Water and ink balance in lithographic printing and its effect on quality

Lithography is the use of oil-water immiscible principle, printing plate graphic part can be lipophilic and hydrophobic, the blank part of the first water in order to have a hydrophilic oleophobic selective adsorption. Another feature of offset printing is the use of indirect printing, which utilizes the dot color rendering method for color reproduction. It is very important to know whether ink painting is suitable in offset printing, the normal transfer of prints, the depth of ink, the accuracy of color registration, and the drying and stickiness of prints. Therefore, whether or not we can properly control and control the balance between water and ink is the key to ensuring the quality of printed products. As a skilled printer, it is necessary not only to understand the relationship between ink and water, but also to master the techniques and essentials of ink and water balance.

1 Relationship between ink and water balance

The ink-water balance refers to adjusting the amount of fountain solution under a certain printing speed and printing pressure, so that the proportion of the volume of the fountain solution contained in the emulsified ink is controlled at 15% to 26% to form a slight degree of water-in-oil ( W / O) emulsified ink, with the smallest amount of liquid to compete with the ink on the plate.

The most important part of the ink balance is the fountain solution and ink control. The fountain solution is composed of water and a surfactant. The fountain solution can fully wet the layout, has cleaning ability and stable pH value, and can ensure that the layout cannot be dirty.

1) Effect of fountain solution

1 Form a uniform water film on the blank part of the plate to prevent the ink on the graphic from infiltrating into the blank part and prevent the dirty plate.

2 The electrolyte in the fountain solution reacts chemically with the metal plate that is exposed due to wear, continuously forms a new hydrophilic layer, and maintains the hydrophilicity of the blank part of the plate.

3 Control the temperature of the layout ink. As the speed of the new printing press is getting faster and faster, the printing speed will increase the temperature of the ink during high-speed transmission, and the ink's fluidity will increase, resulting in the increase of the dot. Therefore, the high-speed machine puts forward a new requirement for the fountain solution, that is, the fountain solution can reduce the temperature of the ink roller and the ink when contacting the ink roller through the layout.

4 Do not enlarge or reduce the graphic. The fountain solution must have the effect of stabilizing the relative positions of the graphic and text portions of the printing plate, ensuring that the graphic is neither enlarged nor reduced throughout the printing process.

The ink is generally composed of a pigment, a filler, a binder, and an auxiliary additive, and the rheological properties and the optical properties of the ink should be fully taken into consideration when selecting the ink.

The rheological properties of inks are the main basis for judging whether inks have good printing performance. The rheological properties of inks are mainly determined by the binder of the ink. Of course, they can also be properly adjusted by auxiliary additives. The evaluation of ink rheological properties can be reflected by the main indicators of ink rheological properties, namely ink viscosity, viscosity, thixotropy, yield, viscoelasticity, and fluidity. In actual production, only by mastering these performance indicators, can we be familiar with the printing properties of various types of inks, so as to achieve the purpose of selecting inks that meet the printing requirements.

The ink and water on the printing plate must be present at the same time to maintain balance. The purpose is to keep the maximum amount of ink in the printing area of ​​the graphic and text, so that the ink is bright, saturated, and the dots are clear and clean. It is also necessary to keep the printing area highly clean and tidy. Pay attention to adjust the relationship between water supply and ink supply in production.

In printing, the darkness of the ink printed on the prints, in addition to the amount of ink on the ink fountain, is directly affected by the amount of water supplied and the ink balance on the layout. When the moisture content of the layout increases, the blotting ink color gradually fades to lighten. On the contrary, reducing the amount of water used to reduce the amount of water on the layout, the ink will be relatively deeper than the original. Therefore, increasing or decreasing ink supply and layout water supply will change the depth of blotting ink. In the printing process, it should be adjusted according to the actual situation. The ink should not be increased simply by reducing the amount of water supply, because the water is too small to lose the balance of water and ink, and it is easy to make the blank part dirty. However, there is a need to prevent the appearance of big ink. When the water is excessive and the ink becomes light, you must not ignore the actual situation and mistakenly believe that the amount of ink is small, and you do not reduce the amount of water supply, but you often increase the amount of ink. The reciprocal increase in the supply of water for the ink supply has created a vicious circle, which has finally led to the emergence of various malpractices in the water and ink, making it impossible for printing to proceed normally.

