What is the principle and method of Relief Printing?

There are many methods of Relief Printing. It can be selenium from a photographic negative on a metal sheet and corroded to form a relief printing plate, or a photographic negative can be sun-dried on a photosensitive resin to form a relief printing plate; and an electronic engraving machine can be used to engrave it into a relief printing plate. Printing plate: The made relief plate can be copied into relief printing plate by casting chip method, and the plate making method can be selected according to the requirements in use.

copper-zinc letterpress

Photographically, copy the graphics and text on the original into a positive negative negative film, and then expose the positive negative negative film to a copper or zinc plate coated with a photosensitive layer, and use iron trichloride after developing a hardened film. Or nitric acid will corrode the blank part of the printing plate layout, and get the embossed printing plate.

Metal plates are generally made of copper plates for high-quality patterns and photos of continuous adjustment and screens. Zinc plates are used for drawing or simple patterns. There are microcrystalline zinc plates in zinc plates, which are suitable for powder-free corrosion and can also be used instead of copper plates. Powdered zinc plate is suitable for powdered corrosion. The powdered zinc plate used to make relief plates is an alloy. Contains 99% zinc, 0.2 to 0.5% lead, 0.2 to 0.35% cadmium, and 0.012 to 0.2% iron. The microcrystalline zinc plate contains 99% zinc, 0.02～0.10% aluminum, and 0.05～0.15% magnesium.

The process flow of plate making is: plate preparation → exposure → corrosion → full page → proofing.

1. Plate preparation

Choose the appropriate copper or zinc plate, cut it into the required size, and remove the oil stains, oxide film, scars, water stains, blisters, etc. on the surface of the board. The removal method can choose light, soft charcoal without knots or cracks for grinding. The polished surface also produces very fine streaks, which can increase the adsorption force of the photosensitive liquid. The polished metal plate is placed in the drying machine and spread with chromium glue photosensitive liquid. After the photosensitive liquid is dried, it can be used for printing.

The chromium glue photosensitive liquid is made by mixing dichromate (ammonium dichromate, potassium dichromate, etc.) and polymer substances (gelatin, polyvinyl alcohol, etc.).

2. Print

Exposure is carried out in an exposing machine, so that the main surface of the metal plate coated with the photosensitive liquid is in close contact with the positive and negative image negative. Some methods of close contact are pressurized with a sponge pad, and some use a vacuum exhaust method. During exposure, only the image on the negative image film can transmit light during exposure, so that the photosensitive layer is exposed to light and becomes a water-insoluble substance, forming an image on the metal plate, and the photosensitive layer on other parts of the plate is not exposed to light Function, the photosensitive layer has not been denatured and can still be dissolved in water.

The exposure time is determined by factors such as the density and transparency of the film, the thickness of the line, the performance of the photosensitive liquid, the thickness of the sensory layer, the composition and strength of the light source, and the distance from the exposure machine to the light source.

After the exposure, the metal plate is washed with water for development, so that the photosensitive layer of the part that is not exposed to light is dissolved by water. After the display, only the shadow formed by the hardened glue layer remains on the surface of the plate. In order to make the image clear and recognizable, The surface of the board can be dyed with methyl violet solution, and can play a role of hardening film to prevent degumming.

In order for the hardened adhesive layer (ie the image part) on the layout to adapt to the corrosion of the next process, the corrosion resistance of the hardened adhesive layer must be increased. Therefore, it should be baked until the adhesive layer is maroon. Baking plate requires uniformity across the entire layout. If the baking plate is insufficient, the adhesive layer is not completely dehydrated and hardened, and it is orange, which will reduce the corrosion resistance and cause local degumming during corrosion. If the plate is baked for too long, the adhesive layer will appear black. The glue layer cannot be closely adsorbed on the plate surface, and the corrosion resistance will be lost, which will cause a total degumming. The baked plate can be repaired and corroded after cooling.

The purpose of retouching is to completely and cleanly remove the unnecessary resist film on the layout, while maintaining and trimming the required image to make it have resist performance.

3. Corrosion

The purpose of the corrosion is to keep the image and text part with the hardened adhesive layer intact, while the blank part without the adhesive layer is dented after being corroded, so that the image and text part is still on the same plane and higher than the blank part, reaching the relief Printing requirements.

