the main application fields of gravure and flexographic printing are roughly the same, but they are not exactly the same. Gravure printing and flexo printing have different advantages in packaging and trademark printing, publishing printing, decorative printing (wood grain paper and transfer printing) and other special printing (such as printing of valuable securities such as banknotes, stamps and lottery tickets).
packaging and trademark printing are the main competitive areas of gravure printing and flexography. In this field, the main application products of flexography are corrugated board, label, flexible packaging and folding carton in turn.
With the social demand for plastic flexible packaging rising year by year, plastic packaging is covering the market with its inherent characteristics of bright colors, novel patterns and convenient use. However, most small and medium-sized packaging color printing enterprises are short of technical materials for manufacturing and material selection, and their professional knowledge has not been popularized, which makes the packaged goods turn south, causing huge losses and making it difficult to expand the packaging business. I put theory into practice and put forward some suggestions for many colleagues in the flexible packaging industry to refer to.
generally, printing methods are classified into relief printing, gravure printing, flat printing, screen printing and so on. Plastic printing is gravure printing, and often mentioned gravure printing machine refers to group (or arrangement) gravure printing machine.
1. Printing machine
The printing speed of the plastic printing unit is determined by the length of the paper feeding distance between the printing color groups of the printing machine and the effect of the drying box. The longer the distance between colors, the better the effect of the oven, the faster the printing speed, otherwise the printing speed will not be up to the canal.
The registration accuracy of a printing machine requires not only the synchronous operation of all parts before and after the printing machine, but also the good stability of the tension control system of all parts. Only when the tension control design is reasonable, the feeding is stable and the winding is stable, the printing will not run off, and the registration accuracy can be maintained.
2. Printing plate
Gravure printing plate can be made in two ways: etching (manual version) and electric engraving (electric engraving). At present, the manual version is cheap, but the printing effect is better than electric engraving, especially the layered version. In practice, it is found that electro-engraving requires higher ink. The quality of printing plate production directly affects the printing effect.
3. Ink
Plastic gravure printing ink can be divided into solvent ink and water-based ink. Solvent inks are divided into surface printing inks and inner printing inks (composite inks).
surface printing plastic gravure printing ink: the surface is smooth and shiny, which is more resistant to friction and smells than the inner printing ink. One-component glue should be used in laminating process, which has poor bonding with two-component glue. Pay attention to that the amount of glue should not be too large. Table printing ink is divided into ordinary ink, ink resistant ink (beverage packaging ink), label ink (calendar ink), transparent ink and so on. Ink-resistant ink is mainly used for printing on milk packaging, beverage packaging and frozen food packaging products. Label ink is very smooth, anti-adhesion and scratch-resistant, and it is the best ink for printing various labels. Transparent ink is a special ink for printing plated aluminum foil and aluminum foil.
The inner printing ink (composite ink) is characterized by bright color, tasteless printed matter and high peeling strength, but the conjunctiva is soft and non-abrasive, so the printed matter is dull, which is beneficial to laminating processing. Two-component glue should be used if gluing dry composite is used. One is to increase the composite firmness, and the other is to have a small smell. However, when one-component glue is used for compounding, the ink on the printed matter will often be dissolved or the finished product after compounding will carry the harmful smell of glue, and the compounding firmness is poor (note that alcohol solvents are not allowed to be added). In addition, there is common composite ink in the inner printing ink, which is the ink printing of OPP film (polypropylene film). When printing PET film (polyester film), polyester ink should be used, which makes the printed pattern firmer and will not stick. In a word, what kind of special ink is used on what film is the principle of plastic gravure printing ink.
water-based ink is usually used for printing paper products, including paper-plastic composite products. Its characteristic is that it can meet the ink absorption of paper printing, making the printed matter full of color, and what is more rare is that its solvent is water and ethanol, which has little pollution to the environment and is known as the green ink for environmental protection.
