The use and printing of tinplate is different from other substrates. It is often necessary to first apply a layer of coating on the surface of tinplate for subsequent printing or use. The coating quality of this layer of tinplate surface is very important for subsequent printing and use. . In order to pursue good printing results and meet the requirements for subsequent use, people pay more and more attention to the quality of tinplate coating. People have always attached great importance to the factors that affect coating quality, especially the influence of tinplate and the coating itself on coating performance. Variety and quality have been greatly improved. The impact of the environment on coating performance is less of a concern. This article is to track the environmental temperature and humidity during coating and tinplate surface temperature changes, combined with the final coating quality, to find out the impact of the environment on the coating quality.
Yin Yin's printing medium - tinplate due to its special physical properties, is more sensitive to the impact of the environment. Changes in temperature and humidity in the environment can cause condensation of water vapor in the air. When these vapors condense on the surface of the tinplate, the moisture vapor that has condensed on the surface of the tinplate will be repelled by the coating during the coating process, resulting in the presence of the final product. Can not be painted, fish eyes, exposed bottom, flowers and other undesirable phenomena. Therefore, the environmental factors that cause condensation and the methods to avoid condensation are the focus of this article.
First, the ambient temperature and humidity, dew point temperature and condensation <br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br> When the humidity in the air reaches a certain degree of saturation . The temperature, relative humidity, and surface temperature of the object in the air are closely related to condensation.
The humidity in the air is ubiquitous, and condensation is only a special phenomenon when the humidity in the air reaches a certain level. Usually, the temperature of the air is lowered so that the relative humidity in the air reaches the temperature of 100%, that is, the dew is initially generated ( The temperature at the time of condensation is called the dew point.
Formula (1) is to interpret the condensation phenomenon in a specific area. For a certain area or region, the absolute moisture content in the humid air can be expressed by formula (1):
In the formula:
H—absolute moisture content in wet air, kg/kg absolutely dry gas;
P——local atmospheric pressure, Pa;
Pf - partial pressure of water vapor in wet air, Pa.
Equation (1) shows that when the atmospheric pressure is constant in a certain fixed area, the absolute moisture content in the moist air is only related to the partial pressure of the water vapor in the moist air. The greater the water vapor partial pressure, the greater the absolute moisture content. When Pf is kept constant, then H also does not change. At this time, if the temperature is raised, the degree of superheat of the steam will increase; and if the temperature is lowered, the water vapor will become saturated. If you continue to cool down to a certain degree, there will be a part of the water vapor condensing into "dew drops" separated from the air. This phenomenon is called condensation. The temperature at which dew condensation begins is called the dew point temperature.
Usually the dew point can be measured by the temperature and humidity in the air, according to formula (2)
After calculation, it can be obtained by looking up the table (see Table 1) to obtain a close dew point temperature.
The dew point temperature formula (2) is:
In the formula:
D—— Dew point temperature
T——indicates the ambient temperature
E——The absolute humidity of the environment C——The relative humidity of the environment (C=E/100)
Through the calculation formula and look-up table, it can be found that the dew point is closely related to room temperature and relative humidity. When the humidity increases, the dew point temperature approaches the actual temperature. When the surface temperature of the object in the environment is close to or lower than the ambient dew point temperature, moisture (condensation) will form quietly on the surface of the object.
Second, the temperature and temperature of the metal skin
There are obvious seasonal changes and wet weather in most parts of China, especially in southern China during the spring and winter seasons. In order to understand the changes of tinplate temperature and ambient temperature, the corresponding temperatures in this season have been followed for a long time.
According to the tracking data of tinplate and ambient temperature, the surface temperature of tinplate will be slightly lower than the ambient temperature of the monitoring area. This situation is very easy to occur on the tinplate surface when the ambient dew point temperature is closer to the surface temperature of the tinplate. Dew condensation.
Third, the relationship between dew point and coating quality <br> Select two indoor areas near the front of the machine, the outdoor semi-open area of ​​the more targeted environment, the temperature and humidity changes in the environment for a long-term tracking record, this The environment of the two areas will directly affect the surface temperature of the tinplate to be coated on the machine. At the same time, statistics were made on the quality of printing and coating during the same period, and comparisons were made to find out the relative influence of environmental factors and printing and coating quality. Figure 1 shows the temperature and humidity recording curves and corresponding dew point temperature curves for the front painter of the indoor paint line in December. Figure 2 shows the temperature and humidity detection and recording curves and the corresponding dew point temperature curves for the outdoor semi-open area in December. Figure 3 and Figure 4 show the temperature, humidity, and corresponding dew point change curves for the above two environments in January. It can be seen from the comparison of indoor and outdoor maps that the dew point temperature curves of the two environments are basically the same, but the difference between the indoor temperature and the dew point is significantly greater than the difference between the two. At the same time tracking and comparing the appearance of dew point peaks and coating quality, it can be found that the peak dew point occurred in December on the 8th, 16th, and 28th, and the coating quality of the same period appeared on the 8th. Coating on the end of the phenomenon. In addition, on the 5th, 12th, and 29th days of the dew point peak that occurred in January, there were similar problems such as the coating of fish eyes.
Through tracking, it can be found that when the dew point peak appears to be the actual temperature of the environment, condensation easily occurs on the surface of the tinplate and coating quality problems such as blooming, fish eyes, and exposed bottom are generated.
IV. Results and Discussion 1. From the surface temperature of tinplate measured when the tinplate is opened, the surface temperature and the storage environment temperature are close to or slightly lower than the ambient temperature, so when the environmental conditions permit, the water vapor easily congeals the surface of the tinplate.
2, when the dew point temperature is close to the ambient temperature, dew point occurs, the surface temperature is lower than the ambient temperature of the tinplate surface is likely to cause condensation, in the subsequent coating process due to the repulsion between the coating and water molecules resulting in fish eyes, The exposed bottom, flowers and other quality problems.
3. When the dew point temperature is low and the ambient temperature is high, the tinplate surface is less likely to be dew, and the resulting quality problems can be avoided.
4, from Figure 1 and Figure 2, Figure 3 and Figure 4 can be seen in the indoor environment, dew point temperature and room temperature gap than in the outdoor, so the surface temperature is lower than the ambient temperature of tinplate, in the outdoor Store more easily on the surface condensation.
5. The dew point temperature is related to the environment humidity and temperature. Controlling the ambient temperature and humidity in a relatively high humidity period can affect the dew point temperature of the environment and avoid dew condensation on the surface of the tinplate, thereby avoiding the resulting problems.
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