The main points of flexographic printing technology (below)

3. Technical points

Here are the main points of production technology, including material use, printing operations and post-press processing.

1. Material use

The flexographic printing materials include printing plate, water-based ink, ceramic anilox roller and its related auxiliary materials.

â‘´ Flexible printing plate

The flexible printing plate uses a photosensitive polymer plate, which is made of rubber-like photopolymer. The basic composition is the main polymer, photoinitiator, acrylic hexene monomer, thermal polymerization inhibitor, additives, etc., under the action of ultraviolet light to achieve a certain rubber hardness.

The brands of flexographic printing plates currently in use are mainly: DuPont Sally version of the United States, BASF version of Germany and GS version of Shanghai.

Printing plate production is generally completed by a flexo plate-making company, and their professional expertise can provide qualified flexo plates for printing manufacturers. According to product requirements, printing companies can make plate-making companies provide corresponding types of plate materials to meet production needs.

Of course, it is not excluded that printing companies make their own plates, but there is a pros and cons relationship. Because printing companies start to engage in flexographic printing, in addition to equipment investment, such as printing plate production, due to the higher technical content of the flexographic printing plate before printing, the investment is greater. For printing companies, if they are willing to spend a large amount of money on flexo pre-press production, it is better to invest funds in the selection of printing materials, concentrate on ensuring the integrity of the printing process, and let flexo play an early economic benefit.

In recent years, the laser CTP direct plate-making technology introduced is worth recommending. The minimum dot of the screen version is 1%, which solves the problem of making high-quality screen printing plates. At present, more sophisticated packaging prints are printed on flexible plates made by CTP technology. These fine products are generally difficult to distinguish between lithographic, intaglio and flexographic printing under visual inspection. It has played a very important role in improving the quality of flexographic printing in China.

⑵ Double-sided tape

The flexible printing plate needs to rely on a special double-sided tape to stick on the surface of the printing plate cylinder to form a complete printing cylinder. The double-sided tape commonly used at present is an elastic pressure-sensitive adhesive material, which is composed of a polyethylene foam substrate, coated with acrylic adhesives of different viscosity on both sides, and protected by single-sided or double-sided release paper. In the flexographic printing process, the selected double-sided adhesive tape will directly affect the print quality of the printed matter.

Flexo double-sided tape has strict performance requirements:

â‘ Ensure that the plate and the plate roller are firmly bonded, and can be easily removed.
②At the printing speed of 150 ~ 250 m / min, ensure the fastness and overprinting precision of the printing plate.
â‘¢The thickness is consistent, the elasticity is uniform, and it is not easy to deform after being compressed to ensure the quality of the imprint.

The thickness of double-sided tape commonly used in flexo plates is 0.38 mm and 0.50 mm; three types of substrate density are divided into three types (taking 3M double-sided tape as an example).

Low density 1115 type, suitable for screen printing and thin line printing;
Medium density 1015 type, suitable for text, line and solid printing;
High-density 411DL type, suitable for field printing;
The new product 1815M type, which is between 1015 and 411DL, can replace 411DL.

Printing companies generally have to prepare three types of double-sided tapes with different densities to suit the printing needs of different plates and graphics.

To select the type of double-sided tape for flexible products, first analyze the network lines and text lines in the product graphics and text, which is the main type in the field; then you must know what the printing material is and choose according to daily experience. Only one type of double-sided tape can be used in the printing of the same product, regardless of the difference between the network line, the text line, and the ground in the set of printing plate graphics. This is because different types of double-sided adhesives have different densities and obvious differences in elasticity. Although the thickness is the same, it is pasted on the plate cylinder of the same diameter. After being compressed as an elastic body, the track length imprinted on the printing material is different. As a result, the length of the prints between the printing plates will change, which makes it impossible to achieve accurate registration between the colors.

⑶Water-based ink

Flexographic printing water-based ink is a key material, it will directly affect the production efficiency of printed products and the practical effect of printed products. At present, there are many brands of water-based ink, and the quality and price are quite different. The reasonable selection of water-based ink is the basic skill that printing personnel must master.

Due to the wide range of application of printing materials, it is very important to choose a suitable water-based ink to match the printing materials. The water-based inks used for absorbent and non-absorbent materials are completely different. Even for absorbent materials, according to the purpose of the printed matter, it is necessary to choose a suitable water-based ink, because the physical properties of the surface of the printing material vary. The choice of water-based ink also needs to consider the actual use of the product, which includes indexes such as printability, viscosity, adhesion, dryness, water resistance, wear resistance, acid and alkali resistance. Such as beer label, it is subject to the friction of the labeling process, the immersion of ice water in the sale and the separation in the bottle washing; the friction resistance of the adhesive label on the packaging line; the adhesion and peel strength of the film prints; These all put forward different requirements for water-based ink.

Therefore, the selection of water-based ink needs to pay attention to the following points:

â‘  Differentiate the type and quality of printing materials;
â‘¡ understand the practical requirements of printed products;
â‘¢The deployment of spot color ink is customized by the manufacturer or the person in charge of the printing plant;
â‘£Stable pH and viscosity of water-based ink;
⑤Control the dosage of additives;
â‘¥Ink and ink of different brands cannot be mixed and mixed.

Domestically produced water-based inks are used more frequently: Shaanxi New Century, Tianjin Tiens, Guangzhou Aksu Nobel, Gao Shi, Dalian Schmidt, etc. High-quality water-based inks have good adaptability and are very easy to use, but the cost is higher. There are also many low-priced products on the market. The selection of this type of water-based ink requires the operator to have some experience in using and can deal with various ink suitability problems that occur during the printing process. It is recommended that printing manufacturers choose water-based inks with good adaptability in the early stage of flexographic printing, so as to at least avoid various printing defects caused by the problem of the suitability of water-based inks, concentrate on mastering skills, and ensure product qualification. Only on this basis can we explore how to reduce the production cost of certain materials.

