1. Huszar, M., Belblidia, F., Alston, S., Wlodarski, P., Arnold, C., Bould, D., & Sienz, J. (2016). The influence of flow and thermal properties on injection pressure and cooling time prediction. Applied Mathematical Modelling, 40(15-16), 7001-7011. https://doi.org/10.1016/j.apm.2016.03.002
2. Fischer, J. M. (2013). Handbook of Molded Part Shrinkage and Warpage. William AndrewPublishing. https://www.sciencedirect.com/book/9781455725977/handbook-of-molded-part-shrinkage-and-warpage