天美影院

Metal injection molding (MIM) is a process in which metal parts are manufactured using powder metal as the raw material. It uses a technique called Powder metallurgy. This process integrates the flexibility of plastic injection molding procedures with the integrity and Strength of metals to produce parts of complex geometries. This process is mostly suited for tiny parts under 100 grams, and the product happens in both large and small volumes.

It is used in spaces that require a high level of precision and quality, such as the aerospace, automotive, and medical devices sectors.

Overview of Process Flow

The production procedure of Metal injection molding is quite similar to that of (PIM) Plastic injection molding since MIM deals with metals, but it’s slightly complicated. A mixture of tiny metal particles and plastic binder-polymerized metal feedstock- is injected into the mold under high pressure. After cooling off, it hardens and is then released from the mold and trimmed if necessary.

However, it’s not over yet! What is produced is what is referred to as the “green part,” which needs to undergo debonding. In the next process, the plastic binder is removed, leaving behind a fragile and porous metal fragment called the “brown part.”

The procedure includes several steps, like feedstock preparation (Compounding), injection molding, debinding, and sintering. Each stage is critical in producing parts with the optimal shape, material properties, and dimensions.

1. Compounding

Also referred to as feedstock preparation, it is the first step in the MIM process. This stage involves a mixture of metal powder with sizes ranging between 4-25? with either wax binders or thermoplastics in the ratio of 60:40 by volume. The mixture is heated and melted in special mixing equipment like the Sigma blade mixer, and particles are distributed evenly throughout. This distribution is essential in ensuring the material viscosity, which affects the injection molding process and the density of the final part. Afterward, the mass is cooled and granulated into a feedstock for the MIM machine.

The metalpowder determines the structural properties of the final part. This binder facilitates the flow during injection molding and also affects the debinding and sintering processes. The feedstock’s consistency is vital to ensure uniform material flow during the injection molding stage, which results in a part with consistent properties throughout.

2. Injection Molding

This process is similar to the plastic injection molding process. It occurs when the prepared feedstock is injected into the mold cavity to produce the desired part. The pelletized feedstock is first heated at a specific temperature and injected under high pressure into the mold cavity.

The rotation of the screw, which is inside the barrel, pushed the feedstock forward, and the pressure allowed the nozzle into the cavity. Once it has filled, it cools and solidifies the binder, retaining the shape of the parts as it is ejected from the solid by either compressed air or ejector pins.

The part that comes out is the “green part,” and the process continues. The mold must incorporate a proper gate and vent location to facilitate consistent filling of the mold chamber to guarantee a high-quality product.

To compensate for the shrinkage that occurs during sintering, the cavity is made to be  20% larger and this shrinkage change depends on each material.

3. Debinding

Debinding is the process of ejecting the binder out of the  “green part” and leaving a porous metal part known as the “brown part.” The process occurs in several steps, and most of the binder is removed to leave behind just enough to hold the parts in the sintering furnace.

Binder removal is accomplished through three categories;

Ⅰ. Solvent Debinding

In this procedure, the green part is dipped in a liquid solvent to dissolve and extract the binder. The binder material determines the type of solvent to be used. For instance, if the binder is water-soluble, an aqueous solvent is used. If it’s not, organic solvents are preferable. The part can be dipped inside the solvent for some time, ranging from a few hours to days.

Ⅱ. Thermal Debinding/Pyrolysis

is one of the easiest methods of debinding. The injection-molded part is heated at a temperature below the sintering temperature of the metal powder. The binder decomposes and then evaporates, leaving a porous metal fragment. Some critical parameters that must be controlled in this case are the heating rate, dwell time, and peak temperature. They ensure the binder is completely removed and reduces defects and distortion.

Ⅲ. Catalytic Binding

This process is very effective but somewhat complex. It involves exposing the green part to an acid vapor, such as oxalic or concentrated nitric acid. The acid vapor in this scenario is a catalyst, which ensures the binder is decomposed from the part’s internal structure. The process occurs under a controlled environment, and the compatibility test of the metals is crucial since the process involves the use of acids.

