Discussion on the flexographic printing process of

2022-08-18
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Discussion on the production process of flexible plastic packaging flexo printing II

II. Common substrates for flexible plastic packaging flexo printing

the types of plastic films commonly used in flexo printing are: Polyethylene (PE, including low density polyethylene (LDPE), medium density polyethylene (MDPE), high density polyethylene (HDPE), linear low density polypropylene (LLDPE)), polypropylene (PP, including biaxial tensile polypropylene (BOPP), cast polypropylene (CPP)), Polyester (PET), polyamide (PA), aluminum foil (AL), polyvinylidene chloride (PVDC), polycarbonate (PC), ethylene vinyl acetate copolymer (EVA) film, etc. Generally, it is suitable for printing and compounding after surface treatment. Among the films used for printing, pet, BOPP, PA, PVC and PE are used in large amounts; PE and CPP are widely used in films used as composite heat sealing layers; The films used as barrier layer generally include PA, pet, PE, EVA, EVOH, aluminum foil, various aluminized films, etc

III. surface treatment of plastic film

for PVC, cellophane, polyester, nylon and other films, because of their large surface tension, they can generally be printed directly when printing products with low quality requirements. The surface tension of untreated polyolefin film is generally very low, and electricity is widely used, especially for thin-walled plastic parts and small gate plastic parts. Due to the fast cooling speed, halo treatment method should be used to shorten the filling process as much as possible. After corona treatment, the film should be used as soon as possible in a short time. Before use, the surface tension of the film should be checked and tested again to promote the development of renewable resources industry. For the cold film, it needs to be treated with a high-power surface electronic processor. The cold film should be preheated with an iodine tungsten lamp before treatment. The film printed on one side only needs to be corona treated on one side, otherwise it is easy to cause adhesion after printing. Under high temperature conditions, if the treatment time is too long, the ink adhesion tends to decrease. Generally, OPP treatment value is required to be greater than or equal to 38 dyne/cm, pet treatment value is required to be greater than or equal to 50 dyne/cm, and PA treatment value is required to be greater than or equal to 52 dyne/cm

Figure 5: Digital flexo under microscope

Figure 6: static electricity in traditional flexo under microscope

plastic film printing will bring a series of problems to the operation, which will directly affect the output and quality of prints. When printing small packaging plastic films, due to electrostatic adhesion, the films are in an anoxic state, which will hinder the curing process of the plastic ink layer. In case of high temperature and high humidity, it is easier to form ink layer adhesion, and if it is light, the printing ink color will shift, increasing the difficulty of printing, slitting, finishing and other processes; Most importantly, the films adhere to each other and cannot be torn apart, resulting in the scrap of prints. In addition, after the bag is made, it will also be continuously discharged during storage and transportation, which will not only affect the heat seal, but also affect the transparency of the physical object and space level in the bag. When printing large-scale films, because of the large amount of static electricity generated, it is likely to cause fire or explosion accidents when the machine speed is high and the resin is not mixed with antistatic agents. The fan blade industry consumes more than 1million tons of composite materials every year, so plastic printing generally adopts the method of adding antistatic agent into the ink to eliminate static electricity

IV. plastic film flexographic ink

solvent based ink is generally used when flexographic printing plastic and other non absorbent substrate materials are used. Prints are dried by the evaporation of solvents in the ink. For different substrate materials, different inks should be selected. In production, in order to adjust the viscosity and volatilization speed of ink, a certain amount of solvent is often added. When adding solvent, we should try to choose a solvent with low surface tension to reduce the surface tension of the whole ink system, so as to improve the wettability of plastic film. In order to solve the contradiction between the adhesion fastness of the ink layer and the evaporation speed of the ink, the resin (rubber) corresponding to the ink or other appropriate compatible bonding resin or rubber can be added to the diluent or the ink, which can also improve the adhesion fastness of the ink layer of chenyunming, deputy general manager of Zhejiang Kangcheng new materials Co., Ltd. to a certain extent

Figure 7: use the chann cup to regularly detect the ink viscosity during printing

(to be continued)

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