Analysis and Research on the printing process of t

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With the development of economy and the improvement of people's living standards, China's dairy industry has developed and expanded rapidly, and people's lifestyle and dietary structure have also undergone major changes. Milk has become a must-have for ordinary people on the table. Among them, milk with plastic film soft packaging accounts for a large proportion, This kind of packaged milk is usually filled by automatic filling machine. Firstly, the milk and packaging film should be sterilized, and then the milk can be aseptically filled into plastic packaging bags. This paper analyzes and discusses the printing process of the milk packaging film

plastic film

1 Film material type

the film material used for milk packaging film is mainly polyethylene (PE) coextrusion film, which can be basically divided into the following three types according to the number of layers:

① simple milk packaging film, generally single-layer film, which is produced by blowing film equipment with various polyethylene (PE) materials added with a certain proportion of White Masterbatch. The milk packaging film is a non barrier structure. It is hot filled by pasteurization (85 ℃/30min). The shelf life is short, only about 3 days

② the black-and-white coextrusion packaging film with three-layer structure, and the black masterbatch added in the heat sealing inner layer can block the light. This packaging film adopts ultra high temperature sterilization (UHT) and hydrogen peroxide (H2O2) sterilization methods, and its shelf life can reach about 30 days at room temperature

③ the black-and-white coextrusion packaging film with five layer structure adds an intermediate barrier layer (composed of EVA, Eval and other high barrier resins) during film blowing. Therefore, this packaging film is a high barrier sterile packaging film with a long shelf life and can be stored for about 90 days at room temperature

2. Film performance requirements

it should be noted that in order to meet the filling requirements of the high-speed automatic filling machine, the polyethylene film for milk packaging must meet the following performance requirements:

① good smoothness

the inner and outer surfaces of the film should have good smoothness to ensure that 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, It is generally required to be between 0.2 and 0.4. The smoothness of the film is mainly realized by adding a smooth agent. After the film is formed, the smooth agent migrates from the film to the surface and accumulates into a uniform thin layer, which can significantly reduce the friction coefficient of the film and make the film have a good smooth effect. The smoothing agent is usually oleic acid amide or erucic acid amide. It is only a mechanical mixture with high molecular polyethylene and cannot be well compatible. The molecular thermal movement makes it gradually migrate to the interface with low free energy, forming a weak interface layer. Therefore, if the content of smoothing agent is too high, it will affect the printability of the film, which is extremely unfavorable to the printing

② strong tensile strength

as the plastic film will be subject to the mechanical tension 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 broken under the tension of the automatic filling machine. LDPE or HDPE particles with low melt index (MI) are used in the blowing process, which is very beneficial to improve the tensile strength of polyethylene film

③ the surface wetting tension value should meet the requirements

in order to enable the printing ink to spread, wet and adhere smoothly 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 and firmness of the ink on its surface, thus affecting the quality of the printed matter. It is generally required that the surface tension of polyethylene film should be above 38 dyne, and it is better if it can be above 40 dyne. As polyethylene is a typical non-polar polymer material, the molecular structure does not contain polar groups, and has high crystallinity, low surface free energy, strong inertness, high crystallinity and stable chemical properties, the film material has poor printability and poor adhesion to ink. In addition, the additives in the film (smoothing agent, plasticizer, etc.) continuously migrate and precipitate to the surface, forming a weak surface layer, which will also affect the printing fastness of the printing ink. Therefore, before printing, the plastic film is generally subject to surface treatment to change the chemical structure of the film surface and improve the surface tension of the film, so as to improve the printing suitability of the polyethylene film and improve the adhesion to the ink, To ensure the smooth progress of production

④ excellent heat sealing performance

the film should have good heat sealing bag making property and good sealing off property, that is, it should not only ensure heat sealing, but also 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 film blowing process can significantly improve the low-temperature heat sealing performance and inclusion heat sealing performance of the milk film, but the addition of LLDPE should not be too large, otherwise the viscosity of the polyethylene milk film will be too high, and knife sticking failure will easily occur during heat sealing

printing ink

1 Requirements for ink performance

milk packaging film is usually printed by surface printing. In addition to the general technical requirements of surface printing ink (such as hue, fineness, viscosity, dryness, etc.), due to the ultra-high temperature sterilization (UHT) and hydrogen peroxide (H2O2) sterilization in the automatic filling process, special sterilization conditions put forward special requirements for the performance of the ink, This is also one of the matters worthy of our attention. Special emphasis should be placed on the following properties:

