Coating, that is, pasting film, is to coat plastic film with adhesive, and make it bond with paper printed matter after heating and pressurizing, forming the processing skills of paper-plastic products. The printed matter of the Film-covering machine has a thin and clear plastic film on its surface, which makes it smoother and brighter, thus improving the gloss and fastness of the printed matter. The pictures and texts are brighter and have a three-dimensional sense. At the same time, they also have the functions of waterproof, antifouling, abrasion resistance, folding resistance and chemical corrosion resistance.
With the development of printing technology, there are more and more coating methods. According to the process of film covering, there are three kinds: dry, wet and oily. According to the different raw materials and equipment, it can be divided into immediate coating film and pre-coating film process. Let's briefly introduce the basic concepts and characteristics of these methods.
1. Coating film
That is to say, when the coating process is operated, the adhesive is first coated on the film and then hot pressed, but the production process is prone to fire.
2. Pre-coated film
This process greatly simplifies the process of film coating because the equipment does not need the binder heating and drying system, and the operation is very convenient. It can be used on-the-fly, and the production flexibility is great. At the same time, it has no solvent odor, no environmental pollution. More importantly, it can completely avoid bubble, delamination and other faults. Its products have a very high visibility and wide application prospects and implementation value.
The structure of pre-coated film is composed of base material and adhesive layer. The base material is usually PET and BOPP film. Considering material cost and processing technology, most of pre-coated film substrates are BOPP film with thickness of 12-20 micron and adhesive layer thickness of 5-15 micron. According to the different processing equipment and process conditions, different thickness of adhesive layer is selected, which is divided into hot melt adhesive and organic macromolecule cryogenic resin. The difference between them is that hot melt adhesives are made by blending and modifying main viscose resins, tackifiers and modifiers, while organic macromolecule resins are low temperature copolymers of single macromolecule.
Due to the limitations of skills, production equipment and raw materials, the pre-coated films produced by domestic equipment (mostly hot melt adhesives) still have quality defects. Because the production process is similar to that of coating film, the colloid is dissolved with organic solvent, and the cot is coated on the base material film with concave reticulated roller, so the solvent evaporation is insufficient, the odor occurs in operation and use, and the solvent evaporates after coating, and the appearance is easy to foam. At the same time, because hot melt adhesives are blended with several kinds of polymer materials, poor temperature control will lead to degradation and crosslinking of polymer in hot melt adhesives, which will make the appearance of the film bad. Some pre-coated colloids are easy to adhere to the surface of the substrate, resulting in poor coiling. Because the adhesive layer is thin and the appearance has not been activated, it is easy to have defects such as insufficient adhesion to printed matter. Because hot melt adhesives are made of several materials, the brightness after coating is significantly lower than that of low temperature pure resin pre-coating.
3. Waterborne Coating
It is a method to process the appearance of printed matter to improve its gloss, strength and aesthetic effect. To measure the quality of the film, we also need to examine its brightness, strength, cohesion and so on. Water-based wet film coating is popular with customers because of its high strength, easy recovery and non-pollution.
4. Curved surface coating
It is a brand-new transfer technology, which can be applied to complex workpieces with different raw materials, and greatly increase the added value of products because of vivid patterns and colors. This skill is to use a specially chemically treated film, which is printed with a certain color pattern and then sent flat to the surface of the water. The pattern is evenly written on the surface of the product by water pressure, while the covered film dissolves automatically. After cleaning and drying, a clear protective coating is added. At this time, the product has shown a completely different visual effect.
According to the conditions of cold and heat, pressure and material, the technology of film coating can be divided into cold (cold) film coating, hot (hot) film coating and liquid film coating.