China's flexible packaging gravure industry should be said to have flourished since the 1990s, and today it has a very clear division of regional and scale development. The more representative are packaging such as Beijing, Tianjin, Yangtze River Delta, and Pearl River Delta. Printing base. These enterprises have relatively advanced equipment, strong technical and economic strength, high product quality, and occupy a large soft bag market share. These conditions have enabled them to develop rapidly in recent years, and the scale of investment and production has grown. At the same time, it is these companies that are constantly guiding China's flexible packaging printing industry to mature, and promote the development of industry standardization and standardization. This article briefly introduces the flexible packaging gravure technology.
I. Brief Introduction to Gravure Printing
Gravure printing is a process technology that uses a printing plate with a graphic portion lower than a blank portion. The graphic part on the layout is lower than the printing plane, and the shades on the surface of the printing plate are used to represent the varying shades of shading on the original. If the graphic part is deeply recessed, the amount of ink filled is large, and the ink layer left on the substrate after imprinting is thick; the concave part is shallow and the amount of ink is small. The ink layer left on the substrate is thin. The amount of ink on the printing plate corresponds to the light and dark levels of the original graphics. When printing, first pass the plate cylinder through the ink tank or use the ink transfer stick to fill the entire printing plate, and then use a squeegee to scrape off the ink attached to the blank part, and fill the ink in the recessed cavity. Under the proper printing pressure, it is transferred to the surface of the substrate. Gravure printing has fast speed (up to 300m / min and above), high printing plate endurance (up to 3 to 4 million impressions), thick ink, rich colors, clear and crisp, moderate contrast, realistic image, and diverse product specifications Etc. Can print on paper, aluminum foil, plastic flexible packaging, thick paper container, liquid paper, trademark and other substrates.
Gravure printing for flexible packaging
(I) Definition of Flexible Packaging
Flexible packaging is also called flexible packaging. Originally referred to plastic film pouch packaging, later it refers to flexible packaging or flexible containers of various single materials or composite materials, and now even includes semi-rigid packaging such as blister packaging, skin packaging and some thermoformed packaging form.
(Two), printing materials
There are many requirements for gravure printing substrates for flexible packaging. Basically: (1) the surface of the printing substrate should be smooth and flat, free of impurities and damage, and have good printability; (2) the surface of non-polar materials should be Treatment to improve the surface energy; (3) the printing substrate should not be too loose, and the shrinkage and elongation deformation rate of the substrate should preferably be less than 1%; (4) possess certain tensile strength and rigidity, good heat resistance and require thermal deformation It is too large; (5) it has friction and puncture resistance, and protects the middle layer; (6) when the double-layer composite film is used, the surface layer also preferably plays a barrier role.
Common surface materials are: BOPP, BOPET, PT, paper, etc. (1) BOPP film is a film produced by longitudinal and transverse stretching of polypropylene after melt extrusion. It has high tensile strength, large amount of elastic film, but low tear resistance; good rigidity and good chemical stability. Features of good water resistance. (2) PET is commonly known as polyester, commonly known as polyester, known for its strength, toughness, and transparency. It has good rigidity, hardness, abrasion resistance, folding resistance, and dimensional stability, and good odor permeability resistance. , Air tightness and moisture resistance, good heat resistance and cold resistance. (3) PA polyamide, commonly known as nylon is a general term for a class of polymers containing many heavy amide groups on the main bond. (4) PT is cellophane, also known as transparent paper, which has excellent transparency, gloss, printability and non-conductivity.
(Three), gravure ink in flexible packaging
Gravure printing ink refers to the ink used when the gravure printing machine prints a variety of different substrates. It is divided into paper gravure ink and plastic gravure ink. In plastic gravure inks, there are two types of surface printing gravure inks and back printing composite inks.
