Most of the lithography used today uses the indirect printing method in which the image on the plate is first transferred to the blanket, and then the blanket is transferred to the paper (substrate). This type of lithography is called offset printing. . Due to the elasticity of the blanket, fine graphics can be printed.
I. Overview
How lithographic plates can make pictures on a plate have good lipophilicity and blank parts have a stable hydrophilicity. Depending on the materials and methods used, there are many kinds of plate making methods.
1. Classification
1 According to the classification of plates: slate, zinc plate, aluminum plate, paper base and so on.
Slate is the earliest lithographic plate. Slate is bulky and can only be printed directly, so it is no longer used.
Zinc and aluminum plates are commonly used plates. The paper base has been developed in recent years, used for electrostatic printing, prints with a small amount of printed matter, and light printing.
2 According to plate-making methods, there are: hand-rendered, transfer-printed, ie coated, pre-coated, multi-layer metal lithographic, plano-intaglio, electrostatic, dry lithography, and so on.
Hand-printing roller The original lithographic printing method uses hand-drawn inks to create a print section on a plate. It is no longer used.
3 According to the beginning of the printing plate surface classification: lithographic, flat convex, flat intaglio.
2. Photosensitive materials for plate making
(1) Dichromate photosensitive material
Dichromate photographic material is a mixture of chromic acid and high molecular substances, and has sensitization properties after drying. The polymeric materials commonly used in lithographic printing are: proteins, polyvinyl alcohol, and the like.
Dichromate: ammonium dichromate. Dichromate is toxic, so be careful when using it.
(2) Diazo photosensitive materials
The diazo photosensitive material is a photosensitive resin composed of a diazo compound or an azide compound and a natural resin. Photoresponsive resin can cause photochemical reaction, which will cause decomposition of molecular structure or change of polymerization and polymerization. When solvent is encountered, physical properties will change. Therefore, it can be used for printing plates and the like.
Second, the homemade lithography law
1, metal lithography method
Metal plates for lithographic printing are mainly made of zinc plates and aluminum plates, and are also used for iron plates or copper plates. This is determined by the characteristics of the lithographic printing. The lithographic plate material must have a high degree of lipophilicity and good hydrophilicity. In fact, there is no material for this treatment. If the general metal has a strong lipophilicity, the hydrophilicity is weak. The order of oleophilicity of commonly used metals is: copper> iron> zinc> aluminum> nickel> chromium.
From the above order, it can be seen that copper has the best lipophilicity and hydrophilicity is the worst. Chromium has the best hydrophilicity, but its lipophilicity is the worst. Both zinc and aluminum have certain oleophilic properties and have a certain degree of pro- Water-based, after special treatment, the lipophilicity and hydrophilicity can be improved, so the use of zinc plate and aluminum plate is mainly used in the lithography, and the copper plate and the iron plate are used to make the multi-layer metal plate.
(1) Plate surface treatment
The surface treatment of the plate is a good basis for establishing the printing part, and the specific surface of the plate is enlarged. The ability to suck is enhanced, and for the blank part, it is beneficial to store moisture and reduce the surface tension. For the used printing plate, there is a role to remove the old graphics on the plate.
Plate surface treatment is to establish the sand, the establishment of sand methods are: mechanical, electrolytic, sandblasting and corrosion, usually using mechanical grinding method.
Mechanical Abrasive Method Plates are ground in a plate-grinding machine to form sand grains. The plate mill consists of a grinding disc, an eccentric shaft and a base. The plate is placed on the plate, and then the ball is placed. The plate is rotated by the eccentric shaft. The ball is rolling on the plate. The water and the sand are sprinkled. The water plays a role of wetting. The ball rubs the plate to the plate. On the surface, grinding occurs, making the surface of the plate fine sand. The depth and thickness of the sand, the thickness and the size of the ball, the specific gravity, and the hardness and fineness of the sand, the amount of water used, the stroke of the plate mill, and the time for grinding are all relevant.
The sand blasting method uses an air compressor to compress air, and the sand particles are sprayed onto the plate material to form sand spots. Sand blasting has high efficiency, but the quality of sand is not easy to control and the uniformity is poor.
