Discussion on the structure and technology of the

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Discussion on the structure and process technology of color printing flexible packaging materials

the so-called composite packaging refers to two or more layers ". film materials are laminated together in a certain way to become a new packaging material. It not only has the advantages of each layer of film, but also overcomes the shortcomings of a single material. It can replace the traditional glass bottle and metal can packaging. It is mainly used for packaging beverages, instant noodles, pickles, fish, sausages, condiments, preserves, biscuits, milk powder, washing powder, drugs, pesticides, etc.

the author takes milk powder bags as an example to analyze and combine other The structure and process technology of color printing flexible packaging materials are described as follows for readers' reference

I. substrate selection and structural design

as we all know, in the 1980s, China's milk powder packaging used bopp20 μ/LDPE50 μ, Its shelf-life is only half a year. In order to extend the fragrance, color, quality and taste retention period, prevent milk powder from caking, seeking materials with high gas resistance and high moisture resistance has become an important development topic of milk powder packaging. Since the vacuum aluminizing of plastic film can greatly reduce the water vapor transmission, which is 45g/m2.24h, which is reduced by about 100 times. The vacuum aluminizing film also has the properties of gas blocking, heat insulation, shading, infrared ray prevention, etc. the composite flexible packaging made of it can maintain the color, aroma, taste and nutrition of food, coupled with its gorgeous metallic luster, is deeply welcomed by consumers

because a single material has its specific advantages and disadvantages, it is impossible to have multiple functions at the same time. The high resistance milk powder bag has been studied and tested to adopt a three-layer film composite structure. The outer layer is BOPP film with certain strength, good transparency, low price and easy printing, and the inner layer is CPE film with good heat sealing performance and low temperature resistance. The key to the milk powder bag lies in the selection of high barrier materials in the middle layer. At the beginning, we chose opp/al/pe structure. Due to the composite of plastic film and aluminum foil, its water vapor transmission is greatly increased, which extends the shelf life. Coupled with its gorgeous metallic luster, it is very attractive to consumers. However, because the aluminum foil is brittle and easy to break and corrode, and its water vapor transmission is inversely proportional to the thickness (see Table 1)

Table 1 Relationship between aluminum foil thickness and water vapor transmission

thickness μ M 913182530 ~ 150 water vapor transmission g/m2 24h 1.08 ~ 10.70 0.60 ~ 4.800 ~ 1.240 ~ 0.460 in order to improve the barrier performance, Al foil must have a certain thickness. With the increase of Al foil thickness, the demand for metal aluminum will increase. At the same time, the production of aluminum foil requires a lot of power consumption, and its cable accounts for more than 50% of the material cost. The vacuum aluminized PET film has excellent barrier performance (see Table 2). The transmission of ultraviolet and visible light is close to zero, which extends the preservation, quality assurance and flavor preservation period by more than 3 times. In addition, it has beautiful metallic luster. After printing on the back of BOPP film, it is compounded with it. Because the light increases the reflection through the surface of plastic film, it can produce a pleasing optical effect, which is especially suitable for the packaging of export products and can improve the added value of products. At the same time, the consumption of aluminum coated film is only a few thousandths of that of aluminum foil, so as to save energy, save metal materials, reduce the weight of packaging bags, and facilitate cost reduction

Table 2 Comparison of barrier properties of several films


Project LDPE 40 μ m BOPP 20 μ m PT 300# PET 12 μ m VM 50~300A PET 12 μ M moisture permeability g/m2 24h 10 ~ 207 1000 ~ 2000 45 1 ~ 3 oxygen permeability cm3/m2 24h 4000 1740 10 ~ 1000 110 3 ~ 5 2. Process flow

according to the special requirements of high resistance milk powder bag (membrane), the following process route is determined. (as follows, especially for projects in line with the national major industrial strategy and emerging industry plan)


BOPP → printing → rewinding → dry compounding → aging → bag making → inspection → packaging and warehousing

ink adhesive

production process flow chart of high resistance milk powder bag

III. selection of auxiliary materials such as ink and adhesive

(1) ink: use plastic inner printing composite ink, and its requirements are: a Good composite fastness; B. Easy to dry, less residual components; C. Good reproducibility; D. The price is moderate

