What do you know about sheet metal materials and surface treatment processes?

1. Commonly Used Sheet Metal Materials

The commonly used sheet metal materials are as follows:

1. Cold-rolled sheet, grade SPCC. The most basic material for sheet metal, widely used, it serves as a substrate for other materials and is relatively prone to rust.

2. Electro-galvanized sheet, grade SECC, widely used, made from cold sheet as a substrate through electroplating, resulting from a chemical reaction.

3. Hot-dip galvanized sheet, grade SGCC, widely used, made from cold sheet as a substrate heated in molten zinc solution to form a zinc layer, the process is a physical change.

4. Hot-rolled sheet, grade SPHC, a low-end material that is prone to rust, generally thicker than 3mm. Usually used for hinge-type products.

5. SD steel plate, a material with excellent rust and corrosion resistance. The installation bracket for outdoor air conditioning units uses this material.

6. Stainless steel and stainless iron, typical grades SUS304 and SUS430, are alloy materials. They have good rust and corrosion resistance and a beautiful surface. Usually used for appearance parts, such as refrigerator door shells and gas stove tops.

7. Aluminum-zinc coated sheet, seems to belong to a type of electro-galvanized sheet, but has an additional aluminum metal element, making the surface smoother and more beautiful, commonly used for appearance parts, such as the back steel plate of a certain model refrigerator.

8. Aluminum sheet or aluminum alloy sheet, also known as aluminum foil. Low density and corrosion-resistant, typical products include refrigerator drip trays and evaporator fins.

2. Price of Sheet Metal Materials

Price comparison:

Under the same thickness:

Hot-rolled sheet < Cold-rolled sheet < Hot-dip galvanized sheet < Electro-galvanized sheet

For the same material, the thicker the thickness, the lower the price.

In the cost of sheet metal parts, material costs account for about 60-70%, sometimes even higher.

Therefore, it is necessary to design a reasonable structure and improve material utilization. If the material utilization is low, it is possible for the product price to be lower than the scrap price.

3. Purpose of Surface Treatment of Steel Plates

Since some sheet metal materials do not possess rust and corrosion resistance, surface treatment is required to achieve this purpose.

The main purposes of surface treatment are twofold:

1. To improve the service life of products in harsh environments.

This refers to: rust and corrosion resistance.

2. To achieve the desired surface effect or function.

This refers to: meeting the appearance requirements of the product.

4. Common Surface Treatment Processes for Steel Plates

1. Electro-galvanizing: This is the process of using electrolysis to form a uniform, dense, and well-bonded metal or alloy deposition layer on the surface of the workpiece.

Compared to other metals, zinc is relatively cheap and easy to coat, belonging to low-value corrosion-resistant electroplating layers. It is widely used to protect steel parts, especially against atmospheric corrosion, and for decoration. Coating technologies include barrel plating (or rack plating), roll plating (suitable for small parts), automatic plating, and continuous plating (suitable for wire and strip materials).

In China, electroplating solutions can be classified into four major categories:

Cyanide zinc plating

Due to the toxicity of (CN), environmental protection has imposed strict restrictions on the use of cyanide in electro-galvanizing, continuously promoting the development of systems that reduce or replace cyanide in electro-galvanizing solutions, requiring the use of low cyanide (micro-cyanide) electroplating solutions. Products plated using this process have good quality, especially colored plating, which maintains good color after passivation.

Zinc salt plating

This process evolved from cyanide zinc plating. Currently, there are two major factions in China: a) the "DPE" series from Wuhan Material Protection Institute; b) the "DE" series from Guangdian Institute. Both belong to alkaline additive zinc salt plating; the pH value is 12.5~13. Using this process, the coating lattice structure is columnar, with good corrosion resistance, suitable for colored galvanizing.

Chloride zinc plating

This process is widely used in the electroplating industry, accounting for as much as 40%. After passivation (blue-white), it can zinc replace chrome (comparable to chrome plating), especially after applying water-soluble varnish, it is difficult for outsiders to distinguish between galvanized and chrome-plated. This process is suitable for white passivation (blue-white, silver-white).

Sulfate zinc plating

This process is suitable for continuous plating (wire, strip, simple, large parts). It is cost-effective.

The electro-galvanizing industry is considered a polluting industry. Although widely used and cost-effective, electro-galvanized products do not meet ROHS standards, so it is advisable to avoid using electroplated products in product design.

2. Chrome plating

The process principle is the same as that of galvanizing.

Compared to traditional electro-galvanizing: chrome coatings have extremely strong corrosion resistance, 7-10 times that of galvanizing, and have a beautiful surface, but the cost is higher.

The most advanced chrome plating process is called DACROMET, which is a revolutionary high-tech metal surface treatment process with no pollution throughout.

2. Powder coating: The powder is polarized and uniformly adheres to the surface of the product with the opposite polarity under the action of the electric field. This is a physical change.

Principle of powder coating process:

Electrostatic powder coating: Mainly polarizes the coating (powder) through electrodes, and then charges the object to be coated with the opposite charge, allowing the powder to uniformly adhere to the surface of the object under the action of the electric field force.

Characteristics of the powder spraying process:

Powder electrostatic spraying does not cause atmospheric pollution, the powder can be recycled to reduce material consumption costs, the coating performance is excellent, resistant to acid, alkali, and salt corrosion, and has high adhesion.

3. Electrophoresis: Under the action of the positive and negative electrodes, charged coating ions move to the product surface and deposit to form a coating. This is a chemical change.

Principle of the electrophoresis process:

Under the action of voltage applied to the positive and negative electrodes, charged coating ions move to the cathode and form insoluble substances with the alkaline action generated on the cathode surface, depositing on the workpiece surface.

Characteristics of the electrophoresis surface treatment process:

Electrophoretic paint films have the advantages of being full, uniform, flat, and smooth. The hardness, adhesion, corrosion resistance, impact performance, and permeability of electrophoretic paint films are significantly better than other coating processes.

Common electrophoretic colors are mostly black, but other colors are also available.

4. Dipping plastic: The product is heated in molten material, and the heated metal combines with the surrounding molten material to form a surface material of a certain thickness. This is a physical change.

Principle of the dipping plastic process:

Metal dipping plastic is a new type of technology treatment process for corrosion protection of metal surfaces. Dipping plastic is a heating process involving preheating the metal, dipping in material, and curing. During the dipping process, the heated metal adheres to the surrounding material; the hotter the metal, the longer the dipping time, and the thicker the material.

Characteristics of the dipping plastic process:

Widely used: It has rich colors, good protective properties, excellent cold resistance and thermal insulation, as well as acid and alkali resistance.