In recent years, plastic film-packed fresh milk, soy milk, and many juice drinks, condiments, etc. have appeared in the market dazzlingly. Because milk, soy milk, beverages, etc. are fresh drinks, the requirements for hygiene and temperature are very strict. Therefore, there are special requirements for the printing of h-bags, which makes the printing of milk and beverage packaging films different. Other technical features of printing.
Characteristics of liquid soft plastic packaging film
The film material used for liquid flexible plastic packaging film is mainly polyethylene (PE) co-extruded film, which must meet the requirements of packaging printing, processing, storage and transportation and sanitation. From the appearance of the film, it can be divided into three categories: soft milky white PE film, black and white co-extruded PE film, and hard high temperature cooking PE film.
Soft milky white PE film is mainly used in simple liquid packaging such as milk, soy sauce, vinegar, etc. The co-extruded film blowing masterbatch is composed of LDPE, LLDPE and milky white material. The film has poor barrier performance, the sterilization temperature does not exceed 90 ℃, and the shelf life is relatively short; it is generally used in manual or low-speed filling equipment. For example, low-grade milk packaging, the general shelf life is up to three days, sold nearby.
The black-and-white film is mostly three layers, with a maximum of five layers. During film blowing, the added middle layer is formed by co-extrusion of the barrier masterbatch. The black masterbatch in the inner layer of the heat seal plays a role of blocking light and blocking ultraviolet rays. Due to the existence of the intermediate barrier layer, it is composed of E-VA, EVAL and other master batches. The shelf life of the contents is longer and can be maintained for more than 30 days. It is composed of barrier masterbatch, so this black and white film also belongs to the first category. Among the three types of films, black and white films are the most unfavorable for printing operations. When the black and white film is formed, the amount of slip agent and brightener contained is the largest, to meet the requirement of small friction coefficient of the black and white film on the high-speed automatic filling machine. The adhesion performance of the ink layer. Most of the high-speed automatic filling machines are directly equipped with a hot hydrogen peroxide sterilization system, which is used for sterilization of packaging films. In this case, we must pay attention to the effect of sterilization conditions on the ink layer, because some pigments in the ink are not very antioxidant. Tests have shown that the oxidizability of hydrogen peroxide significantly increases with increasing temperature. Therefore, when providing packaging materials, color printing manufacturers must clearly understand the post-processing conditions of customers.
The hard high-temperature cooking PE film is mainly used for dairy products such as yogurt and soy milk. When the contents are sterilized after filling, the ultra-high temperature sterilization process is used. The hard co-extruded PE film is mainly composed of HDPE masterbatch, adding some LDPE and colored masterbatch. Therefore, the hard PE film has good high temperature resistance and can meet the requirements of ultrahigh temperature sterilization. Due to the pressure difference during sterilization, it is easy to produce "bulging" phenomenon, so it is required that the rigid PE film not only has good heat seal strength, but also has good tensile strength performance. Color printing manufacturers generally use "three meters high" or "One-footed" test effect.
Printing ink used in liquid flexible plastic packaging film
Liquid packaging films such as milk and beverages are usually printed by surface printing. Most of them are polyamides or modified as connecting materials. This type of ink is the earliest developed in China's gravure ink, and the production process is relatively mature and perfect. This type of ink is suitable for polyethylene and polypropylene films. The ink layer has good adhesion fastness and good gloss; but the polyamide ink has poor water resistance, oil resistance, and freezing resistance. Printing this type of ink in high-temperature summers is prone to stickiness and softening. When printing in low-temperature winters, freezing will occur.
Liquid packs are subjected to high-temperature sterilization or freezing during post-processing such as filling, circulation and storage. Therefore, the inherent defects of polyamide-based surface printing inks cannot meet the requirements of liquid packaging. Color printing manufacturers that produce liquid packs For different types of co-extruded PE films, non-polyamide based surface printing inks must be selected. Currently, the most suitable liquid package inks on the market are polyesters and modified products. There are single-liquid and double-liquid reactive types. Ink.
