In highly competitive manufacturing industry, while intrinsic quality of injection molded products is important, their "surface engineering"—surface treatment process—often determines product's added value, market appeal, and lifespan. From giving a metallic texture and wood grain pattern to achieving vibrant colors and special functions, surface treatment technology is like dressing a product in a custom-made coat. This article will delve into ten mainstream surface treatment processes for injection molded products, providing you with a practical technical guide from principles and processes to application scenarios.

 

Water transfer printing technology excels at achieving full-coverage pattern transfer on irregular or even complex three-dimensional surfaces, making it preferred process for obtaining complex images such as wood grain, stone grain, and camouflage patterns.
Process Principle: Utilizing water pressure, pattern on a special transfer film dissolved in water is activated by an activator and precisely transferred onto product surface.
Core Process: Film Printing → Film Floating and Stretching → Pattern Activation → Water Pressure Transfer → Washing → Drying → Applying Protective Coating.
Application Areas: Automotive interior parts (e.g., gear shift, panels), military products (e.g., helmets, gun stocks), appliance casings, sporting equipment, etc.
Key Technical Points:
Water-based Transfer: Suitable for all-around pattern coverage of the entire product surface; film has high tension and can perfectly adhere to irregular curved surfaces.
Water-based Marker Transfer: More suitable for transferring partial logos, text, and other markings.
Subsequent application of a clear topcoat is crucial, directly determining durability, gloss, and feel of pattern.

Brushed metal finish is a process that uses physical friction to create directional or random textures on the surface of metal (mainly aluminum alloy). It can conceal minor scratches and enhance visual appeal and tactile feel of a product.
Process Principle: Using tools such as nylon wheels and stainless steel brushes, directional or non-directional mechanical friction is applied to metal surface to create continuous, silky textures.
Main Texture Types:
Straight-line Brushed Finish: The most common, with straight lines. It can be continuous or discontinuous, giving a simple and technological impression.
Random-line Brushed Finish: Surface has an irregular matte silky texture, with a delicate feel and elegant appearance.
Swirl Brushed Finish: Concentric circle texture, often used for circular decorative parts, such as signs and knobs.
Wave and Spiral Textures: Present wavy and spiral textures respectively, with stronger decorative effects.
Application Areas: Consumer electronics (phone frames, laptop casings), elevator decoration, kitchen appliances, door and window hardware, etc.

Anodizing is core surface treatment technology for light metals such as aluminum, magnesium, and titanium. It generates a hard, porous oxide film on substrate surface through an electrochemical method.
Process Principle: Using an aluminum part as anode, it is placed in an electrolyte and energized, resulting in formation of a dense Al₂O₃ oxide film on the surface.
Core Values:
Enhanced Protection: Significantly improves corrosion resistance and wear resistance.
Decorative Coloring: Porous structure of oxide film can adsorb dyes, achieving a wide range of colors (such as colorful aluminum alloy bodies of Apple products).
Improved Functionality: Can be used as an insulating layer or to increase surface hardness.
Coloring Methods:
Adsorption Coloring: Immersion in dye after oxidation, offering a wide range of color choices.
Electrolytic Coloring: Coloring through electrodeposition of metal ions, providing excellent weather resistance (commonly used in architectural aluminum profiles).
Self-Coloring: Specific alloys directly generate a colored oxide film during electrolysis.
Development Trends: New technologies such as plasma electrolytic oxidation can achieve ceramic coatings with even superior performance.

Diamond engraving utilizes CNC machine tools with diamond tools to precisely engrave intricate three-dimensional patterns or textures on the surface of aluminum plates.
Process Principle: Path, depth, and angle of diamond tool are controlled by a CNC program to cut designed pattern onto aluminum surface.
Advantages and Features:
High Precision and Consistency: Suitable for mass production of complex logos and delicate textures.
Strong Three-Dimensionality: Achieves strong visual depth through a combination of deep and shallow engraving.
Excellent Material Properties: Combines lightweight and high strength of aluminum with decorative properties of engraving.
Application Areas: High-end electronic product decorative panels, automotive interior parts (dashboards, door panel nameplates), architectural decorative panels, medals and trophies.

Electroplating is a process that deposits a layer of metal or alloy coating on the surface of a product through electrolysis to alter its surface properties.
Process Principle: Workpiece to be plated is used as cathode and immersed in an electrolyte containing target metal ions. When an electric current is applied, metal ions are reduced to metal atoms on cathode surface, forming a plating layer.
Plating Classification and Applications: Common Electroplating Types and Applications

Environmental Trends: Cyanide-free electroplating and the replacement of hexavalent chromium with trivalent chromium are becoming mainstream development direction in the industry.

