Liquid soft plastic packaging film printing analysis
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In recent years, fresh milk, soy milk, and many 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 packaging bags, which makes the printing of milk and beverage packaging films different from others. Technical characteristics of printing. Characteristics of liquid soft plastic packaging film.
The film material used for the liquid soft plastic packaging film is mainly a polyethylene (PE) coextruded film, which must meet the requirements of packaging printing, processing, storage and transportation. 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, 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 inner layer of the heat seal serves 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 yogurt 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 is partially added with LDPE and colored masterbatch. 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 ink layer has good adhesion fastness and good gloss. However, polyamide inks generally have poor water resistance, are not resistant to grease, and are not resistant to freezing and other performance defects. In the summer, this type of ink is printed, which is prone to 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.
(1) Safety and hygiene
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. The general printing uses a two-component polyurethane type surface printing ink, and the varnish treatment is carried out 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 Life 313A printing ink and the Super Life 314 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 curing, it can be both boiled, 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. Chao Li 314 The 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 after detection. 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 properties
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 transfer performance of the ink is relatively 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, in which 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. Solvent or quick-drying solvent to adjust the drying speed of the ink to ensure that the ink can be fully dried to prevent "false dry" phenomenon. Generally, when using the Super Lifu 314 two-liquid reaction type ink, the ink viscosity is generally controlled to about 15 seconds under normal conditions of a 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.
Precautions in the printing process of liquid soft plastic packaging film
(1) Choosing the right materials. In actual production, the appropriate printing film and printing ink should be selected according to the filling speed, sterilization condition and length of product shelf life of the automatic filling machine, and the printing process parameters should be adjusted and controlled according to the actual situation, and the process flow should be standardized. To ensure the printing quality of the film. Polyamide surface printing inks are not suitable for liquid-based inks due to their water resistance and oil resistance. Liquid polyurethane inks are generally not available. Polyurethane inks are also available in different types. They must be targeted when used. New Oriental Super Life 313A inks are generally only used for softening. Co-extruded PE film, while Type 314 is suitable for a variety of co-extruded PE films. The co-extruded PE film used by color printing manufacturers is generally blown by the manufacturer's own film, and the technical level is different. Therefore, it is necessary to grasp the selection of the masterbatch. It is advisable to use the masterbatch without the slip agent type; the selected PE film is passed. After EDM, the surface energy should be at least 40 dynes, preferably more than 42 dynes.
(2) Customer's post-processing conditions or test standards. Color printing manufacturers can only do a good job in knowing the conditions of the food manufacturers at the time of filling and the process of subsequent processing, and even the details of the liquid circulation in the circulation of the market. The main points of attention include: filling speed, sterilization conditions, length of shelf life, storage requirements, etc.
(3) Requirements for the printing process. In order to ensure the printing quality of the liquid packaging film, it is necessary to pay attention to the control of the process conditions such as printing tension, printing pressure, printing speed and drying temperature in the printing process, formulate a set of stable and reliable process standards, standardize the process flow, and ensure scale production. Smooth progress. In addition, the comprehensive quality of the printing workers, the technical information provided by the ink manufacturers, the quality of the thinner materials, and the changes in the workshop environment are also factors that cannot be ignored.
(4) Control the printing environment. The change of temperature and humidity in the printing workshop has certain influence on the material properties of the substrate and the performance of the ink. If the environment is too dry, it will cause static electricity of the film and cause printing difficulties. If the environment is too humid, the ink will not dry enough and affect the firmness of the ink layer. Therefore, the ambient temperature and relative humidity in the printing shop should be strictly controlled to keep it as relatively constant as possible and to ensure good ventilation to reduce the incidence of printing failures. Generally, the temperature control in the workshop is preferably between 18 ° C and 25 ° C, and the relative humidity is preferably between 60% and 70%.
(5) Do a good job in the inspection of finished products. Customer satisfaction is the highest standard and final desire of every printing company, so we must ensure that qualified products are delivered to customers. This not only makes the customer satisfied, but also maintains the image and reputation of the company, which is the basis for maintaining and developing good customer relationships.
In the production process of the liquid package, the film change is frequent, and the performance of the film is likely to be different. The amount of ink used in the liquid package is not much. The use of old ink for a long time may also bring about a decrease in ink performance. Therefore, the liquid package can be produced. There are more variables. Color printing companies can develop a set of appropriate finished product testing standards by simulating the customer's conditions of use to ensure that unqualified products are not shipped out of the factory and avoid unnecessary losses.