In recent years, fresh milk in plastic film flexible packaging has appeared in the market. This type of packaging first sterilizes milk and packaging film, and then uses an automatic filling machine to fill milk into plastic bags. Since milk is a fresh drink, the requirements for hygiene, bacteria, temperature, etc. are very strict. Therefore, there are special requirements for the printing of the packaging bag, which makes the printing of the milk packaging film different from other printing techniques.
The selection of flexible packaging milk plastic film for the selection of milk packaging film must meet the requirements of packaging printing, processing, storage and transportation and sanitation. At present, the commonly used film material is mainly a polyethylene (PE) coextruded film, which is obtained by melt extrusion and blow molding of a polyethylene resin.
The properties of polyethylene films depend primarily on the density of the resin, the melt index and the molding process. Polyethylene is insoluble in various solvents, but when the temperature exceeds 70 ° C, polyethylene can be dissolved in a small amount in a solvent such as toluene or amyl acetate. This property provides conditions for surface finishing. The plastic can be soaked with a heating solvent to swell the surface and destroy part of the crystal to improve the adhesion of the ink film on the plastic surface. In addition, a mixture of fuming sulfuric acid, concentrated nitric acid, chromic acid and sulfuric acid can slowly act on polyethylene at room temperature: sulfuric acid and nitric acid can rapidly destroy polyethylene at 90-100 ° C; use potassium dichromate, sulfuric acid, The mixture of distilled water is immersed in polyethylene plastic at 70-75 ° C, which is beneficial to enhance the adhesion of the ink film on the plastic surface.
The film currently used for milk packaging can be basically classified into three types according to the layer structure.
1 simple packaging film. Generally, it is a single-layer film, which is produced by adding a certain proportion of white masterbatch from various polyethylene materials through blown film equipment. The packaging film is a non-blocking structure, which is hot-filled by pasteurization (85 ° C / 30 min) and has a short shelf life (about 3 days).
2 three-layer structure black and white co-extruded packaging film. It adopts LDPE, LLDPE, EVOH, MLLDPE and other resins, and is combined with black and white masterbatch to form a high-performance composite film. The black masterbatch added to the inner layer of the heat seal serves to block light. The packaging film adopts ultra-high temperature instantaneous sterilization and hydrogen peroxide sterilization method, and the shelf life can reach about 30 days at normal temperature.
3 five-layer structure black and white co-extruded packaging film. When the film is blown, an intermediate barrier layer (composed of a high barrier resin such as EVA or EVAL) is added. Therefore, the packaging film is a high barrier aseptic packaging film having a long shelf life and can be stored for about 90 days at normal temperature. The three-layer and multi-layer black-and-white co-extruded packaging film has excellent heat sealing performance and light and oxygen barrier resistance, and has the advantages of low price, convenient transportation, small storage space and strong practicability.
In order to meet the requirements of milk filling and printing, the polyethylene film mainly has the following requirements.
1 smooth and smooth. The inner and outer surfaces of the film should have good smoothness to ensure smooth filling on the high-speed automatic filling machine. Therefore, the dynamic and static friction coefficient of the film surface is relatively low, and the general requirement is 0.2-0.4. Slip property is mainly achieved by adding 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 friction coefficient of the film and make the film have a good cool. Slip effect.
The slip agent is usually oleic acid amide or erucamide, which is only a mechanical mixture with the polymer polyethylene. It is not well compatible, and the molecular thermal motion gradually migrates to the interface of low free energy to form a layer. A weak interfacial layer that produces and enhances the smoothing effect. Due to the characteristics of the slip agent, it is necessary to pay attention to the amount of the slip agent. If the amount of the slip agent is too small, the smoothing effect will not be achieved, but if the amount is excessively added, the printing suitability of the film will be affected and the film thickness will be lowered. The adhesion fastness of the ink and the like is extremely disadvantageous for printing.
2 tensile strength. Since the plastic film is subjected to 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 being pulled off by the tension of the automatic filling machine. The use of LDPE or HDPE particles with a lower melt index during the blown film process is advantageous for increasing the tensile strength of the polyethylene film.
