The main points of flexographic printing technology (below)
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Third, technical points
Here are the main points of production technology, including material use, printing operations and post-press processing.
Material use
Flexographic materials include printing plates, water-based inks, ceramic anilox rolls and related materials.
(1) Flexible plate
The flexographic printing plate uses a photopolymerizable plate made of a rubber-like photopolymer. The basic composition is a host polymer, a photoinitiator, an acrylate hexene monomer, a thermal polymerization inhibitor, an additive, etc., and achieves a certain rubber hardness under the action of ultraviolet light.
The brands of flexographic plates currently in use are: DuPont Seri Edition, BASF, and Shanghai GS.
Plate making is generally done by flexographic platesetters, whose expertise allows the printer to provide a qualified flexo plate. The printing manufacturer can let the plate making company provide the corresponding type of plate according to the product requirements to meet the production needs.
Of course, it is not excluded that the printing manufacturers make their own plates, but there is a pros and cons here. Because the printing manufacturers began to engage in flexographic printing, in addition to equipment investment, such as re-printing, due to the higher content of flexible printing pre-press technology, the investment is greater. For the printing manufacturers, if they are willing to spend a lot of money on flexo prepress production, it is better to put the funds into the printing materials selection, concentrate on ensuring the perfection of the printing process, and let the flexo printing bring economic benefits as soon as possible.
In recent years, the introduction of laser CTP direct plate making technology is worth recommending. The minimum dot size of the screen-made version is 1%, which solves the problem of high-quality layered screen printing. More sophisticated packaging prints are now printed on flexible plates made by CTP technology. These fine products are generally difficult to distinguish between lithographic, gravure or flexographic printing under visual conditions. It has played a very important role in improving the quality of flexographic printing in China.
(2) double-sided tape
The flexographic plate needs to be adhered to the surface of the plate cylinder by means of a special double-sided tape to form a complete printing cylinder. The double-sided tape currently in common use is an elastic pressure-sensitive adhesive material, which is made of a polyethylene foamed substrate, coated with a different adhesive acrylate adhesive on both sides, and protected by a single-sided or double-sided release paper. In the flexographic printing process, the selected double-sided tape will directly affect the quality of the printed matter.
Flexographic double-sided tape has stringent performance requirements:
1 Ensure that the bond between the plate and the plate roller is firm and easy to remove.
2 Guarantee the fastness and overprinting accuracy of the printing plate at a printing speed of 150 to 250 m/min.
3 The thickness is uniform, the elasticity is uniform, and it is not easy to be deformed after being pressed, thus ensuring the quality of the print.
The thickness of the double-sided tape commonly used in the flexo plate is 0.38 mm and 0.50 mm; it is divided into three types according to the type of substrate density (taking 3M double-sided tape as an example).
Low density 1115 type, suitable for screen printing and fine line printing;
Medium density 1015, suitable for text, line and field printing;
High density 411DL type, suitable for field printing;
The new product, the 1815M, is between 1015 and 411DL and can be used in place of the 411DL.
Printers generally need to prepare three different density double-sided tapes to suit the printing needs of different plates.
Which type of double-sided tape is selected for the flexographic product, first analyze the network cable and text lines in the product graphic, and the field is mainly based on that one; it is necessary to know what the printing material is, and choose according to daily experience. Only one type of double-sided tape can be used in the same product printing, regardless of the difference between the screen, the text line and the field. This is because the density of the different types of double-sided tape is different, and the elasticity is also significantly different. Although the thickness is the same, it is pasted on a plate cylinder of the same diameter, and after being pressed as an elastic body, the length of the track imprinted on the printing material is different. As a result, the length of the print between the plates is changed, which makes it impossible to accurately overprint the colors.
(3) Water-based ink
Water-based inks for flexographic printing are key materials that directly affect the productivity of printed products and the practical effects of printed matter. At present, there are many brands of water-based inks, and the quality and price difference are large. The rational selection of water-based inks is the basic skill that printing personnel must master.
