Common fault analysis and solutions of thermal paper in offset printing

Thermal paper is a special kind of paper processed by coating. Due to the presence of a coating layer on its surface, its printability will be affected.

The structure of thermal paper can generally be divided into three layers, the bottom layer is the paper base, the middle is the heat-sensitive layer, and the top layer is the protective layer.

Thethermallayer is mainly composed of two components: one is a colorless dye (leuco dye), and the other is a color developer. Commonly used as a colorless dye is CVL. BPA is commonly used as a color developer. The two components are separated by a thin film. The thin film melts under heating conditions, and the two substances mix to cause a color reaction, so they develop color when heated.

The protective layer can absorb part of the light that causes the chemical reaction of the thermal coating and slow down the deterioration of the thermal paper. It is mainly composed of some small particles.

During the thermal paper offset printing process, the following failures are prone to occur:

1. The phenomenon of piled rubber. In serious cases, a thick layer of graphics and text will accumulate on the rubber for thousands or even hundreds of sheets as soon as the machine is turned on. If the machine is not shut down in time, not only will the printed products be easily scrapped, but the rubber cloth will also be crushed. The main reason is that during the printing process, due to the peeling tension, the protective layer on the surface of the thermal paper is peeled off, and these fine particles accumulate on the blanket. If they cannot be transferred in time, the phenomenon of rubber accumulation will occur. Solution: ① Reduce ink viscosity. On the premise that normal printing is possible, add ink-adjusting oil to the ink to reduce the viscosity of the ink to the lowest possible level. ② Use the minimum printing pressure on the premise that the printed product meets the requirements. The specific method is: adjust the printing pressure to the minimum, close the pressure in the slow-motion state of the machine, and observe while applying pressure. Stop applying pressure immediately until the graphics and text meet the requirements. ③ Reduce the printing speed. The higher the printing speed, the greater the peeling tension of the paper, and vice versa. Therefore, when printing on thermal paper, the printing speed should not be too fast.

2. The drying speed of the ink decreases. When offset printing is performed on thermal paper, the drying speed of the ink often decreases significantly, causing stains on the back of the printed product and dirty rollers. Reason analysis: Since the color developer in the heat-sensitive layer of thermal paper is an acidic substance, this acidic substance is brought into the ink during the printing process, which reduces the PH value of the ink, causing the desiccant lead (Pb), Cobalt (Co), manganese (Mn), etc. undergo chemical reactions, the driers become ineffective, and the drying speed of the ink is naturally reduced. Treatment method: Appropriately increase the PH value of the fountain solution. The PH value of the dampening solution in offset printing is generally controlled between 5-6. When we print on thermal paper, the PH value of the dampening solution should be appropriately adjusted to the upper limit of about 6, and the dampening solution should be measured frequently during the printing process. The pH value of the liquid can be adjusted at any time.

3. On thermal paper, we often print detection black blocks with thick ink layers or very dark patterns (color tickets). In order to make the printed products dry quickly and completely, we have to use UV ink to print UV When the lamp is dry, due to the physical properties of dark colors absorbing heat, the dark parts of the image and text absorb the heat emitted by the UV lamp and develop color. When encountering this situation, the approach we usually take is: ① Increase the printing speed of the machine appropriately, shorten the residence time of the graphic part in the UV light box, so that it can pass through the UV light box before absorbing enough heat for color development ② In On the premise that the printed matter meets the drying requirements, the power of the UV lamp should be reduced as much as possible. If these two methods are used in place, they can basically solve the color development failure of thermal paper caused by UV drying.