With increasing environmental awareness, higher demands are being placed on the coating of construction machinery. Powder coatings, with their low pollution, high coating efficiency, and high economic benefits, have been widely adopted in the construction machinery field.
1. Characteristics of Powder Coating
The successful implementation of powder coating technology represents a major breakthrough in coating processes for the construction machinery industry. The shift from traditional solvent-based coatings to powder coatings has resulted in significant improvements in production costs, application processes, environmental protection, and automation.
2. Powder coating technology has the following advantages
2.1 Powder coatings do not contain organic solvents, do not emit harmful gases during production, prevent air pollution, and improve working conditions for workers. They are considered environmentally friendly coatings, while traditional paints contain over 40% solvents and large amounts of toxic substances such as toluene and xylene.
2.2 Powder coatings are recyclable, with a utilization rate of over 95%, even reaching 99%, while the utilization rate of paints is generally only 35%–50%.
2.3 A thicker coating (50–300 μm) can be easily obtained in a single application, thus reducing construction steps and saving energy. 2.4 The absence of solvent evaporation reduces the risk of fire.
2.5 Powder coatings possess excellent mechanical and chemical properties.
2.6 Powder coating has a higher yield rate than spray painting. Generally, the scrap rate for spray-painted parts is around 10%, while for powder-coated parts it is around 1%, resulting in a lower rework rate and reduced costs.
2.7 Application is simple, requiring less skilled operation and is easy to master. Thick coatings do not suffer from sagging, facilitating automated production lines.
2.8 As powder coatings are solid, they are easy to store and manage.
3. Requirements for White Parts in the Cab After Powder Coating
Constraints in the production of engineering machinery products, such as limitations in steel, welding processes, and molds, result in numerous surface defects in finished white parts, including weld defects, molding defects, and sanding defects.
Therefore, most companies require extensive puttying, drying, sanding, and topcoat application after electrophoretic primer to ensure optimal appearance.
However, powder coatings require prolonged baking at 180–200°C during film formation, thus necessitating a base coat with high-temperature baking resistance.
Furthermore, due to the electrostatic adsorption principle used in powder coating, the base coat must be conductive. Therefore, both the putty and sealant used to correct defects in finished unfinished parts must possess conductivity and high-temperature resistance.
To address the issue of unfinished parts defects, extensive process experiments were conducted without altering the preceding processes. Various domestic and international brands of putty and sealant were tested, and the results were as follows:
3.1 Putty is unsuitable for large-area and thick application, as it easily leads to defects such as blistering, pinholes, and orange peel texture.
3.2 The mixing ratio of putty requires precise control; otherwise, pinholes are likely to appear in the powder coating.
3.3 Sealant can be used to correct weld defects, but a white base coat is necessary; otherwise, color differences in the powder coating are likely.
3.4 Sealant cannot be used as a material to correct defects in sheet metal.
Based on the above test results, when implementing powder coating, the first step is to address surface defects in the workpieces, starting with the forming and welding processes to eliminate surface defects.
Furthermore, the sealant at weld breaks should be made of high-temperature resistant materials that can cure simultaneously with the powder without causing quality defects.
4. Powder Coating Process Design
4.1 Layout of Electrostatic Powder Spray Guns
Most engineering machinery products are flat or tubular, making high-voltage electrostatic powder coating suitable. The characteristics of electrostatic powder coating technology are that workpieces can be coated at room temperature, and the powder utilization rate is high, reaching over 95%;
The coating film is thin, uniform, and smooth, without sagging. Even on sharp edges and rough surfaces, a continuous, flat, and smooth coating film can be formed. Based on these characteristics, powder coating can be easily automated and continuously operated.
Therefore, based on the production capacity design, the working cycle should be minimized, and reciprocating machines and fixed spray guns can be used to achieve automated spraying operations.
4.2 Powder Coating Booth Design
The design of the powder coating booth mainly focuses on two aspects: First, the size of the chamber openings, including the workpiece inlet/outlet, the reciprocating machine extension/retraction port, and the chain passage. The openings should be as small as possible to reduce the power consumption of the recovery system and lower procurement and operating costs.
Second, the selection of wall panel materials. Different wall panel materials have varying powder adsorption properties. To facilitate cleaning the booth after each shift, materials with poor powder adsorption should be selected.
Of course, the operating conditions must also be considered. Generally, it is recommended to use stainless steel for the bottom and PP or PVC panels for the sides and ceiling.
Generally, engineering machinery products consist of two colors, with balanced production volumes, and cannot be produced in batches by alternating colors. Therefore, two powder coating booths can be designed for simultaneous production, avoiding color-changing operations.
