Tinplate is mainly used as a tin can for packaging. Tinplate printing is mainly lithographic offset printing, which uses the physical phenomenon that oil and water are incompatible to print. Therefore, in addition to the printing suitability of offset printing inks, iron printing inks must also have characteristics such as heat resistance, solvent resistance, processability, cooking resistance, and wear resistance. The coating is also an indispensable material in tinplate processing.
Coatings are mainly composed of film-forming substances, pigments, driers, auxiliary agents, solvents, etc., and can be divided into three coatings: internal coating, external coating and varnish from the coating method and role. It plays a good protection on the ink layer and increases the brightness of the image so that the product has a good metallic color. At the same time, food cans and other coatings can effectively prevent the contents from being chemically and biologically contaminated, reduce the possibility of leaking cans, protect the original flavor and color of the food, extend the storage time and make the contents easy to pour out of the container.
â‘ Primer coating with good performance. Primer coating is an indispensable coating in tinplate surface printing process. It is mainly to improve the adhesion of the tinplate surface, so that the ink can be firmly adsorbed on the iron surface, so that when the tinplate is bent, impacted, stretched, curled, etc., the ink coating is not easy to be damaged. Therefore, when used as a coating, it is required to have a good wet adsorption performance on the metal surface, and a high mechanical resistance after drying to form a film. However, resins such as alkyd and epoxy have good adsorption and resistance properties due to molecular structure.
â‘¡Advanced coating process. To obtain a high-quality primer coating layer, in addition to a primer with good performance, a technologically advanced coating process is also indispensable. High-quality coating means that the tinplate surface should be coated with a thin and uniform coating, good flowability, and smooth surface. The thickness of the coating film should be controlled at about 1.5 ~ 2g / m2. If the coating is too thin, the hiding power, abrasion resistance, adsorption, gloss, etc. cannot meet the requirements. If the coating is too thick, it will cause shrinkage stress and affect the adhesion. At the same time, it will cause increased costs and easily cause incomplete hardening. quality.
The control of the coating process is mainly to control the speed and stability of the coating machine, the uniformity of the coating supply and the level of the baking temperature, and this depends greatly on the advanced nature of the coating process. Equipment accuracy and automation.
â‘¢ Influence of paint viscosity. In addition to coatings with superior performance and advanced coating processes, the viscosity of the tinplate coating layer also has a great influence on the uniformity of the coating thickness, so the control of the coating viscosity also appears to be quite important. Different paint viscosities will have different paint layer thicknesses under the same conditions. The higher the viscosity, the thicker the coating after coating with the same amount of coating; this aspect improves the protective performance of the coating, but the mechanical processing performance will be reduced, which is prone to poor curing, insufficient adhesion, and coating turtles. Cracking and shedding. At the same time, the leveling of the paint is reduced, and the coating film is unevenly coated, which is prone to defects such as "wrinkle". Conversely, the lower the viscosity, the thinner the coating thickness, and the worse the wear resistance and adsorption.
Therefore, whether the viscosity of the paint is too high or too low will affect the quality of the coating, and the control of the viscosity of the paint is mainly achieved by adding a diluent. At the same time, the change in the ambient temperature will also change the viscosity of the paint, and the viscosity will decrease when the temperature increases Otherwise, it will increase. [next]
2. Printing ink
The composition of tinplate printing ink is similar to ordinary ink, mainly composed of pigments, binders, fillers and auxiliary agents. However, due to the smooth surface of tinplate and poor absorption, the ink used for tinplate printing must be different from ordinary offset printing inks. In addition to the printing suitability of general offset printing inks, tinplate printing inks should also have the characteristics of impact resistance, strong adhesion, heat resistance, light resistance, and cooking resistance.
â‘ Good adhesion and mechanical properties. The surface of the tinplate printed substrate is very smooth and does not have absorption capacity, so it should have good adhesion. At the same time, tinplate prints are generally not final products. They must be made into various food cans, toys, metal boxes, and barrels and cans for chemical products. They need to be cut, bent, stretched, and shaped. Has strong mechanical properties.
â‘¡High temperature resistance. The surface of tinplate is smooth, non-absorbent, impermeable to moisture and solvents. Drying mainly depends on the oxidation of conjunctiva and the evaporation of ink solvent, and the drying speed is slow. Generally, it needs to be baked to complete drying, so tinplate should have good heat resistance, and the pigment will not fade after high temperature baking. At the same time, food, medicine and other packaging boxes must be sterilized by high temperature cooking, and the ink should not change color and yellow in this environment.
â‘¢ Dryness of ink. In tinplate printing production, it is necessary to reasonably control the drying speed of the ink to ensure product quality. If the ink dries too quickly, it will reduce the transfer performance of the ink; it will cause the imprint to become weak, the ink color will be pale, and even the ink dry on the surface of the printing plate and the ink roller, and the ink will be blocked. If the ink dries too slowly, it will cause difficulties in overprinting, adhesion, backside stickiness, poor adhesion, and low abrasion resistance; it is easy to cause scratches in the transmission process. Controlling the drying speed of the ink is one of the important factors to improve the adhesion fastness of the ink layer. Therefore, in recent years, a large number of UV inks have been used in metal printing because UV inks can easily control the drying speed and have good adhesion. And wear resistance. [next]
3. Coated with varnish
In tinplate printing, in order to improve the adaptability of post-press processing and improve the appearance gloss of tinplate products, before the printing ink is not completely dried, it should be glazed to form a uniform and smooth coating to avoid bleeding. . Coating with varnish can not only improve gloss, but also enhance the flexibility, impact resistance, abrasion resistance and adhesion of the ink layer surface. In addition, the coating film formed by high-temperature curing of varnish isolates the ink coating from the outside world and protects tinplate products from physical and chemical substances.
Commonly used varnishes are solvent-based, water-based and UV varnishes. The solvent type has certain pollution due to its volatile solvent, so its use is subject to certain restrictions.
Water-based varnish is an environmentally friendly varnish that has been more and more widely used in recent years. It has high gloss, fast drying and good film forming performance, and excellent friction resistance. It is an ideal varnish. However, 30% of the drying process of water-based varnish is through volatilization, 70% is absorbed by the material, and tinplate is not absorbent at all, so it is not yet suitable for printing on tinplate.
UV varnish is mainly composed of photosensitive resin, reactive diluent, photoinitiator and additives. UV curing is to initiate polymerization reaction and cure to form a film under ultraviolet irradiation. Since the surface of tinplate is a non-absorbent surface, the drying problem after printing is one of the main technical problems of tinplate printing. It adopts ultraviolet irradiation for instant drying, fast curing speed, firm film surface after drying, good gloss, wear resistance and protection, does not pollute the environment, it is a very widely used varnish in recent years. UV varnish still has high cost, large energy consumption and irritating smell.
Various varnishes have advantages and disadvantages. In actual production, the varnish type should be selected correctly according to the characteristics and uses of the printed product. But no matter which type of varnish is used, you should understand the solid content of the varnish, the purity of the oil, the viscosity of the coating, the baking temperature and other technical data before use to ensure the hardness (wear resistance) and gloss of the coating film Degree and adhesion.
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