Printing technology of the hottest liquid flexible

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Printing technology of liquid flexible plastic packaging film in recent years, fresh milk, soybean milk, many juice drinks and condiments packaged with plastic film have appeared in the market. As milk, soymilk and beverages are fresh drinks, they have very strict requirements on hygiene and temperature. Therefore, there are special requirements for the printing of packaging bags, which makes the printing of milk and beverage packaging films different from other printing techniques

I. characteristics of liquid flexible plastic packaging film

the film material used for liquid flexible plastic packaging film is mainly polyethylene (PE) coextrusion film, which must meet the requirements of packaging and printing, processing, storage and transportation, sanitation, etc. From the perspective of the apparent properties of the membrane, it can be divided into three categories: soft milky white PE membrane, black-and-white coextrusion PE membrane, and hard high-temperature cooking PE membrane. Soft milky PE film is mainly used for simple liquid packaging such as milk, soy sauce and vinegar. The coextrusion blowing masterbatch is composed of LDPE, LLDPE and milky white materials. In terms of performance, the barrier performance of the membrane is relatively poor, the sterilization temperature is not more than 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 generally has a shelf life of up to three days and can be sold nearby. The black-and-white films are mostly three layers, with a maximum of five layers. During film blowing, the increased intermediate layer is formed by CO extrusion of barrier masterbatch. The black masterbatch in the heat sealing inner layer can block light and isolate ultraviolet rays. Due to the existence of the intermediate barrier layer, which is composed of EVA, Eval and other masterbatches, the shelf life of the contents is long and can be maintained for more than 30 days. However, there are some three-layer co extruded black-and-white films in the market. The intermediate layer is not composed of barrier masterbatches, so such black-and-white films also belong to the first category. Among the three types of film, black-and-white film is the most unfavorable for printing operation. When the black-and-white film is formed, it contains the largest amount of smoothing agent and brightener, so as to meet the requirements of low friction coefficient of the black-and-white film on the high-speed automatic filling machine. The added oily substances are extremely unfavorable to the color printing process, greatly reducing the adhesion of the ink layer. In most high-speed automatic filling machines, the sterilization system of hot hydrogen peroxide is directly installed for the sterilization of packaging film. In this case, we should pay attention to the impact of sterilization conditions on the ink layer, because some pigments in the ink have poor oxidation resistance. The test shows that the oxygen line wear of hydrogen peroxide = the original size of the sample - the dimensionality after wear increases significantly with the increase of temperature. Therefore, when providing packaging materials, color printing manufacturers must be aware of the customer's post-processing conditions. The hard high-temperature cooking PE film is mainly used for yogurt, soymilk and other dairy drinks. When the contents are sterilized after filling, the ultra-high temperature sterilization process is adopted. The hard coextrusion PE film takes HDPE masterbatch as the main body and adds some LDPE and color masterbatch. Therefore, the hard PE film has good high temperature resistance and can meet the requirements of ultra-high temperature sterilization. As the actual stress does not increase with the increase of external load during sterilization, it is easy to produce "bag expansion" phenomenon. Therefore, it is required that the hard PE film not only has good heat sealing strength, but also has good tensile strength performance. Color printing manufacturers generally use "three meters high throwing" or "one foot stepping" to test the effect

II. Printing ink used for liquid soft plastic packaging film

liquid packaging films such as milk and beverage are usually printed by surface printing, and most of them use polyamide or modified as connecting material. This kind of ink is the earliest developed gravure ink in China, and its production technology is also 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; However, polyamide inks generally have performance defects such as poor water resistance, grease resistance and freezing resistance. Printing this type of ink in high temperature summer is prone to sticky and softening, and freezing will occur in low temperature winter. In the post-processing process of filling, circulation and storage of liquid packages, high-temperature sterilization or freezing are required. Therefore, the inherent defects of polyamide surface printing inks cannot meet the requirements of liquid packaging. Color printing manufacturers producing liquid packages must select non polyamide surface printing inks for different types of CO extruded PE films. At present, polyester and modified products are the most suitable inks for liquid packages in the market, There are single liquid and two liquid reactive inks

(I) safety and hygiene

as milk and beverages are for people to eat, packaging films, printing inks, etc. should also meet the requirements of hygienic standards for packaging materials and food packaging regulations. Therefore, it is required that the printing inks used should be non-toxic, odorless, less residual solvents, and free of toxic heavy metals. So as not to pollute milk and beverages and cause harm to consumers' health. According to the above requirements for ink performance, you must select an ink with better performance. Generally, the two-component polyurethane surface printing ink is used for printing, and the varnish treatment is required after the printing ink. The cost is relatively high and the process is 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 of good quality. They do not need to be polished separately. They only need to add a certain amount of hardener into the printing ink, and after 24-hour aging treatment after printing, they can not only be resistant to water boiling, hydrogen peroxide treatment, but also withstand friction. These two inks are special printing inks designed for milk and beverage packaging film. They have good affinity with polyethylene film, good gloss of ink layer, high adhesion fastness, and excellent heat resistance, grease resistance and water resistance. They can be said to be an ideal choice for liquid packaging film printing inks such as milk and beverage. Super Lifu 313A ink is an alcohol soluble polyamide ink, which can withstand boiling below 100 ℃ and hydrogen peroxide sterilization below 60 ℃. Super Lifu 314 surface printing ink is a two liquid reactive polyurethane modified ink, which can withstand 121 ℃/40 minutes of cooking sterilization and 80 ℃ and 35% hydrogen peroxide sterilization. According to the oxidation resistance test of super Lifu 314 double liquid reactive ink, the printing film after 24 hours of aging was soaked in dioxygen water with a temperature of 45~50 ℃ and a concentration of 35% for about 15 minutes. After taking it out, it was detected that the ink had good adhesion and basically no color fading, which could fully meet the requirements of liquid packaging such as milk and beverages