Plastic gravure ink is generally used for printing plastic films such as PE, PP, OPP, NY, etc., but it can also print composite flexible packaging. As long as there are no large color blocks on the printing plate, the composite odor and fastness are still basic. Yes. Plastic gravure surface printing ink is mainly made of binder, polyamide resin, plus pigments and auxiliaries after grinding and filtering. The softening point of the polyamide resin is controlled at 121 degrees. Otherwise, due to the low softening point of the resin, printed products will stick when printing in the moldy rainy season. The resin has a high softening point, and it will freeze when it is used in winter. You need to heat and mix to melt it. The dilution solvents for plastic gravure surface printing inks are toluene, isopropanol, xylene, butanol and the like. You can also add some ester solvents to adjust the viscosity of the ink. Toluene and isopropanol are quick-drying solvents. Xylene and butanol are slow-drying solvents. When using butanol, please note that if the ink dries too fast, which will cause the loss of graphic dots, the proper addition of butanol will make the ink reproducible, and the small characters and dots will be clear, but adding more will cause the ink to dry, especially in the mildew rain season. There will be print adhesion and anti-adhesion.
Plastic gravure printing ink, also called compound ink, refers to the ink printed on the substrate. The binder of this ink is mainly chlorinated polypropylene resin, which has high adhesion to printing films such as OPP, PET, and NY. After compounding, the black ink is sandwiched on the inner layer, which is the ink for printing high-grade plastic soft bags. Its main solvents for dilution are toluene, ethyl ketone, methyl ethyl ketone and ethyl acetate. Methyl ethyl ketone, toluene and ethyl acetate are fast-drying solvents, and methyl ethyl ketone are slow-drying solvents. This type of ink must not be added with alcohol solvents, otherwise the ink will deteriorate. Pay attention to the use of plastic gravure composite ink, because ink manufacturers in the design of ink there is a difference between high-speed machine dry ink and low-speed machine dry ink. When printing, the speed of the gravure printing machine should be used to determine the choice of ink. Otherwise, the speed of the gravure printing machine is slow, and the ink dries in the pits of the gravure dots.
For retort-resistant plastic flexible packaging or printed aluminum foil, select retort-resistant two-component gravure inks and special inks for aluminum foil. Otherwise, the cooking resistance will not be achieved, and the ink in the inner layer after cooking will become thinner, discolored, and the picture and text are not clear. In the production of retort-resistant packaging bags , it is also necessary to choose high-temperature-resistant materials and retort-resistant two-component composite glue.
The surface tension of plastic gravure ink is generally 36 dyne, while the surface tension of PE and OPP is generally only about 32 dyne. This requires that when performing gravure printing, the film undergoes corona treatment, destroying the molecular structure of the plastic surface and improving the film. The surface tension makes it reach above 38 dyne, so that the ink and the film are compatible and firmly adhered, and the ink cannot be removed by rubbing with hands and adhesive tape, so as to meet the printing quality requirements. As plastic gravure inks have an effect on the ozone layer during production, a large amount of toluene is used to dissolve and adjust the viscosity of the ink during gravure printing, which harms the health and safety of workers. This requires ink manufacturers to develop low Toxic, non-toxic alcohol-soluble and water-soluble gravure plastic printing inks. We use plastic flexographic printing inks in the production of paper-plastic composite rolls (for users to pack medicines, tea bags, etc.) and liquid packaging film rolls. This ink is alcohol-soluble, the added solvents are alcohols and esters, low toxicity, and meets the tasteless, benzene-free sanitary requirements.
(IV) Common problems in the production of flexible packaging gravure
Flexible packaging gravure often has many quality problems, of which knife line is one of the most common problems. There are various reasons for the cutting line: (1) The cutting line is related to the viscosity of the ink. The generation of knife lines has a direct relationship with the viscosity of the ink. In actual production, the higher the viscosity of the ink, the easier it is to produce a knife line. The generation of knife lines has a direct relationship with the viscosity of the ink. In actual production, the higher the viscosity of the ink, the easier it is to produce a knife line. (2) The relationship between the cutting line and the thickness of the ink particles. The finer the ink particles are, the better the print quality is, and the chance of cutting lines is correspondingly reduced. There are strict requirements on the particle size and thickness of the ink, and the diameter of the ink fineness is generally required to be 15 μm. The fineness of the ink can not meet the requirements, and the rough particles will lift up the doctor blade, thereby generating a knife line. At the same time, the ink particles are too rough, which will cause great damage to the doctor plate. The thickness of the ink should be moderate, and too thin is not good. (3) The relationship between the cutting line and the squeegee. The generation of the cutting line has a lot to do with the quality of the squeegee, the installation angle, and the actual adjustment. Excellent printing operator, can adjust the scraper to the best position. Not only can reduce the number of knife lines, but also have a bearing on the quality of the entire print. It can be said that blade adjustment and installation are the core issues of gravure. The squeegee blade has a great relationship with the knife line. If it is ground well, it is not easy to produce a knife line, otherwise it is easy to produce a knife line.
