A Comprehensive Guide to Surface Treatment Processes for Materials
Surface treatment is a way to make a layer on the outside of a material that has different properties from the material itself.
The purpose of surface treatment is to meet various requirements, such as corrosion resistance, wear resistance, decoration, or achieving other special product functions. To accomplish this, common surface treatment methods like mechanical grinding, chemical treatment, surface heat treatment, and surface coating are employed. These processes often involve steps such as cleaning, deburring, removing oil stains, and eliminating oxide scale from a workpiece. With this foundation, let’s explore specific surface treatment processes in more detail.
Some of the most widely used surface treatment processes are vacuum electroplating, electroplating, anodizing, electrolytic polishing, pad printing, zinc plating, powder coating, water transfer printing, screen printing, and electrophoresis.
01. Vacuum Electroplating
Vacuum electroplating is a physical deposition phenomenon. It involves injecting argon gas into a vacuum environment. Argon gas interacts with the target material, breaking it down into molecules that are adsorbed by conductive materials, forming a uniform, smooth, metallic-like surface layer.
Applicable Materials:
1. Many materials can be vacuum electroplated, including metals, soft and hard plastics, composite materials, ceramics, and glass. Aluminum is the most commonly used material for surface treatment, followed by silver and copper.
2. Natural materials are not suitable for vacuum electroplating because the moisture content of the natural materials can affect the vacuum environment.
Process Cost: Vacuum electroplating involves spraying, loading, unloading, and re-spraying the workpiece, resulting in relatively high labor costs. However, the cost also depends on the complexity and quantity of the workpiece.
Environmental Impact: Vacuum electroplating has minimal environmental impact, comparable to that of spraying.
02. Electropolishing
Electropolishing is an electrochemical process in which atoms in the workpiece are immersed in an electrolyte, converted into ions, and removed from the surface by the passage of an electric current, thereby removing fine burrs and increasing the workpiece’s surface gloss.
Electropolishing is an electrochemical process in which atoms in the workpiece are immersed in an electrolyte, converted into ions, and removed from the surface by the passage of an electric current, thereby removing fine burrs and increasing the workpiece’s surface gloss.
Applicable Materials:
1. Most metals can be electropolished, with stainless steel being the most commonly used (especially austenitic core-grade stainless steel).
2. Different materials cannot be electropolished simultaneously, and they cannot even be placed in the same electrolytic solvent.
Process Cost: The entire electropolishing process is largely automated, so labor costs are very low.
Environmental Impact: Electropolishing uses less hazardous chemicals, requires only a small amount of water, and is simple to operate. Furthermore, it can extend the properties of stainless steel, slowing down corrosion.
03. Pad Printing Process
This process allows text, graphics, and images to be printed onto irregularly shaped surfaces and is becoming an important specialty printing method.
Applicable Materials:Almost all materials can be printed using pad printing, except for materials softer than silicone pads, such as PTFE.
Process Costs: Low mold costs and low labor costs.
Environmental Impact: Because this process is limited to solvent-based inks (which contain harmful chemicals), it has a significant environmental impact.
04. Galvanizing Process
Galvanizing is a surface treatment technique that coats steel alloys with a zinc coating for aesthetic purposes and rust prevention. The zinc layer is an electrochemical protective layer that prevents metal corrosion. The main methods used are hot-dip galvanizing and electro-galvanizing.
Applicable Materials:Because galvanizing relies on metallurgical bonding, it is suitable only for surface treatment of steel and iron.
Process Cost: No mold costs, short cycle time/moderate labor costs, as the surface quality of the workpiece largely depends on the manual surface treatment before galvanizing.
Environmental Impact: Galvanizing extends the service life of steel parts by 40-100 years and effectively prevents rust and corrosion, thereby positively affecting environmental protection. Furthermore, galvanized workpieces can be returned to the galvanizing bath after their service life expires; the repeated use of liquid zinc does not generate chemical or physical waste.
05. Electroplating Process
Electroplating is a process that uses electrolysis to coat the surface of parts with a metal film. This prevents metal oxidation, improves wear resistance, conductivity, reflectivity, corrosion resistance, and enhances aesthetics. Many coins also have an electroplated outer layer.
