About liquid product packaging and printing technology
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In recent years, fresh milk, soy milk, and many fruit juice drinks and condiments in plastic film packaging have appeared in the market. Since 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 printed technical features.
Characteristics of liquid soft plastic packaging film
The film material used for liquid soft plastic packaging film is mainly polyethylene (PE) co-extruded film, which must meet the requirements of packaging printing, processing, storage and transportation. From the appearance of the film} can be divided into three categories: soft milky white PE film, black and white co-extruded PE film, hard high temperature cooking PE film.
The soft milky white PE film is mainly used for simple liquid packaging such as milk, soy sauce and vinegar. The co-extrusion blown film masterbatch is composed of LDPE, LLDPE and milky white material. The performance of the film is relatively poor, the sterilization temperature does not exceed 90 ° C, the shelf life is relatively short; generally used in manual or low-speed filling equipment. For example, low-grade milk packaging, the general shelf life of up to three days, near sales.
The black and white film is mostly three layers, and the maximum is five layers. When the film is blown, the added intermediate layer is coextruded with a barrier masterbatch. The black masterbatch of the heat-sealed inner layer functions to block light and block ultraviolet rays. Due to the existence of the intermediate barrier layer, it is composed of masterbatch such as E-VA and EVAL. The shelf life of the contents is longer and can be maintained for more than 30 days. However, there are some three-layer co-extruded black and white film on the market, and the middle layer is not It is composed of a barrier masterbatch, so such a black and white film is also in 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 the slip agent and the brightener is the largest, so as to meet the requirement that the black and white film has a small friction coefficient on the high-speed automatic filling machine, and the added oily substance is extremely disadvantageous to the color printing process, and the reduction is greatly reduced. Adhesion properties of the ink layer. On most high-speed automatic filling machines, a sterilization system directly equipped with hot hydrogen peroxide is used for sterilization of the packaging film. In this case, attention must be paid to the effect of the sterilization conditions on the ink layer because some of the pigments in the ink have poor oxidation resistance. Tests have shown that the oxidation of hydrogen peroxide is significantly improved with the increase of temperature. Therefore, color printing manufacturers must clearly understand the post-processing conditions of customers when providing packaging materials.
The hard high-temperature cooking PE film is mainly used for dairy products such as yoghurt and soy milk. The contents are sterilized after filling, and the ultra-high temperature sterilization process is adopted. The hard co-extruded PE film is mainly composed of HDPE masterbatch, and part of LDPE and colored masterbatch are added. Therefore, the hard PE film has good high temperature resistance and can meet the requirements of ultra-high temperature sterilization. Due to the pressure difference during sterilization, it is easy to produce "expansion bag" phenomenon. Therefore, it is required that the hard PE film not only has good heat-sealing strength, but also the tensile strength of the film is good. Color printing manufacturers generally adopt "three meters throw high" or "People step on one foot" test effect.
Printing ink for liquid soft plastic packaging film
Liquid packaging films such as milk and beverages are usually printed by surface printing. Most of them are made of polyamide or modified into connecting materials. These inks are the earliest developed in China's gravure inks, and the production process is relatively mature and perfect. This type of ink is suitable for polyethylene and polypropylene film. The adhesion of the ink layer is good and the gloss is good. However, the polyamide ink has poor water resistance, is not resistant to grease, and is not resistant to freezing and other performance defects. This type of ink is printed in the summer at high temperatures, which tends to cause stickiness and softening. When printing at low temperatures in winter, it will freeze.
The liquid package is subjected to high temperature sterilization or freezing during the post-processing process of filling, circulation and storage. Therefore, the defects inherent in the polyamide-based surface-printing ink cannot meet the requirements of the liquid packaging, and the color printing manufacturer that produces the liquid package Non-polyamide-based surface-printing inks must be used for different types of co-extruded PE films. Currently, the most suitable liquid-packed inks on the market are polyesters and modified products, which are single-liquid and two-liquid reactive. Ink.
Since milk and beverages are for human consumption, packaging films, printing inks, etc. should also meet the requirements of packaging materials hygiene standards and food packaging regulations. Therefore, the inks required for printing should be non-toxic, odorless, and have less residual solvents. Does not contain toxic heavy metals and other requirements. In order to avoid contamination of milk and beverages, and to harm the health of consumers.
According to the above requirements for ink performance, it is necessary to select an ink having superior performance. Generally, the printing is a two-component polyurethane type surface printing ink, and the varnish treatment is performed after printing the ink, which is relatively high in cost and cumbersome in process. At present, the widely used and better quality is the Super Lifu 313A surface printing ink and the Super Life 314 type printing ink produced by Zhejiang New Oriental Ink Group Co., Ltd., which does not need to be separately coated with varnish, only in printing ink. A quantitative amount of hardener is added, and after 24 hours of aging, it can be both water-resistant, resistant to hydrogen peroxide and subjected to friction.
These two inks are special printing inks designed for milk and beverage packaging films, and have good affinity with polyethylene film, good gloss of ink layer, high adhesion fastness, heat resistance, grease resistance and resistance. Water-based properties are also excellent; it is ideal for liquid packaging film printing inks such as milk and beverages. The Super Life 313A ink is an alcohol-soluble polyamide ink that can withstand boiling below 100 °C and can withstand 60% or less of hydrogen peroxide sterilization. Chaolifu 314 surface printing ink is a two-liquid reactive polyurethane modified ink that can withstand retort sterilization at 121 ° C / 40 minutes, and can withstand hydrogen peroxide sterilization at 80 ° C and a concentration of 35%. According to the oxidation resistance test of Chaolifu 314 two-liquid reaction type ink, the printed film after aging for 24 hours was immersed in hydrogen peroxide at a temperature of 45 ° C to 50 ° C and a concentration of 35% for about 15 minutes, and then taken out and detected. The ink has good adhesion and basically no color fading, and can fully meet the requirements of liquid packaging such as milk and beverage.
(2) Adjustment and control of ink performance
1. Viscosity of the ink
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, the control of ink viscosity during printing is very important, generally controlled between 15-20 seconds. The viscosity of the ink is too low, which is easy to cause pinholes, faintness and ink spreading in the graphic part. Conversely, if the viscosity is too high, the performance of the ink is poor, and it is easy to produce paste, dry plate phenomenon and uneven ink on a large area. Faults such as broken thin lines.
2. Dryness of the 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 for obtaining good printing results. The drying speed of the ink is too fast, and the dry plate failure is liable to occur, but the drying speed of the ink is too slow, and the back of the printed matter is dirty. The general principle for selecting the drying speed of the ink is that under the existing conditions of the printing press, the ink should be sufficiently dried before entering the next printing color and before rewinding, but it cannot be dried on the printing plate. The ink used for printing the packaging film is a solvent-based ink, wherein the drying property mainly depends on the volatilization speed 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 specific conditions. A solvent or a fast-drying solvent to adjust the drying speed of the ink to ensure that the ink is sufficiently dry to prevent "false dry" phenomenon. Generally, when printing with Super Lifu 314 two-liquid reaction type ink, the ink viscosity is generally controlled at about 15 seconds under normal conditions of printing speed of about 80/min. According to practice, the basic ratio of the mixed solvent used is : Butanone: Toluene: Butyl ester = 4: 3.5: 2.5, which ensures better print quality.