Printing Notes for Liquid Soft Plastic Packaging Films

In recent years, fresh milk, soy milk, and many juice drinks and condiments packaged in plastic films 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.

First, the 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 and hygiene. From the apparent performance of the film can be divided into three categories: soft creamy white PE film, black and white coextrusion PE film, rigid high temperature cooking PE film. The soft creamy white PE film is mainly used for simple packaging of liquids such as milk, soy sauce, and vinegar. The co-extrusion blown film masterbatch consists of LDPE, LLDPE, and milky white material. Membrane performance in performance, relatively poor barrier properties, sterilization temperature does not exceed 90 °C, shelf life is relatively short; generally used in manual or low-speed filling equipment. Such as low-grade milk packaging, the general shelf life of up to three days, the nearest sale. The black-and-white films are mostly three-layered, with a maximum of five layers. When the film is blown, the increased intermediate layer is a coextruded barrier masterbatch. The black masterbatch of the heat-sealing inner layer blocks light and blocks ultraviolet light. Due to the existence of an intermediate barrier layer, the composition of EVA, eval, and other master batches, the shelf life of the contents can be maintained for more than thirty days, but there are some three-layer co-extruded black and white film in the market, the middle layer is not to block The composition of the masterbatch, so this black and white film is also the first category. Among the three types of films, the black and white film is the most unfavorable to the printing operation. Black and white film in the film, contains the largest amount of smoothing agent and brightener to meet the black and white film in the high-speed automatic filling machine on the friction coefficient of the small requirements, and the added oily material is extremely unfavorable to the color printing process, greatly reduced Ink adhesion properties. In most high-speed automatic filling machines, a direct sterilization system with hot hydrogen peroxide is used to sterilize the packaging film. In this case, it is necessary to pay attention to the influence of the sterilization conditions on the ink layer because some of the pigments in the ink have poor oxidation resistance. Tests have shown that hydrogen peroxide oxidation increases significantly with increasing temperature. Therefore, color printing manufacturers must be clear about the customer's post-processing conditions when providing packaging materials. The rigid high-temperature cooking PE film is mainly used for milk beverages such as yoghurt, soy milk and so on. When the contents are sterilized after filling, an ultra-high temperature sterilization process is used. The rigid co-extruded PE film uses HDPE masterbatch as the main component, adding some LDPE and color masterbatch. Therefore, the rigid PE film has good high-temperature resistance and can meet the requirements of ultra-high temperature sterilization. Due to the pressure difference during sterilizing, it is easy to produce "bulging bag" phenomenon. Therefore, it is required that the hard PE film not only has a good heat seal strength, but also the tensile strength performance of the film is better, and color printing manufacturers generally use "three meters throw high" or "Man's single foot step" test effect.

Second, printing ink for liquid soft plastic packaging film

Liquid packaging films such as milk and beverages are usually printed by means of surface printing. Most of them are polyamides or modified into binders. These inks were first developed in gravure printing inks in China, and the production process is relatively mature and complete. This type of ink is suitable for polyethylene, polypropylene film, ink layer adhesion fastness, good gloss; but polyamide inks generally have poor water resistance, not resistant to grease, not resistant to freezing and other performance defects at high temperatures In the summer, printing of this type of ink tends to produce stickiness and softening, and freezing occurs during printing at low temperatures in the winter. Liquid packaging in the post-processing of filling, distribution, storage, etc., must be sterilized by high temperature or frozen, so the inherent defects of the polyamide type table printing ink can not meet the requirements of liquid packaging, production of liquid packaging manufacturers of color printing Must be for different types of co-extruded PE film, non-polyamide type of printing ink, the most suitable liquid package ink in the market is polyester and modified products, single-liquid type and two-liquid reaction type Ink.

(I) Safety and health

Since milk and beverages are for human consumption, packaging films, printing inks, etc. should also meet the hygienic standards for packaging materials and food packaging regulations. Therefore, the inks used for printing should be non-toxic, odorless, and have less residual solvents. Does not contain toxic heavy metals and other requirements. So as not to contaminate milk and drinks, and harm the health of consumers. According to the above requirements for ink performance, it is necessary to select an ink with superior performance. The general printing is the use of two-component polyurethane type surface printing ink, and after the ink is printed, the gloss oil treatment is performed, the cost is relatively high and the process is cumbersome. At present, the more widely used and better quality is Chao Lifu 313A type surface printing ink and Chaolifu 314 type surface printing ink produced by Zhejiang New Oriental Ink Group Co., Ltd. It does not need to be separately treated with a varnish, but only in the printing ink. The addition of a fixed amount of hardener, after curing for 24 hours after printing, can achieve both resistance to boiling, hydrogen peroxide treatment and abrasion. These two kinds of inks are special printing inks designed for milk and beverage packaging films, and have good affinity with polyethylene film, good glossiness of ink layer, high attachment fastness, heat resistance, oil resistance, and resistance Water-based and other properties are also very good, can be said to be ideal for liquid packaging film printing inks such as milk and beverages. Chao Lifu 313A ink is an alcohol-soluble polyamide ink that can withstand boiling below 100°C and can withstand hydrogen peroxide sterilization below 60°C. Chao Lifu 314 printing ink is a two-reaction reactive polyurethane-modified ink, capable of resistance to 121 °C / 40 minutes retort sterilization, can withstand 80 °C, a concentration of 35% hydrogen peroxide sterilization. According to the oxidation resistance test of Chaolifu 314 two-liquid reactive ink, the printed film after curing for 24 hours was soaked in a concentration of 35% in hydrogen peroxide at a temperature of 45 to 50°C for about 15 minutes, and was taken out for testing. Adhesion of ink is good, almost no discoloration phenomenon, fully able to meet the requirements of liquid packaging such as milk and beverages.

