Application of Near Infrared Technology in Powder Coating of Household Appliance Shells

       Near-infrared heating and curing technology is widely used in powder coating of mobile phone and household appliance shells. This technology is used in the curing of powder coatings, which can shorten the curing time and have an effect on the curing of the coating and the performance of the cured coating. When the coating is cured, the heating process experienced by the coated workpiece and the curing temperature have an important impact on the performance of the coating.



1 Influence on coating adhesion



   For good adhesion, the coating should completely wet the surface of the workpiece, replacing air and all other substances that attract the surface of the workpiece. For pre-treated workpieces (for example, the surface shell of the freezer has undergone degreasing, surface adjustment and phosphating treatment, and a dense phosphating film is formed on the surface), in order to ensure that the pores are completely penetrated, the coating must have a minimum The surface tension of the coating should be as low as possible to make the coating melt sufficiently. During the curing process, the temperature of the coated workpiece gradually increases, and the molten layer begins to melt and wet the surface of the workpiece when it reaches a certain temperature. On the one hand, the higher the temperature, the lower the viscosity; on the other hand, the higher the temperature, the longer the heating time, the higher the degree of crosslinking of the solution layer and the higher the viscosity. The use of near-infrared heating can accelerate the temperature of the coated workpiece and reduce the melt viscosity of the coating, which is conducive to the wetting of the coating on the workpiece, thus increasing the adhesion of the coating.



2 Influence on the smoothness of the coating film



The horizontal fluidity of powder coatings is an important indicator to measure the smoothness of the coating film. The horizontal fluidity is large, the coating fluidity is good, and the coating film smoothness is strong. The horizontal flow of powder coatings is greatly affected by the curing temperature. When the curing temperature is low, although the gelation time is longer, the low temperature melt viscosity of the coating is high, the fluidity is poor, and the horizontal flow is low: when the temperature gradually increases, the melt viscosity is low, and the fluidity is better. However, due to the shorter gelation time of powder coatings at higher temperatures, that is, the short flow time before curing or film, the horizontal fluidity is also low. The use of near-infrared heating technology can quickly increase the coating temperature to the curing high temperature region, reduce the time spent in the temperature region with high melt viscosity below the curing temperature, which is beneficial to increase the horizontal fluidity of the powder coating, and improve the gloss of the coating. Flatness.



3 Reduce blistering and volatilization pores on the coating surface



Powder coatings contain certain volatile substances such as water, low-molecular-weight organic compounds, etc. (generally less than 1%), and some water and other low-molecular-weight substances are sometimes adsorbed on the surface of the substrate. These water or low-molecular substances will volatilize during heating and curing. If the powder coating is gelled or about to be gelled, these substances will continue to volatilize, which will form volatilization pores on the surface of the coating, which will cause the coating to foam in severe cases. The thicker the coating, the more severe this phenomenon is. With near-infrared curing, the temperature of the workpiece can reach above the boiling point of these volatile substances in the shortest possible time. These substances will volatilize before the coating gels, and will not form volatilization pores on the surface of the coating.



4 Improve the curing degree of the coating



At present, the curing furnaces of some domestic manufacturers generally have a large difference between the upper and lower furnace temperatures, and the lower end of the workpiece is not fully cured or cured due to fast chain speed. The coating is brittle. The use of near-infrared curing technology shortens the curing time and makes the temperature of the workpiece surface uniform, which reduces the quality problems of poor adhesion and brittle powder layer caused by insufficient curing degree.



 



Compared with the original curing furnace, infrared heating technology has the following characteristics:



(1) The heating method used in the traditional curing furnace is convective heat transfer, which uses the heat of the air to heat the workpiece, so that the thermal efficiency is low and the energy consumption is large. Near-infrared technology directly heats the workpiece in the form of radiation with high thermal efficiency.



(2) The near-infrared heating element has a high-energy output and can efficiently generate high-intensity infrared heat. The near-infrared heating element can radiate 80% of the effective radiant energy within 1s after power-on, while the far-infrared heater needs 1 It takes ~3min to achieve a similar output, and the near-infrared components are also much faster than other components in terms of cooling after power failure. Therefore, the original curing furnace needs to be turned on 1 hour in advance to heat up to reach the curing temperature, while the near-infrared curing furnace can be started when it needs to be used, and the temperature can be heated up immediately; Avoid the yellowing of the paint film due to the long residence time of the workpiece in the high temperature curing furnace.



(3) Due to the application of near-infrared technology, the curing time has been shortened. The curing time of the original paint film was 15 minutes, and now it is shortened to 4 minutes, correspondingly shortening the furnace body by one-third, saving the floor space of the production equipment.



(4) In the case of strong radiation, the heating is instantaneous, so the quality of the workpiece has nothing to do with the temperature uniformity in the furnace, but mainly depends on whether the surface temperature of the workpiece is uniform, because the curing is carried out on the surface of the workpiece. Because the elements in the furnace are symmetrically arranged, the density is consistent, and the power of each element is equal, the uniformity of irradiance is ensured, which overcomes the temperature difference between the upper and lower temperatures in the original furnace, which leads to poor adhesion at the lower part of the workpiece and the frying powder. Phenomenon.