3 surface wetting tension. 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 be up to a certain standard, and it is necessary to rely on corona treatment to achieve a higher wetting tension, otherwise the ink will be affected in the film. The adhesion and firmness of the surface affects the quality of the print. Generally, the surface tension of the polyethylene film should be above 38 dyne, and more preferably 40 dyne or more. Since polyethylene is a typical non-polar polymer material, the molecular structure does not contain polar groups, and the crystallinity is high, the surface free energy is low, the inertness is strong, and the chemical properties are stable. Therefore, the printing suitability of the film material is relatively better. Poor, the adhesion to the ink is not ideal.
In addition, the additives (slip agents, plasticizers, etc.) in the film continuously 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. The surface treatment is carried out to change the chemical structure of the surface of the film, and the surface tension of the film is improved, thereby improving and improving the printing suitability of the polyethylene film, and improving the adhesion to the ink to ensure smooth production.
4 heat sealability. The most worrying film automatic packaging is the problem of broken bag and loose seal, which leads to bag breakage. Therefore, the film should have good heat-sealing bagging property, good sealing property, and a wide heat sealing range for packaging. When the speed changes, the heat sealing effect is not greatly affected, and MLLDPE is often used as the heat sealing layer to fully ensure the stability and heat sealability of the heat sealing condition. That is to say, it is necessary to ensure heat sealing and to be able to cut smoothly so that the molten resin does not stick to the knife.
Adding a certain proportion of LLDPE during the blown film process can significantly improve the low-temperature heat-sealing performance of the film and the heat-sealing performance of the inclusions, but the amount of LLDPE added should not be too large, otherwise the viscosity of the polyethylene film will be too high, when heat-sealed Sticky knife failure is easy to occur. For the structural design of the film, the packaging film of the corresponding structure can be selected according to the content of the package and its shelf life.
Selection of printing inks 1. Requirements for ink performance Milk packaging (analysis of development of new packaging systems) Films are usually printed by surface printing, except for the general technical requirements of surface printing inks (such as hue, fineness, viscosity, dryness, etc.) In addition, because the milk is subjected to ultra-high temperature instantaneous sterilization and hydrogen peroxide sterilization during the automatic filling process, this special sterilization condition puts special requirements on the performance of the ink, and in particular, the following aspects are particularly emphasized. 1 has a strong adhesion to plastic film. Since the surface of the plastic film is dense and smooth, the ink is impermeable on the surface thereof, so the adhesion between the ink and the plastic film is required to be very strong, which is a key problem of the ink for printing the plastic film, that is, the ink used for the packaging film. It should have high affinity and adhesion to polyethylene film substrates, strong adhesion and good printing fastness, in order to ensure high quality prints and good printing results. 2 good wear resistance and scratch resistance. Since the surface printing ink is printed on the outer surface of the film, it will be subjected to friction during the circulation of the product, and must be able to withstand the irregular friction of the surface, so that the ink will not be in the process of packaging processing, storage, transportation, sales, etc. Deinking. Therefore, the printing ink used is required to have strong abrasion 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 defective printing patterns. Incomplete, which affects the appearance and quality of the product.
3 superior oxidation resistance. On most high-speed filling machines, a hot hydrogen peroxide sterilization system is used to sterilize the packaging film. Hydrogen peroxide has strong oxidizing properties, and its oxidizing property is also significantly increased with the increase of temperature, which is suitable for printing. The anti-oxidation ability of the ink puts forward higher requirements. If the oxidation resistance of the pigment in the ink does not meet the requirements, ink fading will occur, which will affect the quality of the product.
4 excellent water resistance. The milk packaging film is exposed to water or water vapor during the ultra-high temperature (140 ° C) transient (1 ~ 3 s) sterilization and hydrogen peroxide sterilization, so the printing ink used must have excellent water resistance, water contact No color loss will occur.
5 excellent heat resistance and freeze resistance. As a printing film for milk packaging film, it must have a certain heat resistance, and must be able to withstand the requirements of pasteurization or hydrogen peroxide treatment and boiled treatment to ensure that the ink does not soften and become sticky after being subjected to ultra-high temperature sterilization treatment. The phenomenon of fading, discoloration, etc. In addition, the milk after filling generally needs to be refrigerated in the process of storage, circulation and sales. Therefore, the printing ink used must also have good freeze resistance, and the ink layer will not freeze at low temperature. And shedding.
