Surface tension analysis of the most popular gravu

2022-07-27
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Gravure printing ink surface tension analysis

so the single rod piston cylinder should be selected. The air inlet of the active air supply device installed in the workshop of this factory is just close to the exhaust port of the extruder. When it is produced in a high temperature environment above 300 ℃, the volatile low molecular polymer is just discharged out of the room and immediately sucked into the printing workshop, However, the static electricity generated by high-speed printing of printing machine is very easy to make these low molecular substances attached to the substrate and cause shrinkage. Such as defoamer and leveling agent can reduce the surface tension of ink

the ink viscosity is not appropriate

the drying speed of ink thinner is too fast

the lower temperature at night will lead to lower ink temperature and higher ink surface tension, which will also lead to shrinkage failure

the change of ink surface tension will have the following effects on the printing process:

there are two reasons for the increase of ink surface tension in the printing process: first, the gradual reduction of solvents with low surface tension, and second, the volatilization of solvents, especially fast drying solvents, which leads to the decrease of ink temperature and the increase of ink surface tension

improve the levelness of the ink

after the machine is stopped for a period of time, the air flow in the workshop decreases or disappears. After the machine is started again, the workshop gradually forms a large negative pressure, and the air flow becomes larger, so the shrinkage cavity occurs again

why does the shrinkage disappear when the machine is turned on again after shutdown

if the two conditions exist at the same time, it is easier to cause shrinkage failure. At the same dilution ratio, thinner with low surface tension is easier to reduce the surface tension of ink than thinner with high surface tension. There is a large amount of inks in the printing text and on the spot, and the ink layer is continuously thick. The solvent absorbs a lot of heat when volatilizing. If the external heat can not make up for the heat lost in the ink layer at this time, the decrease of the ink surface temperature will increase the ink surface tension, while the lower layer of the ink has a higher temperature, and the surface tension is relatively low (or the substrate surface tension is low). When working at night, the shrinkage often appears. After stopping the machine for 30 minutes, the shrinkage disappears when the machine is started again, but it occurs again after 30 minutes. The causes of shrinkage and related cases are listed below. In the second stage, the ink moves at the same speed as the substrate, which belongs to the static wetting state

the contact angle s < 90 can be wetted, and s > 90 can not be wetted. Figure 1 shows the static wetting state of the ink. The reason is basically the same as above. High liquid temperature and low surface tension; Low temperature and high surface tension. For example, at 20 ℃, the surface tension of toluene is 2.85 n/m, that of ethanol is 2.28 n/m, and that of acetone is 2.37 n/m

the dilution ratio of the original ink and the diluent used. According to Bernal's eddy current theory, the ink with low surface tension spreads to the ink layer with high surface tension nearby, and the ink film leveling becomes worse, which leads to shrinkage when it is severe. The wind generated by the negative pressure blows onto the ink film. The solvent has volatilized a lot before the ink layer enters the drying oven. The sharp drop in the temperature of the ink layer increases the surface tension of the ink layer. At this time, the ink layer is still active, so it is easy to form shrinkage holes on the printed matter. The wetting degree of the ink on the substrate will affect the adhesion with the printing substrate. The lower the surface tension of the ink, the better the wetting degree of the printing substrate

the base material is not clean and greasy. Generally, the surface tension of ink resin is greater than that of printing substrate, and the surface tension of common diluent is less than that of printing substrate. The surface tension of most oil stains is lower than that of the substrate. As long as any stage of poor wetting, ink transfer can not be carried out normally

in a packaging factory, the extrusion workshop is firmly adjacent to the printing workshop, and shrinkage cavities often appear during the operation of the extrusion machine. The formation of shrinkage cavity needs to meet the requirements of thermodynamics and dynamics at the same time. Thermodynamics requires that the surface tension of ink is too high to wet the substrate; Dynamics requires ink to be active and have enough time to shrink

external reasons make the solvent volatilize too fast before the ink layer enters the drying oven, and the ink surface tension increases. Thinner with low surface tension can reduce the surface tension of ink

when the surface tension of the printing substrate does not match the surface tension of the ink, for example, when the surface tension of the substrate is low or the surface tension of the ink is high, the ink cannot be fully spread on the surface of the substrate, forming a dewdrop shape, that is, causing shrinkage failure

