Common Surface Treatments and Their Effects in Metal Injection Molding Process

Common Surface Treatments and Their Effects in Metal Injection Molding Process

Introduction

Metal Injection Molding (MIM), as an advanced near-net-shape forming technology, can mix metal powders with binders and then inject them into molds. After processes such as debinding and sintering, high-precision and complex-shaped metal parts can be obtained. However, the surface condition of the formed parts often cannot directly meet the requirements of practical applications and requires appropriate surface treatment. Surface treatment can not only improve the appearance of parts but also significantly enhance their corrosion resistance, wear resistance, hardness and other properties, thus broadening the application fields of metal injection molded parts.

Common Surface Treatment Methods

Mechanical Polishing

Mechanical polishing is to use tools such as polishing wheels and sandpapers to grind and polish the surface of parts, so as to remove surface scratches and defects, reduce surface roughness and improve surface gloss. In the surface treatment of metal injection molded parts, mechanical polishing is a relatively basic and commonly used method. For some decorative parts with high requirements for appearance, such as jewelry accessories and electronic product housings, mechanical polishing can make their surfaces present a mirror-like luster, enhancing the aesthetic appearance of products. Its operation is relatively simple and the cost is low, but it requires high technical skills from operators. Moreover, it is difficult to treat all surfaces of complex-shaped parts, and uneven polishing is likely to occur.

Application Case: In the manufacturing of high-end watches of a well-known brand, the metal injection molded watch cases and bracelet parts are treated by mechanical polishing. After fine mechanical polishing, the surfaces of the watch cases and bracelets are as bright as mirrors and can clearly reflect the surrounding objects, significantly enhancing the overall luxurious texture and refinement of the watches, meeting the strict aesthetic requirements of consumers for the appearance of high-end watches.

Electroless Plating

Electroless plating is a process in which metal ions in the plating solution are reduced and deposited on the surface of parts to form a coating without applying an external current. In the metal injection molding process, electroless nickel plating is a relatively common method. The electroless nickel coating has good corrosion resistance, wear resistance and uniformity. Regardless of how complex the shape of the parts is, a uniform-thickness coating can be obtained on its surface. For some parts used in humid and corrosive environments, such as automotive parts and medical devices, electroless nickel plating can effectively extend the service life of parts. In addition, the electroless nickel coating also has a certain hardness and low friction coefficient, which can improve the surface properties of parts and enhance their working efficiency.

Application Case: The fuel injector parts in an automotive engine are manufactured by the metal injection molding process and then treated with electroless nickel plating. In the harsh environment of high temperature, high pressure and fuel corrosion in the automotive engine, the fuel injectors treated with electroless nickel plating show excellent corrosion resistance. Their service life is extended by about 30% compared with untreated parts. At the same time, the low friction coefficient of the electroless nickel coating makes the fuel injection process more smooth, optimizing the fuel injection effect and improving the overall performance of the engine.

Electroplating

Electroplating is a surface treatment method that deposits a layer of metal or alloy on the surface of parts using the principle of electrolysis. Common electroplated metals include chromium, zinc, copper, etc. Chromium plating can endow parts with good decorative and corrosion-resistant properties. Its coating is bright and hard, which can effectively improve the surface hardness and wear resistance of parts and is often used in the surface treatment of hardware tools, sanitary ware and other products. Zinc plating is mainly used to prevent the corrosion of steel parts. The zinc layer can form a dense oxide film in the atmospheric environment, providing good protection for the base metal and is widely used in industries such as construction, automobiles and household appliances. The electroplating process is mature and there are various types of coatings. Appropriate electroplated metals and process parameters can be selected according to different usage requirements. However, certain environmental pollution problems will occur during the electroplating process and strict wastewater treatment is required.

Application Case: In the construction field, the metal injection molded connecting parts of staircase handrails are often treated with zinc plating. These connecting parts are installed in outdoor staircases and other locations and are exposed to the natural environment for a long time. The zinc plating layer effectively prevents the corrosion of the connecting parts. Even in areas with frequent rain, after years of use, the surfaces of the connecting parts still remain in good condition without obvious rust marks, ensuring the structural stability and safety of the staircase handrails. In terms of sanitary products, faucets adopt the chromium electroplating process. After chromium plating, the surface of the faucet is bright and beautiful, which not only improves the decorative property of the product, but also due to the high hardness and corrosion resistance of the chromium plating layer, the faucet is not easy to be worn and corroded during daily frequent use, extending the service life of the product.

Anodizing

Anodizing is mainly aimed at metal materials such as aluminum and aluminum alloys. In a specific electrolyte solution, the part is used as the anode, and an oxide film is formed on its surface through electrolysis. The anodized film has high hardness, good insulation and corrosion resistance. At the same time, rich colors can be obtained through subsequent treatments such as dyeing, enhancing the decorative property of parts. In metal injection molded aluminum parts, anodizing is often used in fields such as electronic device housings and aerospace parts, which can not only improve the protection performance of parts but also meet the diverse needs of products for appearance colors. The thickness and performance of the anodized film can be adjusted by controlling process parameters such as electrolysis time and current density.