In certain instances, the process known as two-step debinding, which involves the combination of thermal and solvent binding, is used to minimize part deformation.

The remaining “brown part” after the debinding process is a fragile porous structure made of linked metal powder particles. At this point, the part is ready for the final process of sintering, which imparts the particle with the desired mechanical properties and consolidates them.

4. Sintering

The sintering process is when the brown metal is subjected to a temperature below the metal powder’s melting point. The debind parts are loaded into a high-temperature, atmosphere-controlled sintering furnace and placed on ceramic setters. Once the binders are near the melting point, the binders liquefy and evaporate. The metal part is then heated to a high temperature, and the void space between the particles is eliminated, making them fuse. The part shrinks, transforming into a dense solid to its desired dimensions. The shrinkage rate of the part can be up to 20% during the sintering stage. However, this is accounted for in the design and mold production stage.

Materials for Metal Injection Molding

The metal materials suitable for MIM are quite common. In theory, any powder material that can be cast at high temperatures can be formed into parts through the MIM process, including hard-to-machine materials and high-melting-point materials in traditional manufacturing processes. Metals that can be processed by MIM include low-alloy steels, stainless steels, tool steels, nickel-based alloys, tungsten alloys, hard alloys, titanium alloys, magnetic materials, Kovar alloys, precision ceramics, and more. Additionally, MIM can also customize material formulations based on the user’s performance requirements.

MIM forming of non-ferrous alloys like aluminum and copper is technically feasible, but they are usually processed by other more cost-effective methods, such as die-casting or machining. Examples of materials include SUS316L, SUS420J2, SUS440C, SUS630, SNCM415, SKD11, SKH51, Ti alloys, etc.

Differences between Metal injection molding (MIM) and Traditional Injection Molding (TIM)

Traditional Injection Molding (TIM) and Metal Injection Molding (MIM) are both manufacturing processes used to produce high-precision, complex parts. However, they have significant differences in materials, equipment used, and production processes.

Table for Comparing MIM and TIM

Differences and Advantages of MIM Compared with Other Metal Manufacturing Processes

Each manufacturing process has specific application areas with advantages and limitations. MIM combines the versatility and cost savings of other manufacturing processes with the endurance and robustness of metals. To provide insights into whether MIM is the optimal manufacturing pathway, we shall delve into its key advantages and how it differs from other manufacturing processes.

1. Complex Geometries: Applications that require components with complex shapes and material properties that are not possible with traditional metalworking.
2. Cost-Effectiveness: MIM is a highly automated process that reduces the need for manual labor compared to other metal forming methods. Negligible excess material is produced, reducing the amount of waste. Its precision also reduces the need for secondary operations.
3. Consistent Quality: The highly automated and repeated process ensures that the parts produced are similar. Since this process is carried out in a controlled environment, defects and contaminants are minimized.
4. Material Selection: An extensive choice of materials, such as steel, titanium, and some alloys, ensures flexibility and suitability for a particular application. This property often outperforms processes like casting.
5. Density & Strength: Metal injection molding parts produced have high mechanical properties, such as hardness and Strength, making them suitable for components that need to endure extreme wear and high-stress conditions.

Comparison Between MIM and Other Manufacturing Processes

Other manufacturing processes used in the production of metal parts include Traditional Powder Metallurgy, Forging, 3D Printing, and LQMT (Liquid metal Technologies). The following chart compares various aspects of production between the MIM and the listed manufacturing processes.

The Role Of Material Characteristics In Product Function And Appearance Design

Material selection is an important factor in the metal injection molding process, which can directly influence the appearance, design, performance, and functionality of the products. Here’s the outlook on how the material selection impacts design.

1. Mechanical Properties And Functionality

When used together, materials such as steel and titanium have strength and corrosion resistance properties. They are suitable for parts requiring mechanical durability. Components designed to use such materials can incorporate thicker wall geometries or be reinforced with lower-strength materials.

