Talking about Several Factors Affecting Printing Contrast (K Value) in Tinplate Printing

Print contrast is the most important indicator for evaluating the quality of printed matter. It is the ratio of the difference between the density of the field and the density of the screen density to the density of the field. Usually, the dot density of the dot of the dot area is 75%. It is mainly displayed on the level of printed matter. The higher the printing contrast, the more open the print level is, and the greater the visual impact on people. On the contrary, it makes the print level hierarchical, no spirit, color bias, giving the impression of the human eye very bad.

Print contrast is so important for prints. How can we control it? In the following, based on the problems and solutions encountered in the production process and some theoretical knowledge, we briefly analyzed the factors affecting the printing contrast of tinplate printing products, hoping to provide some help for future production control.

First, the quality of the original

The influence of the original quality on the K value is mainly reflected in the linearity of the printing film and the development of the film.

The linearity of the film refers to the consistency of the number of actual dots on the film pattern and the number of dots required by the pattern itself, which can be detected using a standard step scale. If the linearity is not correct, the produced product cannot correspond to the original document at all, and the K value data will not have any guiding significance.

The development of the film is mainly due to the influence of the developing quality on the quality of the dots during the output of the film. If the image is not developed sufficiently, the value of the dot will be unclear. The value of K will be smaller than that under normal conditions. Excessive development will cause the dots to be lost or lost. K value Will be bigger than normal. Observe the test strip with a 40x magnifying glass to check it out.

Second, printing quality

The effect of printing quality on K value is mainly reflected in the quality control during exposure and development. Excessive exposure and over-development will lead to the loss of network points, which will cause the K value to become larger, and the fine-tuning (low-forming dots) part of the level will not be fine enough; insufficient exposure and development will lead to a higher K-value of the higher-order dot layer level and a lower level of darkness. . Through the printing plate can accurately determine the quality of the printing plate.

Third, the ink adaptability

The influence of ink suitability on the K value mainly lies in the emulsifying property of the ink. Excessive emulsification will cause the ink on the ink path and the water path to accumulate, causing the paste plate to cause a decrease in the K value; insufficient emulsification will deteriorate the transferability of the ink, and the ink cannot be uniformly transferred. On the substrate, the K value is unstable. The emulsification performance of the ink is mainly affected by the quality of the ink itself, which is related to the binder and auxiliaries of the ink. The differences in the suitability of different inks are very different.

Fourth, printing pressure

The effect of printing pressure on the K value is mainly reflected in the two aspects of ink delivery and dot quality. The printing pressure includes the pressure between the ink roller on the ink path, the pressure between the platen roller and the plate, the pressure of the water path, the pressure between the plate and the blanket, and the pressure between the blanket cylinder and the impression cylinder. Any abnormal pressure above will cause the outlets to expand abnormally, causing the K value to drop.

Fifth, white canning color

From the experiments done at present, the color difference of white cocoa can affect the K value of one or two colors. The reddish phase of Keding has a greater influence on the K value of the red ink. The K value of the red ink will decrease more, but the impact on other colors will be relatively small. The reason for this is that we think that the red phase of K At the bottom of the red, when the overall red reaches the desired density, the density of the red ink does not really reach, that is, when we remove the red, its density is less than the density of the bottom red (that is, the same density. Underneath, the thickness of the bottomed red ink layer is thinner than that of the bottomless red ink layer. At the same time, the effect of the bottom red on the dots is greater than that of the bottom layer. The same can be said for other colors of cocoa, but the effect of the color is different.

Sixth, fountain solution

The pH of the fountain solution has a great influence on the ink transfer of our offset printing, and is a necessary condition for generating inorganic salt layer in the blank. In theory, the ability of the printing plate to form the inorganic salt layer is greatly weakened when the pH of the fountain solution is >6. Promote the blank part of the hydrophilic capacity greatly reduced, the ink to the graphics and more than part of the expansion, even leading to the blank part of the dirty, so at the same field density, the dot gain rate value than the normal printing conditions, dot gain rate, corresponding to Its K value also decreased. Similarly, if the pH of the fountain solution is < 4.5, the ink is easily emulsified, resulting in a paste plate, causing an increase in the dot gain rate, resulting in a decrease in the K value.

The influence of the alcohol in the fountain solution on the K value is no less than the effect of the pH value on the K value. As we all know, the main role of alcohol in printing is mainly to reduce the surface tension of water and reduce the temperature of the ink roller. If the concentration of alcohol is too low, the surface tension of water cannot be effectively reduced, and the fountain solution cannot be spread well on the blank part of the printing plate, and the ink can easily enter the blank area, resulting in an increase in dot gain rate, and secondly, it cannot be effective. The cooling effect causes the ink to emulsify excessively, and the dot easily expands, all of which cause the K value to drop.

Seven, blanket

Blanket is the intermediate of our lithographic (offset) ink transfer. During printing, the blanket comes in contact with the ink of the graphic portion of the printing plate while also coming into contact with the water in the blank portion of the printing plate. Therefore, the blanket is mainly composed of rubber of a non-polar material, so as to ensure that the blanket absorbs the ink to the maximum extent and transfers the ink, and absorbs water to the minimum. If this performance of the blanket is poor, the ink will not transfer well to the sheet metal, causing an irregular change in the K value.

In order to obtain the ideal K value for the printed matter, the performance of the blanket is largely determined by the properties of the blanket. The performance indexes of the blanket include: ink transferability, elongation, elasticity, flatness, etc. If these properties of the blanket are satisfied. If the index is not good, it will not be able to print out shaped dots, which will directly affect the K value of the product.

At present, the blankets we use are air-cushion blankets, and there are two types of air cushions. In general, the air cushion is thicker than the printed dot products, and its K value is also higher, but it is not conducive to the printing of products in the field. On the contrary, the blanket with thin air cushion is very suitable for printing products in the field but is not suitable for printing dot products. Its K value is also not very high.

Eight, the temperature and humidity of the environment

When the temperature of the printing shop increases, the viscosity of the ink decreases, and the flowability increases. At the same time, the dry vegetable oil also separates more free fatty acids, resulting in a decrease in the hydrophilicity of the blank part. The ink of the graphic part spreads outward, resulting in the outlets. The abnormal expansion caused a decrease in the K value. If the temperature of the printing shop decreases, the ink yield value increases, the transfer ability decreases, and the water evaporation capacity of the printing plate decreases, causing the K value of the printed product to change irregularly.

When the humidity of the printing shop is not stable, the production operator cannot control the water volume of the printing plate correctly, resulting in large and small printing plate water volume, causing print defects such as chromatic aberration, and at the same time, the K value of the printing product will change irregularly.

To sum up, in order for a printed product to have an ideal K value, it must be strictly controlled from all the above aspects. Poor control at any point will result in unsatisfactory K value and affect the quality of the final product.