Because milk and beverages are for human consumption, packaging films, printing inks, etc. should also meet the requirements of packaging material hygiene standards and food packaging regulations. Therefore, the inks used for printing should be non-toxic, odorless, and have little residual solvents. Does not contain toxic heavy metals and other requirements. So as not to contaminate milk and beverages, and cause harm to consumers' health.
According to the above requirements on the performance of the ink, it is necessary to choose an ink with better performance. Generally, the two-component polyurethane type surface printing ink is used for printing, and the varnish treatment is performed after printing the ink, the cost is relatively high and the process is relatively cumbersome. At present, Chaolifu 313A surface printing ink and Chaolifu 314 surface printing ink produced by Zhejiang New Oriental Ink Group Co., Ltd. are widely used and have better quality. A certain amount of hardener is added to it, and after curing for 24 hours after printing, it can achieve both boiling resistance and hydrogen peroxide resistance treatment, and can withstand friction.
These two inks are special printing inks designed for milk and beverage packaging films, and have good affinity with polyethylene films, good gloss of the ink layer, high adhesion fastness, heat resistance, grease resistance, resistance Water-based and other properties are also very good; it can be said to be an ideal choice for printing inks for liquid packaging films such as milk and beverages. Chaolifu 313A ink is an alcohol-soluble polyamide-based ink that can withstand boiling below 100 ° C and can withstand sterilization with hydrogen peroxide below 60%. Chaolifu 314 surface printing ink is a two-liquid reactive polyurethane modified ink, which can withstand 121 ℃ / 40 minutes of cooking sterilization, and can withstand 80 ℃, 35% concentration of hydrogen peroxide sterilization. According to the oxidation resistance test of Chaolifu 314 two-liquid reactive ink, the printed film after aging for 24 hours was immersed in hydrogen peroxide water at a temperature of 45 ° C to 50 ° C and a concentration of 35% for about 15 minutes, and then taken out for testing The ink has good adhesion and basically no color fading, which can fully meet the requirements of milk and beverages and other liquid packaging.
Adjustment and control of ink performance
1. Ink viscosity
Viscosity is the most important control index in ink application. It directly affects the transfer performance of ink and the quality of printed matter. Therefore, it is very important to control the viscosity of ink during the printing process, generally between 15-20 seconds. If the viscosity of the ink is too low, it is easy to cause pinholes, blurring, and ink spreading in the graphics; on the contrary, if the viscosity is too high, the ink transfer performance is relatively poor, and it is easy to cause the phenomenon of paste, dry plate and large areas of ink unevenness , Broken thin lines and other faults.
2. Dryness of ink
The drying performance of the ink is also one of the important indicators to measure the quality and performance of the ink. Controlling the drying performance of the ink is a necessary condition to obtain a good printing effect. The drying speed of the ink is too fast, and it is easy to cause dry plate failure, but the drying speed of the ink is too slow, and the failure of the backside of the printed matter will occur. The general principle of selecting the ink drying speed is that under the existing conditions of the printing press, the ink should be sufficiently dry before entering the next printing color and before rewinding, but it cannot be dried on the printing plate. The ink used for printing and packaging film is a solvent-based ink, and the drying property mainly depends on the evaporation rate of the solvent. Therefore, in actual production, the ratio of the solvent must be strictly controlled, and an appropriate amount of slow drying can be added to the ink according to the specific situation. Solvent or quick-drying solvent to adjust the drying speed of the ink to ensure that the ink can be fully dried to prevent the phenomenon of "false drying". Generally, when printing with Chaolifu 314 two-liquid reactive ink, under normal circumstances where the printing speed is about 80 / minute, the viscosity of the ink is generally controlled at about 15 seconds. According to practice, the basic ratio of the mixed solvent used is : Butanone: Toluene: Butyl ester = 4: 3.5: 2.5, so as to ensure better printing quality.
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