4. Compounding
Compounding of soft plastic packaging, laminating extrusion compounding, dry gluing compounding and glue-free hot pressing compounding, etc. Extrusion compounding means that PE or PP material is hot-melted, extruded into a film, and then hot-pressed and compounded on a printed film to form a composite finished product. It is characterized by low cost and reduced glue pollution. However, the effect will be better if the extrusion materials are selected well, the extrusion temperature is controlled well, and a small amount of glue or AC agent is added as coupling agent between films.
drying tunnel length of dry gluing laminating machine. Heating mode, gluing mode, gluing quantity control, tension control of receiving and releasing materials, temperature control of hot drum and whether it has cooling setting device constitute its price and grade. In the process of gluing and compounding, firstly, users should understand that the glue must be diluted in order to glue evenly, and the solvent on the coating must be evaporated in order to restore the viscosity of the glue (that is, the concentration process), so the gluing amount on the compound is the key to compounding processing. Special attention should be paid to reducing the amount of glue when compounding. So it is not recommended to use it in food packaging.
One-component glue is difficult to eliminate because it is used with surface printing ink, so it is recommended not to use it in food packaging, and special attention should be paid to reducing the amount of glue when compounding.
If two-component glue is used for compounding, attention should be paid to the content of curing agent in addition to the proportion of glue according to the technical data provided by the manufacturer, and the amount of glue should be reasonably controlled to reduce the odor and glue spots on the laminating products.
in addition, if you want to use PE film as the lining of laminating, you should pay special attention to the surface tension of PE film, which should usually be above 45-52 dynes to ensure the firmness after laminating (it is recommended to install an EDM processor on laminating machine).
5. Plastic film
Because of the different requirements of the package, different packages need different plastic films. The following will briefly introduce the properties and uses of the most commonly used plastic films.
PE film, that is, polyethylene film, is widely used. It is characterized by low cost, good waterproof, but poor gas barrier. Low-pressure polyethylene has small extensibility and high tensile strength, and the packaging bag can be made very thin, but its heat sealing performance is poor, so it is not suitable for composite inner film. High-pressure polyethylene film has high ductility and good impact resistance, but it is not suitable for fried food and real and retort pouch composite inner film.
PP film, also called polypropylene film, has high transparency, high strength, good gas and water resistance, but poor oxygen resistance. This film should not be used as fabric for vacuum, cooking, medicine, beverage and other packaging. Among them, CPP uniaxially stretched polypropylene film and heat-sealable biaxially stretched polypropylene film are very good and commonly used composite linings for vacuum packaging or cooking packaging.
PET, also known as polyester film, has high tensile strength, poor impact resistance, high transparency and good oxygen and gas barrier properties, but it is not resistant to sunlight and has low water vapor permeability, so it can be used as fabric and substrate for ordinary vacuum bags, medicine bags and aluminized bags. Not suitable for retort pouch.
PA film, commonly known as nylon film, is currently used as the fabric of cooking packaging bags. Because of its high cost, vacuum bags are generally not used.
other films, such as PE preservative film for packaging vegetables, fruits and meat, water-soluble PVA film, PVCD high barrier film, PVC, PP and PE heat shrinkable film.
producers and users should choose the necessary fabrics and lining materials for packaging according to the inherent characteristics of various film materials, and strive to produce qualified packaging bags with economical benefits, high quality and low price.
6. Appendix Common Diluent Ratio of Plastic Gravure Ink for reference:
1. Diluent Ratio of Linyin Plastic Gravure Ink:
Quick-drying solvent: 2% ethyl acetate, 8% toluene
Medium-drying solvent: 2% butyl acetate, 8 or 1% toluene
Slow-drying solvent: 2% butyl acetate, 8 xylene. The ratio of diluents used in surface printing plastic intaglio ink:
Fast-drying solvent: ethyl acetate 2%, toluene 8%, isopropanol 4%
Medium-drying solvent: ethyl acetate 25%, toluene 5%, isopropanol 45%
Slow-drying solvent: xylene 5%, isopropanol 5%
Extra-slow-drying solvent: xylene 5%.