Due to the wide range of substrates, it is important to choose the right water-based ink to match the substrate. The aqueous inks used for the absorbent material and the non-absorbent material are completely different. Even for absorbent materials, depending on the intended use of the print, compatible aqueous inks should be used because the physical properties of the surface of the substrate vary. The choice of water-based inks also takes into account the actual use of the product, including printability, viscosity, adhesion, dryness, water resistance, abrasion resistance, acid and alkali resistance. Such as beer label, it has to withstand the friction of the labeling process, the soaking of ice water in the sale and the separation in the washing bottle; the abrasion resistance of the self-adhesive label on the packaging line; the adhesion and peeling strength of the film-like printed matter; These all have different requirements for water-based inks.
Therefore, the following points should be noted when selecting water-based inks:
1 to distinguish the type and quality of the substrate;
2 understand the practical requirements of printed products;
3 The spot color ink is prepared by the manufacturer or by the printing factory.
4 stabilize the pH and viscosity of the aqueous ink;
5 control additive dosage;
6 different brands of ink can not be mixed and blended.
Domestically produced water-based inks are used more: Shaanxi New Century, Tianjin Tiens, Guangzhou Akzo Nobel, Gaoshi, Dalian Smith and so on. High quality water-based inks are very good, very good, but costly. There are also many low-priced products on the market. The selection of such water-based inks requires operators to have certain experience in using them to deal with various ink suitability problems that occur during the printing process. It is recommended that the printing manufacturer select a water-based ink with good suitability in the early stage of flexographic printing, so as to avoid at least the various printing defects caused by the suitability of the water-based ink, concentrate on mastering the skills, and ensure the product is qualified. On this basis, we can find out how to reduce the production cost of certain materials.
(4) Ceramic anilox roller
It is said that the ceramic anilox roller is the "heart" of the flexo printing machine, and its importance can be seen. The reasonable matching of the ceramic anilox roller and the doctor blade can accurately and stably transfer the aqueous ink. Therefore, ceramic anilox rolls, also known as metering rolls or metering rolls, keep the flexographic ink layer within the selected BCM value throughout the ink transfer. The same is true for the flexo product "no color difference".
Selection of ceramic anilox roller: Firstly, according to the color of the printed matter of the printed matter, it is estimated by daily experience how to select the number of lines per inch of the anilox roller to meet the ink volume of the imprint. Secondly, according to the ink absorption amount of the printing material, in the preliminary proof printing process, the hue of each color imprint and the saturation degree of the ink amount are observed, and then the anilox rolls of different line numbers are exchanged until the requirements of the imprinted ink layer are met.
Recommendation: Data logging during each printing process is an important basis for future production.
The number of commonly used ceramic anilox rolls is as follows:
Field version: 250 to 400 lines/inch,
Text line version: 400 to 600 lines / inch,
Reticulated version: 550 ~ 800 lines / inch (suitable for 133 ~ 150 network line / inch), 700 ~ 1000 lines / inch (for 175 network line / inch)
For the selection of the anilox roller for the ink quantity requirement of a certain color printing plate, not only the matching of the number of lines but also the comparison of the BCM values in the same number of anilox rolls is required, so that the selection is reasonable. It is generally required to equip the number of anilox rolls to be more so as to select different product and the amount of ink to be printed. It is not necessary to use the ink color to make up the ink color. It is difficult to achieve the optimum amount of ink and ink layer, and the hue is difficult to achieve the best effect. It will not only waste time and ink, but also cause the quality of the print to be unqualified. Over time, the damage it will bring will far exceed the price of several ceramic anilox rolls.
The maintenance of ceramic anilox rolls is also very important work. The price of a ceramic anilox roller is still relatively expensive. Careful protection of the anilox roller is not only a matter of service life, but more importantly it directly affects the quality of ink delivery and imprinting. Tiny cells are easily blocked by dry ink, so the anilox rolls should be cleaned after each print. The anilox roller that has been blocked by the ink can only be cleaned with a professional cleaning agent, or the original BCM value can be restored.
