The Importance of the Cooling Section of the Curing Oven in the Powder Coating Process

The Importance of the Cooling Section of the Curing Oven in the Powder Coating Process

With the continuous development of the construction machinery coating industry and the continuous improvement of environmental protection requirements in various countries, the environmental protection and energy-saving advantages of powder coating technology in the construction machinery industry have gradually emerged, and various construction machinery manufacturers have established powder coating production lines.

In practical applications, powder coating has the advantages of high construction efficiency, low VOCs emissions, and excellent coating performance. In particular, it has great advantages in reducing VOCs emissions. It can avoid the construction of large waste gas treatment equipment, thereby greatly reducing construction costs and operating costs. However, due to the characteristics of engineering machinery spraying workpieces, the cooling method after powder solidification has become a prominent issue restricting the operation of the production line.

1 Characteristics of engineering machinery powder solidification

Powder coating has gradually developed from thin plate parts and small structural parts to large structural parts in the field of engineering machinery. Nowadays, large structural parts are increasingly used. Compared with thin plate parts and small parts, the characteristics of large structural parts are more significant: large structural parts are usually welded with thick plates, shaped steel, etc. The thickness of the workpiece steel plate is much higher than that of small parts, usually the thickness is 10~20 mm. Some workpieces will be above 20 mm. Due to the larger thickness of the plate and the increased heat absorption, large structural parts take longer to solidify the powder and absorb more heat, which brings difficulties to the subsequent cooling process. On the one hand, the temperature required for powder solidification is relatively high (usually above 200°C), and the workpiece cannot be lowered to the lower temperature in a short time. On the other hand, the cooling section usually uses cold air blowing to speed up the heat dissipation on the surface of the workpiece. Due to the thick plates of large structural parts, the internal temperature can only rely on conductive heat dissipation through the base material, which leads to the phenomenon of "heat recovery". After the workpiece is cooled to the target temperature, it heats up again in the buffer zone, making it impossible to unload the workpiece.

2 Design principle of cooling section of curing furnace

The design of the cooling section after drying generally requires that it reaches room temperature after cooling. From the buffer area to the unloading area, the surface temperature recuperation does not exceed 10°C. The length of the cooling time has a great impact on the cooling effect. Generally, the production unit of the engineering machinery coating line is long, the cooling section is usually set up at 2 to 3 stations, and the cooling time is 30 to 45 minutes. The forced cooling section consists of a chamber body, air supply and exhaust devices, air supply and exhaust ducts, and blower hoods. In the construction machinery industry, most use direct outside air cooling. This method mainly collects the air outside the factory building, filters it through the filter in the air supply and exhaust device, and blows it directly to the surface of the workpiece through the blower hood for cooling. No refrigeration device is used. . The strong cooling chamber (cooling section) is generally composed of a profiled steel frame and a 50 mm thick composite sandwich rock wool board. The purpose of using rock wool boards is generally to keep the appearance consistent with the drying room. If the appearance does not need to be consistent, it can also be made of galvanized sheets, aluminized sheets and other materials. The blowing hood of the forced cooling room adopts an adjustable high-speed air outlet nozzle. Usually the wind speed at the outlet of the nozzle is about 15 m/s, the wind speed in the air supply and exhaust duct is 8~12 m/s, and the number of air changes in the room is usually 6 times. /min, if the strong cooling effect is considered, it can be increased appropriately. The air supply and exhaust device is the core component of the forced cooling room. Our company, QINGDAO Jieruixin Machinery& Technology Co., Ltd., can customize this part. This kind of equipment filters the outdoor air through a high-efficiency filter (filtration level is usually not lower than F5) and then sends it into the room through the blower. At the same time, the exhaust fan empties the hot air in the room in time. The device room adopts a double-layer steel structure, lined with galvanized steel plates on the inside, galvanized corrugated plates on the outside, and filled with thermal insulation cotton in the middle, giving it a beautiful appearance and structure.

3 Ways to improve cooling effect

3.1 Increase air volume

The forced cooling chamber is generally designed to directly use outside air for cooling. Since this design is highly sensitive to external temperature, increasing the wind speed of the blow nozzle to accelerate the air flow speed on the surface of the workpiece has a good effect on improving cooling efficiency and reducing cooling time. . For example, the design air speed of the mouthpiece is 14 m/s. When the air supply volume is increased to 16 m/s, the cooling speed will be greatly improved. For some modified equipment, the fan pulley can be replaced, the speed of the fan impeller can be increased, and the design margin of the cooling section fan can be used to increase the air supply volume, thereby achieving the effect of increasing the wind speed of the mouthpiece.

3.2 Make reasonable use of the cache area

The usual way to use the buffer area to solve the heat recovery problem is to allow the workpiece to dissipate heat naturally and let the heat dissipate into the workshop. This method will increase the temperature in relevant areas of the workshop, thus reducing the working environment of the workshop. At the same time, due to the lack of effective heat dissipation means, the heat dissipation efficiency of the workpiece is low. In view of the above shortcomings, now more and more closed buffer areas are used in the use of buffer areas, combined with exhaust fans (negative pressure fans or axial flow fans) to continue to maintain the air flow speed on the surface of the workpiece, and at the same time, prevent the dissipated heat from Affect the work production environment.

3.3 Improve the performance of heat exchange medium

The design concept of the cooling section is usually to blow air to the surface of the workpiece and take away the heat of the workpiece to achieve cooling. During this process, the air flow rate on the surface of the workpiece increases, taking out heat. Due to the low specific heat capacity of the air, the heat taken away by the air per unit volume is very limited. At the same time, due to the increase in air temperature, the ability of the air to take away heat is further reduced. . Therefore, the heat exchange medium is replaced to increase the specific heat capacity, so that the heat that can be taken away by the medium per unit volume increases, thereby improving the effect of the cooling section.

Water is an economical medium with high specific heat capacity. Therefore, an atomizing device can be added to the air supply and exhaust device. After the water is atomized, the mixed medium of air and water mist can be blown evenly to the surface of the workpiece with the blower to cool the workpiece. The evaporation of water mist takes away a large amount of heat, thereby achieving better cooling effect. This method generally increases the cost of construction and use. It is rarely used in the construction machinery industry. It is usually used when there is no buffer area after cooling and parts need to be released in a shorter time.

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