 

Nickel plating serves as a decorative surface and is an indispensable intermediate layer and functional coating in electroplating systems.
Process Principle: Similar to electroplating, it primarily uses plating solutions with nickel sulfate, nickel chloride, or nickel sulfamate as main salt.
Key Applications:
Underlayer/Intermediate Layer: Plating nickel before chromium or gold plating improves the overall corrosion resistance and smoothness of coating.
Functional Coatings: Such as electroless nickel plating (nickel-phosphorus alloy), offering high hardness, wear resistance, and uniform corrosion resistance, used for precision parts.
Decorative Coatings: Combined with brighteners, they achieve a mirror-like finish.
Technological Development: Advanced technologies such as pulse nickel plating and nano-composite nickel plating allow for precise control of coating properties such as internal stress and hardness.

Chromium plating, with its extremely high gloss, hardness, and wear resistance, has become a classic final stage in protective and decorative electroplating.
Process Principle: Primarily uses chromic acid solution, electroplating is performed under strictly controlled temperature and current density.
Core Classifications:
Decorative Chromium: Extremely thin (approximately 0.5μm), mirror-like shine, requires a copper or nickel underlayer.
Hard Chromium: Thick (up to hundreds of micrometers), directly plated onto the substrate, used for dimensional repairs and extreme wear resistance applications.
Key Challenges and Trends:
Environmental Pressure: Traditional hexavalent chromium is highly toxic. Trivalent chromium plating technology has developed rapidly due to its greater environmental friendliness, but performance of hard chromium still mainly relies on improved hexavalent chromium processes.
Hydrogen Removal Treatment: Significant hydrogen evolution occurs during chromium plating. High-strength steel parts must undergo baking to remove hydrogen and prevent hydrogen embrittlement.

Spraying paint onto surface of plastic parts is the most direct method to change their color, texture, and provide protection.
Process Principle: Liquid or powdered paint is evenly applied to plastic surface using a spray gun, and after leveling and curing, a continuous coating film is formed.
Coating System Selection:
Based on Plastic Material: ABS and PC adhere easily; PP and PE, non-polar plastics, require pre-treatment with flame or corona discharge.
Based on Performance Requirements: UV coatings cure quickly and are wear-resistant; PU coatings offer good flexibility; epoxy coatings have strong adhesion.
Environmental Trends: Water-based coatings, high-solids coatings, and powder coatings are gradually replacing traditional solvent-based coatings to reduce VOC emissions.

Pad printing is specifically designed for printing precise patterns on small, irregularly shaped surfaces (such as buttons, toy eyes, and medical devices).
Process Principle: Ink is first filled into the recessed image on a steel plate, then picked up by a silicone tip and pressed onto product surface like a stamp.
Core Advantages:
Strong Adaptability to Curved Surfaces: Silicone tip is deformable, conforming to various irregularly shaped surfaces.
High Precision: Enables extremely fine lines and precise registration.
Application Areas: Electronic buttons (character printing), toy figurines (finishing touches), cosmetic containers (logos), sporting goods (signage).
Automation Trends: Fully automatic CNC pad printing machines combined with vision positioning systems significantly improve production efficiency and consistency.

Heat transfer is suitable for transferring complex multi-color patterns onto flat or regular curved surfaces in one go, offering high efficiency and excellent color reproduction.
Process Principle: A special transfer film with printed pattern is placed on workpiece. Heated and pressurized by hot rollers, dye in pattern layer sublimates or melts and adheres to workpiece surface.
Main Types:
Sublimation Transfer: Dye sublimates and penetrates coating or fabric fibers, resulting in a smooth feel and high durability. Commonly used for synthetic fabrics and coated metal sheets.
Melt Transfer: Adhesive is applied through a hot melt adhesive layer. Suitable for various materials, such as patterned labels and some plastic decorations.
Application Areas: Advertising signs, personalized water cups, custom phone cases, and wood grain decoration for some home appliance panels.

 

Faced with a variety of surface treatment processes, how to choose? The key lies in prioritizing product needs:
For decorative appearance and complex patterns: Water transfer printing (3D curved surfaces), heat transfer printing (flat multi-color), or pad printing (small area precision) are preferred.
For metallic texture and wear resistance: Anodizing (aluminum-magnesium alloy), electroplating (chrome/nickel plating), or brushed metal are classic choices.
For special functions: For example, choose hard chrome or electroless nickel plating for high hardness; consider silver/copper plating for conductive shielding; consider powder coating or fluorocarbon coating for outdoor weather resistance.
Considering cost and environmental protection: Spraying processes offer flexibility, and water-based systems are more environmentally friendly; anodizing has more controllable pollution compared to electroplating.
In actual projects, multiple processes are often combined (e.g., anodizing followed by laser engraving, or electroplating followed by clear coat) to achieve the best results. Understanding core capabilities and limitations of each process is foundation for making the best decision.