Discussion on the problem of no light hardening of carbon paper

Carbonized paper has been sensitized to ensure that the adhesive layer's solvency and ease of exposure are not affected by light. The so-called dull hardening (that is, dark reaction) is the sensitized carbon paper, which has a thicker adhesive layer thickness after development without being exposed to light. The concentration of potassium dichromate solution is large, the free acid is large, and the animal gum is different. Therefore, in the actual production process, care must be taken to promote the photohardening process of the carbon paper layer. This requires the actual operator not only to pay attention to the indoor temperature and humidity during the preservation of the sensitized carbon paper, but also to pay attention to the storage time of the sensitized carbon paper, because it is related to whether the corrosion process can be achieved. The problem of stability and access to high-quality printing.

The reaction process inside the gel layer is indicated as follows. Potassium dichromate is in an ionic state in aqueous solution: K2Cr2O7→2K++Cr2O7--, wherein: Cr2O7--+H2O—2CrO4--+2H+. That is, potassium dichromate has different ion forms in acid and alkaline solutions: When the solution is basic, dichromate ion reacts in the direction of chromic acid generation ions. The oxidation of colloids by dichromate ions is stronger than the chromate oxidation of chromate ions. Therefore, in order to weaken the oxidation of dichromate ion and slow down the dark reaction, an appropriate amount of ammonia can be added to the potassium dichromate solution to enhance the alkalinity of the solution. There are three roles:

1. The reaction proceeds in the direction of the formation of chromate ions. At this time, the orange potassium dichromate develops a pale yellow color, and the dark reaction inside the carbon paper layer is weakened.

2, to improve the adhesion of colloidal substances, so that the adhesive film can closely adhere to the layout.

3. Inhibit the activity of microorganisms to prevent the photoreceptor solution from corrupting and deteriorating.

After the photorelution solution is added with ammonia, the pH is usually between 7.5 and 8.8 and it is slightly alkaline. During the drying process, ammonia is gradually reduced, become slightly acidic, and therefore the film and photographic performance gradually increased, does not affect the results. Since higher temperature will lead to separation from pure chromic acid chromate out settings required for this purpose freezer. It must be noted that the humidity inside the refrigerator is the same as the relative humidity in the studio.

First, carbon paper continues to harden after exposure

The exposed carbon paper can not be developed after it is stored for 20 hours under certain humidity conditions. Practice has proved that if the carbon paper is stored in the sun for a day after the wet season, and then goes to the sun, this carbon paper is not easily adhered to the plate cylinder. Even if it has been attached to the roller, it is obviously No image comes. Therefore, it is required that the carbon paper that has sunrayed through the mesh and the positive image be immediately printed, and the time between the printing and the overprinting should not be prolonged. Prolonged delay will bring certain difficulties to development and corrosion work. Plate-making workers must cooperate with each other to enhance the work plan, clean the drums in a timely manner to do a good job of degreasing work;

Another method to prevent the dark layer of the adhesive layer is to add 0.3% potassium citrate (K3C6H5O7.H2O) as a cleaning agent in the 4% sensitization solution to prevent the latent image of the adhesive layer. The photosensitivity of the adhesive layer can be improved by increasing the acidity of the potassium dichromate (K2Cr2O7) solution in an appropriate amount, but the dark hardening of the adhesive layer can be accelerated accordingly.

Second, new carbon paper substitutes - pre-photogravure resist film

Several pre-sensitized gravure resist films have been tried in the past. There is a new pre-sensitized carbon paper; there is also a carbon paper substitute, and the back is a film of Rotax film (Rotofilm DuPont product), which is a patented film for printing. Germany's pre-sensitized carbon paper has been tested in China, but its effective performance has not yet been reported enough. On the other hand, Rotofilm has been used in many printing factories for several years and can be replaced by continuous positive images and magenta or gray contact screens. (Dorgen's Dultgen mesh method has two positive images. Edition method). It is a positive color film, so it must be washed under dark red security lights. The magenta screen used in conjunction with the rota film is exposed through a K2 or #3385 frosted glass yellow filter, while the continuously adjusted image is exposed through a frosted glass #5840 UV filter. After washing, the diaphragm has the following advantages: 1, can be stored for use. 2, can be shipped without refrigeration. 3. There is no dark reaction or continuous reaction like carbon paper. 4. Density meter can read out its density value to check the accuracy of exposure and development. 5, only one continuous adjustment of the Yang picture, without the need for a positive screen like the positive picture in the two positive drawing methods for the transverse network, it has a greater contrast characteristics, so it can not be copied as high light like carbon paper detail. In order to make up for this shortcoming, it is necessary to have a sufficiently high highlight on the continuous adjustment pattern, otherwise it will not be correctly and satisfactorily corroded at a density of 0.30 or 0.35. It is also more sensitive than the carbon paper in the modification of dyes and UV factors. After exposure and development, Rohto sheet is basically the same as carbon paper in terms of printing, hot water development and etching.

Kroneville is another corrosion resistant film recently invented by DuPont to replace the existing photosensitive resist film. After exposure, it can be plated onto copper or other surface layers for corrosion after exposure on a positive chart. . After the plate is transferred to the cylinder, all operations are the same as the carbon paper overprint technology used now. The back of the membrane can be peeled off with fingers. Only the corrosion-resistant film on the copper surface can be subjected to ferric chloride corrosion. Hot water or chemical developer is required. After the etching is completed, the remaining corrosion-resistant film can be washed with acetic acid and an aqueous solution, eliminating the need for developing time with hot water, and eliminating all the disadvantages due to hot water development. And ensure that it can achieve a uniform corrosion effect. This special Koronaville corrosion resistant film is currently only suitable for high contrast and line images. Emulsions used for continuous positive images cannot yet be used for all printing units because the emulsion produces dust particles that may cause halos or flares when corroding the copper surface. According to the manufacturer's explanation, the emulsion film is too thin, and this defect has been corrected to increase the thickness so that it can adhere well to the copper surface. It is expected that the improved Croneville Corrosion Resistant Film will replace other corrosion resistant films such as carbon paper.

The high-contrast Kronewell corrosion-resistant film can also be applied to positive images on long-distance horizontal meshes to achieve the effect of direct corrosion. As for the reproduction curve, it cannot be compared with the results obtained by the two positive drawing methods and the carbon paper corrosion-resistant film, but it can also be obtained a fairly good reproduction, if it does not emphasize the requirements of the main tone levels and details, etc., in the corrosion The reproducibility can be confirmed without any problems.