The printing suitability, printing method and compound method of food flexible packaging materials are the essential knowledge for employees of flexible packaging factories. This article explains in detail for reference.
1. Printability of common food flexible packaging materials
The adhesion of the PE film ink is poor, and it needs to be improved by electrical treatment. The index of wetting tension is commonly used to identify the quality of corona treatment. Generally, LDPE and LLDPE films are 38 ~ 420mN / m, HDPE films are 40 ~ 44mN / m, and they are usually printed with polyamide ink.
The expansion strength of LDPE film is low, the tensile elongation is large, and the color registration is difficult. Therefore, the tension control should be small during printing, and the tension should be constant everywhere. For multi-color overprinting, in order to ensure the accuracy of overprinting, it is best to use a large roller printing machine. The LDPE film will soften when heated a little. It is very easy to stretch and deform. When printing and drying for this reason, the film surface temperature should not exceed 55 ℃. In order to prevent ink from sticking, the film temperature should be cooled below 35 ° C during winding.
Similar to PE, the adhesion of PP film to ink is poor, 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 printing PP film with polyamide ink, the ink adhesion is low, and it shows good adhesion after about one day of aging, and if the polyurethane adhesive composite film is used at the same time, there will be a problem of small peeling force.
PET film is not easy to stretch, and the requirements for tension control are lower than that of PE film. It is often printed by the Liyin process and co-cooperated with CPP to cook the bag. The insulation of PET film is good, during the printing process, it is easy to generate static electricity, and it is not easy to disappear. In severe cases, the film may also be attracted to 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 the temperature of its film surface should not exceed 85 ℃. When winding, the film should be cooled to room temperature or slightly above room temperature.
2. Printing methods of food packaging materials
Food flexible packaging printing is mainly based on gravure and flexographic printing, but compared with the general gravure and flexo printing used in publishing and commodity printing, there are many differences. For example: flexible packaging printing is to print on the surface of the roll-shaped substrate. If it is a transparent film, the pattern can be seen from the back. Sometimes it is necessary to add a layer of white paint or use the inner printing process.
Linyin refers to a special printing method that uses a reverse image graphic plate to transfer the ink to the inside of the transparent printing material, thereby expressing the positive image graphic on the front of the printed matter. The inner printed product is brighter and more beautiful than the surface printed product, with bright colors, no fading, and is moisture-proof and wear-resistant; and due to the compound processing after the inner printing, the ink layer is sandwiched between the two films, so it will not pollute the packaging items.
The printing color sequence of the Indian printing process is opposite to that of ordinary surface printing. For example, in ordinary printing, the background color is printed first, and the inside printing is the final background color. Liyin is a unique process for plastic composite packaging printing. It has become diversified, multifunctional and serialized in printing equipment. For example, the introduction of various models of plastic packaging gravure printing production lines has linked multiple processes such as blow molding, printing, compounding, slitting, and bag making. The second is to use a flexographic printing machine to print plastic films. Most of the flexographic printing machines form a production line with a dry laminating machine.
3. The compound method of food packaging materials
Flexible packaging materials generally do not use a single material, but mainly composite films. The main compound methods are as follows:
â‘ Wet composite method
A layer of water-soluble adhesive is coated on the surface of the substrate (plastic film, aluminum foil), and is combined with other materials (paper, cellophane) through the pressing roller, and then dried into a composite film through a hot drying tunnel. This method is suitable for packaging dry food.
â‘¡Dry compound method
First, the solvent-based adhesive is evenly coated on the substrate, and then sent to the hot drying tunnel, after the solvent is fully volatilized, it is immediately compounded with another film. Such as stretched polypropylene film (OPP), it is generally combined with other materials after dry printing with a typical composite structure: biaxially oriented polypropylene film (BOPP, 12μm), aluminum foil (AIU, 9μm) and unidirectional Stretched polypropylene film (CPP, 70 μm). The process is to use a roller coating device to evenly 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 compound it with another layer of film with a compound roller.
â‘¢ Extrusion compound method
The curtain-shaped molten polyethylene extruded from the slit of the T mold is pressed by a pinch roller and drooled onto the paper or film, coated with polyethylene, or supplied with another film from the second paper feed section, using polyethylene as the adhesive The layer is bonded.
â‘£Hot melt compound method
The polyethylene-acrylate copolymer, ethylene acid-ethylene copolymer, and paraffin wax are put together and heated and melted, then coated on the substrate, immediately compounded with other composite materials, and then cooled and made.
⑤Multilayer extrusion compound method
A variety of plastic resins with different properties are passed through multiple extruders and extruded into a mold to form a film. No adhesive or organic solvent is used between the layers of this process, and the film has no odor and no harmful solvent penetration. It is suitable for food packaging with a longer shelf life. If the general structure LLDPE / PP / LLDPE has good transparency, the thickness is generally 50 ~ 60μm, if the shelf life is longer. More than five layers of high-barrier coextruded film are required, and the middle layer is high-barrier materials PA, PET and EVOH.
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