In the general term for national packaging (GB4122-83), a flexible package is defined as: A flexible package is a package whose shape may change after the contents are filled or removed. All kinds of bags, boxes, sleeves, and envelopes made of paper, aluminum foil, fibers, plastic films, and their composites are all flexible packaging.
Sheet plastics with a thickness of 0.25 mm or less are generally called films. Plastic film is transparent, flexible, has good water resistance, moisture resistance and gas barrier, good mechanical strength, chemical stability, resistant to grease, easy to print beautiful graphics, can be heat sealed bag. It can meet the packaging requirements of various items, and is an ideal material for packaging easily-stored, easily-placed convenience foods, daily necessities, supermarkets and small packaged goods. Flexible packaging based on plastic film plays an important role in packaging and printing. According to statistics, since 1980, the plastic packaging of some advanced countries in the world has accounted for 32.5% to 44% of the total package printing.
In general, because the protection of the content of a single film material is not ideal, more than one composite film is used to combine two or more films into a layer to meet the requirements of food preservation, aseptic packaging technology. The outer layer material of the composite film is selected from materials that are not easily scratched and grinded, has excellent optical properties, and has good printing properties, such as paper, cellophane, stretched polypropylene, and polyester; and the interlayer is a barrier polymer, such as : Aluminum foil, vapor-deposited aluminum, poly-di-N, N-divinyl-based electric inner layer materials are mostly made of non-toxic, odorless polyethylene and other thermoplastic resins. The flexible packaging manufacturing process is shown in Figure 1.
First, the pre-press treatment of plastic film Plastic film is generally used gravure printing, flexographic printing and screen printing and other processes, usually after the printing but also for composite processing, in order to achieve good results. The surface properties of various plastics vary greatly depending on their molecular structure, polar groups, crystallinity, and chemical stability. These factors have a great influence on the binding fastness of the printing ink and the bonding strength of the composite film. Therefore, before printing or composite processing, depending on the surface properties of different plastics, the surface treatment should be appropriately performed in order to obtain excellent processing suitability.
1. To improve the printability of the film, in theory, to obtain good printability on the substrate, the surface tension of the substrate should be higher than the surface tension of the printing ink.
PP and PE films are non-polar high-molecular materials and have stable chemical properties and small surface tension. In addition, openings, antistatic agents, and anti-aging agents added to synthetic resins are difficult to adhere to ink. The surface tension and printing performance of several commonly used plastic films are shown in Table 1.
In order to improve the surface tension of the plastic film, improve its wetting performance and adhesion performance, thereby improving the fastness of the printing ink, commonly used surface treatment methods are as follows.
1 Corona treatment. Using a high-frequency (intermediate frequency) high-voltage power supply, a corona release phenomenon occurs between the discharge blade holder and the blade, and the plastic film is subjected to surface treatment. This method is called corona treatment, and is also referred to as electronic shock or electric spark treatment. Its processing role is:
a. Through the discharge, the oxygen between the two poles is ionized to produce ozone. Ozone is a strong oxidant that can immediately oxidize the surface molecules of plastic films, converting them from non-polar to polar, and increasing their surface tension.
b. After the electron impact, the surface of the film is generated with dimples and dense holes, so that the plastic surface is roughened and the surface activity is increased.
2 chemical treatment. The surface of the polyolefin film is treated with an oxidizing solution such as potassium dichromate-sulfuric acid to produce hydroxyl groups, carbonyl groups, and the like, and a certain degree of roughening is obtained.
3 photochemical treatment. Select the appropriate wavelength of ultraviolet light to irradiate the polymer surface, causing it to crack, cross-link and oxidation and other chemical changes.
At present, corona treatment methods are often used in production. The chemical treatment method has a long processing time, and the treatment solution is chemically aggressive. It is only used when it is inconvenient to use other treatment methods. The effect of the photochemical treatment method is not ideal, it takes a long time and the cost is high.
2. The static electricity of the anti-static treatment film is due to the excellent dielectric properties of PE and PP, high electrical resistance and poor electrical conductivity. The static electricity generated by the friction of the film in the process of extrusion and winding brings about static electricity generation and accumulation in the printing process, and it is not easy to release, so that a large amount of electrostatic charges accumulate on the surface of the film. The static electricity in thin film printing will bring a series of problems to the operation and directly affect the quality of printed products. Due to electrostatic adhesion, the film is in an anoxic state, which will hinder the curing process of the inner layer of the plastic; if it encounters a high temperature and high humidity environment, the adhesion of the ink layer will be more easily formed, and the printing ink will be transferred to the printing area to increase printing and cutting. Difficulties in the process of finishing and the like, when the film is interlinked, tearing does not open, causing the printing product to be scrapped.
At present, the domestic anti-static treatment method for plastic film pre-press is: The antistatic agent is directly incorporated into the resin and stirred by a high-speed mixer, and then directly poured into the extruder to extrude the film. According to information, Japan uses ionized air electrostatic treatment devices to produce cationic gas and anion gas through an ion generator, and then sprays on the surface of plastic film through nozzles to achieve the effect of handling static electricity.