(5) Squeegee
The squeegee and the ceramic anilox roller are combined to form an ink transfer system. The flexographic printing press is now equipped with a plug-in reverse squeegee structure that is easy to operate and has a good squeegee effect. The material of the squeegee is steel and plastic. The blade edge is ground into a bevel to help clean the ink. The pressure adjustment of the squeegee is based on the removal of excess ink on the surface of the anilox roll, and the pressure is not too large to avoid damage to the ceramic anilox roll. For worn wiper blades, they should be replaced in time, otherwise the quality of ink transfer cannot be guaranteed.
It must be reminded that the squeegee can only be installed if the ceramic anilox roller is inked.
2. Printing operation
The printing operation is the focus of the entire flexographic printing. It will reflect the true technical level of printing equipment, material selection, process development, and operation level in the actual production process. All operating procedures are performed by humans. Correct operation of each process is the most basic requirement for the operator.
The usual sequence of operations is as follows:
Rolling material→feeding paper→adjusting and correcting deviation→installing anilox roller→upper roller→adjusting three-roller pressure→inking→imprinting→registration→tension control→drying→die cutting→cutting→winding.
Among them, paper feeding, adjusting pressure, controlling tension, adjusting pH and viscosity, and die-cutting products are the main points of operation.
(1) paper feeding
After the winding, the center position of the coil is also the central axis position of the printing. This is important because the plate making, pasting, winding, rectification, die cutting, slitting, etc. are all based on the central axis. The printing material passes through the respective paper guiding rollers, the correcting device, the tension roller, the printing embossing roller, the drying box, the die cutting roller, the slitting roller, etc. according to the printing paper feeding line, and the winding material is wound. After the paper is worn, the machine can be started to make the printing material run smoothly, and the tension should be adjusted to make the printing material bear a certain tension control. Adjust the deviation and let the edge of the printing material pass through the center of the probe sensor. When adjusting, the corrector should be kept in the middle position of the left and right swing to ensure accurate correcting action.
(2) Adjust the pressure of the three rollers
The adjustment of the flexographic printing pressure is a key step to ensure the quality of the product, and it must not be sloppy. The three-roller pressure adjustment can achieve the parallelism between the three rolls; and the two-end pressure between the three rolls can be equal.
The pressure between the three rolls must be adjusted for each new product print. The adjustment of the printing pressure requires a careful and meticulous work by the operator.
The most important are two aspects of pressure regulation:
1 pressure between the ceramic anilox roller and the plate cylinder;
2 The pressure between the plate cylinder and the impression cylinder.
First adjust the spacing between the three rollers. In the combined state, the spacing between the anilox roll and the plate roll, the spacing between the plate roll and the platen roll is the same: 0.38 + 1.70 = 2.08 mm, (0.38 is the thickness of the double-sided tape, 1.70 For the plate thickness). Two standard feeler gauges of 2.08 mm thickness can be used. Under the combined pressure of the press, the standard thickness of the feeler gauge is placed at the two ends (without printing plates or substrates) between the two rollers, and the netting is manually adjusted. The distance between the roller and the plate roller, the plate roller and the platen roller is such that the standard feeler resistance of the respective two ends is the same. The spacing at this time is the ideal pressure value in printing, but it is slightly different from the actual embossing force, and requires the operator to fine-tune in normal printing.
In the slow running of the flexo printing machine, starting from the first color group, firstly observe the ink transfer of the anilox roller to the surface of the printing plate. Fine adjustment can be made from both ends to achieve the best ink transfer effect. The pressure of the anilox roller on the printing plate is light, which is good for the correct transfer of ink, to ensure the quality of the graphic imprint and to protect the printing plate from damage.
Next, observe the transfer of the print. The degree of clarity of the print on the surface of the substrate is a testament to the correctness of the transfer pressure. The embossing force is adjusted by the plate roller fine-tuning screw, and the two ends are gradually performed by light weighting until the blot is completely clear.
Flexographic printing is a light pressure printing process that is much smaller than lithographic and gravure printing. The ink transfer force of the anilox roller to the printing plate or the imprinting force of the printing plate on the printing material, we all require small as the main, that is, "point to the end". This will ensure the quality of the print, especially the dot quality of the screen printing. Excessive pressure on flexographic printing is a drawback and must be overcome.
