Analysis and Exploration of Milk Plastic Film Packaging Printing Process

With the development of the economy and the improvement of people’s living standards, our country’s dairy industry has grown rapidly and has grown. People’s lifestyles and dietary structures have also undergone major changes. Milk has become a must-have item for ordinary people on the table. Plastic film flexible packaging of milk accounts for a considerable proportion, this packaging of milk is usually used automatic filling machine for filling, first to sterilize the milk and packaging film, and then aseptically filling the milk into the plastic bag . This article analyzes and discusses the printing process of this milk packaging film.
Plastic Films 1. Film Material Types The film materials used in milk packaging films are mainly polyethylene (PE) coextruded films, which can be basically divided into the following three according to the number of layers:
1 simple milk packaging film, generally a single film, it is a variety of polyethylene (PE) material is added to a certain proportion of white masterbatch, produced by blown film equipment. This milk packaging film has a non-barrier structure and is pasteurized (85°C/30min) for hot filling. The shelf life is short, only about 3 days.
2 Black and white co-extruded packaging film with three-layer structure. The black masterbatch added in the heat-sealing inner layer plays a role in blocking light. This packaging film uses Ultra High Temperature Sterilization (UHT) and hydrogen peroxide (H2O2) sterilization methods, and the shelf life at room temperature can reach about 30 days.
3 five-layer structure of black and white co-extruded packaging film, blown film increases the middle barrier layer (by EVA, eval and other high barrier resin composition), therefore, this packaging film is a high barrier aseptic packaging film, a long The shelf life can be stored at room temperature for about 90 days.
2. The film performance requirements need to be particularly pointed out, in order to meet the filling requirements of high-speed automatic filling machine, polyethylene film for milk packaging must meet the performance requirements of the following aspects:
1 Good Slickness The inner and outer surfaces of the film should have good smoothness, ensure it can be filled smoothly on the high-speed automatic filling machine, therefore, the dynamic and static friction coefficient of the film surface should be relatively low, and the general requirement is 0.2~ Between 0.4. The smoothness of the film is mainly achieved by the addition of a slip agent. After the film is formed, the slip agent migrates from the film to the surface and accumulates into a uniform thin layer, which can significantly reduce the coefficient of friction of the film and make the film have Good smoothness effect. Slip agent is usually oleic acid amide or erucic acid amide, which is just a kind of mechanical mixing with macromolecule polyethylene. It is not well compatible. The molecular heat movement makes it gradually migrate to the interface of low free energy to form a layer. The weak interface layer, therefore, if the lubricant content is too high, it will affect the printability of the film, which is extremely unfavorable for printing.
2Strong tensile strength As the plastic film is subjected to the mechanical pulling force from the automatic filling machine during the filling process, it is required that the film must have sufficient tensile strength to prevent it from being pulled by the automatic filling machine. It was pulled off. The use of LDPE or HDPE particles with a low melt index (MI) in the blown film process is advantageous for increasing the tensile strength of the polyethylene film.
3 The surface wetting tension value must meet the requirements In order for the printing ink to be able to spread, wet, and adhere on the surface of the polyethylene plastic film, the surface tension of the film should reach a certain standard, otherwise it will affect the adhesion of the ink on the surface. And firmness, which affects the quality of prints. It is generally required that the surface tension of the polyethylene film is preferably 38 dynes or more, more preferably 40 dynes or more. Since polyethylene is a typical non-polar high molecular material, its molecular structure does not contain polar groups, and its crystallinity is high, its surface free energy is low, its inertness is strong, its crystallinity is high, and its chemical properties are stable. Therefore, the film material The printability is poor and the adhesion to the ink is not ideal. In addition, the additives (slip agents, plasticizers, etc.) in the film constantly migrate to the surface to form a weak surface layer, which also affects the printing fastness of the printing ink. Therefore, the plastic film is generally required before printing. Surface treatment, change the chemical structure of the film surface, improve the surface tension of the film, so as to achieve the purpose of improving and improving the printability of the polyethylene film, improve the adhesion of the ink, in order to ensure the smooth progress of production.
4Excellent heat-sealing performance The film should have good heat-sealed bag-making property and good sealing property, that is, it must guarantee the heat-sealing, and it must be able to cut off smoothly so that the molten resin will not stick to the knife. Adding a certain proportion of LLDPE in the blown film process can obviously improve the low temperature heat sealing performance of the milk film and the heat sealing performance of the inclusions, but the addition amount of the LLDPE can not be too large, otherwise the viscosity of the polyethylene milk film is too high, and the heat sealing is performed. Sticking knife failures can easily occur.
Printing inks 1. Requirements for the performance of inks Milk packaging films are usually printed by means of table printing, in addition to the general technical requirements (such as hue, fineness, viscosity, drying, etc.) of table printing inks, due to the automatic filling process. In the middle, ultra-high temperature sterilization (UIHT) and hydrogen peroxide (H2O2) sterilization are required. Therefore, special sterilization conditions impose special requirements on the performance of the ink. This is also one of the issues that deserve our attention. What needs special emphasis is the following aspects of performance:
1 It has strong adhesion to polyethylene film. Because the surface of plastic film is dense and smooth, and the ink cannot penetrate on its surface, the adhesive force between ink and plastic film must be very strong. This is the printing ink for plastic film. The key issue, that is to say, the ink used for printing milk packaging film should have high affinity and adhesion to the substrate material - polyethylene film, strong adhesion, and good print fastness, this is to ensure high quality Print and good printing effect.