(II) 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 ink viscosity in the printing process, which is generally controlled within 15~20 seconds. The viscosity of the ink is too low, which is easy to cause pinholes, weakness and ink spreading in the image and text; On the contrary, if the viscosity is too high, the transfer performance of the ink is relatively poor, which is easy to cause the phenomenon of paste and dry version, as well as the failure of uneven inking and fine line fracture in large-area field parts

2. Ink dryness

the drying performance of ink is also one of the important indicators to measure the quality and performance of ink. Controlling the drying performance of ink is a necessary condition to obtain good printing effect. The drying speed of the ink is too fast, which is prone to dry plate failure. However, if the drying speed of the ink is too slow, the back of the printed matter will be sticky and dirty. The general principle for selecting the ink drying speed is that under the existing conditions of the printing machine, the ink should be fully dried before entering the next printing color rule and before rewinding, but it cannot be dried on the printing plate. The ink used for printing and packaging film is solvent based ink, in which the dryness mainly depends on the volatilization speed of the solvent. Therefore, the proportion of the solvent must be strictly controlled in actual production. An appropriate amount of slow drying solvent or fast drying solvent can be added to the ink according to specific conditions to adjust the drying speed of the ink, ensure that the ink can be fully dried and prevent "false drying". Generally, when the super Lifu 314 double liquid reactive ink is used for printing, under the normal condition that the printing speed is about 80/min, the ink viscosity is generally controlled at about 15 seconds. According to practice, the basic ratio of the mixed solvent used is: butanone: toluene: butyl =4:3, 5:2.5, which can ensure better printing quality

III. precautions in the printing process of liquid flexible plastic packaging film

(I) select appropriate materials. In actual production, appropriate printing film and printing ink shall be selected according to the filling speed, sterilization conditions, product shelf life and other specific conditions of the automatic filling machine, and the printing process parameters shall be adjusted and controlled according to the actual situation to standardize the process flow and ensure the printing quality of the film. Polyamide surface printing ink can not be used because of its poor water resistance and grease resistance; Polyurethane inks also have different types, which should be targeted when selecting. New Oriental super Lifu 313A inks are generally only used for soft coextrusion PE films, while 314 inks are suitable for various coextrusion PE films. The coextrusion used by color printing manufacturers, such as the PE film of graphene based high-density porous carbon material, is generally blown by the manufacturers themselves, with different technical levels. Therefore, we must grasp the selection of masterbatch, and should use masterbatch without smoothing agent; After the selected PE film is treated by electric spark, the surface energy should reach 40 dyne at least, and it is better to exceed 42 dyne

(II) customer's post-processing conditions or test standards. Only when the color printing manufacturers fully understand the conditions of the food manufacturers when filling, the subsequent processing technology, and even the detailed requirements of the circulation link of the liquid package in the market, can they be targeted and have a clear idea. The main points for attention include: filling speed, sterilization conditions, shelf life, preservation requirements, etc. from the perspective of fixed investment expenditure and the acceleration of plastic product production

(III) requirements for printing process. In order to ensure the printing quality of liquid packaging film, we must also pay attention to the control of printing tension, printing pressure, printing speed, drying temperature and other process conditions during the printing process, formulate a set of stable and reliable process standards, standardize the process flow, and ensure the smooth progress of large-scale production. In addition, the comprehensive quality of the printing workers, the technical data provided by the ink manufacturer, the good quality of the thinner and the change of the workshop environment are also factors that can not be ignored

(IV) control the printing environment. The change of temperature and humidity in the printing workshop has a certain impact on the properties of substrate materials and inks. Too dry environment will cause film static electricity and make printing difficult; Too humid environment will make the ink drying insufficient, affecting the firmness of the ink layer. Therefore, the ambient temperature and relative humidity in the printing workshop should be strictly controlled to keep it relatively constant as far as possible and ensure good ventilation to reduce the incidence of printing failures. Generally speaking, the temperature in the workshop should be controlled between 18 ℃ ~25 ℃, and the relative humidity should be maintained between 60%~70%

(V) do a good job in finished product inspection. Customer satisfaction is the supreme principle and ultimate desire of every printing enterprise. Therefore, it is necessary to ensure that qualified finished products are delivered to customers. This not only makes customers satisfied, but also maintains the image and reputation of the enterprise, which is also the basis for maintaining and developing good customer relations. During the production of liquid package, the film is changed frequently, and

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