(3) Tension control
The external tension and resistance of the printing material during the printing process can be referred to as printing tension. Tension control is an important mechanism for any web press, and the tension control is accurate, which largely determines the overprint quality of the print. Similarly, in addition to the precise precision of the various plates, the flexographic printing requires the precise positioning of the plates, and the tension control device ensures the constant printing tension during the printing process, which is the key to ensuring accurate overprinting. .
The tension control device is mainly disposed in the unwinding portion, the printing portion, and the winding portion. The output of the coil requires tension, the rotation of the coil has inertia, and the tension inside the coil itself also has the tension of the coil. In addition, the pulling force of the printing speed, the resistance of each color overprinting to the printing material during the printing process makes the tension control of the unwinding, printing and winding parts It is very important.
The magnitude of the tension control value should be determined by the thickness and texture of the substrate. The thicker the substrate, the greater the tension value, and the stronger the texture. Conversely, the thinner the substrate, the smaller the tension value. For ultra-thin materials, tension control is more demanding because it takes into account the wrinkling and stretching of the substrate.
The suitable point for printing tension control is to register all the “crosshairs” of the multi-color printing overprint, and will not “move” back and forth as the standard. If it is found that the "crosshair" overprinting is unstable during the printing process, the tension of the unwinding or winding portion can be appropriately adjusted to stabilize the "crosshairs" of each color. Of course, the printing speed is also a factor that affects the tension stability. The tension control under low speed, medium speed and high speed is not exactly the same. It is recommended to adjust the tension control under the premise of the normal printing speed.
In the printing process, overprinting is often inaccurate, or the phenomenon of “walking” often occurs. People are used to adjusting the plate roller, or looking for equipment, accessories, materials, etc. This is a must. In fact, if you start with a slight adjustment from the tension control, the problem of overprinting will be solved.
(4) pH and viscosity
Effective control of the pH and viscosity of the aqueous ink during the printing process is the primary operational step to ensure the quality of the print.
The pH of the aqueous ink is determined by a special test chart. Generally, the pH value is about 8.5, and the aqueous ink is relatively stable within this value. During the use, as the temperature rises and the ammonia in the ink evaporates, the pH value changes, which affects the printability of the aqueous ink. A small amount of stabilizer can be added to this to control the pH. In normal printing, it is usually required to add 5 ml of stabilizer every half hour and stir it evenly, and the aqueous ink can maintain a relatively stable normal printability. However, it is not allowed to add stabilizers arbitrarily, otherwise the quality of the print will be affected and various printing defects will follow.
The viscosity is determined by the No. 2 Caien Cup. Different brands of water-based inks use slightly different viscosities, usually around 25 to 30 seconds, and the viscosity can be slightly higher when printing dots. When the viscosity is high, it can be adjusted with a small amount of purified water. Of course, it is also possible to reduce the viscosity when a stabilizer is added. However, as the temperature rises and the printing speed changes during the printing process, the viscosity of the aqueous ink changes, and the operator should always check the viscosity of the aqueous ink in the ink fountain and make appropriate adjustments.
(5) Die cutting
Three die-cutting stations are arranged on the narrow-width flexographic printing press, and after printing, the indentation is performed online, and the desired printed product is obtained by die-cutting. It can be used for the processing of finished products such as stickers, computer forms, tags, kits, cigarette packets, etc. The die-cutting cutter adopts a round-pressed indentation roller and a combination of a profile cutter roller for indentation die-cutting. It is different from the flat-pressed, round-flat die-cutting indentation tool, which is formed by inserting the blade and the cutter wire into the same stencil sew. The former is made of integral metal roller, the indentation and die-cutting of the die-cutting tool are separated, and the integral type is divided; the split type is a pair of indentation rollers (die roller, punch roller), and a profile roller Combined. The integral type is that the indentation line and the knife line are formed on the same pair of die-cutting rolls to form a die-cutting tool.