In addition, inaccurate overprinting of text and text is also one of the common faults. The phenomenon is that in the process of multi-color Fengmei printing, each color cannot completely overlap, and there is a certain deviation. If the deviation exceeds the specified requirements (GB7707-87 front pattern ≤0. 3mm, back pattern ≤ 0.5mm), it becomes a defective product. Material is wasted due to inaccurate overprinting. So it has attracted great attention from the industry. To understand the reasons for overprinting, it is necessary to master the method of elimination. There are many reasons for the inaccurate overprinting of plastic film gravure, including mechanical equipment (the influence of the roll diameter, the charging of the guide roller, the incorrect plate loading, the gear wear, and the improper selection of the rubber roller); the ink suitability (the viscosity of the ink is too Large will also cause displacement of the image, so the viscosity value should be adjusted according to the actual situation); ambient temperature (temperature control at 25 ℃, humidity control at 40% is appropriate, too high, too low will affect the overprint accuracy); printing plate Increasing amount (In gravure printing, taking into account the stretching of the plastic film, according to the color sequence, the printing plate has an increasing amount, which is gradually increased. If there is an error in the increasing amount, which exceeds the allowable range, the registration accuracy will also decrease) Rewinding tension (in the process of running plastic film, the rewinding tension needs to be stable. As the rewinding diameter gradually increases, the rewinding tension should also increase accordingly, while paying attention to the range of reeling jitter). The impact also includes the influence of subjective factors such as the quality awareness of the operator.
Again, mimeographs are also common defects in gravure printing on flexible packaging.
The factors that cause the above errors are personnel factors, equipment factors, printing plate factors, and operating techniques, because these errors exist objectively. To control them within a certain range, it is necessary for the operating staff to have a strong working ability.
Third, the elements of flexible packaging gravure process
(I) Printing color sequence
Gravure printing generally has two methods: surface printing and back printing. Because of its physical properties, paper is basically printed, and plastic films are available in two ways. In the process of printing color restoration, the famous color plate can obtain the original color by superimposing colors with its corresponding color ink. According to this color registration principle, based on the actual experience in the production process, the general printing color sequence of the surface printing is: white → yellow → magenta → cyan → black; the printing color sequence of the inner printing is: black → cyan → magenta → yellow → white. Spot colors can be placed behind black or behind the three primary colors. Sometimes, appropriate adjustments can be made according to production plan arrangements.
(Two), printing tension control
Printing tension is an important process parameter of the printing process, and its tension system is also the core content of the entire printing equipment. The setting of printing tension is determined according to the properties of the printing substrate. The general tensile value of BOPP, PE, etc., which is easy to stretch, is set at 100N, the paper, paper-aluminum composite materials, etc., which are not easy to stretch, are generally set at about 120N, and NY, PET, PT, etc. are generally set at about 110N. For different models and materials with different specifications, the tension setting value will be different.
(Three) drying temperature
The control of the drying temperature is also determined based on the properties of the substrate. There is also a certain relationship between drying temperature, printing speed and solvent boiling point. The temperature is constant. If the boiling point of the solvent is high, the speed should be reduced, otherwise the speed can be high; when the boiling point of the solvent is constant, the speed should be increased, and the temperature should be appropriately increased; otherwise, the temperature can be lowered. If the temperature is high, the temperature will increase, otherwise the temperature will decrease. In addition, the control of drying temperature is also related to process conditions, graphic area, ink properties and ink thickness.
(IV) Printing pressure
The printing pressure is not enough, the ink transfer is not good, and the graphic and text defects are easy to occur; if the pressure is too high, the marks are easily pressed, and the film is wrinkled. Different substrates have different printing pressures. In the specific production process, appropriate adjustments must be made according to the effects of the printing layout and ink and other related factors.
