Especially in recent years, with the increasingly fierce market competition and the continuous improvement of customer's consumer taste, manufacturers have given unprecedented attention to the surface finishing process of product packaging. Although the coating process is relatively simple, there are many influencing factors and it easily leads to some quality problems. Moreover, since its quality problem is generally irreversible, the addition of a light coating process is a post-printing process. In the event of a quality accident, the losses incurred are considerable. Therefore, paper mills should pay enough attention to this process and have enough professional knowledge to avoid quality problems.
Polishing process Polishing refers to the process of applying a layer of varnish to the surface of prints to dry them to form a colorless transparent film. The glazing process plays a role in protecting the printed product and increasing the surface gloss of the printed product.
Polishing process classification
According to the glazing method, it can be divided into three types: printing machine direct glazing, special glazing machine glazing alone and online glazing. Printing machine directly glazing means that after the printed matter is dried, the varnish is applied to the surface of the printed matter with a printing machine; the separate glazing process is the use of a special glazing machine to polish the printed matter after the initial drying, also known as the offline process. Online coating refers to the process of directly coating the varnish when the varnishing machine is directly connected to the printing station of the printing machine and the ink is not dried yet. It is also called the online process. According to the coating method of varnish, it can be divided into direct coating and indirect coating. The direct coating is divided into direct coating of a roller and direct coating of an offset plate.
Roller direct coating refers to a method in which a suitable amount of varnish is applied to a roller and the varnish on the roller is directly transferred to a print. Offset direct coating refers to the process of applying varnish to an offset plate and then transferring the offset plate to the printed product. Indirect coating refers to the process of first transferring the varnish onto the roller, and then transferring from the roller to the blanket. Finally, the varnish is transferred to the surface of the printed product through the blanket. According to the glazing part, it can be divided into all the glazing and local glazing.
All glazing means that the surface of the print is coated with varnish, while the partial glazing only selectively applies varnish to the surface of the printed product. According to the glazing dry state can be divided into wet touch dry process and wet touch wet process. Dry wet touch refers to the fact that the printed surface is dry when it is applied. Wet touch wet refers to when the light, the printed surface of the ink has not yet dried, on-line coating is wet wet touch process. According to the gloss effect of varnish, it can be divided into bright type and matt type. After the bright coating, the surface gloss of the printed product increases and the color becomes more vivid. After matt type coating, the surface gloss of the printed product disappears and the color is more elegant. According to the material of the varnish, it can be divided into solvent varnish coating, water varnish coating and UV varnish coating.
Solvent-based Varnish Varnish Solvent Varnish Varnish, also known as oily Varnish, is a high-grade, catalyzed polymerization of advanced polyurethane and acrylic resin that dissolves the film-forming resin to evenly coat the surface of prints. For toluene, xylene and other chemical solvents, it is called solvent-based Varnish. The main components of the solvent-based varnish are film-forming resins, solvents and additives. Among them, the resin functions as a film, an adhesion, and a highlight, and the solvent functions as a solution, a permeation, and a quick-drying.
The basic principle of the solvent-based varnishing process is that the varnish is evenly applied to the surface of the printed product through a roller or a blanket, and then rapidly spreads on the surface and penetrates into the surface of the printed product to form a wet film. After drying, the solvent is volatilized. After the solvent is removed, a protective film with gloss, abrasion resistance and water resistance is formed. (Title 2) How to ensure the stability of the solvent-based coating quality First, quality inspection must be performed on each batch of incoming materials to ensure the stability of batch-to-batch quality. Incoming inspection items generally include viscosity, solids content and pH. Secondly, ensure the stability of the proportion of open and dilute optical oil. Since the solvent of the varnish is extremely volatile during the cycle, the ratio of the film-forming resin and the day water will change, so the solvent must be added at intervals. However, for how often the solvent is replenished and how much it is replenished, many factories rely solely on the experience of operating workers to judge.
Here, it is recommended that you use the method to test the viscosity to determine the amount of supplementation and interval time. The method is to test the viscosity after each time the varnish is dispensed, and the measured viscosity indicates the standard viscosity of varnish and solvent. Viscosity is measured every 20 to 30 minutes during the coating process, and if the increase in viscosity exceeds the specified range, the solvent is replenished to the standard viscosity. It should also be noted that due to different seasons, the viscosity of the same open dilute varnishes is not the same, so the viscosity standard value should change with the temperature change. The viscous cup can be coated with a 4# cup, or a slightly higher viscosity cup such as the Japanese RIGOSHA 4# cup. The permissible range of viscosity fluctuations also depends on the vapour's volatility, air temperature and the quality of the coating. In addition, the operator must adjust the speed, the roll distance, the drying tunnel temperature, and the air volume according to different types of varnish, different quality requirements and the condition of the equipment.
Analysis of Causes of Common Quality Problems and Improvement Measures
Environmental and safety features
Solvent-based varnish has irritating effects on the respiratory tract and the eyes. Frequent contact causes damage to the human liver and reproductive system. During the use, it should be noted that the workshop is well ventilated and the remaining odors are pumped away as much as possible. At the same time pay attention to labor protection, do not directly contact with the human body. Solvent-based varnish is a flammable substance and precautions must be taken during storage and use.
Waterborne Varnish Coating
Waterborne varnish is also called water-based varnish. It is a mixture of water-soluble resin, emulsion resin, anti-wear agent, and other additives in a certain proportion by high temperature catalytic polymerization reaction. The water in the waterborne varnish plays a role of volatilization, permeation, and quick drying, while the resin and the emulsion act as a film, adherence, and high light. Waterborne varnish has high filming speed, smoothness, high abrasion resistance, no yellowing, good elasticity, and high gloss. It is also waterproof, moisture-proof and oil-repellent. Its overall performance is greatly improved. Solvent based coating oil. The basic principle of varnishes for waterborne varnish is the same as solvent varnish, but its curing mechanism is more complex.
Waterborne Varnish Curing Mechanism
After the water-based varnish covers the surface of the printing product through the wetting process, diffusion and permeation functions. The free wear-resistant particles and emulsion molecules in the varnish combine with the water-soluble resin to form a wet film. After drying to eliminate moisture, the film-forming resin and the emulsion are cured into a protective film, and the wear-resistant particles are also fixed in the film. (to be continued)
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Triple C International (ShenZhen) Co.,Ltd. was established in 2018. It`s a professional foreign trade company engaged in import and export trade, and carries out business operations such as international trade, cooperative production and intermediary trade. The company mainly provides high-quality OEM-product and ODM-product services for customs in Japan, Southeast Asia, Europe and Unite States, and has a professional technical team that provides hundreds of OEM-product services for dozens of customers, and has dozens of excellent suppliers. The company mainly provides sewing products, plastic products, paper products, wooden products, household products and other OEM services, including handbags, gift boxes, Storage Boxes, stationery boxes, kid toys, Paper Boxes, school supplies, office supplies and other products import and export business.