There is a magnetic roller die-cutting tool, which embeds high-strength permanent magnet material in the integral metal roller, and a precision-processed thin metal plate-shaped knife can be adsorbed on the surface of the magnetic roller to form a shape cutter, which is mainly used for self-adhesive products. Die-cutting. Metal sheet cutters are easy to use. Flexographic printing On-line die-cutting If magnetic roller is used, it must be equipped with several types of magnetic rollers to form a series of specifications, which can meet the die-cutting requirements of most different sizes of self-adhesive products.
Another die cutter is called a single roll. It is a knife roll that is inserted into a die-cutting blade. It can be used to cut one, two, four or more sipe rolls, which are mainly used for cutting a single finished product. Its blades can be exchanged frequently. The sheet roll can be used in the same size as the printing roll or twice as large.
Custom-made die-cutting tools need to be designed, typeset, and determined to repeat the circumference of the tool under the guidance of professional technicians. The tool processing drawings can be drawn by professional technicians and submitted to the manufacturer for correct proof before they can be put into production.
The outer cutter roller must be affixed with a special knife sponge before use to ensure the quality of the die cut and the protection of the knife edge. The use of pressure to cut off the substrate as the optimum pressure extends the life of the die cutter.
3. Postpress processing
With the development of the market economy, the demand for the appearance of packaging is getting higher and higher, and the post-press decoration processing of packaging printed matter is becoming increasingly prominent. The post-press processing equipment of the flexographic printing machine enables the flexo printing machine to form a complete integrated printing production line, which is incomparable to other printing equipment. The main post-press processing steps are as follows:
(1) glazing
The flexo printing machine is finished by the printing unit, so it can also be called "printing light". Aqueous varnish can be used, or a UV drying system can be added, using UV varnish. There is also a special glazing device for light on both sides, and it can also be used for coating work of self-adhesive glue, and it can be combined into a self-adhesive printed product. The glazing coating is thicker than the printing ink layer to achieve better gloss or other effects.
There are quite a lot of glazing (printing) materials, and choosing the appropriate varnish is a major factor in improving the quality of flexo products.
(2) Laminating
The way of film lamination is very convenient and labor-saving. More previously used is the hot pre-coated film, which must be heated to bond to the substrate. In the printing, the film and the printing material directly enter the hot pressing roller to be compositely formed, and the laminating process is completed.
Another type of laminating method is a cold pre-coated film, the film is similar to a non-dry tape, and the layer with a glue on the surface is a composite layer. It enters the rubber pressure roller along with the printing material to complete the laminating process, without the need for other heating auxiliary settings. It is an online lamination method that is now promoted, which is convenient to use and low in cost.
(3) Hot stamping
Flexographic printing online bronzing requires the addition of a special bronzing device. If the continuous bronzing is in the paper feeding direction, its structure is relatively simple, and only needs to be composed of a thermostatic control device, a heating device, and a custom hot stamping roller. If the gap type hot stamping trademark or text is used, in order not to waste the electrochemical aluminum material, a stepping bronzing device is required, which is complicated and expensive, and is rarely used in China.
Since the bronzing method is a circular pressing structure, it is still difficult for the anodized aluminum material to be subjected to a hot pressing area for a large area of bronzing. If you need a large-area bronzing pattern design, for flexographic printing, it is better to change the printing material and printing process, and also achieve the bronzing effect.
(4) Die cutting
Three sets of die-cutting stations are generally arranged on the flexographic printing unit. The first two groups can be used for bronzing, indentation or embossing, indentation, and the latter group completes forming or cutting a single sheet. Die-cutting tools should be used with extreme care. First, it is heavy. The tool needs to be protected and loaded into the die-cutting station by the lifting frame. The second set of paired die-cutting rolls requires the upper and lower rolls to be placed in the die-cutting station, otherwise the die-cutting rolls are easily damaged. The adjustment principle of the die cutter roller is that after the upper and lower rollers are registered, the pressure is gradually pressurized from light to heavy until the finished product is just cut through. Too much pressure will only shorten the life of the tool.
Post-press processing also has punching, folding, slitting, etc., which are not introduced here.
Conclusion
This article is only a brief description of the process technology of flexographic printing, I believe it can become a stepping stone for the printing staff to engage in flexographic printing. In the face of ever-changing packaging prints, flexographic technology does not have a fixed model for reference.