2 Strong abrasion resistance and scratch resistance As the surface printing ink is printed on the outer surface of the film, it will be subject to friction during the circulation of the product. Therefore, it is required that the printing ink used must have strong abrasion resistance. And scratch resistance, the surface of the ink film after printing is high in strength, good in firmness, and resistant to rubbing. Otherwise, the phenomenon of ink layer peeling may occur, resulting in incomplete print patterns, thus affecting the appearance and quality of the product.
3 Excellent oxidation resistance Most high-speed filling machines are equipped with a hot hydrogen peroxide (H2O2) sterilization system to sterilize the packaging film. Hydrogen peroxide (H2O2) has strong oxidative properties, and its oxidizing properties also With the increase of the temperature, the temperature is significantly increased, which puts a higher requirement on the anti-oxidation capability of the printing ink. If the oxidation resistance of the pigment in the ink is not very good, ink fading occurs and the quality of the product is affected.
4 Excellent water repellent milk packaging film In the ultra-high temperature instant sterilization (140 °C / 1 ~ 3 seconds) and hydrogen peroxide (H202) sterilization process, the surface will be exposed to water or water vapor, so the required printing ink must be used With excellent water resistance, water will not fade.
5 heat resistance and freeze-resistance The printing ink used in milk packaging film must also have a certain degree of heat resistance, in order to ensure that after the ultra-high temperature sterilization treatment does not occur ink softening, stickiness and imprinting fade, discoloration and other phenomena. In addition, the frozen milk is generally required to be refrigerated at low temperatures during storage, circulation, and sales. Therefore, it is required that the printing ink used must also have good freezing resistance, and the ink layer will not occur under low temperature conditions. Freezing and shedding.
6 Good hygienic performance For milk packaging film, it should also meet the hygienic standards for packaging materials and food packaging regulations. Therefore, the ink used for printing should be non-toxic, odorless, with low residual solvent and no toxic heavy metal. So as not to contaminate milk, and harm consumers' health.
At present, the Chaoli 313A surface printing ink and the Chaolifu 314 surface printing ink produced by Zhejiang New Oriental Ink Co., Ltd. are widely used in the field of milk packaging film printing. These two inks are designed for milk packaging film. Special printing inks, they have good affinity with polyethylene film, good gloss, fast adhesion, excellent heat resistance, grease resistance, water resistance, etc., can be said that milk packaging film printing Ideal for inks. 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 Li Fu 314 surface printing ink is a two-liquid reactive polyurethane modified ink, can withstand 121 °C / 40min retort sterilization, can withstand 80 °C, a concentration of 35% hydrogen peroxide sterilization. For example, our factory used the Chaolifu 314 two-liquid reactive ink when printing milk film. The oxidation resistance test and the results were as follows: The printed film after curing for 24 hours was at a temperature of 45°C～50°C. , The concentration of 35% soaked in hydrogen peroxide for about 15 minutes, after removal of the test, the ink adhesion is good, basically no discoloration phenomenon, fully able to meet the requirements of milk packaging.
2. Ink performance adjustment and control 1 ink viscosity viscosity is the most important control indicator in ink applications, it directly affects the ink transfer performance and print quality, therefore, in the printing process of the ink viscosity control 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 may be poor, and it may be easy to produce stencil printing and dry printing. The insufficiency of ink may occur in large areas. Broken lines and other ills.
2 Drying of inks The drying performance of inks is also one of the important indicators to measure the quality and performance of inks. Controlling the drying performance of inks is a necessary condition for obtaining a good printing effect. The drying speed of the ink should be appropriate, the drying speed of the ink is too fast, and dry-type failures are likely to occur. However, the drying speed of the ink cannot be too slow, otherwise, the back-printing phenomenon of the printed matter may occur. The general principle for 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 group and before rewinding, but it cannot be dried on the printing plate.
The ink used for printing milk packaging film is a solvent type ink, and the drying property is mainly determined by the volatilization rate of the solvent. Therefore, in the actual production, the ratio of the solvent must be strictly controlled, and an appropriate amount of slowness can be added to the ink according to the specific conditions. Dry 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.
For example, when the factory uses Chaolifu 314 two-liquid reactive ink to print the milk film, under normal conditions (printing speed is about 80 meters/minute), the ink viscosity is generally controlled at about 15 seconds, and the mixed solvent is basically used. The ratio is: butanone:toluene:butyl ester=4:3,5:2.5, which can ensure better printing quality and printing effect.
In order to ensure the printing quality of milk packaging film, the printing process must also pay attention to the control of printing tension, printing pressure, printing speed and drying temperature and other process conditions, formulate a set of stable and reliable process standards, and standardize the process flow to ensure The scale of production goes smoothly.
1. Printing tension should not be too high During the printing process of polyethylene film, the control of printing tension is a key link. Because the stretch rate of the polyethylene film is relatively large, it is prone to tensile deformation under the action of the drawing tension of the printing press. If the tension value is too large, the polyethylene film will have a large tensile deformation, and the overprint may not occur. The quasi-failure affects the quality and accuracy of the printed matter. Therefore, the value of the printing tension should be set to be small to prevent overprinting and graphic distortion due to overstretching of the polyethylene film.
2. Grasp the printing pressure According to the characteristics of the printing method, the performance of the printing ink and the properties of the substrate material, the printing pressure is controlled within an appropriate range. If the printing pressure is too large or too small, the quality of the printed matter may be adversely affected. Printing pressure is too high, the ink is easy to spread, the graphic is easy to get thicker and wider, and even the stencil, bar code can not be read; the printing pressure is too small, the amount of ink is not enough, the graphic is easy to thin, narrow, and even defective.
3. The speed of printing must be properly printed to increase the production efficiency.