Influencing factors and Countermeasures of surface

2022-08-08
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Factors influencing the surface tension of gravure printing ink and Countermeasures

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 substrate, and the change of ink surface tension is greater in printing. Gravure printing ink is a kind of liquid ink, which is easy to produce some printing quality problems related to ink surface tension

factors affecting surface tension

there are four main factors affecting the surface tension of gravure ink:

1) the formula of raw ink, such as the selection of resin, dispersant, etc

2) dilution ratio of raw ink and diluent used. 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. For example, at 20 ℃, the surface tension of toluene is 2.85 × N/m, the surface tension of ethanol is 2.28 × N/m, the surface tension of acetone is 2.37 × N/m。 Thinner with low surface tension can reduce the surface tension of ink. Obviously, it depends on the speed and explosive force of the aircraft. However, if the dilution ratio is high, the ink viscosity is low, and the ink surface tension is also low. 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

3) temperature. High liquid temperature and low surface tension; Low temperature and high surface tension

4) additives. Such as defoamers, leveling agents, etc. can reduce the surface tension of the ink industry, which has also faced the stereotyped problem of excess capacity in recent years

effect of surface tension on printing

ink dilution is a process of reducing surface tension. The higher the dilution rate, the lower the surface tension. After the ink is transferred to the substrate, with the volatilization of the solvent, the surface tension of the ink gradually increases, reaching the highest when it is dried

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

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

1) affect the leveling of ink. Inks with low surface tension level better

2) affect the adhesion of ink. The wetting degree of the ink on the substrate will affect the adhesion force with the printing substrate. The lower the surface tension of the ink, the better the wetting degree of the printing substrate

3) lead to printing faults in printing, such as shrinkage, etc

printing fault analysis and solution

after the normal dilution of gravure printing ink, the surface tension is low, which is conducive to the leveling and adhesion of ink; 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)

1. The ink transfer process

in terms of ink wetting, it can be divided into two stages: the wetting of the ink to the printing plate (that is, the wetting of the ink to the hole) and the wetting of the ink in the hole to the printing substrate. As long as the wetting at any stage is poor, the transfer of ink cannot be carried out normally. The definition principle of ink "wetting" is:

contact angle θ S wettable at 90 °, θ S do not wet at 90 °

but there is also dynamic wetting in the printing process. When the droplet moves to the left (or the wetted object moves to the right), two contact angles, namely dynamic contact angle, will be generated. One contact angle is greater than θ s. Is the forward angle θ a; The other contact angle is less than θ s. Is the backward angle θ r。 When the velocity is constant, the smaller the surface tension of the droplet, the smaller the advance angle; Otherwise, the larger. The faster the droplet moves, the greater the angle of advance θ A is larger; When the speed reaches a certain degree, the dynamic contact angle is greater than 90 °, resulting in the wettable system becoming the non wettable system (that is, the hydrophilic system becomes the non wettable hydrophobic system). Under the condition of wetting, the maximum allowable interface velocity is called the wetting critical velocity

in the first stage of the printing process, the ink in the ink tank is static, and the plate cylinder is rotating, which belongs to a dynamic wetting state. In the second stage, the ink moves at the same speed as the substrate, which is in a static wetting state. Due to the extremely weak electric signal output by the extensometer, the dynamic wetting of the oil with high viscosity may not be completed, that is, the ink cannot be filled (5) the friction time and load time experimental curves are recorded in real time to separate the wet holes; 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

2. Analysis of shrinkage failure

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 completely spread on the surface of the substrate, forming dew drops, which leads to shrinkage failure. The causes of shrinkage and related cases are listed below

1) the surface tension of the substrate is low

2) the base material is not clean and greasy. The surface tension of most oil stains is lower than that of the substrate

3) the substrate may be attached with some foreign matters with low surface tension

in a packaging factory, the extrusion workshop is closely adjacent to the printing workshop, and shrinkage often occurs during the operation of the extrusion machine. Later, the extrusion workshop moved away and the shrinkage cavity disappeared. Originally, 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, and the static electricity generated by the high-speed printing of the printing machine is very easy to cause these low molecular substances to attach to the substrate and cause shrinkage

4) the ink formula is unreasonable

5) the ink viscosity is not suitable. 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 high enough to not wet the substrate; Dynamics requires the ink to have fluidity and enough time to shrink. The printing image and text are dark and on-site, the ink volume is large, the ink layer is thick, and the solvent absorbs a lot of heat when volatilizing. If the external heat at this time cannot make up for the heat lost by the ink layer, the decrease of the ink surface temperature will increase the ink surface tension, while the lower layer temperature of the ink is higher, and the surface tension is relatively low (or the substrate surface tension is low). According to Bernal vortex theory, the ink with low surface tension spreads to the surrounding ink layer with high surface tension, and the ink film leveling becomes worse, which will lead to shrinkage when it is serious. If the viscosity of the ink is increased, the solvent content in the ink is less, the evaporation is less, and the temperature reduction is small; The fluidity of the ink becomes poor, and the time required from the transfer of the ink to the substrate to the loss of fluidity of the ink is shortened. The ink does not have enough time to shrink, resulting in shrinkage failure on the printed matter

6) the drying speed of ink thinner is too fast. The reason is basically the same as above. It can be solved by reducing the drying speed

7) external reasons make the solvent volatilize too fast before the ink layer enters the drying oven, and the ink surface tension increases

one nine color gravure press and one dry compound press are installed in the same workshop of a printing plant, and the air inlets of the drying oven are in the workshop, resulting in a negative pressure of the workshop with a fan power of about 70kw. When working at night, shrinkage often occurs. After stopping the machine for 30 minutes, the shrinkage disappears again, but it occurs again after 30 minutes. There are two main reasons for this phenomenon

first, the large negative pressure in the workshop leads to the generation of large airflow, 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. The wind generated by negative pressure blows onto the ink film, and 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 still has fluidity, so it is easy to form shrinkage holes on the printed matter

second, low temperature at night leads to lower ink temperature and higher ink surface tension, which also leads to shrinkage failure

if the two conditions exist at the same time, shrinkage failure is more likely to occur

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

because after shutdown for a period of time, the air flow in the workshop decreases or disappears, and after restart, the workshop gradually forms a large negative pressure, and the air flow becomes larger, so shrinkage occurs again. It can be solved by improving the viscosity of the ink, reducing the drying speed of the ink and eliminating the airflow in the workshop, but the best way is to eliminate the negative pressure, such as moving the air inlet of the gravure printer and the dryer to the outside, and then installing the active air supply device in the workshop

3. improve the leveling property of ink

low surface tension of ink is conducive to ink leveling. 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

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