Discussion on the problem of non-photohardening of carbon paper
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The sensitized carbon paper is used to ensure that the gel layer is melted and sensitive to light without being affected by light. The so-called matt hardening (ie, dark reaction) is a sensitized carbon paper that has a thicker layer thickness after development without exposure. The concentration of the potassium dichromate solution is large, the free acid is too much, and the gelatin of the animal is also different, and the photohardening is also added. Therefore, in the actual production process, attention must be paid to the process of photohardening of the carbon paper layer. This requires the actual operator not only pay attention to the indoor temperature and humidity during the process of preserving the sensitized carbon paper, but also pay attention to the storage time of the sensitized carbon paper because it is related to whether the corrosion process can be achieved. Stable and high quality printing.
The reaction process inside the rubber layer is indicated as follows. Potassium dichromate exhibits an ionic state in aqueous solution: K2Cr2O7→2K++Cr2O7—where: Cr2O7—+H2O—2CrO4—+2H+. That is to say, the ionic form of potassium dichromate in acid and alkaline solutions is different: when the solution is alkaline, the dichromate ions react in the direction of generating chromic acid ions. The oxidation of the colloid by the dichromate ion is stronger than the oxidation of the colloid by the chromate ion. Therefore, in order to reduce the oxidation of dichromate ions and slow down the dark reaction, an appropriate amount of ammonia water may be added to the potassium dichromate solution to enhance the alkalinity of the solution. There are three functions:
1. The reaction is carried out in the direction in which the chromate ion is generated. At this time, the orange potassium dichromate exhibits a pale yellow color, and the dark reaction inside the carbon paper layer is weakened.
2. Improve the adhesion of the colloidal substance so that the film can adhere tightly to the plate.
3. Inhibit the activity of microorganisms to prevent the sensitizing solution from decaying and deteriorating.
After the photosensitive solution is added with ammonia water, the pH is usually between 7.5 and 8.8, which is slightly alkaline. During the drying process, the ammonia water gradually decreases and becomes slightly acidic, so the photographic performance of the film is gradually increased, which does not affect the use effect. Because higher room temperature also causes pure chromic acid to separate from the chromic acid, a freezer is required for this purpose. It must be noted that the humidity inside the refrigerator should be consistent with the relative humidity of the studio.
First, carbon paper continues to harden after exposure
The exposed carbon paper can be stored in a humid environment for 20 hours and then passed through the plate. Practice has proved that if the carbon paper is stored in the wet season for one day and then goes to the sun, the carbon paper is not easy to adhere to the plate cylinder, even if it has been attached to the drum. Nothing comes out. Therefore, it is required that the carbon paper that has been sun-dried and the positive image should be immediately printed, so that the intermittent time between the printing and the printing is not prolonged. Prolonged delays can cause difficulties in development and corrosion. The plate-making workers must cooperate with each other, strengthen the work plan, clean the drums in a timely manner to do the degreasing work; prepare the development of the development of hot water and other developments in advance.
Another way to prevent dark hardening of the adhesive layer is to add 0.3% potassium citrate (K3C6H5O7.H2O) as a cleaning agent to the 4% sensitizing solution to prevent the latent image of the adhesive layer. The sensitizing property of the adhesive layer can be increased by appropriately increasing the acidity of the potassium dichromate (K2Cr2O7) solution, but the dark hardening of the adhesive layer is also accelerated accordingly.
Second, the new carbon paper substitute - pre-photosensitive gravure resist film
There have been several pre-photosensitive gravure resist films that have been tried in the past. There is also a new pre-sensitized carbon paper; there is also a carbon paper substitute, and the back is a film of Rope film (DuPont Rotofilm), which is a patented film for the version. Germany's pre-sensitized carbon paper has been tested in China, but there is not enough data to report on its effective performance. On the other hand, Rotofilm has been used in many printing factories for several years, and it can be replaced by a continuous yang image and a magenta or gray contact screen instead of the two images (Durgen Dultgen mesh method) Version law). It is a positive color film, so it must be washed under dark red safety light. The magenta screen used with the Luotu film is exposed through the K2 or #3385 frosted glass yellow filter, while the continuous tempering image is exposed through the frosted glass #5840 UV filter. After rinsing, the diaphragm has the following advantages:
1. It can be stored for use. 2, can be shipped without refrigeration. 3. There are no dark reactions and continuous reactions like carbon paper. 4. The density value can be read on the densitometer to check the accuracy of exposure and development. 5. Only one continuous illuminating picture is needed, instead of the screened positive image used in the two positive image methods of the horizontal net, it has a large contrast characteristic, so it cannot be reproduced as often as carbon paper. detail. In order to make up for this shortcoming, you need to There is a relatively high specular position on the continuous tempering map, otherwise the correct satisfactory corrosion is not obtained at a density of 0.30 or 0.35. It is also more sensitive than carbon paper in the process of dyeing and UV. After exposure and development, the Rope film is basically the same as carbon paper in terms of processing, hot water development, and etching.
Kronaville is another corrosion-resistant film recently invented by DuPont to replace the existing photosensitive resist film. After exposure on the positive image, it can be etched on copper or other surface layer without development. . After the plate is pressed to the roller, all the operations are the same as the carbon paper-passing technology used now. The back of the film can be peeled off with a finger. Only the corrosion-resistant film is on the copper surface, and the iron chloride corrosion can be performed. Hot water or chemical developer is required. After the corrosion is completed, the corrosion-resistant film can be washed away with acetic acid and an aqueous solution, eliminating the need for hot water development time, and eliminating all the shortcomings caused by hot water development. And to ensure a uniform corrosion effect. This special Kronaville anti-corrosion film is currently only available for high contrast and line images. Emulsions used in continuous positive images are not yet available for all printing units because they produce dust particles that may cause halos or spots when corroding the copper surface. According to the manufacturer's explanation, because the emulsion film is too thin, this defect has been corrected to increase the thickness to achieve better adhesion to the copper surface layer. It is expected that the improved Kronaville corrosion resistant film can replace other corrosion resistant films such as carbon paper.
The high-contrast Kronaville anti-corrosion film can also be used to add a positive image to a long-distance transverse dot to achieve the effect of direct corrosion. As for the reproduction curve, although it can not be compared with the results obtained by the two positive method and the carbon paper anti-corrosion film, it can also obtain a fairly good replica, if it does not emphasize the requirement of the main tone level and details, etc. The reproducibility on the top can be determined to be no problem.