6 safety and hygiene. Since milk is for human consumption, packaging film, printing ink, etc. should also meet the requirements of packaging materials hygiene standards and food packaging regulations. Therefore, the ink used for printing should be non-toxic, odorless, less residual solvent, and free of Heavy metals and other requirements. In order to avoid contamination of milk, and harm to the health of consumers.
According to the printing ink packaging film requirements for ink performance, the printing mainly uses polyamide and two-component polyurethane type printing ink. The polyamide printing ink has good affinity with polyethylene film, and the ink layer has good gloss. It has high adhesion fastness, basically no color fading phenomenon, and better heat resistance, grease resistance and water resistance. It can be said to be an ideal choice for milk packaging film printing ink. The two-component polyurethane type surface printing ink is used for varnish treatment after printing ink, and the cost is relatively high and the process is cumbersome.
2. Adjustment of ink color For plastic film printing, it is necessary to pay attention to the adjustment of color in order to print a colorful film and improve the competitiveness of sales.
In the color adjustment, it is necessary to pay attention to using the sizing inks of the same color and color close to the same manufacturer as the main color to carry out the grading, and try not to use the inks produced by different manufacturers, and it is not possible to mix inks of different manufacturers and different kinds of colors. When color matching, you should use the original color ink, do not use the color ink, try to reduce the variety of ink. When the color ink is formulated, the complementary color phase is added to the color reduction according to the color subtractive complementary color principle, so care should be taken to avoid and utilize this principle in the coloring.
For example, when blending the color or diluting the original color ink, the addition of the complementary color ink should be avoided as much as possible, because the more the complementary colors are mixed, the more the color-removing components are added, and the chroma of the ink color is lowered to make the color dull. However, in printing, sometimes the principle of complementary color is also used to solve some practical problems. For example, in order to make the whiter whiter, a small amount of ultramarine (about 1% or so) is added to the white ink to eliminate the yellow tone in white.
Adding a small amount of titanium cyan in the black ink also eliminates the yellow tint in the black ink, making the ink blacker. Also, when the blue ink is reddish, as long as a small amount of green ink is added to the ink, it can become a yellowish blue ink. If the blue ink is yellowish, it can be corrected by adding a small amount of purple-blue ink. When the ink is bluish, a small amount of orange or the like may be added in order to make the yellow ink reddish.
For the preparation of light pastel inks, pay attention to the white ink, add a small amount of primary ink, because the plastic film is a non-absorbent material, therefore, when the ink color is too deep to dilute, you can not use thinner and use white ink to dilute, In color matching, it is necessary to accurately select colors to reduce the variety of inks to obtain accurate and ideal colors. According to practical experience, some light color inks are formulated as follows: 1 pink ink: mainly white ink, adding a small amount of gold red, red, magenta, fluorescent orange and so on.
2 jade color ink: mainly white ink, supplemented by fluorescent orange red, slightly added medium yellow.
3 m color ink: mainly white ink, slightly added medium yellow, magenta, or medium yellow orange supplement.
4 light blue ink: mainly white ink, adding a small amount of medium blue.
5 lake blue ink: mainly white ink, supplemented by green ink, slightly add sky blue or peacock blue.
6 lake green ink: mainly white ink, green, medium yellow, light yellow ink as a supplement, slightly add sky blue or peacock blue.
7 gray ink: mainly white ink, adding a small amount of black ink.
8 silver gray ink: mainly white ink, add a small amount of black ink, add a small amount of silver paste.
9 Snow Cyan Ink: Add a small amount of light red and royal blue to the white ink.
3. Adjustment and control of ink properties 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, the control of ink viscosity during printing is very important, generally controlled between 15 and 20s. The ink viscosity 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 dry before entering the next printing color set and before rewinding, but not dry on the printing plate.
