Application Research of Gas Radiant Heating Technology in Powder Coatings Curing

Abstract: The advantages and disadvantages of convective hot air heating and the problems in its application are summarized; the principle of gas radiant heating and its application and benefits in powder curing.

introduction

Curing (drying) is one of the three basic processes of an industrial coating line and an important process for coating film formation.

Therefore, whether the selection of curing methods and equipment is reasonable, and whether the selection and implementation of drying and curing specifications are correct will directly affect the quality of the coating and the cost of coating.

The global greenhouse effect and energy saving and emission reduction have become the main issues for human survival.

The home appliance industry is mainly powder coating.

The thermal efficiency of the convection hot air heating chamber is 20% to 30%, and most of the heat is used for heating the drying/curing furnace device and the heat loss in the air.

The hot air convection curing method commonly used in the home appliance industry has a longer curing time (generally at least 20 minutes in the furnace); higher discharge temperature and greater heat loss (generally heat loss is as high as 30% or more), resulting in lower overall energy efficiency; hot air Flow can easily lead to wrinkles and even orange peel of the coating.

Through the study of coating curing mechanism, the gas radiant heating device is applied to the powder coating drying/curing process.

1.

Convective hot air circulation heating is the most basic heating method for coating line drying/curing.
According to the principle of heat flow from high temperature to low temperature, the heat balance calculation in the drying/curing process generally regards the heat actually carried by the coated object as heat energy The effective utilization rate is thermal efficiency.

The thermal efficiency of the convective hot air heating chamber is 20%~30% (see Thermal Efficiency Table 1), most of which are used for device heating and heat loss in the air.
If the heat required for the heating and curing of the coating film is used as the thermal efficiency, the thermal efficiency is lower.

The specification for curing powder by convection hot air circulation heating is 180~220℃/15~25min, and the total heat required for curing is 5×105kcal/hr×2.

Only 30% of this total heat is used for heating and curing of parts, while 40% of the heat is consumed by the heating of the conveyor chain and the heat dissipation of the furnace body, and the remaining 30% is lost by the heat dissipation of the furnace body, the air duct heat and the direct exhaust air.
Drop.

It can be seen that the heat actually used for parts heating and other purposes only accounts for 70% of the total heat, and the actual wasted heat is 30%.

The outdoor powder mainly used in the home appliance industry is polyester thermosetting powder coating, and the curing principle is cross-linking and curing.
The main heating methods used in the powder coating curing furnace are hot air circulation, infrared and infrared heating air circulation.

The advantage of the hot air stove is that the temperature in the furnace is uniform, which is suitable for coatings of various shapes; the disadvantage is that the heat transfer of the coating is gradually heating up from the surface to the inside.
Not only the heating rate is slow, but also pig pores or pig pores are likely to appear after the coating reaches a certain thickness.
Disadvantages such as pinholes.
Hot air circulation curing is widely used in the home appliance industry.

The infrared heating method used in the home appliance industry is infrared radiant energy, which generally does not require a blower to circulate hot air.

The advantage of this kind of baking furnace is that the coated object and the coating film are heated at the same time, and the temperature of the coating film is fast, and it also avoids the pollution of the coating film caused by the dust brought into the furnace during the blast; the disadvantage is the temperature distribution in the baking furnace Non-uniformity, the distance between the heat source and the coated object has a great influence on the temperature.
For coated objects with complex shapes, the temperature difference between different parts is relatively large.

Therefore, this kind of baking furnace is only suitable for simple coated objects such as plates or cylindrical shapes, or when the distance between the heat source and different parts of the coated object is very close (for example, when the cylindrical coated object is rotated) , The distance between each part and the heat source is not much different) before it can be used.
It is not suitable for coated objects with complex shapes.

2.
Basic principles of gas radiant heating technology

When the temperature of any substance is above K=0 (-273℃), the phenomenon of radiation will be produced.

The essence of the radiation phenomenon is that the electrons in the atoms in the substance are in a state of non-stop motion (rotation, vibration, and mysticism) in the presence of temperature, so alternating electric and magnetic fields (ie, electromagnetic waves) are generated, and the electromagnetic waves move into space.
The propagation is radiation.

