Paper moisture management

Paper and ink are the main materials for printing. The quality of paper and ink printing determines the quality of the print. For paper, the change in moisture before printing, during printing, and after printing does not underestimate the quality of the product. Often, some printers do not pay enough attention to it, resulting in printing failures that cannot be ruled out. Especially when it comes to abrupt changes in climate, it is even more helpless. This article attempts to provide some superficial views on the management of paper moisture for your reference.

First, the paper phase inspection when the paper enters the factory

Paper is required to maintain a stable normal paper phase before it is processed, during printing, or after printing. The so-called paper phase refers to the paper with a certain moisture content, which is present in a certain temperature and humidity. The basic feature of normal paper is that it has good flatness. For a piece of paper, whether the paper is normal or not is relatively difficult to judge, unless the piece of paper has undergone more serious deformation. The usual method is: unwrap the paper bag, and stack the paper one type of high to see if the paper phase is normal. The normal paper is a paper with a flat surface and a smooth end surface. On the contrary, when the paper phase is not normal, it presents a variety of shapes, such as mountain-type, valley-type, angular-warp-type, bag-type, tilt-type, valley-type, curl-type, wave-type, saddle-type.

The reasons for the poor paper phase are as follows: (1) The moisture content on the entire surface of the paper sheet is uneven, showing uneven local shrinkage. (2) The thickness of the paper formed during the papermaking process is not uniform, and the force during papermaking is not uniform. (3) The front and back sides of the paper are inconsistent, especially for single-sided coated paper.

After the paper is loaded in, the end of wrapping paper is already open, and the elongation of the detected paper in the water is too large. Such paper is preferably rejected. If the paper phase is normal and the paper has an elongation of less than 2% in water, the dimensional stability of the paper when printed is ideal.

Second, the printing shop temperature and humidity control

The printing environment generally speaking refers to the temperature and humidity of the printing shop. The moisture of the paper changes with the change of the environment's wetness and humidity. The moisture in the paper changes, and the paper also deforms accordingly.

The humidity of the air refers to the degree of humidity of the air. The air contains much moisture, and the humidity of the air is large. On the contrary, the humidity of the air is small. The humidity of the air is usually expressed in absolute humidity and relative humidity. Absolute humidity refers to the total amount of water vapor contained in the unit volume of air, expressed as the weight of moisture contained in each cubic meter of air, ie, g/m2, which is not affected by changes in humidity. Relative humidity refers to the percentage of saturated water vapor contained in the air with the same volume and the same temperature in the unit volume of air, and the relative humidity reflects the degree of saturation of the water vapor in the air. The moisture content of the paper varies with the relative humidity of the air, not a definite value. Table 1 is the actual measurement of the relationship between the moisture content of some papers and relative humidity.

Under the condition that the relative humidity of the air is constant, the water content of the paper decreases as the temperature increases. Table 2 shows the actual measured value of the relationship between the moisture content of a certain paper and the temperature when the relative humidity of the air is stabilized at 45%.

The relative humidity of the environment is very close to the temperature, and usually the relative humidity decreases as the temperature increases. Table 3 shows the change in relative humidity due to changes in temperature. Thus, relative humidity is not a constant value for a specific day.

The temperature and humidity measurements of the printing environment can be used for wet bulb hygrometers, hair hygrometers, hair hygrometers, and automatic temperature and humidity recorders. To maintain the printing shop constant humidity and humidity, the purpose is to make the paper moisture content and the printing environment to maintain a balance and stability at a certain value. Therefore, conditional work should ideally add air-conditioning equipment to maintain a constant printing environment. Of course, this does not require that the temperature and humidity of the four seasons are the same. All that is required is for each batch of printed matter, from white paper to printing, and the temperature and humidity of the workshop should not exceed the prescribed changes. Table 4 shows the range of temperature and humidity control for printing shops in different seasons.

When a piece of paper is moved from one place to another, it often occurs that the paper is wavy, especially in winter or in early spring and late autumn. This is the effect of the so-called “dew point” on the moisture content of the paper. To avoid this situation, it is necessary to measure whether the temperature difference between the paper and the workshop has reached the “dew point” value. If the measured temperature has reached the “dew point” value, it cannot be eager to disassemble the cold paper in the high-temperature workshop. For bags, the whole box of paper should be placed in the workshop for a period of time before the temperature difference is less than the “dew point” value before unpacking. In general, it takes 10 hours for the temperature difference between paper and the shop to be 5°C, and 20 hours for the temperature difference of 10°C to be unpacked. For the “dew point” phenomenon of paper that is prevented from moving from the paper-jet workshop to the printing shop, the most appropriate solution is to make the temperature of the two workshops the same or similar, and never allow the temperature difference to reach the “dew point” value.

Third, the humidity of the paper

There are usually three ways to adjust the humidity of a paper:

1. It is carried out in the printing workshop or the paper-hanging workshop, using the temperature and humidity of the printing workshop or paper-hanging workshop to adapt the moisture content of the paper.

Table 1 Relationship between moisture content and relative humidity of paper

Table 2 Relationship between water content and temperature in the paper

Table 3 Relationship between temperature and relative humidity