The hottest film covering and specific measures fo

Film coating and its specific measures for quality control

the concept and significance of film coating

film coating, that is, coating plastic film with adhesive, and bonding it with paper printed matter after heating and pressing, forming the processing technology of paper plastic integrated products. Because of a thin and transparent plastic film on the surface of the coated printed matter, the surface is smoother and brighter, so as to improve the gloss and fastness of the printed matter. The color of the pictures and texts is more bright, full of three-dimensional sense, and at the same time, it plays a role of waterproof, antifouling, wear-resisting, folding resistance, chemical corrosion resistance and so on

II. Introduction to film coating classification

according to the process of film coating, it is divided into three types: dry, wet and oily. According to the raw materials and equipment used, it can be divided into instant coating and pre coating processes

1. Namely coating film

namely, during the operation of coating film process, the adhesive is first coated on the film, and then hot pressed, but its production process is prone to fire

2. Precoating and laminating

this process greatly simplifies the laminating process because the laminating equipment does not need to be bonded and can withstand a greater force agent heating and drying system before fracture, and the operation is very convenient. It can be used and started at any time, with great production flexibility, no solvent odor, no environmental pollution, and more importantly, it can completely avoid bubbles, delamination and other failures. Its finished products are highly transparent, and have broad application prospects and promotion value

the pre coating structure is composed of substrate and adhesive layer. The substrate is usually pet and BOPP film. Considering the material cost and processing technology, most of the pre coating substrate adopts BOPP film with a thickness of microns and the adhesive layer thickness of microns. According to the different processing equipment and process conditions, choose different thickness of adhesive layer, which is divided into hot melt adhesive and organic polymer low-temperature resin. The difference between the two is that the hot melt adhesive is made by blending and modifying the main viscosity resin with several materials such as tackifier and modifier, while the organic polymer resin is a single polymer low-temperature copolymer

due to the limitations of technology, production equipment and raw materials, the pre coating film (mostly hot melt adhesive) produced by domestic equipment still has quality defects. Because the production process relies on a process similar to that of instant coating, which dissolves the colloid with organic solvent and applies the glue roll on the substrate film with concave corrugated roller, there is insufficient solvent volatilization, peculiar smell during operation and use, solvent volatilization after coating, and the surface is easy to blister. At the same time, because the hot melt adhesive is blended by several kinds of polymer materials, poor temperature control will cause the degradation and crosslinking of the polymer in the hot melt adhesive, resulting in poor film surface. Some pre coated colloids are easy to adhere to the outer surface of the substrate, resulting in poor coiling. Because the adhesive layer is thin and the surface has not been activated, it is easy to produce defects such as insufficient adhesion to the printed matter. Because the hot melt adhesive is made of several materials, the transparency after coating is significantly worse than that of the pre coating of low-temperature pure resin

3. Water based coating

is a method to process the uneven brush surface of printing coating, so as to improve the gloss and strength of printing surface and enhance the aesthetic effect. To measure the quality of film covering, we also need to check its brightness, intensity, adhesion, etc. Water based wet lamination is favored by customers for its high strength, easy recycling, pollution-free and other characteristics of lamination printing

4. Curved surface coating

is a new transfer technology, which can deal with workpieces with complex appearance, and is suitable for different materials. It also greatly improves the added value of products due to vivid patterns and colors. This technology is a film with special chemical treatment. After printing a certain color pattern, it is sent to the surface of water, and the pattern is evenly written on the product surface by using the action of water pressure. The covering film is automatically dissolved, cleaned and dried, and then coated with a layer of transparent protective paint. At this time, the product has presented a completely different visual effect

film covering process can be divided into cold (cold press) film covering, hot (hot live) film covering and liquid film covering according to cold and hot, pressure conditions and materials. Thermal lamination technology refers to the heating and activation of the adhesive precoated on the diaphragm, through which the diaphragm and printed matter are bonded together; The cold lamination is through pressure, directly rely on the adhesive to bond the film and the print together. These two methods can produce various special films such as UV protective film, waterproof or anti friction film. Recently, there has been a new technology called "liquid film coating" method. That is to brush a layer of film on the surface of the product directly with a brush, spray gun, roller or special film covering machine. Although the technical operation is simple and the price is low, it is still immature

third, the material of film coating is

the commonly used plastic films are: polyvinyl chloride (PVC), polypropylene (BOPP) and polyester (PET) films. BOPP film (PM) is flexible, non-toxic, transparent and cheap. It is an ideal composite material in the film covering process

according to the performance and purpose of the film itself, the film thickness is 0 02mm2. How to reduce the error of the pressure testing machine is appropriate. It must be treated by corona or other methods. The surface tension of the treated surface should reach 4pa, so as to have better wettability and bonding performance. The corona treatment surface should be uniform, and the higher the transparency, the better, so as to ensure the best clarity of the covered print. The transmittance of pet is generally%, and that of other films is usually between%. The film should have good light resistance, not easy to change color under long-term light irradiation, and the geometric size should be stable

since the film must be in contact with solvents, adhesives, inks, etc., it must have a certain chemical stability. The appearance of the film surface should be flat, without bumps and wrinkles, and the film should be free of bubbles, shrinkage cavities, pinholes and pockmarks. Due to the limited adjustment capacity of the laminating machine, it is also required to rewind neatly, with the same tightness on both sides, so as to ensure the uniformity of gluing. Of course, the cost should be low