Application Case: The metal injection molded housing of a new tablet computer is treated with anodizing. After anodizing, a hard and well-insulated oxide film is formed on the surface of the housing, effectively preventing scratches on the housing caused by friction during daily use and enhancing the electromagnetic shielding effect on the internal electronic components. In addition, through dyeing treatment, the housing of this tablet computer presents a unique deep space gray color, which not only conforms to the current simple and fashionable design trend of electronic products but also enhances the product's recognition and texture, meeting the dual pursuit of consumers for the appearance and performance of electronic products. In the aerospace field, some aluminum alloy parts of aircraft engines are anodized after being formed by metal injection molding. The high hardness and good corrosion resistance of the anodized film enable these parts to operate stably in the complex high-altitude environment, ensuring the safety and reliability of aircraft flight.

Painting

Painting is a surface treatment method in which coatings are sprayed onto the surface of parts through tools such as spray guns to form a protective film. For metal injection molded parts, painting can provide good anti-corrosion and decorative effects. There are a wide variety of coatings, such as epoxy resin coatings and polyurethane coatings, which can be selected according to different usage environments and decorative requirements. The painting process is simple and the cost is relatively low, and it is suitable for parts of various shapes and sizes. In some products with high requirements for appearance and certain protection performance, such as furniture, toys and electrical appliance housings, painting is widely used. By selecting different colors and texture coatings, diverse visual effects can also be presented on the surface of parts.

Application Case: In the manufacturing of children's toys, the bodies of many metal injection molded toy cars are treated with painting. Environmentally friendly colored polyurethane coatings are selected, which not only endow the toy cars with bright and rich colors to attract the attention of children, but also the protective film formed by painting can effectively prevent the metal of the car body from rusting. Even when children play frequently and come into contact with sweat, etc., the appearance and performance of the toy cars can still be maintained well. In the furniture industry, the metal injection molded chair and table legs adopt the painting process. By spraying epoxy resin coatings with wood grain textures, the metal chair and table legs present an imitation wood appearance effect, which not only meets the overall design style requirements of furniture but also takes advantage of the firmness of metal materials. At the same time, the painting layer also plays a role in anti-corrosion and wear resistance for the metal chair and table legs.

Evaluation of Surface Treatment Effects

Appearance Effect

After surface treatment, the appearance of parts will change significantly. The surface of the parts after mechanical polishing is smooth like a mirror with high gloss, which can present the metallic texture; the surfaces of the parts after electroless plating and electroplating have uniform and bright coatings, and different coatings have different colors, such as the bright white of the chromium plating layer and the silver white of the zinc plating layer, which not only increase the aesthetic appearance but also enhance the product grade; the anodized aluminum parts can obtain rich colors, from common silver and black to bright colors, meeting the personalized appearance needs of different users; the parts after painting can present various color and texture effects according to the selection of coatings, such as high gloss, matte and sand grain, providing more creative space for product design.

Corrosion Resistance

Corrosion resistance is one of the important indicators for measuring the surface treatment effect. The electroless nickel coating, electroplating layer and anodized film can all improve the corrosion resistance of parts to a certain extent. The corrosion resistance of parts can be evaluated by methods such as salt spray tests. For example, the parts treated with electroless nickel plating can withstand corrosion for a long time in salt spray tests without obvious rusting, and their corrosion resistance is significantly improved compared with untreated parts. The zinc plating layer effectively prevents the corrosion of the base metal through its sacrificial anode protection effect and has good protection performance in harsh atmospheric environments. The anodized film can prevent corrosion media such as oxygen and moisture from contacting the base metal due to its dense structure, thus improving the corrosion resistance of parts.

Wear Resistance

For some parts working in a friction environment, wear resistance is of great importance. Although mechanical polishing can reduce surface roughness, its effect on improving wear resistance is relatively limited. The electroless nickel coating and chromium plating layer have high hardness and can significantly improve the wear resistance of the part surface. In practical applications, the wear rate of parts treated with electroless nickel plating or chromium plating is significantly reduced and their service life is extended. The anodized film also has a certain hardness and can play a protective role in some light-load friction occasions. In addition, by selecting appropriate coatings and coating processes, painting can also improve the wear resistance of the part surface to a certain extent.

Hardness

The hardness of parts usually changes after surface treatment. The electroless nickel coating, chromium plating layer, etc. can all increase the surface hardness of parts. For example, the hardness of the electroless nickel coating is generally between 500 - 1000HV, which is a significant increase compared with the base metal, making the parts less likely to deform and wear when subjected to external forces. The anodized film also has a relatively high hardness, which can enhance the surface hardness of aluminum and aluminum alloy parts and improve their scratch resistance. While painting has a relatively small effect on improving the hardness of parts, it can buffer the impact of the outside world on the part surface to a certain extent and protect the base metal.

Conclusion

The combination of the metal injection molding process and appropriate surface treatment methods can significantly improve the performance and appearance quality of metal parts. Surface treatment methods such as mechanical polishing, electroless plating, electroplating, anodizing and painting have their own characteristics and application scopes. Through the rational selection and application of these methods, the diverse needs of different fields for metal injection molded parts can be met. When evaluating the surface treatment effect, multiple factors such as appearance effect, corrosion resistance, wear resistance and hardness need to be comprehensively considered to ensure that the surface-treated parts can exert the best performance in actual use and provide strong support for the development of related industries. With the continuous progress of science and technology, the metal injection molding process and surface treatment technologies will also continue to innovate and improve, bringing more possibilities for the manufacturing and application of metal parts.