Key points for quality control of the hottest wate

Key points for quality control of water-based film coating products

film coating process, also known as post press overmolding, post press glue mounting and post press film sticking, is one of the most important processes in post press processing technology and an effective means to improve the grade and commodity value of prints. It is often used in the decorative processing technology of pre printed carton paper, carton, handbag, cover and other products. Printed products can be waterproof, antifouling, anti fading, heat-resistant, friction resistant and other properties after being coated. At the same time, it can enhance the gloss, aesthetic feeling, puncture strength and tensile strength of products, and improve the service life and packaging quality of products. In the film covering process, the solvent type, that is, the coating type, is damaged to the health of operators due to the use of solvents containing benzene, pollutes the workshop environment, and the increasing concentration of organic solvents leads to fires. The paper and film materials after film covering are difficult to recover, wasting resources and other shortcomings, and are gradually replaced by the water-based film covering process, which is favored by customers because of its high strength, high grade, easy recovery, pollution-free and other characteristics. Water based film coating can not only improve the environmental problems in the production process, avoid physical injury to operators, but also reduce production costs. It is easy to recycle and widely used because of the high strength of film coated products. Its process flow is shown in Figure 1. The quality of printed matter after water-based film coating is also affected by many factors. This paper will discuss the key points of film coating product quality control in water-based film coating process

1. Self quality of film coating adhesive

water based film coating adhesive is a white lotion adhesive composed of water as dispersant and polymer as dispersed phase. The water-based film-forming resin of water-based film-forming adhesive affects and dominates the gloss, adhesion, wear resistance, water resistance, flexibility and drying performance of the film-forming adhesive film layer, and plays a protective role on the surface of paper and ink. Functional additives are used to improve the transfer, leveling, drying and smoothness of water-based film covering adhesive, adjust viscosity and improve gloss. The water-based adhesive has a certain powder eating ability for the powder spraying on the printed matter, and can improve the light of the coating. As shown in the photo, there are many varieties of gloss, from general-purpose fixtures to special fixtures for wire rod experimental films. However, if too much powder is sprayed, snowflake like white spots will appear. At the same time, when printing with gold and silver ink, if the amount of glue is too large, the water will soak the gold and silver ink, causing the gold and silver ink imprint to blacken. Therefore, the water-based film covering adhesive with good quality should not contain organic solvents, and the residual monomers should be less than 0.1%. It smells odorless or slightly with the smell of alcohol or ester. After the film covering, the paper fiber absorbs and emits water to make the adhesive dry and transparent, and firmly adheres to the plastic film. The film covering product has good gloss, but there is still no obvious improvement, and the adhesive force is strong

2. Paper material

after water-based coating, due to the poor initial viscosity of the water-based adhesive, the viscosity is very small before drying, and the paper deforms due to the adsorption of water. After coating, the paper will be separated from the film at the concave and convex places due to the deformation of the paper, resulting in bubbles after drying. At the same time, when the paper is thick or stiff, the water-based film trailer with film is slightly longer, which will affect the finishing of the subsequent process. Therefore, the water-based film coating process requires that the paper and paperboard quantity be 100 ~ 400g/m2. It is relatively difficult to film coated paper with a quantity of less than 100g/m2 and paperboard with a quantity of more than 400g/m2, especially not easy to be used for sizing offset paper. Practice has proved that the products covered with film and water-based film on offset paper products are easy to cause separation of paper and plastic. Sometimes, the film can be completely separated from the print with a gentle tear, which is easy to cause quality accidents. At the same time, the coated paper with low ration is easy to discount and overflow when laminating, and the cardboard with high ration is easy to degumm at the indentation during die cutting after laminating

