How much does environmental temperature and humidity affect paper?

In the printing process, we often encounter such problems: the same type of paper, although the overprinting effect was good yesterday, is much worse today, and often requires adjusting the equipment for a long time to ensure the printing overprinting effect; Sometimes, after a period of time, the paper is prone to tearing and other problems during the printing process. Has there been any change in the paper? Why does such an abnormal situation occur?

In printing production, paper is one of the consumables with high environmental requirements. Because paper is a porous structure composed of plant fibers and various auxiliary materials, it has strong hydrophilicity. If the environmental temperature and humidity are not well controlled, it will cause paper deformation and affect normal printing. Therefore, maintaining appropriate environmental temperature and humidity is the key to improving the quality of paper prints and improving production efficiency.

The Effect of Temperature and Humidity on Paper Size

We sampled and tested four commonly used papers in two production workshops, and obtained the specific changes in size of papers of different types and sizes placed in different temperature and humidity environments, as shown in Tables 1-4.

Through the analysis of the data listed in Tables 1 to 4, we found that no matter what type of paper is placed in different temperature and humidity environments, it will cause abnormal situations of paper size reduction or increase, but the appropriate temperature and humidity for different papers are different. As shown in Table 1, for the ordinary 71g/m2 plain grain aluminum plated paper in Workshop 2, the horizontal dimensions of the paper measured in three sampling tests have all decreased, especially for the paper collected in the workshop environment with a temperature and humidity of 21 ℃/35% after 72 hours, the horizontal dimensions have decreased by 1.5mm; However, for the same type of paper in Workshop 1, only the paper collected at a temperature and humidity of 25 ℃/55% after entering the workshop for 24 hours showed a lateral increase of 0.2mm and a slight increase in moisture content. However, in the two sampling tests with an ambient temperature and humidity of 27 ℃/55% and 27 ℃/52%, although the moisture content of the paper decreased, the size did not change. The matte aluminum plated paper in Table 2 has only been collected in a temperature and humidity of 26 ℃/40% after entering workshop 2 for 24 hours. The horizontal size of the paper has been reduced by 1mm, and the size of the paper sampled and measured when the humidity is not less than 50% has not changed. The single-sided copper coated paper in Table 3 only showed no change in size when sampled at a temperature and humidity of 25 ℃/40%, while others showed varying degrees of increase. The lateral size of the paper measured at a temperature and humidity of 22 ℃/60% even increased by 2.5mm. The waterproof coated paper in Table 4 shows that only the paper size measured at a temperature and humidity of 22 ℃/45% in Workshop 1 remains unchanged, while others may decrease or increase, but the change amplitude is significantly smaller than that of single-sided coated paper.

Don't underestimate the 1-2mm variation, as it can affect the final printing quality of large batch assembled printing products. Of course, the sensitivity of different papers to the environment is not the same. Ordinary coated paper is more hydrophilic than waterproof coated paper and aluminum coated paper, but paper that is no longer sensitive to moisture can also cause changes in paper size after being placed in different environments, especially in environments with significant changes in humidity. The only difference is the length of time it takes to place the same changes, and these subtle changes are enough to affect the overprinting of the print.

The reasons why temperature and humidity affect paper size

The reason why temperature and humidity affect paper size is because paper fibers have characteristics such as porosity and hydrophilicity, and are very sensitive to water. If there is a slight change in environmental temperature and humidity, the water content of the paper will change. Usually, the water content of paper increases with the increase of relative humidity, and the corresponding paper size will increase, usually with significant lateral changes. On the contrary, if the relative humidity decreases, the water content of the paper will decrease, and the corresponding paper size will decrease. The dimensional stability of paper is closely related to the suitability and registration of printing operations. Especially for multi-color overprinted products, significant changes in paper size can lead to inaccurate overprinting, resulting in defective and scrap products. Therefore, the size change of paper caused by temperature and humidity changes is one of the most important issues affecting product quality. In addition, changes in paper moisture content can also cause changes in other paper properties. If the water content increases, the tensile strength and surface strength of the paper decrease; If the water content is too low, the paper becomes brittle and hard, and static charges are more likely to occur during printing.

Suitable temperature and humidity environment for different papers

Through years of work experience and repeated experimental observations, some experiences can be summarized: the suitable humidity for printing aluminum coated paper is between 45% and 60%, and the storage environment for copper coated paper is between 40% and 50%. Of course, this is not absolute either. It is necessary to adjust the environmental humidity while ensuring the stability of various indicators of the raw materials. Because some raw materials have high or low moisture content when leaving the factory, which can also affect the printing effect, it is necessary to air or wet them in advance.

In summary, temperature and humidity control, especially humidity control, in the printing workshop is an important issue that cannot be ignored. Maintaining stable workshop humidity within a reasonable range is one of the requirements of the printing process. When selecting heating equipment, it is recommended to use direct heating in the workshop and install a temperature and humidity meter at an appropriate location in the workshop to monitor environmental temperature and humidity, and make corresponding adjustments according to the situation. It should be emphasized that after returning from holidays (after a period of downtime), the humidity and temperature of the printing workshop should be adjusted three hours in advance to facilitate a faster and smooth start to printing. Of course, the temperature and humidity requirements for printing different prints are not exactly the same. We need to adjust them according to the suitability of different papers during the printing process, which requires us to repeatedly demonstrate and experiment during the printing process to master. In short, any printing data is closely related to the accumulation and analysis of various types of data in the process.