Anti-rust coating technology for construction machinery

Construction machinery refers to the machinery that serves the construction of various basic projects such as urban and rural construction, railways, highways, ports and docks, farmland water conservancy, power, metallurgy, mining, and sea and air bases. All the necessary operation machinery and equipment in the comprehensive mechanized construction of earthwork, stonework, concrete work and various building installation projects are generally referred to as construction machinery. In China, construction machinery mainly includes ten categories: excavating machinery, earth-moving and transportation machinery, engineering lifting machinery, road machinery, compaction machinery, pile driving machinery, reinforced concrete machinery, rock drilling machinery, forklifts, and special-purpose construction machinery. Its classification in other countries is largely the same. For instance, in the United States, construction machinery is called building machinery; in Japan, it is known as construction machinery; and in Russia, it is referred to as construction and road-building machinery. Common and specific construction machinery includes: hydraulic excavators, crawler bulldozers, wheel loaders, rollers, motor graders, pavers, forklifts and truck cranes, etc.

As is well known, construction machinery is an outdoor working machine that is constantly exposed to harsh environments such as the outdoor atmosphere (including Marine atmosphere, industrial atmosphere, urban atmosphere and rural atmosphere) and different water qualities (seawater, river water). At all times, it is subject to varying degrees of corrosion from different regions and environments. This has led to various "chronic diseases" and "frequently-occurring diseases" in construction machinery. Painting of construction machinery is an important link in the manufacturing process of construction machinery products. The quality of anti-rust and anti-corrosion coating is one of the important aspects of the overall quality of the product. The appearance quality of a product not only reflects its protective and decorative performance, but is also an important factor constituting the product's value.

1 Coating process

The anti-rust coating process for construction machinery can generally be divided into surface treatment process before coating and anti-rust primer coating process.

1.1 Surface treatment

Surface treatment is one of the important processes in anti-rust coating. The quality of anti-rust coating for construction machinery largely depends on the quality of surface treatment methods.

According to Imperial Chemical Company of the UK, the lifespan of coatings is restricted by three factors: surface treatment, accounting for 60%; Coating construction, accounting for 25%; The quality of the coating itself accounts for 15%.

Surface treatment methods for different components in the construction machinery industry.

Mechanical cleaning can effectively remove rust, welding slag and oxide scale from workpieces, eliminate welding stress, increase the adhesion between the anti-rust coating film and the metal substrate, and thus greatly improve the anti-rust quality of construction machinery parts. The mechanical cleaning standard requires reaching Sa2.5 level of ISO8501-1:1988. The surface roughness should reach one third of the thickness of the anti-rust coating. The steel shot used for shot blasting and shot peening shall meet the requirements of GB6484.

The surface treatment of thin plate stamping parts is generally called chemical surface treatment. The process flow is

Pre-degreasing → degreasing → hot water wash → cold water wash → acid wash → cold water wash → neutralization → cold water wash → surface adjustment → Phosphating → cold water wash → hot water wash → pure water wash → drying

The above-mentioned process can also be appropriately adjusted according to the oil and rust conditions of the thin plate stamping parts, or the acid washing process can be omitted, or the pre-degreasing process can be omitted. Degreasing and phosphating are key processes in the chemical treatment process, and these two processes directly affect the quality of the chemical treatment of the workpiece and the quality of the anti-rust coating. The process parameters and related auxiliary equipment are also factors that cannot be ignored in affecting the quality of surface treatment.

1.2 Coating process

Due to the wide range, diverse specifications, heavy overall weight and large components of construction machinery, spraying is generally adopted for coating. The spraying tools include air spray guns, high-pressure airless spray guns, air-assisted spray guns and portable electrostatic spray guns. The spraying efficiency of air spray guns is low (about 30%), and high-pressure airless spray guns waste paint. The common feature of both is that they cause relatively serious environmental pollution. Therefore, they have been and are being replaced by air-assisted spray guns and portable electrostatic spray guns. For instance, the world's leading construction machinery company, Caterpillar (CAT) of the United States, uses air-assisted spray guns for spraying and portable electrostatic spray guns for thin plate covers such as engine hoods. The coating equipment used in construction machinery generally adopts the relatively advanced water-jet spray booth. For small and medium-sized components, water curtain spray booths or pumpless spray booths can also be used. The former has advanced performance, while the latter is economical, convenient and practical. Due to the heavy weight and large heat capacity of the entire machine and its components of construction machinery, the drying of its anti-rust coating is generally carried out by using a uniform hot air convection drying method. Heat sources can be adapted to local conditions, such as steam, electricity, light diesel oil, natural gas and liquefied petroleum gas, etc.

2. Main coating equipment

2.1 Through-type shot blasting chamber

The through-type shot blasting chamber is composed of the cleaning chamber body, shot blasting machine, shot circulation system, ventilation and dust removal system, bottom foundation and electrical control system. Non-standard designs of varying sizes can be carried out according to different specifications of workpieces, and standardized equipment can also be selected.

The through-type shot blasting chamber is suitable for surface cleaning and strengthening of structural parts and forgings and castings of construction machinery, to achieve a certain roughness and smoothness of the surface, increase the adhesion of the coating film, and improve the anti-corrosion effect of the workpiece. When the workpiece enters the shot blasting chamber, the shot blasting machine driven by the motor throws out high-speed shot by centrifugal force, which strongly impacts the rust and oxide scale on the surface of the workpiece. After the workpiece moves at a constant speed or rotates several times, it is cleaned thoroughly, and the surface quality can reach the Sa2 to SA2.5 grade standard.

