How to control the environmental conditions of the offset printing workshop
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Environmental condition control, for offset printing work, is mainly environmental temperature and humidity control (actually dust and solvent evaporation, etc.).
We all know that in the printing work, the change of temperature and humidity of the environment will directly affect the shape stability of the paper, 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, and the amount of water in the printing process. The control of the amount of ink, as well as static electricity, etc., may even affect the normal operation and operation of electrical equipment and mechanical equipment of printing machinery. From this point of view, the control of the temperature and humidity of the environment plays a very important role in the smooth printing and the quality of the printed matter.
Humidity concept
First let's figure out a question: What is humidity?
This problem seems to be simple. In fact, there is a lot of learning inside. 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 a corresponding adjustment and change based on 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 dryness and humidity of the air. The amount of water vapor contained in the air changes with environmental conditions. At a certain temperature, the less air in a certain air, the more dry the air; the more water vapor, the more humid the air. Generally, the method of measuring the humidity of air is absolute humidity and relative humidity.
Absolute humidity is the density of water actually containing water, 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 by the strong water pressure.
But just knowing the absolute humidity is not enough for the actual 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, and is often not directly related to the absolute content of water vapor in the air (for example, when the pressure of water vapor in the air is also 12.79 mm high) In the case of mercury column, people do not feel wet in the summer of 35 °C, but in the autumn of 15 °C, people will feel very wet, because at this time the water vapor has reached saturation, the water can not only evaporate, but also condense into water) . This requires understanding a different physical quantity, which is relative humidity.
The relative humidity is: the water vapor density actually contained in the air, and the percentage of saturated water vapor density at the same temperature, called relative humidity. Of course, the ratio of the vapor pressure can also be used to indicate the relative humidity (see formula below).
Where: RH is relative humidity; p1 is the actual vapor pressure of air; P is the saturated vapor pressure at the same temperature; d1 is the actual water vapor density per unit volume of air; d is the density of saturated water vapor at the same temperature.
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 through the above table. The saturated water vapor P at normal temperature can be found through “Table 1”.
T(°C) p T(°C) p T(°C) p T(°C) p T(°C) p
-30 0.3 14 11.99 22 19.83 30 31.82 38 49.69
-20 0.77 15 12.79 23 21.07 31 33.70 39 52.44
-10 1.95 16 13.63 24 2.38 32 35.66 40 55.32
0 4.85 17 14.53 25 23.76 33 37.73
10 9.21 18 15.48 26 25.21 34 39.90
11 9.84 19 16.48 27 26.74 35 42.18
12 10.52 20 17.54 28 28.35 36 44.56
13 11.23 21 18.65 29 30.04 37 47.07
Because of the same temperature and water vapor content in the air, it may reach saturation or even supersaturation in a low temperature environment, but in a high temperature environment, it will be lower, even lower than the saturation value.
Therefore, the relative humidity plays an absolute role in the water content of the paper. Since the moisture content of the paper is closely related to 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, we can easily measure the relative humidity directly using a dedicated instrument.
Workshop temperature and humidity control
In order to accurately print, 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 to 8% higher than that of the printing shop, and because many machines in the printing shop emit heat during operation (including the friction of the ink roller, the friction of the motor, the air pump, and the components between the machines) It will increase the temperature), as well as the evaporation of moisture from the surface of the layout and the blanket, which will increase the humidity of the environment. Therefore, controlling the temperature and humidity of the production environment is very important for printing work.
In order to give full play to the role of air-conditioning equipment and improve economic efficiency, it is necessary to prevent the humidity from being too high to reduce the mechanical strength of the paper surface and to delay the drying of the print; and to prevent the problem of static electricity caused by low humidity. Therefore, according to different seasons, it is very reasonable to specify the range of temperature and humidity control.
The data shown in “Table 2” is the temperature and humidity control range of the workshop in a certain area. Different regions can be used as reference according to the local climatic conditions.
Season
Summer season winter season spring and autumn season
Temperature (°C) 26～30 16～20 21～25
Relative humidity (%) 60～65 45～50 53～58
To maintain the stability of the paper's water content, it is not required to maintain the same temperature and humidity throughout the year. However, it is required that in the unified batch printing, from the white paper input to the printing completion, the temperature and humidity of the workshop should not have a big change.
When changing the setting of the temperature and humidity of the workshop according to the seasonal changes, pay attention to the placement of the finished and semi-finished products in the workshop. Do not change the one-time large-scale conversion. Otherwise, the temperature and humidity change too much, so that the geometric dimension of the printing product is more than the allowable range.
In the workplace where there is no air conditioning equipment, the windows must be opened and closed as needed, and an exhaust fan is installed. In the case of mildew rain or when the outside climate is not normal, the semi-finished products are covered with plastic film. A more positive approach is to use a local air conditioning method to separate the machines that often print high quality products from the outside world, and to 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 plant must adopt a scientific method, from the plant structure to the air conditioning setup, 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 paper moisture content
In the winter or in the early spring and autumn season, the paper that has just been unpacked and unpacked is often very flat, but after one or two hours, the paper is found to have “flank” and the water content of the paper is seriously uneven. Also, the paper that has been hanged and cut and stored in the white paper preparation workshop is flat and intact, but soon after being transported to the printing workshop, the "leaf edge" will occur, making the paper unprintable or even printed. Wrinkling arches occur. When checking the relative humidity of the two places, there is no obvious difference. Why is this?
In order to explain the above reasons, from 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 dew point. .
Based on the relative humidity and temperature of the known air, the dew point is reached when the temperature drop is measured.
It is known that the ambient temperature is 25 ° C, and when the relative humidity is 60%, how much does the temperature drop to reach the dew point? First, the absolute humidity at 25 ° C was calculated (by looking up the table, the saturated vapor pressure at 25 ° C was 23.76 mmHg). According to the formula given above, the absolute humidity is 14.3mmHg, and then the temperature corresponding to P=14.3mmHg is found in Table 1, which is 16~17°C, which is the dew point we require. That is to say, when the temperature drops from 25 ° C to 16 ~ 17 ° C, condensation will occur.
To understand the effect of dew point on the moisture content of paper, it can be explained by some phenomena in daily life: there will be a layer of water droplets on the wall of the water pipe in summer; a layer of water mist will be formed on the glasses from the outdoor room in winter. The JY203 proofing machine uses the method of freezing plates to make the plates get enough moisture from the air to wet the plate without wiping the water. This is because the temperature of the utensils and the plate is lower than the ambient temperature. When the dew point is reached. reason.
Similarly, when the cold paper enters the workshop with high temperature, if the temperature difference reaches the dew point, the water droplets will be condensed on the edge of the paper pile and absorbed by the edge of the paper, so that the water content of the paper edge is larger than the middle, and the lotus edge appears.
In order to avoid condensation around the paper that is removed from the cold carton (paper bag), the carton (package) can be stored for a few days, so that the temperature difference is lower than the "dew point".
To solve the condensation phenomenon of white paper from the preparation workshop into the printing workshop, the most suitable method is to make the temperature of the two workshops close, 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, the temperature of the paper pile can be measured first, and the temperature difference can be calculated. When the dew point temperature is not reached, the white paper is transported to the printing workshop. For example, don't pick up the paper early in the morning, and transport 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, and we will also find that in the temperature and humidity of the environment, we must be connected to each other to control, but not in isolation. In the actual work, if we can control the environment of our printing work well, then we will definitely benefit from the work.