4.3 Recovery System Design
Powder coating equipment recovery systems are generally divided into two types: single-stage filter cartridge recovery devices and large cyclone filter cartridge two-stage recovery devices. The exterior coating colors of engineering machinery products are generally relatively simple, mostly consisting of two colors.
Therefore, selecting a single-stage cartridge-type recovery device is beneficial for reducing investment and operating costs, and also reduces the amount of powder chamber cleaning required after each shift. A single-stage cartridge-type recovery device consists of the following components: powder feeding device, rotary screen (or vibrating screen), powder supply tank, filter cartridge, blower, and final filter cartridge.
Because a stable airflow is smoothly blown into the powder chamber from various openings around its perimeter, the oversprayed powder is ultimately carried into the recovery assembly, preventing powder spillage and improving powder recovery efficiency.
Furthermore, the powder concentration inside the powder chamber must be below the powder's explosive limit. Therefore, the actual airflow design of the powder chamber is as follows.
1) Calculation formula for powder coating chamber exhaust volume:
Q1=3600K·S·V
Where: Q1 is the powder coating chamber exhaust volume, m3/h; S is the total area of all openings in the powder coating chamber, m2; K is the powder coating chamber efficiency coefficient, taken as 1.8～3.6; V is the air velocity in the powder coating chamber after deducting airflow velocity, m/s, with the air intake velocity at the opening being 0.4～0.7m/s.
2) Calculation formula for recovery device exhaust volume:
Q2=D/n·p
Where: Q2 is the recovery device exhaust volume, m3/h; D is the total powder ejection rate, g/min; n is the powder coating efficiency; p is the lower explosive limit concentration of the powder, 30g/m3.
Therefore, the actual air volume of the powder coating chamber Q≥Q1. Q2. The air velocity at the opening of the automatic powder coating chamber is higher than that of the manual powder coating chamber to accommodate multi-gun spraying and large powder supply, ensuring no powder overflow and that the powder concentration is below approximately 50% of the explosive concentration. Near the workpiece being coated, the velocity should not exceed 0.3 m/s to minimize airflow disruption to the spray pattern and ensure a high deposition rate.
5. Design of Powder Drying Oven
Generally, powder coatings require a workpiece temperature maintained between 180 and 200°C to cure and achieve their performance specifications; otherwise, the weather resistance and other properties of the powder coating will be significantly reduced.
Therefore, powder drying consumes a relatively high amount of energy. Ensuring high energy utilization and implementing proper insulation measures are key design considerations for powder drying ovens.
Drying ovens are generally classified into three types: box ovens, tunnel ovens, and bridge ovens. In bridge ovens, the density of hot air at the inlet and outlet sections is lower; hot air rises while cold air sinks, creating an air seal that effectively prevents heat loss and the entry of external dust.
The drying oven chamber panels typically employ a modular labyrinthine male-female slotted plate structure, assembled on-site. To prevent heat loss due to conduction, the inner surface is welded with silicon bronze using argon-shielded welding. A fiber rock wool insulation layer is filled between the inner and outer walls (some designs incorporate a 10mm layer of aluminum silicate board for insulation).
The joints on the sides of the inner and outer wall panels, as well as the joints on the bottom and top panels, are fully welded. Temperature control employs multi-point measurement and control, using a constant temperature control method.
Once the workpiece surface temperature reaches the set operating temperature within the effective temperature zone of the drying oven, the temperature deviation at the top, middle, and bottom points within the workpiece height range should be ≤ ±5℃. A slight negative pressure should be maintained within the drying tunnel to prevent heat leakage from the tunnel inlet and outlet.
6. Overlay Painting on Powder Coated Surfaces
In the construction machinery industry, overlay painting has been gradually replaced by adhesive labeling due to its high cost, complex process, and susceptibility to defects in secondary coating. However, to improve the overall aesthetics of the machine, some products still retain overlay painting. There are two methods for overlay painting on powder coatings:
6.1 The surface to be overlaid should be thoroughly sanded with 240# sandpaper to ensure a rough surface. After cleaning, paint can be applied, and then allowed to air dry or bake.
6.2 If the surface is not sanded, it can be painted directly, but the paint must be dried in a 70-90°C environment. If the above steps are not performed, and the powder coating is simply painted and allowed to dry naturally, the adhesion between the coating layers will be very poor.
7. Conclusion
For the construction machinery industry, powder coating technology, as a new environmentally friendly technology, is an important manifestation of green manufacturing technology and has high promotional value.
Several large domestic construction machinery manufacturers have not only widely adopted powder coating technology at their OEM plants, but have also required most of their external suppliers to promote its use, recognizing its significant economic benefits.
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