Causes and control of pulling hair
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Pulling is one of the common faults in printing. During the printing process, the ink on the printing plate or blanket is partially transferred to the surface of the paper and then separated at the very short moment of imprinting. If the peeling force of the ink is greater than the surface strength of the paper during the separation, the ink layer will be peeled off. Particles such as fine fibers, rubber, paint, etc. on the paper surface cause blistering and blistering on the paper surface, which may cause peeling of the paper in severe cases.
First, the impact of the phenomenon of printing on printing
The phenomenon of pulling hair will cause pollution in the graphic part and affect the quality of the printed matter. On the one hand, the position of the peeled particles on the paper surface is white spots due to the transfer of the ink; on the other hand, the peeled particles are adhered to the printing plate by the transfer of the blanket, and the printing plate is first contacted with the dampening solution. Being wetted. In the subsequent contact with the ink, the ink is repelled, so that the printing image portion is not inked, resulting in a large number of white spots on the surface of the printed product. Or the peeled small particles are not found at first, and will accumulate on the surface of the blanket and the printing plate as the printing progresses, causing the printing plate to wear and increase the number of times the blanket is cleaned. Some small particles can even be transferred to the ink tank and the water tank, making printing impossible.
Second, the analysis of the causes of the phenomenon of pulling
In the short moment of imprinting, there is a very complicated force between ink, paper and blanket. The viscosity of the ink itself creates an adhesion force FI to the surface of the paper when it is separated. The blanket is also viscous and will also produce an adhesion force FN. The interaction between the ink, paper, and blanket also produces a peeling force FT. If the surface tension of the paper is T, then its surface strength must be met: TFI+FN+FT
If the above formula is not satisfied, the pulling occurs, which affects the printing quality and increases the frequency of cleaning the blanket.
Third, the factors affecting the speed of pulling
The complex effect of ink, paper and blanket on the imprinting makes the surface strength of the paper sometimes not low, but it still produces hair, which indicates that the pulling speed of the paper is not only affected by the printing speed, but also with the ink. Viscosity, thickness of ink layer, printing pressure, temperature, etc.
1. Relationship with printing speed
The pulling speed is based on the theory that the separation force of the fluid between the planes is proportional to the speed at the time of separation, that is, the greater the printing speed, the greater the separation force. For the ink fluid, as the printing speed is increased to a certain extent, the separation force generated by the ink is greater than the bonding force of the paper surface, the tearing and fluffing visible to the naked eye is generated, and the minimum speed is called the pulling speed. .
2. Relationship with the radius R of the drum
When the printing speed and the like are fixed, if the radius of the drum is larger, the smaller the relative acceleration, the smaller the peeling force generated between the ink layer and the blanket and the paper surface, and the less likely the paper is to be shed. Experiments have shown that the lint speed of the paper is substantially proportional to the square of the radius of the drum, as shown in Figure 1.
3. Relationship with ink fluidity
The pulling speed of the paper generally decreases as the viscosity η and the adhesion (Tack value) of the ink increase. That is, the greater the adhesion of the ink, the smaller the pulling speed. As shown in Figure 2 and 3. In addition, the temperature also affects the ink, which affects the pulling speed. As the temperature rises, the viscosity and adhesion of the ink decrease, and the pulling speed increases.
4. Relationship with the thickness δ of the ink layer
When the ink layer is very thin, the blanket cylinder is almost in contact with the paper surface, and the amount of ink is small, and the peeling force is small, but as the ink layer is increased, the pulling speed is greatly reduced. When increased to a certain extent, the trend decreases and tends to be flat, as shown in Figure 4.
5. Relationship with printing pressure P
As shown in Fig. 5, the AB segment is in a relationship of insufficient pressure. As the pressure increases, the napping speed drops sharply, and the BC segment is the normal printing pressure, but the napping speed is reduced to a minimum. The CD segment is in a situation of excessive pressure, and the pulling speed is increased due to the decrease in ink transfer.
Fourth, the method of controlling paper pulling
1. Master the ink and water balance during offset printing. Under the premise of not being dirty, make the paper on the surface of the printing plate to reduce the wetting as much as possible, so as to reduce the oil resistance of the paper and increase the amount of ink.
2. Appropriately adjust the adhesion and viscosity of the ink used, and add appropriate amount of the dispersing agent and diluent to the ink to reduce the adhesion and viscosity in the ink, thereby improving the influence of the adhesion of the ink on the paper. Strictly control the indoor temperature to prevent the temperature from decreasing.
3. Before printing, the paper can be air-pressed. Under the action of pressure, some paper and paper powder are adsorbed onto the blanket to reduce the phenomenon of paper pulling and falling off. At the same time, after the paper fibers swell and swell, the paper structure becomes tighter under pressure, and the printability is improved.
4. When the paper is out of powder and the hair is pulled more seriously, you can also print a layer of primer on the surface of the paper, and then print it in color after drying. Due to the improved permeability of the paper surface, the quality of the printed product, especially the gloss of the printed product, is significantly improved.
5. When the delivery deadline is not affected, the printing speed can be appropriately reduced, so that the printing speed is much smaller than the pulling speed. It is also possible to adjust the printing pressure so that the embossing force between the blanket cylinder and the impression cylinder is appropriately reduced.
6. Do a good job of daily cleaning and maintenance of the blanket and keep it in good condition. In addition, the blanket is used for a long time, causing aging due to the vulcanization reaction, especially in summer, when the old blanket is sticky and causes lint and powder removal. At this point, a layer of sulfur powder can be applied to the blanket. If it still doesn't work, I have to replace it with a new one.
7. If it is impossible to adjust successfully, you should negotiate with the paper supplier to replace the paper with good surface strength.