(2) adhesive: it must have the characteristics of strong adhesion, good compatibility with ink, uniform coating, transparent color, and meet the requirements of food hygiene. Therefore, we choose huahuangyan New Oriental food grade two-component polyurethane adhesive pu-180

(3) toluene (1) montmorillonite (MMT), xylene: used as ink solvent. Because coking xylene contains carcinogens, and is often mixed with benzene, toluene, ethylbenzene... And other mixtures in coking production, sometimes it has unpleasant Coke without odor, and is not suitable for food packaging as a solvent, so petroleum xylene is selected. The same is true. Toluene is also better than petroleum toluene (produced in oil refinery) without coking toluene

(4) ethyl acetate: as a two-component polyurethane adhesive and solvent, the purity of ethyl acetate must be ≥ 98%, and the content of water and alcohol must be ≤ 0.2%. Otherwise, the curing agent isocyanate reacts with Oh, destroying the active gene in the adhesive, reducing the efficiency of the adhesive and affecting the composite fastness

IV. key points of technology

(I) printing

1 Plastic film and its printing performance: plastic film has many varieties and specifications, including thickness, thickness, width and width, and the properties of various substrates are also different. Therefore, the tension of materials changes differently during the printing process. The change of material tension has a direct impact on the overprint accuracy of printed patterns. In order to make the printing pattern chromatic accurately, the printing operator must properly control the tension of the film. In addition, in order to improve the printing fastness, the film surface must be corona treated to make it reach more than 38dyn

2. Ink and solvent: printing ink must have less impurities, good reproducibility, easy drying, less residual components, and high composite strength. Chlorinated polypropylene internal printing ink is generally used for color printing of milk powder bags

flexible packaging ink solvents usually include toluene, dimethyl benzene, butanone (methyl ethyl ketone), ethyl acetate, isopropanol, butanol, etc. Because the solvent has different solubility and volatility to the resin, there are requirements for what film to print, what ink to use, and what solvent to dilute in printing. The correct use of solvents is closely related to the improvement of film printing quality

the mixing ratio of ink and solvent varies with temperature, printing and ink quality

3. Drying: in order to completely volatilize the solvent in the film printed with various inks, the film needs to pass through the oven after each color is printed. At this time, special attention should be paid to process parameters such as air volume, wind speed and drying temperature to prevent printing adhesion, unclear appearance, product odor, etc. Generally, the drying temperature is 40 ~ 80 ℃

4. Printing factors

plate roller accuracy, tension control, overprinting technology, film grade, drying degree

(II) compound

1 Selection of polyurethane adhesive for dry compounding

(1) dry food packaging: Huangyan New Oriental pu-180

(2) boiling: Anji 175 and Huangyan New Oriental pu-8850

(3) transparent cooking bag: domestic cooking glue

(4) aluminum foil Cooking: imported cooking glue, such as American Morton company 585k, Japanese Toyo ink chemical company ad-502, Takeda pharmaceutical company A-310, German Henkel company uk3640/6800

2 Dilution and coating

(1) the dilution solvent is ethyl acetate. The preparation sequence of polyurethane adhesive is as follows: gradually add solvent to the main agent according to the dosage, stir it fully, add curing agent, and then use it after it becomes a uniform solution. The use concentration of adhesive is usually 20% - 5% (solid content on dry basis), and 3 viscosity cup (chann cup) is 18 ~ 25s

(2) generally, coating rollers and polishing rollers are selected, and the required coating amount depends on the packaging purpose. Insufficient coating amount often leads to quality accidents, such as poor appearance and adhesion of the composite film, easy delamination, and the decline of heat sealing, boiling, cooking and other endurance... The best coating of the adhesive is based on the substrate structure, the size of the printing surface, the post-processing conditions of the composite film and the end use. The standard coating amount of dry composite adhesive is shown in Table 3

Table 3 standard coating amount of dry composite adhesive

classification film composition and application standard coating amount g/m2 general purpose

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