IV. composite strength

refers to the adhesion fastness between plastic film and paper (board) print in film covered products. Its size depends on the adhesion between plastic film, paper (board) print and adhesive (or hot melt adhesive). When laminating, the adhesion mainly comes from the mechanical and physical-chemical adhesion between plastic film, paper (board) printing products and adhesives (or hot melt adhesives)

the key to film fastness lies in the affinity between glue and printed matter (ink and substrate). As a film coating industry, printing ink is not optional, that is, only the film coating that adapts to the ink market is the only choice, so as a pre coating, only the performance that adapts to all inks can be considered as high quality. The composite strength of film coated products is affected by the process parameters in the film coating production process, such as the temperature, pressure, speed of the film coating process, the coating condition of the adhesive, film tension and environmental factors. At the same time, it is also affected by the printing process, such as the surface state of the printed matter formed after printing, such as the thickness of the ink layer, the area and distribution of graphics and texts, the characteristics of the ink, the use of diluent and powder, and the drying of the ink layer

v. factors affecting film covering quality

1 The ink layer condition of printed matter

refers to the paper property, ink performance, ink layer thickness, graphic area and printed graphic density. When the ink layer of the printed matter is thick and the image and text area is large, many paper fiber pores will be closed, preventing the penetration and diffusion of the adhesive, making it difficult for the printed matter to adhere to the plastic film, and prone to delamination, blistering and other failures

2. Environmental humidity

the water content between the adhesive and plastic film and printed matter changes with the change of air relative humidity. Printing materials sensitive to humidity will produce internal stress due to dimensional changes. For example, the longitudinal expansion rate of printed matter is 0.5%, and the longitudinal thermal shrinkage of BOPP film is 4%. If the printed matter is stretched due to excessive water absorption, it will cause internal stress between the printed matter and the heating shrinkage of the film, which will lead to curling, wrinkling and poor adhesion of the film covered products. In addition, in a high humidity environment, the equilibrium moisture value of the printed matter will also change. A large amount of moisture absorbed from the air medium will be released from the surface during the hot pressing process, stagnate at the bonding interface, and locally form a non bonding phenomenon. Moreover, the change of the equilibrium moisture value of the printed matter (moisture absorption from the air medium or moisture release to the air medium) mostly occurs at the edge of the printed matter, making it form a lotus leaf edge or tight edge, which is not easy to form a good adhesion with the film and produce wrinkles in the hot pressing composite, so that the production cannot proceed smoothly

3. Temperature control

heating can soften the film and make the paper adhere to the plastic film. However, the temperature is too high, and the film is easy to break, which seriously affects the product quality. Therefore, measures such as fan cooling and turning off the electric heating wire should be taken

VI. green development trend of plastic film coating

the paper after film coating becomes a kind of white pollution because it cannot be recycled, and toxic substances such as toluene and Tianna water volatilize during the film coating process, damaging people's health. Today, when we advocate "protecting the environment and resources", the plastic film covering process has been eliminated in Europe and the United States, and Europe and the United States even refuse to import all kinds of packaging materials with plastic film. Although there are water-based plastic film coating process and hot-melt pre coating dry plastic composite process, the former still fails to solve the problem that the waste of film coated printing materials cannot be degraded and recycled. The latter requires a lot of heat in the production process, and the waste can be separated from paper and plastic only in 60 ℃ water, so the cost is high

with the improvement of environmental protection awareness, new scientific and technological achievements will eliminate the products of the old process and make qualitative changes in the film covering process. "Non toxic, harmless, green and environmental protection" has naturally become the inevitable development trend of the film covering process. In addition, the glazing process is therefore favored. Glazing is to add a layer of colorless transparent paint, namely glazing oil, to the surface of printed matter by coating, spraying or printing. After calendering and drying, it increases the gloss and stiffness of the surface of printed matter. At the same time, the printed matter waste after glazing can be recycled or decomposed by itself, which does not pollute the environment. Therefore, glazing is a processing process with great development potential, which may gradually occupy the plastic coating process market. Although the diluting machines used in the oil-based glazing and alcohol-based glazing on the market are toluene and alcohol respectively, which are harmful to human body and environmental protection, water-based glazing has gradually become the mainstream of glazing process. At present, the water-based UV glazing process that has been developed is an ideal environmental friendly glazing process, and it is also the development direction of glazing process in the future

reprinted from: keyin

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