Corrosion method Etching the plate surface with a chemical etching solution to form a sand spot. The advantage of the corrosion method is that it avoids the noise of mechanical plate-making and reduces the physical labor intensity. However, it has high cost, low efficiency, unstable quality, and less utility.
Electrolytic aluminum plates use electrolytic sand, the use of alternating current, electrolyte with dilute hydrochloric acid, when the aluminum plate is the cathode, the hydrogen ions in the solution in the aluminum plate to accept the electrons, generate hydrogen and escape, when the aluminum plate is the anode, aluminum atoms released The electrons generate aluminum ions and enter the electrolytic solution. Under the repeated action of the alternating current, the reaction proceeds alternately, and the sand is produced by this method. In this way, fine and uniform sand can be obtained.
(2) Protein version printing
The printing method uses eggs as a high molecular material, and is mixed with dichromic acid as a photosensitive liquid, which is suitable for printing with an anisotropic negative film. Therefore, it is called a protein plate, and the process is also known as negative image printing.
After grinding, the metal plate of the sand is set up, and then the front etching is performed. The purpose is to remove mud and oxide film remaining on the plate after grinding. At the same time, the pre-etching solution is an acidic solution, which can cause slight erosion of the plate with sand, forming a very fine uneven, further increase the specific surface area of ​​the layout, increase the adsorption capacity.
After the metal plate surface is subjected to grind plate and pre-etching, the flow cloth protein photosensitive solution is dried, and is intimately exposed with the negative film. Since the transparent part of the negative film is a graphic part, the photosensitive layer of the graphic part is exposed to light, and photochemistry occurs. The reaction causes the photosensitive layer to harden, forming a printed portion on the printing plate. After coating the developing ink, the hardened adhesive layer absorbs the developed ink and has a lipophilic property. The blank part is an opaque part of the negative film and can block the passage of light. Therefore, the photosensitive layer is not exposed to light and the gum does not degenerate during exposure. Can still be dissolved in water, dissolved in water during development, leaving only the hardened adhesive layer on the layout, the printed part.
Proteinboard printing process flow is: before corrosion → flow cloth photosensitive solution → exposure → rubbed developing ink → development → after corrosion → gluing. As shown in Figure 4-25.
The base of the printing part of the protein printing plate is a photosensitive adhesive layer coated with a strong developing ink to form a lipophilic printing part. The printing part is slightly higher than the blank part, and the foundation is only a hardened adhesive layer, so the wear resistance and the resistance to the printing force of this printing plate are low, and in particular it cannot meet the requirement of a high-speed printing press. In addition, the graphic is prone to hypertrophy, so it is less used now. Its advantages are simple operation, low cost and so on.
(3) flat gravure printing
Flat intaglio printing is the use of polyvinyl alcohol and dichromate mixed as a photographic solution, the use of positive photo negative plate printing version of the production, it is also known as the PVA version, the process is also known as Yang Shai Ban. The plate-making method is to enhance the image-resistance of the image part and make the image concave 3 to 5 microns, which is called deep-etched plate.
The gravure plate engraving process flow is: before the corrosion → flow cloth photosensitive solution → exposure → development → corrosion → rinse → dry → squeegee → on the base paint → wipe the developing ink → in addition to the film → after corrosion → sizing. As shown in Figure 4-26.
The development is to remove the undamaged adhesive layer on the plate, so that the graphic part can be completely exposed on the metal plate. In order to not only remove the adhesive layer of the graphic part, but also protect the adhesive layer acting on the light, the developer is composed of calcium chloride, zinc chloride, and lactic acid. The length of development time is related to the ambient air temperature, humidity, and developer concentration.
Corrosion is to expose the printed part of the metal plate, the corrosion with the acid, so that the graphics part of the formation of micro-concave, the composition of the etching solution and the liquid phase near, only acidic, is the use of strong acid (hydrochloric acid) instead of weak acid ( Lactic acid).
After the metal plate has been rinsed, the etching solution is removed and dried to check whether the metal surface of the plate has a gap. If there is any contamination, the exposed metal surface must be covered with glue, which is called a glue.
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