(II) Adjustment and control of ink properties

1, the viscosity of the ink

Viscosity is the most important control index in the ink application. It directly affects the ink transfer performance and print quality. Therefore, the control of the ink viscosity in the printing process is very important, generally controlled between 15 to 20 seconds. If the viscosity of the ink is too low, it may easily cause pinholes, hair voids, and ink spreading in graphic parts. On the other hand, if the viscosity is too high, the transfer performance of the ink is poor, and it is easy to produce stencil, dry printing, and uneven ink on a large area. Faults such as broken lines.

2, the drying 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 for obtaining a good printing effect. The drying speed of the ink is too fast, and it is prone to dry printing failure. However, the drying speed of the ink is too slow, and the problem of sticky printing on the back side of the printed matter occurs. The general principle of choosing the ink drying speed is that under the existing conditions of the printing machine, the ink should be able to be fully dried before entering the next printing color and before rewinding, but it cannot be dried on the printing plate. The ink used for the printing and packaging film is a solvent type ink, and the drying property mainly depends on the volatilization 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 actual 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 “sham dry” phenomenon. When using Chao Lifu 314 dual-liquid reactive inks, the ink viscosity is generally controlled at about 15 seconds under the normal printing speed of 80/min. According to practice, the basic mixing ratio of the mixed solvents used is: Butanone: Toluene: butyl ester = 4:3, 5:2.5, which ensures better print quality.

Third, the liquid soft plastic packaging film printing process precautions

(a) Choose the right material. In actual production, suitable printing films and printing inks should be selected according to the specific conditions such as filling speed, sterilization conditions, and product shelf life of the automatic filling machine, and the printing process parameters should be adjusted and controlled according to the actual situation to standardize the process flow. , to ensure the printing quality of the film. Polyamide watch ink because of its water resistance, oil resistance is not good, liquid ink can not generally be selected; polyurethane inks are also different types, the selection must be targeted, New Oriental Chao Li 313A ink is generally used only for soft Coextruded PE film, and 314 is suitable for a variety of coextruded PE film. The co-extruded PE film used by the color printing manufacturers is generally manufactured by the manufacturers themselves, and the technical level is not the same. Therefore, it is necessary to grasp the selection of the masterbatch. It is advisable to use the masterbatch that does not contain the slip agent; the PE film used passes through After EDM treatment, the surface energy must be at least 40 dyne, preferably more than 42 dyne.

(b) The customer's post-processing conditions or test standards. The color printing manufacturers can only achieve a targeted and well-informed knowledge if they fully understand the conditions of the food manufacturers during filling, and the subsequent processing techniques, and even the detailed requirements for liquid packaging in the market. The main points of attention include: filling speed, sterilization conditions, length of shelf life, preservation requirements, etc.

(C) the requirements of the printing process. In order to ensure the printing quality of the liquid packaging film, it is also necessary to pay attention to the control of printing tension, printing pressure, printing speed and drying temperature during the printing process, formulate a set of stable and reliable process standards, standardize the process flow, and ensure the scale production. The smooth progress. In addition, the overall quality of printing workers, the technical information provided by ink manufacturers, the quality of thinner materials, changes in the environment of the workshop are also factors that cannot be ignored.

(D) control the printing environment. The change of temperature and humidity in the printing shop has a certain impact on the properties of the substrate materials and inks. Too dry environments can cause static electricity in the film and cause difficulties in printing. Too much humidity in the environment can result in insufficient drying of the ink and affect the firmness of the ink layer. Therefore, the ambient temperature and relative humidity in the printing shop should be strictly controlled, as far as possible to maintain a relatively constant, and to ensure good ventilation, in order to reduce the incidence of printing failures. In general, the temperature in the workshop is controlled between 18°C ​​and 25°C, and the relative humidity is preferably between 60% and 70%.

(e) Do a good job of inspection of finished products. Customer satisfaction is the highest criterion and final desire of each printing company, so it is necessary to ensure that qualified products are delivered to customers. This will not only satisfy customers, but also safeguard the company's image and reputation. This is the basis for maintaining and developing good customer relationships. In the production process of liquid packs, the changing of the film is relatively frequent, and the performance of the film is likely to vary. The liquid pack uses a small amount of ink, and the long-term use of the old ink printing may also bring about a decline in the performance of the ink. Therefore, the liquid pack may be produced. Many factors are changing. Color printing companies can work out a set of suitable testing standards for finished products by simulating the conditions of use of customers, so as to ensure that the production of non-conformities does not leave the factory and avoid unnecessary losses.

Author/Li Suzhen

Source: Chinese Packaging