The ink used for printing the packaging film is a solvent-based ink, and 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. A solvent or a fast-drying solvent to adjust the drying speed of the ink to ensure that the ink is sufficiently dry to prevent "false dry" phenomenon. When using Superflux 314 two-liquid reactive ink for printing, the ink viscosity is generally controlled at about 15s under normal conditions of printing speed of about 80m/min. According to practice, the basic ratio of mixed solvent used is: methyl ethyl ketone. : Toluene: Butyl ester = 4: 3.5: 2.5, which ensures better print quality.
Requirements of the printing process In order to ensure the printing quality of the milk packaging film, the printing process must also pay attention to the control of the printing conditions such as printing tension, printing pressure, printing speed and drying temperature, and develop a set of stable and reliable process standards and standardize the process flow. To ensure the smooth production of scale.
1 Adjust the printing tension. In the printing process of polyethylene film, the control of printing tension is a key link. Because the stretch ratio of the polyethylene film is large, the tensile deformation is easy to occur under the tension of the printing machine. If the tension value is too large, the polyethylene film will have a large tensile deformation, and the overprint may not occur. Quasi-fault, which affects the quality and precision of the printed matter. Therefore, the printing tension value should be set smaller to prevent overprinting and graphic deformation due to excessive stretching of the polyethylene film.
2 Control the printing pressure. The printing pressure is controlled within an ideal range according to the characteristics of the printing method, the performance of the printing ink and the performance of the substrate material. If the printing pressure is too large or too small, the quality of the printed matter may be adversely affected. Printing pressure is too large, the ink is easy to spread, the picture is easy to become thicker, wider, and even paste, the barcode can not be read; the printing pressure is too small, the amount of ink is insufficient, the picture is easy to become thinner, narrower, and even defective.
3 printing speed should be appropriate.
Printing speed will increase production efficiency and output, shorten the production cycle, but the printing speed is not as fast as possible, otherwise it will cause printing failure. The printing speed should match the drying speed of the ink, the viscosity of the ink, the temperature of the drying oven, and the temperature and humidity in the workshop.
4 Set the drying temperature reasonably. The temperature setting of the drying oven of the printing machine should not be too high, generally around 40 °C, otherwise it will easily cause shrinkage deformation of the polyethylene film, thus affecting the overprinting accuracy. Therefore, when setting the temperature of the drying oven, it is necessary to comprehensively consider the characteristics of the substrate material, the printing speed, the printing tension, and the size of the printing pattern to ensure that the ink solvent can be sufficiently volatilized and dried to enhance the adhesion fastness of the ink.
Problems that should be paid attention to during production 1 Do a good job in the quality inspection of raw and auxiliary materials (films, inks, solvents, etc.), and strictly control to ensure that only the raw and auxiliary materials that have passed the inspection and inspection can enter the production process, and the unqualified raw and auxiliary materials will be Control is outside the production process, which is a prerequisite for ensuring the quality of milk packaging film printing.
2 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.
3 The plastic film after corona treatment should be used as soon as possible. It is best to use it immediately. After storage for a period of time, the corona treatment effect of the film will be worse. The surface tension value of the plastic film will gradually increase with the extension of the standing time. Attenuation, it is likely that the requirements will not be met when printing on the machine. Therefore, the plastic film stored for a period of time must be carefully tested for surface tension before use. Once the surface tension is found to be unsatisfactory, it must be re-corroded. Otherwise, the printing quality will be affected.
4 Choose the right material. 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. The process ensures the printing quality of the film.
5 When formulating ink, it should be used with the adjustment, and observe the printing effect at any time, pay attention to maintain the consistency of the ink hue, and control the color difference of the printed matter. When printing the same job, if the printed material changes, it will inevitably lead to chromatic aberration. Even if the same type of ink produced by the same ink manufacturer, the color often has a certain difference if the batch is different. Therefore, In the printing process of long-run printing or large-volume jobs, the same batch of film and ink produced by the same manufacturer should be used for printing.
6 control the printing environment. The change of temperature and humidity in the printing workshop has certain influence on the performance of the substrate material and ink. If the environment is too dry, it will cause static electricity of the film, which will make printing difficult. 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 in the workshop is controlled at 18 to 25 ° C, and the relative humidity is preferably maintained at 60 to 70%.
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