2.2 Water jet spray booth

The water spin spray booth is also known as the hydraulic spin spray booth. This booth adopts a gas laminar flow suppression method to prevent the diffusion of paint mist and filters the paint mist with hydraulic rotary tubes. It is currently one of the more technologically complete spray booths.

The water-jet spray booth is composed of the booth body, electric door, paint mist filtering device, air conditioning supply air device and exhaust air device. The lighting fixtures are installed on both sides of the room, with an illuminance of over 500 Lux, and can be controlled in groups. Flammable gas concentration detection and alarm devices are installed indoors. The paint mist filtration device consists of a washing plate, a hydraulic spinning device, an underground water tank and an exhaust device, with a paint mist removal rate of over 98%. The air conditioning supply air device is the main component that makes the gas form a laminar flow state. The air sent into the air supply chamber is filtered and heated, and then evenly distributed into the room through the air supply ceiling, making the indoor air velocity uniform. When there are no workpieces, the vertical air velocity can reach 0.35 to 0.4 m · s ⁻ ¹. The exhaust device uses a fan to filter the paint mix-containing gas in the room through a hydraulic spinning tube and then discharge it between the water tank and the washing plate.

2.3 Hot air drying room

The drying method is hot air circulation convection. Its greatest advantage is uniform baking, especially suitable for construction machinery parts with large heat capacity. The heating heat sources include steam, electricity, light diesel oil, natural gas, liquefied petroleum gas, etc. Where conditions permit, liquefied petroleum gas should be given priority. This is a clean and efficient energy source. Its greatest advantage is that the production and operation cost is only 35% to 40% of the electricity cost.

2.4 Conveying System

For the components (structural parts) of construction machinery, the use of self-propelled electric hoists is an effective choice.

The self-operated electric hoist conveying system consists of the portal frame column, load-bearing track, sliding guide rail and collector, cargo car, electric hoist, and a central control cabinet controlled by PC.

This system integrates the essence of four major technologies: mechanics, electronics, computers and automatic control, and concentrates the four major logistics links of warehousing, transportation, loading and unloading, and process operation. Not only can workpieces be continuously or step by step conveyed according to the preset program, but it is particularly convenient that in places with manual operation, such as the upper and lower parts, and the spray painting room, automatic or manual free lifting can be achieved.

3 Coatings

Coatings for construction machinery should possess the following characteristics:

It has extremely low permeability to water, oxygen and corrosive media. ② It has strong and long-lasting adhesion to the substrate. ③ It has good anti-rust and decorative properties; ④ It has special properties, such as insulation, electrical conductivity, heat insulation, etc. ⑤ It has an appropriate coating supporting system.

With the need for industrial energy conservation and environmental protection, in addition to ensuring the above-mentioned performance, the coating should also have the properties of quick drying at low temperatures, environmental protection and energy conservation. Such coatings are particularly suitable for the coating of construction machinery parts and complete machines with small production batches.

At present, the coatings generally used in domestic construction machinery are epoxy ester-based primers and polyurethane-based topcoats. The former has good anti-rust performance, while the latter has better decorative performance.

4. Environmental protection and labor safety and hygiene

4.1 Environmental Protection

Environmental protection is a very important issue in the coating design of construction machinery. The basis for this is

Gb8978-7996 "Integrated Wastewater Discharge Standard"

② GB16297-1996 "Integrated Emission Standard of Air Pollutants"

③ GB12348-90 "Noise Standard for Industrial Enterprises"

④ JBJ16-2000 "Code for Environmental Protection Design of Machinery Industry", etc.

We must resolutely implement the relevant design and construction measures for preventing pollution and other public nuisances, and enforce the "three simultaneous" system, which stipulates that the design, construction and commissioning of the main project should be carried out simultaneously.

For instance, in the anti-rust coating project of construction machinery, the regularly discharged coating wastewater and the acid and alkali wastewater discharged from chemical treatment are collected and regulated, and then treated by a dedicated wastewater treatment unit to meet the national standards of pH = 6 ~ 9, SS = 70 mg/L, and COD = 100 mg · L ⁻ ¹. Petroleum products can only be discharged at a concentration of less than 5 mg/L.

In addition, certain measures should also be taken to treat waste gas, waste residue, noise, etc., to meet the standards.

4.2 Labor safety and hygiene

In terms of labor safety and hygiene, the national standards that serve as the basis are:

Gbj16-87 (2001 Edition) "Code for Fire Protection Design of Buildings"

② JBJ18-2000 "Code for Design of Occupational Safety and Health in Machinery Industry"

③ GB6514-1995 "Safety Regulations for Coating Operations - Safety of Painting Processes and Ventilation Purification"

④ GB14443-1993 "Safety Regulations for Coating Operations - Safety Technical Provisions for Coating Drying Rooms"

⑤ GB14444-1993 "Safety Regulations for Coating Operations - Safety Technical Provisions for Spray Booths", etc.

Strengthen the management of labor safety and hygiene in the workshop, and continuously carry out education and training on safety knowledge and labor hygiene. Strictly implement all national and enterprise regulations on labor safety and industrial hygiene.