Corrosion Generally, copper plates are corroded with ferric chloride solution, and the chemical reaction formula is:

2FeCl3+Cu→2FeCl2+CuCl2

The zinc plate is corroded with dilute nitric acid solution, and the chemical reaction formula is as follows:

4Zn+1一10HNO3→4Zn(NO3)2+N2O+5H2O

The corrosion results of these solutions on the metal printing plate not only corrode the blank part vertically downwards; but also produce side corrosion to the image part, thereby reducing the printing resistance of the printing plate. In order to prevent side corrosion, powder ( Red powder) corrosion and powder-free corrosion are two methods.

(1) Powder corrosion process

The powdery corrosion process is: before the corrosion starts, acid-resistant paint should be painted on the back of the metal plate to protect the back. Put people into the corrosion machine for initial corrosion. When the graphics and text begin to appear on the side, the side must be protected to avoid side corrosion during the continued corrosion process, which will make the small lines thin and deformed. After the initial corrosion is completed, the side will be cleaned. , Pay attention to the ammonium chromate solution in all aspects. The effect of the ammonium chromate solution is that ammonium dichromate is hydrophilic and oleophobic. It plays a role of oil resistance in the blank part of the recess and does not make the recessed part stick to ink. Then roll the ink on the protruding image and text part with the adhesive film to further protect the image and text part.

Side protection, using red powder on the sides of the graphics. After baking, the red powder is dissolved into a corrosion-resistant layer. The red powder is a bright red powder made from the red resin of the Kylin blood tree fruit. It has a low melting point and is insoluble in water. It is soluble in ethanol, ether and oils, and has corrosion resistance. .

After the top red powder is cooled by the baking plate, the second corrosion can be carried out. When the new side continues to produce, repeat the previous method, again brushing, corrosion, and trimming. Generally, it needs to be topped with powder 4 to 5 times to achieve The depth of corrosion is required. Due to the result of repeated application of red powder, the side of the figure is stepped. Such a printing plate is not suitable for printing. For this reason, it is necessary to perform complete corrosion at the end to remove the steps on the side of the figure and text, forming a slope. The side surface can be used for printing. Due to the complex operation and poor working conditions of the process, the powder-free corrosion process has been switched to.

(2) No powder corrosion process

Powder-free corrosion is carried out in a powder-free corrosion machine, which is a corrosion process in which chemical additives are added to the corrosive liquid as a side protection agent. The additives consist of organic solvents and surfactants. The organic solvent is mainly composed of diethylbenzene, refined benzene, kerosene, etc. It has acid resistance, but cannot be mixed with water. After adding the surfactant, under mechanical stirring, the water and the active agent form an oil-in-water emulsion liquid. The tension of the oil is reduced after the film is formed on the plate, and a protective film is formed during the corrosion process. The adhesion of the protective film on each part of the plate is different. In the graphic part, the adhesion of the protective film is large due to the presence of the photosensitive film layer. , And the adhesion of the blank part is small. During the corrosion process, the vertical spray force of the corrosive liquid is very large, which destroys the protective film of the layout and performs deep corrosion, and the side of the graphic part is protected, and the slope is formed. Uniform side.

Powder-free corrosion can be completed at one time, and the graphics and text sides remain intact. Therefore, the powder-free corrosion has the advantages of fast plate making speed, good slope of the graphics and text sides, low labor intensity, and high quality of plate making. Therefore, the powder corrosion process is gradually reduced to powder-free corrosion. Corrosion is replaced by art.

4. Full page

The whole page is the corroded copper-zinc relief plate, and the unnecessary parts are drilled or drilled deeply with a drill to avoid ink staining and dirt during printing. As long as the blank part of the drill can be accommodated, the drill must be drilled. The drill is based on the graphics and text. The closer it is, the better. The drilled plane must be flat, uniform and beautiful, and the depth of the drill must meet the requirements of printing.

For example, if the plates are assembled together, the graphics should be separated one by one. In addition, the plates must be assembled and printed with movable type. Therefore, the plates must be equipped with a base. The base has a wooden base or a metal base. The metal base can be recycled. The angle of the bottom bracket should be 90, which is easy to make up, and the thickness of the bottom bracket should be the same. The height of the plate after adding the bottom bracket should be the same as the height of the type, but it should be low instead of high. If it is low, you can paste cardboard on the back of the bottom bracket to correct it.

Paste the pattern on the metal base or nail it to the wooden base for printing.

5. Proofing

The purpose of proofing is to check and prevent the graphics from being reversed or placed in position, and to check the quality, such as the quality of corrosion. For the problems exposed in the proofing, adopt corresponding methods to solve them, or take the whole corrosion, or take the partial corrosion, or use needles, knives, etc., to make the quality meet the requirements of the original manuscript.