Influence of Ambient Temperature and Humidity on Offset Printing Quality in Printing Workshop

According to the requirements of the standardization standards, the environment of the offset printing workshop should be constant temperature and humidity. This will not only help prevent the deformation of the paper, improve the accuracy of product overprinting, but also ensure the printability of the ink. However, in fact, due to the different climatic conditions in different regions, enterprises have different investment conditions in the offset printing shop environment, which makes it difficult for most manufacturers to guarantee the constant temperature and humidity of the offset printing shop environment, which brings some practical difficulties and process failures to the offset printing production.

First, the effect of changes in temperature and humidity on paper

The water content of the paper changes with the change of the temperature and humidity of the environment. The water absorption and the water loss ability of the paper are related to its own nature, structure and raw materials. In general, the higher the density of the paper, the longer the fibers, the higher the degree of sizing, the lower the water content, otherwise the opposite. The water content of the paper produced by the paper mill is usually 5% to 7%. During the transportation, storage, and printing of the paper, the water content will increase or decrease with different environments. This change in water content will affect the offset printing. It is often harmful and useless.

Paper suction

Because the temperature and humidity of the paper itself is lower than the ambient temperature and humidity, the paper must absorb moisture from the air in order to achieve a balance with the environment, resulting in reduced density around the paper, increased fiber gap, paper deformation is "foil", four sides Rippling warping. The difficulty of this type of deformed paper to offset printing is that, for machines with a pendulum-type front gauge, it is difficult to enter the front gauge because the paper is wavy and undulating. The operator must intensify the pressure of the paper pressure plate to achieve its purpose. The consequence is that the paper is not in place or overprinted. Even the paper will go over the front gauge directly into the printing unit to cause bad printing plates, blankets, etc.; for the front of the pendulum, the raised part of the paper

When entering the front gauge, it collided with the front paper sheet and the pad pressure sheet, causing the paper not to reach the bottom, affecting the overlay accuracy. At the same time, the paper's drag is somewhat uneven, which also brings difficulty to the normal separation of paper by Feida. This change in the paper can be solved by the normal paper-hanging method for the offset paper, and for the coated white paper of 250 g/m2 or more, the paper-hanging paper is hardly effective. Our solution is to use as much as possible the paper produced in the same area as our factory, pile up the cut paper into the offset printing shop a few days in advance, and let it have a process that adapts to the environment of the workshop. We strictly control the moisture in the printing process. , neatly stacking semi-finished products.

2. Paper loss of water

Loss of water from paper causes shrinkage, which creates a “tight edge”, ie, a depression in the middle (or raised) and a curled edge (or voltaic). When printing, it is difficult to separate the paper and the second is when it enters. After the impression cylinder, the stress is not even, and it is easy to wrinkle and cause the product to be scrapped. In addition to the above methods, we also closed doors and windows and installed curtains to ensure that the temperature and humidity of the workshop are not too low. When loading paper, rub the board paper and press it.

Second, the temperature of the ink

The viscosity and fluidity of the ink change with temperature. The higher the temperature, the lower the viscosity and the more fluid. This characteristic of ink has a great relationship with the production process and the quality of the printed matter. The performance is: whether the viscosity is appropriate or not, directly affects the transfer of the ink, the solidity of the print, the ink penetration, and the gloss of the printed product. If the fluidity is too small, the ink will not spread smoothly and unevenly, resulting in unevenness in the ink on the same printing surface; if the fluidity is too large, the printing will not be transferred accurately. The graphic level is not clear, the ink is not full, and the color is not sufficient. Not bright, reducing the quality of printed products. There is a close relationship between the viscosity and fluidity of the ink, which will reduce the viscosity of the ink while at the same time reducing its fluidity. Otherwise, the opposite is true. It can be seen that according to temperature changes, the correct adjustment of ink viscosity, fluidity is of great importance for printing. In the cold winter, our factory suffered from the following two process failures due to the low temperature.

1. If the temperature is too low, ghosting may be caused by incorrect ink adjustment. When printing at 枉8-10°C, due to the small ink flowability, we added a large amount of No. 6 varnish to reduce the viscosity of the oil, forming a floating ink on the printing plate graphic and transferring it. The water supply roller, after the water roller turned one week later, inked in the blank area of ​​the printing plate. As a result, the ink roller and the water roller simultaneously supplied ink to the printing plate to form a special ghost image, and the ghost image was exactly the circumference of the water roller from the actual printing. Later, the temperature rose, the car wash refilled, ghosting troubleshooting.

2. The temperature is too low, the viscosity of the ink is too high, and production is forced to stop. One year in January and mid-January, the temperature dropped sharply, the temperature is generally -25 ~ -27 °C, although the workshop has a heating supply, but the night temperature is only 5 °C. Under such operating conditions, it is difficult to properly adjust the ink viscosity and fluidity so that it has a good printability. Adding various auxiliary agents to reduce the viscosity of the ink will inevitably increase its fluidity and will result in process failures with excessive fluidity. When we printed a batch of color printing in the night shift, we added ink that had been placed in advance on the heating side to the ink fountain, and added some de-sticking agent and No. 6 varnish, but in less than half an hour, the ink became hard again. Because the temperature is too low, the internal cohesion of the ink is increased, the activity of the oil molecules is weakened, the discrete force is reduced, and the ink is not easy to pull and break. In the printing, the pressure of the embossed teeth is insufficient to overcome the combined force of the adhesive force and the peeling force of the blanket on the paper. As a result, a fault occurs where the paper is stuck on the blanket. Although we have taken measures such as lightening the printing pressure, replacing the blankets, and reducing the printing speed, we still cannot eliminate the malfunction and stop production. On the following day, after the temperature of the shop picked up again, there was no failure of the paper sticky blanket and production returned to normal.

In summary, changes in temperature and humidity do have a major impact on offset printing production. It is recommended that qualified manufacturers should try their best to improve the status of the offset printing plant and provide a relatively stable temperature and humidity environment for offset printing to ensure normal production.