Brushing is one of the common faults in printing. In the printing process, the ink on the printing plate or blanket is partly transferred to the surface of the paper and separated at the very short moment of imprinting. When the separation force is greater than the surface strength of the paper, the ink layer will peel off The fine fibers, rubber, paint and other particles on the paper surface can cause the paper surface to be visually broken, blistered and blistered. In severe cases, paper peeling may occur.

1. The effect of the hair pulling phenomenon on printing

The pulling phenomenon will cause pollution of the graphic parts and affect the quality of the printed products. On the one hand, the position of the peeled particles on the paper surface caused white spots due to the lack of ink transfer; on the other hand, the peeled particles adhered to the printing plate through the transfer of the blanket, and first contacted with the fountain solution with the rotation of the printing plate Wetted. In the subsequent process of contact with the ink, the ink is repelled, so that the graphic part of the printing plate is not inked, causing a large number of white spots on the surface of the printed product. Or the small particles that have been peeled off are not found at first, and will accumulate on the surface of the blanket and the printing plate as the printing progresses, causing the abrasion of the printing plate and increasing the number of times the blanket is cleaned. Some small particles will even be transferred to the ink tank and water tank, making printing impossible.

2. Analysis of the causes of hair pulling

In the brief moment of imprinting, there are very complex forces between ink, paper and blanket. The viscosity of the ink itself will produce an adhesive force FI on the paper surface when it is separated. The blanket is also viscous and will also produce adhesion FN. The interaction between the ink, paper, and blanket also produces a peel tension FT. If the surface tension of the paper is T, then its surface strength must meet: TFI + FN + FT

If the above formula is not satisfied, the roughening occurs, which affects the printing quality and increases the frequency of cleaning the blanket during shutdown.

3. Influencing factors of the pulling speed

The complex effect of ink, paper, and blanket during imprinting makes the surface strength of the paper sometimes not low, but it will still produce roughness, which shows that the roughness 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 drawing speed is based on the theory that the separation force between the fluids in the plane is proportional to the separation speed, that is, the higher the printing speed, the greater the separation force. For the ink fluid, as the printing speed increases to a certain degree, when the separation force generated by the ink is greater than the binding force of the paper surface, tearing and fluffing will be visible to the naked eye. The minimum speed at this time is called the pulling speed . [next]

2. Relationship with drum radius R

When the printing speed and other conditions are fixed, if the radius of the cylinder is larger, the relative acceleration is smaller, the peeling force generated between the ink layer and the blanket and the paper surface is smaller, and the paper is less likely to fluff. Experiments show that the paper lint speed is basically proportional to the square of the drum radius.

3. Relationship with ink fluidity

The drawing speed of paper generally decreases with the increase of ink viscosity η and tack (Tack value). That is, the greater the adhesion of the ink, the lower the pulling speed. In addition, the temperature will also affect the ink, which affects the speed of the hair. As the temperature increases, the viscosity and viscosity of the ink decrease, and the pulling speed increases.

4. Relationship with ink layer thickness δ

When the ink layer is very thin, the rubber cylinder is almost in contact with the paper surface. Due to the small amount of ink, the peeling force is very small, but with the increase of the ink layer, the pulling speed decreases greatly. When increasing to a certain degree, the trend decreases and tends to be flat.

5. Relationship with printing pressure P

As the pressure increases, the pulling speed drops sharply, reaching the normal printing pressure, at this time the pulling speed has dropped to a minimum. When the pressure is too large, due to the reduction of ink transfer, the pulling speed has increased.

Fourth, the method of controlling paper shaving

1. Master the ink and ink balance during offset printing, and make the paper wool on the surface of the printing plate less wet as much as possible on the premise of not dirtying the plate, so as to reduce the oil resistance of the paper wool and increase the amount of ink.

2. Properly adjust the viscosity and viscosity of the ink used, add an appropriate amount of spreading agent and thinner to the ink to reduce the viscosity and viscosity of the ink, thereby improving the effect of the ink's adhesion on the paper. Strictly control the indoor temperature to prevent the temperature from decreasing.

3. Before printing, the paper can be air-compressed. Under the pressure, some paper wool and paper powder can be adsorbed on the rubber blanket, which can reduce the phenomenon of paper shaving and powder falling. At the same time, after the paper fibers absorb water and swell, under pressure, the paper structure will become tighter, which will improve the printability.

4. When the paper is powdered and frayed more seriously, you can also print a layer of primer on the surface of the paper first, and then perform color printing after drying. Due to the improved permeability of the paper surface, the print quality, especially the gloss of the printed product, will be significantly improved.

5. When the delivery time is not affected, the printing speed can be reduced appropriately so that the printing speed is much less than the pulling speed. The printing pressure can also be adjusted so that the embossing force between the rubber cylinder and the embossing cylinder is appropriately reduced.

6. Do the daily cleaning and maintenance of the blanket to keep it in good condition. In addition, the rubber blanket has been used for a long time, and it has vulcanization reaction and aging, especially in the hot summer weather. The old blanket is easy to stick and cause hair loss and powder removal. At this time, a layer of sulfur powder can be coated on the blanket. If it still doesn't work, you have to replace it with a new blanket.

7. When it is really impossible to adjust successfully, you should consult with the paper supplier to replace the paper with good surface strength.

Source: "Guangdong printing" 2009 4 Author: Li Junfeng Zhu Xu Yao

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