2. Shrinkage and Dimensional Accuracy

The shrinkage rate of MIM ranges between 15% and 20% during the sintering process. However, this depends on the material properties and behavior. Designers ought to account for this shrinkage in the mold dimensions by scaling up proportionately for better accuracy.

3. Corrosion Resistance.

Components subjected to harsh environments require corrosion-resistant materials, such as stainless steel (316L) or titanium. Designers incorporate these materials to minimize the need for protective coatings and preserve the geometries.

4. Thermal Properties

Copper alloys have high thermal conductivity properties and can be used in heat-sensitive applications. Designers can place features like vents and fins that are more effective for heat dissipation in such materials.

5. Aesthetic and Surface Finish

Materials like stainless steel have excellent finishing properties. They are easier to coat, plate, and polish. Products such as consumer electronics use such materials since they require smooth surfaces and a premium appearance.

Product Design Optimization Strategies Based on MIM and Taboos for MIM

Simplify Complex Geometries

MIM allows intricate geometries, which are sometimes challenging or hard to achieve. This increases the risk of defects and costs. To minimize the risk, a designer can optimize this by employing strategies like thin features, radii, or fillets to reduce sharp corners. Also, multiple components can be integrated into one to eliminate assembly.

Optimize Wall Thickness

Designing parts with uniform thickness enhances material flow and prevents warping, cracking, voids, and sink marks. Using a method such as coring can reduce material and processing time.

Incorporate Draft Angles

Draft, or a slight taper, is needed to eject parts from the mold cavity. When a draft angle is required, an angle of 0.5° to 2° on vertical walls is sufficient for smooth ejection.

Incorporate Functional Features

MIM optimization can be integrated with functional features to enhance performance and reduce assembly. These features may include snap fits, self-joining elements, or alignment tabs. Designing for multi-functionality, such as structural elements and aesthetic highlights.

Avoid Undercuts and Complex Mold Actions

Undercuts can be internal or external and are required for part functions. However, depending on the location and type, they increase the tooling costs and prolong the cycles. Redesigning undercuts into simple geometries and using side actions is recommended.

Disassembly Of The Full-Process Design Of Case Products Using MIM

The design process of case products occurs in several stages, from conceptualization to final assembly/disassembly. Disassembly is very important for product repair, maintenance, and recycling considerations. The following is a breakdown of the disassembly process and considerations in using metal injection molding to design case products.

Concept Development: This is the initial design analysis that identifies the functional requirements of the case products for design—for instance, decorative features in consumer products or lightweight enclosures for aerospace components.

Material Selection: The material used in the product is critical to ensuring ease of assembly and disassembly. A durable material will withstand without cracking or degrading when designing a case product that will require frequent disassembly.

Modular Design for Disassembly: This is the breakdown of products into modular components for easier production and simplified disassembly. Features such as self-locating pins, dovetail slots, and threaded connections are incorporated directly into the MIM parts.

Mold Design: When designing the mold, you have to consider aspects such as the case’s geometry, wall thickness, and functional requirements. Gates and vents must be placed strategically for material to flow easily and minimize defects like voids and weld lines.

Prototyping: Physical samples can validate the viability of the design. 3D printing creates prototypes for testing before the real product to ensure the final products meet the stipulated goals.

MIM Applications

Metal Injection Molding (MIM) has proved to be successful in numerous applications. Some of the key industries where MIM is adopted are:

1. Automotive Applications: Manufacture of lightweight, high-strength performance gears for transmission systems.
2. Medical Device Components: Manufacture of tiny and intricate surgical equipment for minimally invasive devices. The material used should have properties like biocompatibility and corrosion or sterilization resistance.
3. Aerospace Components: The industry demands high-strength and lightweight materials to lower the weight and maximize fuel efficiency. Engine fuel ejector nozzles have intricate geometries for optimal fuel mixing, which is crucial for engine efficiency.
4. Consumer Electronics: MIM’s ability to produce complex, high-precision products makes it irresistible in this industry. It’s used to create key products in devices like smartwatches, computer hardware, and smartphones.