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The so-called environmental condition control, for offset printing work, is mainly the temperature and humidity control of the environment (in fact, there are dust and solvent evaporation, etc.).
We all know that in the printing work, changes in the temperature and humidity of the environment will directly affect the paper's form stability, the viscosity of the ink, the viscosity of the ink, the transfer of the ink, the drying of the ink on the surface of the substrate, the amount of water in the printing process The control of ink volume and static electricity will even affect the normal operation and operation of electrical equipment and mechanical equipment of printing machinery. From this, it can be seen that the control of the temperature and humidity of the environment has a very important role in the smooth printing and the quality of the final printed matter.
This question seems to be simple, but in fact, there are a lot of knowledge. Many printing workers only mechanically control the humidity of the environment according to an inherent regulation, and actually do not understand the true meaning of humidity. This will result in the inability to make corresponding adjustments and changes according to the changes in conditions when the required process and conditions change. Let us first understand the concept of humidity.
The so-called humidity is the degree of dry and wet air. The amount of water vapor in the air changes with changes in environmental conditions. At a certain temperature, the less water in a certain air, the drier the air; the more water vapor, the more humid the air. Generally, there are two methods to measure the humidity of the air: absolute humidity and relative humidity.
Absolute humidity refers to the density of water vapor actually contained in the air, usually expressed in grams of water vapor contained in one cubic meter of air. The law of gas shows that the pressure of air increases with the increase of air density, so the absolute humidity of air can also be expressed with water pressure.
But just knowing the absolute humidity is not enough for practical work. Because the degree of dryness and humidity of the air is related to whether the amount of water vapor contained in the air is close to saturation, but it is often not directly related to the absolute content of water vapor in the air (for example, when the pressure of the water vapor in the air is also 12.79? High In the case of mercury column, people do not feel wet in the summer of 35 ℃, but in the autumn of 15 ℃, people will feel very humid, because the water vapor has reached saturation at this time, the water cannot only evaporate, but also condense into water . This requires understanding a different physical quantity, that is, relative humidity.
The relative humidity is: the density of water vapor actually contained in the air, and the percentage of the density of saturated water vapor at the same temperature, called relative humidity. Of course, the relative humidity can also be expressed by the ratio of water vapor pressure (see the formula below).
In this way, as long as we know the temperature of the environment and the absolute humidity of the air, we can calculate the relative humidity from the above table, and the saturated water vapor P at normal temperature can be found through "Table 1."
Because the temperature and water vapor content in the same air may be saturated or even supersaturated in a low temperature environment, but under a high temperature environment, it will be lower, or even significantly lower than the saturation value.
Therefore, relative humidity plays an absolute role in the moisture content of the paper. Because the moisture content of paper has a close relationship with temperature and relative humidity, the temperature and humidity of the workshop should be controlled, and the controlled humidity should be relative humidity.
In fact, it is not necessary to obtain the relative humidity from the water vapor pressure in production. In actual production, it is easy to directly measure the relative humidity using special instruments.
Workshop temperature and humidity control In order to overprint accurately, the offset printing workshop should have air conditioning equipment to strictly control the temperature and humidity. If there is a special paper drying workshop, the temperature and humidity of the paper drying workshop should be controlled accordingly.
Because the relative humidity of the paper drying workshop is required to be 5-8% higher than that of the printing workshop, and because many machines in the printing workshop emit heat during operation (including the friction of the ink roller, the motor, the air pump, and the friction between the parts of the machine Will increase the temperature), and the evaporation of water on the surface of the plate and the blanket will increase the humidity of the environment. Therefore, controlling the temperature and humidity of the production environment is very important for the printing work.
In order to give full play to the role of air conditioning equipment and improve economic benefits, it is necessary to prevent the problem of excessively high humidity reducing the mechanical strength of the paper surface and delaying the drying of the blot; and to prevent the problem of paper static electricity caused by too low humidity. Therefore, according to different seasons, it is very reasonable to regulate the range of temperature and humidity accordingly.
The data shown in "Table 2" is the temperature and humidity control range of the workshop in a certain area. Different areas can be used as a reference according to the local climate conditions.