① strong adhesion to polyethylene film

as the surface of the plastic film is dense and smooth, and the ink cannot penetrate on its surface, it is required that the adhesion between the ink and the plastic film must be very strong, which is the key problem of the ink for plastic film printing, Also, Mr. Nicholas Smith, general manager of Bayer materials technology textile coating global business, said: "For all industries using textiles, the ink used for printing milk packaging film should have high affinity and adhesion to the substrate material polyethylene film, with strong adhesion and good printing fastness, in order to ensure high-quality prints and good printing effect.

② strong wear resistance and scratch resistance

as the surface printing ink is printed on the outer surface of the film, it will be rubbed in the process of commodity circulation As a result, the printing ink used must have strong wear resistance and scratch resistance. The surface strength of the ink film after printing is high, the firmness is good, and the friction is resistant. Otherwise, the ink layer may fall off, resulting in incomplete printing patterns, which will affect 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 a strong oxidizability instead of the composite wall, and its oxidizability also increases significantly with the increase of temperature, which puts forward higher requirements for the oxidation resistance of printing ink. If the oxidation resistance of the pigment in the ink is not very good, the ink will fade, affecting the product quality

④ excellent water resistance

during the process of ultra-high temperature instantaneous sterilization (140 ℃/1 ~ 3 seconds) and hydrogen peroxide (H202) sterilization, the surface of the milk packaging film will contact with water or water vapor, so the printing ink used must have excellent water resistance and will not fade in case of water

⑤ heat resistance and freezing resistance

the printing ink used for milk packaging film should also have certain heat resistance to ensure that after ultra-high temperature sterilization treatment, the ink will not soften, become sticky, and the imprint will fade and change color. In addition, the filled milk generally needs to be refrigerated at low temperature in the links of storage, circulation and sales. Therefore, the printing ink used must also have good freeze resistance, and the ink layer will not freeze and fall off under low temperature

⑥ good hygienic performance

for milk packaging film, it should also meet the requirements of the hygienic standards for packaging materials and food packaging regulations. Therefore, it is required that the printing ink used should be non-toxic, odorless, less residual solvents, and free of toxic heavy metals. So as not to pollute the milk and do harm to the health of consumers

at present, super Lifu 313A surface printing ink and super Lifu 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 special printing inks designed for milk packaging film. They have good affinity with polyethylene film, good gloss of ink layer, high adhesion fastness, and excellent heat resistance, grease resistance, water resistance and other properties, It is an ideal choice for milk packaging film printing ink. 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 ℃/40min cooking and sterilization, and can withstand 80 ℃ and 35% hydrogen peroxide sterilization. For example, when printing milk film, our factory uses super Lifu 314 double liquid reactive ink. The oxidation resistance test and results of the ink are as follows: today, Shandong Sida high tech first introduced the elimination device to eliminate the error caused by the device: immerse the printing film after 24 hours of aging in dioxygen water with a temperature of 45 ℃ ~ 50 ℃ and a concentration of 35% for about 15 minutes, and then take it out for detection. The ink has good adhesion and basically no color fading, Fully meet the requirements of milk packaging

2. adjustment and control of ink performance

① viscosity of 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, it is very important to control the viscosity of ink 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, and the ink unevenness, fine line fracture and other defects occur 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 should be appropriate. If the drying speed of the ink is too fast, it is prone to dry plate failure. However, the drying speed of the ink should not be too slow, otherwise 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 press, the ink should be fully dried before entering the next printing color deck and before rewinding, but it cannot be dried on the printing plate

the ink used for printing milk packaging film is solvent based ink, and its dryness mainly depends on the volatilization speed of the solvent. Therefore, the solvent ratio 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"

for example, when our factory printed milk film with super Lifu 314 double liquid reactive ink, the

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