Experienced printers, in actual production, often ensure that the printing plate can not be dirty under the premise of the water supply to control the smallest possible range, and the water supply and ink supply in a relatively stable state , This will ensure that the printed ink is consistent in depth before and after printing and the print job is stable. Advocating the use of small water and ink thick practices, where the water little finger is the minimum amount of water on the premise that the blank part of the layout is not dirty; the so-called ink thickness is also based on the small water, the water caused by excessive ink emulsification, The ink layer cannot be thick.

2 Conditions for producing ink balance

In the printing process, in addition to well-controlled ink and wash, half-way stoppages, changes in temperature and humidity in the printing room, printing machine operating speed, ink, paper type, and quality of the media can directly affect the ink-water balance.

1) The machine should run at a constant speed. In the production process, the machine must run at a constant speed and run at a constant speed. The machine is fluctuating, and it is easy for the ink and ink balance to lose control. The faster the machine speed, the smaller the water squeezing drops, the smaller the water consumption, and vice versa.

2) The temperature and humidity of the printing shop should be constant. In principle, the workshop is required to have a constant temperature and humidity, because changes in temperature will cause changes in the viscosity and fluidity of the ink. The workshop temperature is high, the moisture evaporates quickly, and the amount of water is large; otherwise, it is small. The humidity in the workshop is high, the water evaporation is slow, the water consumption is small, and vice versa, so that the ink ink loses its balance. Therefore, it is best to install air conditioners and humidifiers in the workshop to control the temperature and humidity within the specified range.

3) Ink properties. The water resistance of the ink determines the water content of the ink. Water resistance is strong, water consumption is small, and water consumption is small; conversely, water consumption is relatively large. Different brands of inks are of different quality and have different water resistance properties.

4) Paper properties. Paper is rough, loose texture, low smoothness, paper absorbs more water, and the amount of ink used is large. The paper has a dense texture, a high degree of smoothness, a small amount of water, and a small amount of ink.

5) Plate category. Different printing plates, layout, different size of trachoma. The layout of coarse sand eyes, stored more water, the layout of the water layer is relatively thick, relatively large amount of water, and vice versa is small. Therefore, the surface of the printing plate must be kept uniform and dense, with a certain physical strength, so that the graphic and blank parts have a solid attachment base and have good water storage conditions.

6) layout area. The large area of ​​graphic and text, the large amount of ink carried, the large consumption of ink and water, and the large amount of water; on the contrary, the amount of water used and the amount of ink used are small.

7) Fountain solution. The pH of the fountain solution should generally be controlled at about 4.5 to 5.5, and must be made constant to avoid wide-range fluctuations. Therefore, when adding wetting powder, it is appropriate to avoid too much or too little.

8) Adjust pressure properly. The press must have precise roller pressure, ink roller pressure, and water roller pressure. These three pressures directly determine the ink balance.

3 Identification of water size

Before printing, according to various conditions, only a small amount of water can be roughly estimated. In actual printing, various methods must be used to identify the size of water. The most commonly used method is to identify the amount of water based on the operator's visual measurement of the strength of the reflected light. Because of a certain thickness of water film, it can fill the plate sand day by day, reduce diffuse reflection and increase the amount of reflected light. However, this method also has some limitations. It is influenced by the area ratio of the layout graphic, the influence of the printing plate, the influence of the light intensity, the angle of observation, and so on. Therefore, in the daily printing, we often judge whether the water is too large from the following points:

1 The ink under the scooping with the ink knife, the ink color fades the hair cream, the ink color is rough and dull, then shows the water is big.

2 Use a knife to scoop ink on the ink roller. There are tiny drops of water on the ink blade. There is also water in the ink fountain.

The 3 layouts often appear to be dirty, or the surface of the layout is still not dry.

4 The imprinted dots were empty, and the mouth markings were wavy and light, and the ink was dark and dull.

5 printed curls soft and weak, delivery is uneven.