reducing the surface tension of ink can be solved by adding leveling agent, using thinner with low surface tension and selecting ink with appropriate viscosity. If you improve the viscosity of the ink, the solvent content in the ink is less, the amount of volatilization is less, and the temperature reduction range is small; The ink activity becomes poor. The time required from the transfer of ink to the substrate to the loss of ink activity is shortened. The ink does not have enough time to shrink, resulting in shrinkage failure on the printed matter

temperature. When the velocity is constant, the smaller the surface tension is, the smaller the advancing angle is; Otherwise, it will be larger

affect the adhesion of ink. A contact angle greater than s is the forward angle a; The other contact angle is less than s, which is the retreat angle R

the substrate may have some foreign matter with low surface tension

the formula of raw ink, such as the selection of resin and dispersant

ink dilution is a process of decreasing the overall prosperity of the industry. The higher the dilution ratio, the lower the surface tension

one nine color gravure printer and one dry compound printer are installed in the unified workshop of a printing plant, and the air inlets of the drying oven are in the workshop, resulting in a negative pressure of about 70kw. Low surface tension ink leveling better

first, the large negative pressure in the workshop leads to the generation of large air flow, especially in the evening printing when only the workshop door is open, the wind at the color deck of the printing machine is very strong, and the wind plays a major role in the drying of ink

affects ink leveling. Obviously, if the dilution ratio is high, the ink viscosity is low, and the ink surface tension is also low

there are four main factors that affect the surface tension of gravure printing ink:

surface tension factors

the surface tension of liquid is the ability of the liquid level to automatically shrink in the air. For printing, it is as important as the surface tension of the substrate, and the change of the ink surface tension is greater in printing

analysis of shrinkage failure

in the first stage of the printing process, the ink in the ink tank is static, and the plate cylinder is rolling, which belongs to a dynamic wetting state

however, there is dynamic wetting during the printing process, as shown in Figure 2

the ink formula is unfair. Gravure printing ink is a kind of liquid ink. It is easy to produce some printing quality problems related to ink surface tension. Later, the shrinkage cavity disappeared after the extrusion workshop was removed. Because the high viscosity of the ink surface tension, may lead to dynamic wetting can not be completed, that is, the ink can not fully wet the hole; It may also cause that the static wetting cannot be completed, that is, the ink in the hole cannot fully wet the printing substrate, and eventually the ink transfer is poor

the wetting of ink can be divided into two stages: the wetting of ink to printing plate (that is, the wetting of ink to cavity) and the wetting of ink in cavity to printing substrate. On the premise of wetting, the maximum allowable interface velocity is called the wetting critical velocity. In the figure, when the liquid drop moves to the left (or the wetted object moves to the right), two contact angles, namely the dynamic contact angle, will be generated. The definition principle of ink wetting is:

ink transfer process

after normal dilution of gravure printing ink, the surface tension is low, which is conducive to ink leveling and adhesion; After the ink is dried, the surface tension of the ink film is high, which is conducive to the overprint of the next color ink or the smooth progress of the next process (such as compounding and gluing)

the surface tension of the substrate is low

analysis and solution of printing faults

causes printing faults in printing, such as shrinkage. The greater the moving speed of the droplet, the greater the advance angle a; When the speed reaches a certain degree, the dynamic contact angle will be greater than 90, resulting in the wettable system becoming the non wettable system (that is, the manual ultrasonic flaw detection method and flaw detection result classification GB 11345 (8) 9 for liquid friendly steel welds become the non wettable hydrophobic system). The low surface tension of the ink is conducive to ink leveling. It can be solved by improving the viscosity of the ink, reducing the drying speed of the ink and eliminating the air flow in the workshop, but the best way is to eliminate the negative pressure, such as moving the air inlets of the gravure printer and the dryer to the outside, and then installing the active air supply device in the workshop. There are two main reasons for this phenomenon

effect of surface tension on printing

additives. After the ink is transferred to the substrate, with the volatilization of the solvent, the surface tension of the ink increases gradually and reaches the maximum when it is dried. It can be solved by reducing the drying speed

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