(V) Ink viscosity
The accuracy and consistency of ink viscosity during gravure printing is also one of the key points, especially in high-speed rotary printing, the control of ink viscosity is related to a series of printing failures, which is an important factor affecting print quality. When gravure printing, the viscosity of the ink must be adjusted first, and the solvent is added to the ink to reduce its viscosity to the degree required to meet the performance and conditions of the printing machine. During the production process, due to the continuous circulation of the ink, the solvent is constantly volatilizing. The viscosity of the ink should be tested at intervals. If it exceeds the control range, a new solvent should be added immediately to adjust it to control it within the normal range.
The use of the scraper is a major feature of gravure printing and one of the important factors affecting the quality of gravure printing. In the printing process, the requirements for the selection, installation and adjustment of the scraper are very high. Generally, the contact angle of the scraper is 55 °. In addition, the scraper pressure has a great influence on the print quality. Excessive pressure will reduce the transfer rate of the ink, which will cause greater wear on the printing plate and blade; excessively small pressure is prone to dirty plates or scraper lines. The abrasion of the scraper mainly occurs in the northwest corner of the graphic area. Therefore, when printing different images, even after a long period of use, the same scraper is still used, which may cause scratches and dirty plates due to scraping clean ink.
(VII) Plate cylinder
As the gravure printing mainly relies on the ink stored in the mesh cavity of the gravure printing plate cylinder to transfer the printing material to realize the printing pattern, the storage and cleaning of the plate printing cylinder are particularly important. Ink accumulation for a long time is used in mesh holes, which affects the use.
Fourth, gravure printing equipment
Gravure printing is currently the most widely used printing technology in China's flexible packaging industry . Of all flexible packaging printing equipment in the country, gravure printing equipment accounts for more than 94%. Gravure printing technology is widely used in various films and composite films such as PVDC, PET, PE, NY, CPP, OPP, BOPP, etc. The printed packaging products cover food, beverage, pharmaceutical and other industries related to life . Investigation shows that more than 95% of flexible packaging manufacturers use gravure printing equipment for printing. As the cost of domestic high-speed gravure printing machines continues to decline, the market share of gravure printing will continue to increase.
However, as a manufacturer of flexible packaging equipment, with the development and maturity of the flexible packaging printing industry, it is gradually facing new tasks and challenges. Although China's flexible packaging equipment has developed a lot after years of development, and even some manufacturers have built equipment that can make foreign manufacturers feel pressure, this does not mean that our flexible packaging equipment can meet the basic requirements of the domestic flexible packaging printing industry demand. There are many domestic manufacturers of flexible packaging equipment, including Shaanxi manufacturing represented by Shaanxi Beiren, Zhejiang manufacturing represented by Ningbo Xinda, and Guangdong manufacturing represented by Songde. Among these many domestic equipment suppliers, each enterprise has a different business philosophy, different purpose, and production capacity and design technology level are very different. Manufacturers of flexible packaging equipment need continuous guidance and support from flexible packaging printing companies in order to obtain rapid, sustainable and healthy development.
Compared with developed countries (especially Europe), the domestic gravure packaging equipment manufacturing industry still has a considerable gap, mainly reflected in electronic shaft drive technology, printing trolleys, sleeve technology, machine management systems and long-distance technical support systems. aspect. Moreover, in the field of flexible packaging in China, the proportion of solvent-based inks used in gravure printing is still quite large. The problem of solvent residues has caused flexible packaging to face the strict requirements of the food and pharmaceutical industries, as well as the constraints of a series of environmental regulations issued by the country. However, people are studying the use of environmentally-friendly water-based inks and alcohol-soluble inks for gravure printing equipment, and have achieved promising results.
With the continuous breakthrough of new technologies in the Chinese printing industry and the increasing market demand, gravure printing equipment will continue to develop in the direction of multi-purpose, multi-color, high-speed, automation, linkage and environmental protection.
Concluding remarks: The representative of the European Gravure Association (ERA) believes that from the grand occasion of the European Food Exhibition, the situation of soft food packaging is gratifying, and it is also favorable for gravure flexible packaging, especially those large-scale, high-quality brand-name food soft packaging still favors Seal. China is a big consumer of soft bag products, and paper, plastics, and composite materials are widely used in many fields. The flexible packaging printing industry will have a lot of room for development, and as a popular gravure, of course, there are unprecedented development opportunities. I hope that gravure technology will play a greater and greater role in the flexible packaging industry.