Applicable Materials:
1. Most metals can be electroplated, but different metals have different levels of purity and electroplating efficiency. The most common are: tin, chromium, nickel, silver, gold, and rhodium.
2. ABS is the most commonly used plastic for electroplating.
3. Nickel should not be used for electroplating products that come into contact with skin because it is irritating and toxic.
Process Costs: No mold costs, but fixtures are needed to hold the parts in place. Time costs depend on temperature and metal type. Labor costs (medium to high) depend on the specific type of part being plated. For example, electroplating silverware and jewelry requires highly skilled workers due to the high requirements for appearance and durability.
Environmental impact: A large amount of toxic substances is used in the electroplating process, so professional diversion and extraction are required to minimize environmental impact.
06. Water Transfer Printing
This method uses water pressure to print colored patterns from transfer paper onto the surface of a three-dimensional product. With increasing demand for product packaging and surface decoration, water transfer printing is becoming increasingly widespread.
Applicable Materials:All hard materials are suitable for water transfer printing, and materials suitable for spray painting are also suitable. The most common applications are injection-molded parts and metal parts.
Process Cost: There are no mold costs, but multiple products require simultaneous water transfer using fixtures. The time cost is generally no more than 10 minutes per cycle.
Environmental Impact: Compared to product spray painting, water transfer printing makes fuller use of printing inks, reducing the possibility of waste leakage and material waste.
07. Screen Printing
Screen printing uses a squeegee to force ink through the mesh of the printing plates onto the substrate, creating an image identical to the original artwork. Screen printing equipment is simple, easy to operate, and the printing and plate-making processes are straightforward, inexpensive, and highly adaptable.
Common printed materials include: color oil paintings, posters, business cards, book covers, product labels, and printed textiles.
Applicable Materials: Almost all materials can be screen-printed, including paper, plastics, metals, ceramics, and glass.
Process Costs: Mold costs are low, but still depend on the number of colors, as each color requires a separate plate. Labor costs are relatively high, especially when multi-color printing is involved.
Environmental Impact: Light-colored screen printing inks have a smaller environmental impact; however, inks containing PVC and formaldehyde contain harmful chemicals and require timely recycling and disposal to prevent water pollution.
08. Anodizing
Primarily used for anodizing aluminum, this process utilizes electrochemical principles to create an Al2O3 (aluminum oxide) film on the surface of aluminum and aluminum alloys. This oxide film possesses unique properties, including protection, decoration, insulation, and wear resistance.
Applicable Materials: Aluminum, aluminum alloys, and other aluminum products.
Process Costs: Water and electricity consumption are substantial during production, especially in the anodizing process. The machine itself generates heat, requiring continuous cooling with circulating water; electricity consumption per ton is often around 1000 kWh.
Environmental Impact: Anodizing is not particularly energy efficient. Furthermore, in aluminum electrolysis, the anode effect can produce gases that damage the ozone layer.
09. Metal Brushing
Metal brushing is a surface treatment method that creates lines on a workpiece’s surface through grinding, creating a decorative effect. Based on the patterns after brushing, it can be divided into straight, random, wavy, and swirl brushing.
Applicable Materials: Almost all metal materials can be processed using metal brushing.
Process Cost: The process is simple, the equipment is basic, and material consumption is minimal, resulting in relatively low cost and high economic benefits.
Environmental Impact: Pure metal products, with no paint or any chemical substances on the surface, do not burn at 600 degrees Celsius, do not produce toxic gases, and meet fire safety and environmental protection requirements.
10. In-Mold Decoration
This method involves placing a pre-printed patterned film into a metal mold, then injecting molding resin into the mold to bond with the film, allowing the printed film and resin to form a unified whole and cure into the finished product.
Applicable Materials: Plastic surfaces
Process Cost: Only one mold is needed, reducing costs and labor time. Highly automated production simplifies the process, enabling a one-time injection molding method that simultaneously performs molding and decoration.
Environmental Impact: This technology is green and environmentally friendly, avoiding the pollution caused by traditional spray painting and electroplating.
