The printability of common food packaging bag
materials the poor adhesion of PE film ink, it needs to be treated with electricity to improve it. The index of wetting tension is often used to identify the quality of corona treatment. The three general LDPE and LLDPE films are 38~420mN/m, and HDPE films are preferably 40~44mN/m. Polyamide inks are commonly used for printing. LDPE film has low expansion strength, high tensile elongation, and difficult registration. Therefore, the tension control during printing should be small, and the tension should be constant everywhere. If it is multi-color overprinting, in order to ensure the overprinting accuracy, it is best to use a large-roller type printing machine. LDPE film will soften when exposed to a small amount of heat. It is easily stretched and deformed. For this reason, the surface temperature of the film should not exceed 55°C when printing and drying. In order to prevent the ink from sticking, the film temperature should be cooled to below 35°C during rewinding. Same as PE, PP film has poor adhesion to ink, and corona treatment should be done before printing. Its wetting tension is preferably 10~14mN/m. Commonly used polyamide ink, chlorinated polypropylene ink (composite) printing. When using polyamide ink to print PP film, the ink adhesion is low. After about one day of aging, good adhesion is shown. If the polyurethane adhesive composite film is used at the same time, the failure of the peeling force will be small. PET film is not easy to stretch, and the tension control requirement is lower than that of PE film. It is often printed by the lining process and combined with CPP for cooking bags. The PET film has good insulation properties, it is easy to generate static electricity during the printing process, and it is not easy to disappear. In severe cases, the film may be sucked on the roller and cannot be printed. Therefore, in high-speed printing, a static elimination device is required. The drying temperature of PET film can be controlled higher, but its film surface temperature should not exceed 85℃. When winding, the film should be cooled to room temperature or slightly above room temperature. Printing methods of food packaging materials Food flexible packaging
printing is mainly based on gravure and flexographic printing. However, compared with general gravure and flexographic printing used in publishing and commercial printing, there are many differences. For example: Flexible packaging printing is to print on the surface of a roll-shaped substrate. If it is a transparent film, the pattern can be seen from the back, and sometimes a layer of white paint is required to be printed, or a lining process is used. Back printing refers to a special printing method that uses a reverse image printing plate to transfer the ink to the inner side of the transparent printing material, so that the positive image and text are displayed on the front side of the printed matter. The printed matter on the lining is brighter and more beautiful than the printed matter on the surface, the color is bright, the color is not fading, and it is moisture-proof and wear-resistant; and the ink layer is sandwiched between the two films after the lining is compounded, so it will not pollute the packaged goods. The printing color sequence of the lining process is opposite to that of ordinary surface printing. For example, in ordinary printing, the background color is printed first, and the inner color is printed last. In printing is a unique process of plastic composite packaging printing, and it has tended to be diversified, multi-functional and serialized in printing equipment. For example, the introduction of various modes of plastic packaging gravure printing production lines has linked multiple processes such as blow molding, printing, compounding, slitting, and bag making. The second is the use of flexographic printing machines to print plastic films. Most flexographic printing machines are combined with dry laminating machines to form a production line. The composite method of food packaging bag materials Generally, the food packaging bag materials do not use a single material, but mainly composite films. The main compounding methods are as follows: ① Wet compounding method. Coat a layer of water-soluble adhesive on the surface of the substrate (plastic film, aluminum foil), compound it with other materials (paper, cellophane) through a pressing roller, and then dry it into a composite film through a hot drying tunnel. This method is suitable for packaging dry food. ② Dry compound method. First, the solvent-based adhesive is evenly coated on the substrate, and then sent to the hot drying tunnel, so that the solvent is fully volatilized, and it is immediately composited with another layer of film. For example, oriented polypropylene film (OPP), after lining, the dry composite process is generally used to composite with other materials. The typical structure is: biaxially oriented polypropylene film (BOPP, 12μm), aluminum foil (AIU, 9μm) and unidirectional Stretched polypropylene film (CPP, 70μm). The process is to use a roller coating device to uniformly coat the solvent-based 'dry adhesive powder' on the base material, and then send it to the hot drying tunnel to fully volatilize the solvent and then use a composite roller to composite with another film. ③Extrusion compound method. The curtain-shaped molten polyethylene extruded from the slot of the T mold is pressurized by a pinch roller and cast onto paper or film for polyethylene coating, or other film is supplied from the second paper feeding part, and the polyethylene is used as the adhesive The junction layer is bonded. ④Hot-melt composite method. Put polyethylene-acrylate copolymer, ethylene acid-ethylene copolymer, and paraffin wax together to heat and melt, then coat it on the substrate, immediately composite with other composite materials and then cool to make. ⑤Multi-layer extrusion compounding method. A variety of plastic resins with different properties are passed through multiple extruders and extruded into the mold to form a composite film. There is no need for adhesives and organic solvents between layers of this process, and the film has no peculiar smell and no harmful solvent penetration. It is suitable for food packaging with longer shelf life. For example, the general structure LLDPE/PP/LLDPE has good transparency, and the thickness is generally 50~60μm, if the shelf life is longer. Need to use more than five layers of high-barrier co-extruded film, the middle layer is high-barrier materials PA, PET and EVOH.
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