3. Film material

plastic film used for water-based coating is basically the same as traditional oil-based coating, mainly polypropylene film, but also PVC film, polyethylene film, polyester film, etc. The film thickness is generally 0.01 ~ 0.02mm, which requires high transparency, light transmittance of more than 90%, light resistance, chemical corrosion resistance, stable geometric dimensions, certain mechanical strength, etc. In the film covering process, the adhesive used for water-based film covering is water-based film-forming resin, which has low requirements for corona treatment of the film and long storage time after film covering. At the same time, the initial viscosity of the water-based film covering adhesive is poor, and the tension and pressure on the film are small. If the wrinkles and rewinding tightness on the film are different, it will cause problems such as hollowing at the lap of the paperboard during film covering, which will affect the quality of the film covered products

4. Printing ink layer condition

printing ink layer condition mainly refers to the size of the graphic area on the paper, the thickness of the ink layer and the performance of the ink, etc. their influence on the coating is mainly manifested in the influence on the adhesion strength between the printing and the film. When the printed matter is on the ground, the ink layer is thick and the image and text area is large, the ink changes the surface characteristics of the paper with many gaps, hinders the penetration and diffusion of the adhesive, and makes the printed matter difficult to bond with the plastic, prone to delamination, difficult to dry and other quality problems. In the water-based film covering process, the film covering adhesive contains a certain amount of water. When it is used for large-area (on-site) graphic products, it will show a large area of snowflake spots on the surface of the graphic, resulting in color change of the graphic and affecting the appearance quality of the product. Therefore, water-based lamination is not suitable for large-area printing lamination, especially for printing gold and silver ink

5. Film covering equipment

the water-based film covering machine is composed of paper feeding part, paper conveying and positioning part, gluing part, pressing part, winding part, slitting part and so on. After laminating, it does not need to be heated and dried, so it can be rolled directly, so the machine speed is faster, and the maximum speed can reach 80 meters/minute (about 7000 pieces/hour). The machine speed is equivalent to that of ordinary offset printing brush machines. The paper feeding part and the paper conveying positioning part are the same as that of a general paper offset press. The water-based film laminating machine has no heating and drying channel, and the distance between the gluing part and the pressing part is relatively close. The gluing part is relatively simple. The glue liquid is mainly dried by osmosis. The pressing part is at room temperature, and there is no pressure roller. The pressure is also small, so it is not suitable for film lamination of gold and silver cardboard and hot stamping products. The special water-based laminating machine is equipped with a drying channel, which is similar to the oil-based laminating machine. It can film gold and silver cardboard, but the speed is low

6. Concave convex embossing

concave convex embossing is also one of the important processes of printing surface finishing, but it is not easy to carry out concave convex processing after water-based film coating. When pressing concave convex embossing on water-based film coated products, white spots, bows and wrinkles will appear on their contact surface, the paste is not firm, and it will blister in serious cases, which is more prominent on coated paper. For example, some adhesive book covers, The rubber bar line on the side of the cover after water-based coating is not very straight, and there are uneven impression marks, which seriously affects the appearance of the product

7. Other factors

in the water-based film covering process, the factors that affect the film covering quality of products also include the drying of diluents and inks. Usually, if white ink is used as diluent in the ink, there are obvious powdery particles in the white ink, which make it not tightly combined with the binder and hinder the adhesion, which will cause weak adhesion and affect the quality of the print. At the same time, when the ink is not completely dry, it will also cause foaming and delamination of the product after coating, which will affect the appearance of the product

in a word, in the film covering process, water-based film covering uses water-based environmental friendly glue without heating and drying device, which is high-speed, energy-saving and high gloss. It is welcomed by many manufacturers and is gradually replacing the traditional oil-based film covering process. However, the effect of water-based film coating is not perfect, and the print quality after film coating is also affected by many factors. The related research on water-based film coating process still needs to be continued. Today, the post press processing technology has attracted much attention. With the improvement of the film covering process, the completion of the production equipment, and the improvement of the relevant properties of the printing paper and film materials, the printed products after water-based film covering will show higher quality and better enhance the product value, and the water-based film covering process, which represents the development direction of the film covering process, will usher in a broader tomorrow