The wavelength range of electromagnetic waves (as shown in the spectrum diagram), from several nanometers to several kilometers, including: cosmic rays, X-rays, ultraviolet (wavelength less than the wavelength of violet light outside 0.
38μm), visible light (0.
38~0.
78μm), infrared (10- 3～7.
8×10-7), microwave (0.
3～10-3 m), radio wave (radar wave, video wave, broadcast wave, the wavelength ranges from several kilometers to about 0.
3m).

Humans use electromagnetic waves of different wavelengths to achieve different application purposes.

Radiant heating technology is a technology that uses electromagnetic waves (called heat rays) with a wavelength of 0.
1 to 100 μm for heating.
Heat rays are electromagnetic waves emitted due to the temperature and thermal movement of substances, including visible rays, some ultraviolet rays and some infrared rays.

Heat rays have wave characteristics and quantum characteristics, the so-called "wave-particle duality".
The transmission of electromagnetic waves follows the law of the fourth power (E=εEb=ελEb).

As a part of electromagnetic waves, heat rays also obey the 4th power law of motion.
The sun (surface temperature 6000℃, center temperature 20000℃) can transmit light and heat from the distant sky to the earth because of this mysterious law of the fourth power.

Gas radiation heating technology equipment can generate electromagnetic waves with a wavelength of 1~40μm (part of the heat rays, we call it soft and strong radiation).
Using this mysterious 4th power law, it effectively solves the rapid heating of various spaces and production processes.
Problems, applied to the heating of tall spaces, heating of painting processes, etc.

The gas radiant heating system simulates the principle that the sun provides heat to the earth, and directly converts the heat energy in the gas into electromagnetic waves with a wavelength of 1-40μm, directional beam radiation, and sends it to the heating destination to directly heat the target object.

Since the air is not heated directly, the thermal efficiency can reach 95%, and the heat energy is dissipated from the parts to the air, which is exactly the same as the volatilization direction of small molecules when the coating is cured, which helps the coating to cure and the appearance quality of the coating is better than convective hot air circulation heating Curing method.

The gas radiant heating technology can directly heat the workpiece without fan convection, no coating quality problems caused by hot air blowing, reducing exhaust gas emissions, reducing heat loss, and reducing powder coating curing costs.

3.
Application case of gas radiant heating technology in powder coating curing

A certain company's powder spray curing line body is not completely uniform in the specifications and thickness of the coated parts.
When making thick parts, the coating cannot be cured completely at the normal line speed.
Therefore, the line speed is limited to 2m/min to meet the curing requirements of parts with different thicknesses.
, Which seriously affected production efficiency.

After repeated research and testing, a certain length of gas radiant heating device is added at the entrance of the wire, so that the workpiece will first undergo powder melting and crosslinking reaction in the gas radiant heating device to shorten the curing time.

The line speed of the parts is increased to 3m/min, and the production efficiency is increased by 50%.
Moreover, the use of a gas radiant heating device reduces the coating drop caused by the vibration of the conveyor belt and the coating defects caused by the hot air blowing, which improves the uniformity of the coating and effectively improves the appearance quality of the coating.

The gas radiant heating system is a closed conversion of gas heat energy to radiant heat energy.
It transfers energy in the form of electromagnetic waves and does not require any media.
Therefore, it has the characteristics of small heat loss, and the energy utilization efficiency is much higher than that of the convection heating method of direct combustion of fuel.
Incomplete combustion produces nitrogen oxides, CO, etc.

The thermal efficiency can reach 95%, and it can save energy by 30% when applied to the powder coating curing process.
According to the national energy-saving standard, it can reduce nearly 200t standard coal every year.

4.
Conclusion

The transformation and innovation of gas radiant heating technology has fundamentally realized energy conservation, emission reduction, and efficiency enhancement, provided technical support for the transformation of production methods, and realized upgrading and transformation, and played a leading role in the latest development of the powder coating industry.