6 The blanket roller has water shadows or drops at the tip.

7 There is a big gap between the ink and the printed product before shutdown when printing is continued.

8 When the ink is applied to the ink roller, the presence of a water film on the surface of the ink roller makes it difficult to make the ink even. When the ink roller is put on the ink, there is a slip phenomenon.

4 Improper capture of ink and ink balance affects print quality

In the printing process, improper control of ink and wash will have a great impact on the quality of printed products, resulting in inaccurate overprinting of prints, distortion of color reproduction, and appearances of “flower pattern” and “burning paste”, as follows:

1) Impact overprint accuracy

Because the layout of water is too large, the transfer paper absorbs excess moisture and stretches. When the next color is to be overprinted, the paper will shrink due to moisture spreading on all four sides of the paper, resulting in irregular expansion and contraction of the paper, resulting in overprinting. Not allowed.

2) Causes the ink to emulsify excessively, hinders the normal ink transmission, affects the product quality

In the printing process, due to the large amount of water in the layout, the surface water is squeezed into the ink under the action of the mechanical pressure and the ink roller to cause the ink to be over-emulsified. Excess moisture will also accumulate on the surface of the ink. In severe cases, an aqueous layer exists between the ink roller and the ink roller, the ink roller and the plate, and the layout and the blanket. When the water layer reaches a certain level, it will seriously affect the normal transmission of ink, thus affecting the quality of the product.

3) The ink of the printed product becomes lighter, and the gloss of the printed product decreases

When the printing is over-printed, it tends to be distributed on the surface of the ink roller in the form of fine beads, and is dispersed in the ink by the mechanical force, thereby reducing the number of pigment particles per unit area and decreasing the color saturation of the ink. The emulsification value increases and the ink becomes lighter. Not only make the product grayish, but also the printing plate "flower", "floating dirty" and other phenomena have occurred, resulting in printing failures. At the same time, when the ink contains excessive water, the ink film of the printed product cannot oxidize the conjunctiva sufficiently, the surface is rough, and the surface is not smooth, so that the ink film is dull and the glossiness of the printed product is lowered, which affects the color and color of the printed network. Restore effect.

4) Dirty paste, affect product quality

In addition, when the layout of the ink water, causing the layout of the ink too much, the ink layer is too thick, under the effect of the extrusion force, graphic network spread out, dirty version from the paste, resulting in print quality problems.

5 Daily production to maintain ink balance considerations

In the daily operation, in addition to the actual situation of the actual layout of the layout, the size and distribution of the layout, the nature of the printing paper, and the performance of the ink, special attention should be paid to the following points:

1 Before starting up, you must add enough water to the dry water roller.

2 When the machine is idling, the water supply should be stopped immediately to prevent the ink from losing balance due to large ink and water.

3 The new exchangeable cashmere cover is tight and flat, and the two ends must be sewn firmly. Before the machine is on, we must first fully wet the water roller, and when printing, we must also control the moisture to a minimum. Newly replaced water rollers tend to have higher water content than older water rollers, which can easily cause excessive water failure.

4 Never adjust the ink too thin when adjusting the ink. If the ink is too thin, its fluidity is large, and the viscosity and cohesion force are naturally reduced, resulting in a drop in water resistance and easy to cause severe emulsification, so that the ink trace spreads under the pressure of the roller, and in severe cases, a paste plate will be produced and the product quality will be affected. Therefore, the amount of varnish should be controlled below 10%.

5 In the printing process, the ink tends to adhere to the surface of the water roller, and the thicker and thicker the area, the larger the area. After shutting down, the ink will quickly dry and congeal. If it is not removed in time, it will cause unevenness in the water supply, which will cause the ink and ink to lose their balance and affect the service life of the printing plate and the water roller. Therefore, sufficient attention should be paid to it.

6 Correctly adjusting the pressure of the water roller and the ink roller is a prerequisite for ensuring the balance of water and ink. If the pressure is improperly adjusted, it often causes poor water transmission.

In short, in the printing process, the operator must adhere to the Sanqin operation, that is, diligently check the printed samples and observe the layout moisture, diligently ink in the ink fountain, diligently wash the blankets and printing plates, and keep the layout moisture to the minimum (the blank part does not hang Dirty), so that the ink is always in a normal state of stability, the only way to ensure the quality of the entire printing process and the smooth printing work.

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