Second, the printing process of plastic film Flexible packaging printing using the printing process are letterpress printing, lithographic printing, gravure printing, flexographic printing and screen printing. According to the printing materials, the amount of ink per unit area, printing quality requirements, pattern patterns, product volume, printing color number, inner layer thickness, frequency of change, cost budget and other factors to choose. The various printing characteristics are shown in Table 2.
As can be seen from Table 2, flexible packaging printing is mainly based on gravure and flexographic printing, but there are many differences compared with general gravure and flexo printing used in publishing and commercial printing. For example: Flexible packaging printing is to print on the surface of the roll of the substrate, if it is a transparent film, you can see the pattern from the back, sometimes the cream is printed with a layer of white paint, or use the Indian printing process.
Liyin refers to a special printing method that utilizes reverse-image-printing plates to transfer ink to the inside of a transparent printing material so that the front surface of the to-be-printed object behaves like a graphic.
Printed in India is lighter and brighter than surface prints, with vivid colors, no fading, and moisture-proof and wear-resistant. Also, due to the composite processing after printing, the ink layer is sandwiched between two layers of film, so it does not contaminate the packaged items and is in compliance with hygiene. Law requirements.
The printing color sequence of the Indian printing process is the opposite of the ordinary surface printing, for example, the ordinary printing first prints the background color, and the Indian printing is the last printing background color.
In recent years, with the continuous development of the Indian printing technology, the newly introduced ink printing ink has gradually replaced the general surface printing ink. This is because most of the products in India are used for composite packaging. Special inks for printing can meet the requirements for the bonding of ink layers and composite materials after printing. Even for large-area ink layers, the adhesive bond is very strong after composite processing.
Liyin is a unique technology for plastic composite packaging printing. It has become more diverse, multi-functional, and serialized in printing equipment. Such as the introduction of various models of plastic packaging printing production line, has blown, printing, compounding, cutting, bag making and other multi-process linkage. At present, most of the plastic film printing and compounding processing in China uses satellite gravure web presses. In modern printing technology, this type of machine has a tendency to be replaced by a unit gravure printing press. Since the printing stations of the unitary printing press are installed on the same plane of the production line one after another, the distance between the stations is large. Therefore, each unit has a separate drying system, which can not only speed up the printing speed, but also print more. The color, and at the same time, the change of printing and ink are all convenient for animal husbandry, and most of the newly introduced equipment is. Followed by the use of flexible printing press printing plastic film, flexo printing presses and dry laminating machine production line.
Third, film composite process Flexible packaging materials generally do not use a single material, but mainly composite film. The main composite methods are the following:
1. Wet compound method. A layer of water-soluble nootropic agent is applied on the surface of the substrate (plastic film, aluminum foil), and is compounded with other materials (paper, cellophane) through a pressure roller, and then dried by a hot drying tunnel to form a composite film. This method is suitable for packaging dry foods.
2. Dry compound method. Firstly, the solvent adhesive is evenly coated on the substrate, and then it is sent to the hot tunnel so that after the solvent is fully volatilized, it is immediately compounded with another film. Extrusion compound method. The curtain-shaped molten polyethylene extruded from the slit of the T mold is pressed by a nip roll, cast onto a paper or a film, coated with polyethylene, or supplied with another film from the second paper feeding portion, and polyethylene is used as The adhesive layer is adhered.
4. Hot melt compound. The polyethylene-acrylic acid ester copolymer, the ethylene acid-ethylene copolymer, and the paraffin wax are put together and heat-melted, and then coated on a substrate and immediately compounded with other composite materials and cooled.
5. Multilayer extrusion composite method. A variety of different properties of the plastic resin through a plurality of extruders, squeeze into the mold composite film. This process does not require fillers, organic solvents, film no odor, no harmful solvent penetration, suitable for food and pharmaceutical packaging, is an advanced composite technology.
At present, the most used material in China is the stretched polypropylene film (OPP), which is generally compounded with other materials after dry printing. The typical structure is: biaxially oriented polypropylene film (BOPP, 12mm). Aluminum foil (Al, 9mm) and uniaxially stretched polypropylene film (CPP, 70mm). In the process, a solvent-based "dry adhesive" is evenly applied to the base material by a roll coater, and then sent to a hot tunnel so that the solvent is fully volatilized and then composited with the other film using the composite roll.
China's introduction of plastic printing dry composite production lines and domestic composite machines, most of the use of polyurethane adhesives. The adhesive has strong bonding ability to different substrates, and is excellent in cold resistance, heat resistance, oil resistance, acid resistance, and abrasion resistance, but has certain toxicity and cannot be used as an adhesive for inner layer composites. Nowadays, non-toxic dry adhesives are being developed at home and abroad. In China, some achievements have been made in scientific research in this area. For example, APA adhesives are not only non-toxic, but also have better adhesive properties than polyurethane adhesives.

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