Analysis and Discussion on the printing process of

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Analysis and Discussion on the printing process of milk plastic film packaging (Part I)

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 changed significantly. Milk has become a must-have on the table of ordinary people, among which, milk with plastic film soft packaging accounts for a considerable proportion, This kind of packaged milk is usually filled by automatic filling machine. First, the milk and packaging film should be sterilized, and then the milk should be aseptically filled into plastic packaging bags. This paper analyzes and discusses the printing process of this 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, which is generally a single-layer film, which is produced by blowing film equipment with a certain proportion of White Masterbatch added to various polyethylene (PE) materials. This kind of milk packaging film is a non barrier structure. It is hot filled by pasteurization (85 ℃/30 can be introduced in the new year? Min). The shelf life is short, only about 3 days

② black and white coextrusion packaging film with three-layer structure, and the black masterbatch added in the heat sealing inner layer plays a role of blocking 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, Generally, it is required to be between 0.2 and 0.4. The smoothness of the film is mainly achieved 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, which 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 detrimental to printing

② strong tensile strength

because the plastic film will be subjected 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. Choosing LDPE or HDPE particles with low melt index (MI) during film blowing is very beneficial to improve the tensile strength of polyethylene film

③ the surface wetting tension value should meet the requirements

in order to make the printing ink spread, wet and adhere smoothly on the surface of 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 printed matter. It is generally required that the surface tension of polyethylene film should be above 38dyne, and it is better if it can be above 40dyne

as polyethylene is a typical non-polar polymer material, its molecular structure does not contain polar groups, and its crystallinity is high, its surface free energy is low, its inertia is strong, its crystallinity is high, and its chemical properties are stable. Therefore, the printability of this film material is poor, and its adhesion to ink is not ideal. In addition, the additives in the film (smooth agent, plasticizer, etc.) continue to 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 surface treatment of the plastic film is generally required to change the surface chemical structure of the film and improve the surface tension of the film, so as to improve the printing suitability of 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 property, that is, it should not only ensure heat sealing, but also be able to cut 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 cannot 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 instantaneous sterilization (UHT) and hydrogen peroxide (H2O2) sterilization in the automatic filling process, special sterilization conditions put forward special requirements for the performance of ink, which is also one of the matters worthy of our attention. Special emphasis should be placed on the following properties:

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, it is required that the adhesion between ink and plastic film must be very strong, which is the key problem of ink for plastic film printing, that is to say, 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

because the surface printing ink is printed on the outer surface of the film, it will be affected by friction in the process of commodity circulation, so it is required that the printing ink used must have strong wear resistance and scratch resistance. After printing, the surface strength of the ink film is high, the firmness is good, and the friction resistance is good, otherwise the ink layer may fall off, resulting in incomplete printing patterns, Thus affecting the appearance and quality of products

③ excellent oxidation resistance

when the tension of metal material tensile testing machine increases, most high-speed filling machines are equipped with hot hydrogen peroxide (H2O2) sterilization system, which is used to sterilize the packaging film. Hydrogen peroxide (H2O2) has strong oxidation, and its oxidation 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 pigments in ink is not very good, ink fading will occur, affecting the quality of products

④ excellent water resistance

during the process of ultra-high temperature instantaneous sterilization (140 ℃/1 ~ 3S) and hydrogen peroxide (H2O2) sterilization, the surface of the milk packaging film will contact with water or steam, 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 a 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 at low temperature

⑥ good hygienic performance

for milk packaging film, it should also meet the requirements of the hygienic standards of packaging materials and food packaging regulations. Therefore, it is required that the ink used for printing should be non-toxic, odorless, less residual solvents, and free of toxic heavy metals. So as not to pollute the milk and cause D2 harm to the health of consumers - the average thickness of the test piece after the experiment

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 can be said to be 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 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 unit is n/15mm and the results are as follows: soak the printing film after aging for 24h in dioxygen water with a temperature of 45 ℃ ~50 ℃ and a concentration of 35% for about 15min, take it out and test it. The ink has good adhesion and basically no fading phenomenon, which can fully meet the requirements of milk packaging

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