To keep the moisture content of the paper stable, it is not required that the temperature and humidity of the workshop be consistent throughout the year. However, it is required that in the unified batch of printed products, from the input of white paper to the completion of printing, there should be no major changes in the temperature and humidity of the workshop.
When you want to change the temperature and humidity settings of the workshop according to the seasonal changes, pay attention to the delivery of the finished and semi-finished products in the workshop. Don't make a large-scale conversion at one time, otherwise the temperature and humidity will change too much, and the geometric size of the product being printed will expand and contract beyond the allowable range.
In a workplace without air-conditioning equipment, the windows must be opened and closed as needed, and an exhaust fan must be installed. In case of the rainy season or when the outside climate is abnormal, the semi-finished products should be covered with plastic film. A more active approach is to use local air conditioning to isolate machines that often print high-quality products, as well as appropriate surrounding space, from the outside world, install window air conditioners and dehumidifiers (and air humidifiers) The temperature and humidity of the regional space are controlled.
The design and construction of the new workshop must adopt a scientific method, from the structure of the workshop to the setting of air conditioning, careful design and strict management, so that the printing workshop has the ideal conditions of constant temperature and humidity.
The effect of "dew point" on the moisture content of paper in winter or early spring and late autumn, it often happens that the paper that was just unpacked and unpacked was originally very flat, but it was found that the paper produced a "lotus edge" after an hour or two, The moisture content of the paper is severely uneven. In addition, the paper that has been hung and cut and stored in the white paper preparation workshop is flat and intact, but soon after being transported into the printing workshop, "lotus edge" will occur, making the paper unable to be printed or even printed. Wrinkles occur sometimes. When checking the relative humidity of the two places, there is no obvious difference. Why?
In order to explain the above reasons, it is necessary to talk about the "dew point" of air. In the nature of saturated water vapor, it can be known that lowering the temperature can make unsaturated water vapor with a certain mass and volume become saturated water vapor, and this temperature is called the dew point. .
According to the relative humidity and temperature of the known air, measure how much the temperature drops, that is, reach the dew point.
It is known that when the ambient temperature is 25 ° C and the relative humidity is 60%, how many degrees does the current temperature drop to reach the dew point? First, calculate the absolute humidity at 25 ℃ (the saturated vapor pressure at 25 ℃ is 23.76mmHg by looking up the table). According to the formula given above, the absolute humidity can be calculated to be 14.3mmHg, and then the corresponding temperature when P = 14.3mmHg is found from Table 1 is 16 ～ 17 ℃. This temperature is the dew point we require. In other words, when the temperature drops from 25 ° C to 16-17 ° C, condensation will form.
To understand the effect of dew point on the moisture content of paper, it can be explained from some phenomena in daily life: a layer of water droplets will form on the wall of the water pipe in summer; a layer of water mist will form on the glasses from the outdoor into the room in winter. JY203 proofing machine uses the method of freezing plate, so that the plate can get enough moisture from the air to wipe the plate without wiping the water. This is because the temperature of the utensil and the plate is lower than the ambient temperature. When the dew point is reached reason.
Similarly, when cold paper enters a workshop with a higher temperature, if the temperature difference reaches the dew point, a large amount of water droplets will condense at the edge of the paper pile and be absorbed by the edge of the paper, so that the water content of the paper edge is greater than the middle, and the lotus edge appears.
In order to avoid condensation of moisture around the paper removed from the cold carton (paper bag), the carton (bag) can be stored for several days to keep the temperature difference below the "dew point".
To solve the condensation phenomenon of white paper entering the printing workshop from the preparation workshop, the most appropriate method is to make the temperature of the two workshops similar, at least not to make the temperature difference reach the dew point.
If the conditions are limited, it is impossible to increase the temperature of the white paper preparation workshop, then you can first measure the temperature of the paper stack and calculate the temperature difference. When the dew point temperature is not reached, the white paper is shipped to the printing workshop. For example, do not ship paper early in the morning, but ship the paper when the temperature difference between the two workshops is small at noon.
Through the above introduction, we have a more comprehensive understanding of the environmental control of the printing workshop. At the same time, we will also find that when the temperature and humidity of the environment must be linked to each other, rather than isolated control. In actual work, if we can control the environment of our printing work well, then we will definitely benefit greatly from our work.
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