â‘·Ceramic anilox roller

People call the ceramic anilox roller the "heart" of the flexo printing machine, which shows its importance. The reasonable matching of ceramic anilox roller and doctor blade can transfer water-based ink accurately and stably. Therefore, the ceramic anilox roller, also known as quantitative roller or metering roller, keeps the flexographic ink layer within the selected BCM value range during ink transfer. The reason for "no color difference" in flexo products is the same.

Selection of ceramic anilox rollers: Firstly, according to the inked area of ​​various colors of the printed matter, based on daily experience, it is estimated how many lines per inch anilox roller can meet the ink volume of the imprint. Secondly, according to the amount of ink absorbed by the printing material, during the preliminary proofreading process, observe the hue of each color imprint and the saturation level of the ink amount, and then change the anilox roller with different number of lines until it meets the requirements of the imprint ink layer.

Suggestion: It is an important basis for future production to make good data records during each printing process.

The line numbers of commonly used ceramic anilox rollers are as follows:

Field version: 250 ~ 400 lines / inch,
Text line version: 400 ~ 600 lines / inch,
Reticulated version: 550 ~ 800 lines / inch (suitable for 133 ~ 150 network lines / inch), 700 ~ 1000 lines / inch (suitable for 175 network lines / inch)

If the anilox roller is selected for the inking requirement of a certain color printing plate, it is necessary not only to select the matching of the line number, but also to compare the BCM value in the same line number anilox roller, so that the selection can be reasonable. It is generally required that the number of anilox rollers is better, so that the size of the blotting ink varies for different products. Do not make use of the adjustment of the ink hue, it is difficult to achieve the best amount of ink and ink layer, and the hue is difficult to achieve the best results. It will not only waste time and ink, but also cause unqualified print quality. In the long run, the losses it will bring will far exceed the price of buying more ceramic anilox rollers.

The maintenance of ceramic anilox rollers is also very important. The price of a ceramic anilox roller is still relatively expensive. Care for the anilox roller is not only a problem of service life, but more importantly, it directly affects the quality of ink transfer and imprint. The tiny mesh holes are easily blocked by dry ink, so the anilox roller should be cleaned after each printing. For anilox rollers that have been clogged with ink, only professional cleaning agents can be used for cleaning, or the original BCM value of ink transfer can be restored.

⑸Squeegee

The ink scraper and ceramic anilox roller are matched to form an ink transfer system. At present, the flexographic printing machine is equipped with a plug-in reverse squeegee structure, which is easy to operate and has good squeegee effect. There are two kinds of materials for the squeegee blade: steel sheet and plastic sheet. The edge of the blade is ground into an inclined plane, which is beneficial to clean the ink. The pressure adjustment of the squeegee is based on the removal of excess ink on the surface of the anilox roller. The pressure is not too large to avoid damage to the ceramic anilox roller. The worn scraping blade 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 filled with ink

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 formulation, and operation level in the actual production process. All operating procedures are performed by humans, and correct operation of each process is the most basic requirement for operators.

The usual sequence of operations is as follows:

Winding material → feeding paper → adjusting deviation correction → installing anilox roller → upper roller → adjusting the pressure of three rollers → using ink → imprinting → registration → tension control → drying → die cutting → slitting → winding.

Among them, paper feeding, adjusting pressure, controlling tension, adjusting pH and viscosity, and die-cutting finished products are the main points of operation.

â‘´Paper feeding

After the roll material is loaded, the center position of the roll material is also the position of the central axis of printing, which is very important, because the plate making, paste plate, roll material, correction, die cutting, slitting, etc. are based on the center axis. The printing material passes through the paper guide rollers, rectifiers, tension rollers, printing embossing rollers, drying ovens, die-cutting rollers, slitting rollers, etc. according to the printing paper path, and is wound by the winding shaft. After passing the paper, the machine can be started to make the printing material run smoothly. At the same time, the tension should be adjusted to make the printing material withstand a certain tension control. Adjust the deviation correction so that the edge of the printing material passes through the center of the probe sensor. During the adjustment, the deviation correction device should be kept in the middle position of the left and right swings to ensure that the deviation correction action is accurate.

⑵Adjust the pressure of the three rollers

The adjustment of the flexographic printing pressure is a key step to ensure the quality of the product, which must not be sloppy. The three-roller pressure adjustment can achieve the parallelism between the three-rollers; and the two-end pressure between the three-rollers can be equal.

Each new product printing must adjust the pressure between the three rollers. The adjustment of printing pressure requires the operator to carry out a job carefully.

The most important is the pressure regulation in two aspects:

â‘  The pressure between the ceramic anilox roller and the plate cylinder;
â‘¡ The pressure between the plate roller and the platen roller.

First adjust the spacing between the three rollers. In the state of combined pressure, the distance between the anilox roller and the plate roller is the same as the distance between the plate roller and the platen roller: 0.38 + 1.70 = 2.08 mm, (0.38 is the thickness of double-sided tape, 1.70 Is the thickness of the printing plate). Two standard feeler gauges with a thickness of 2.08 mm can be selected. With the press combined, the feeler gauge of the standard thickness is placed at the two ends between the two rollers (where there is no printing plate or substrate) to manually adjust the texture The distance between the roller and the plate roller, and the plate roller and the platen roller, so that the pulling resistance of the standard feeler gauge at the two ends of them is the same. The pitch at this time is the ideal pressure value in printing, but there is a slight difference from the actual use of embossing force, and the operator needs to make fine adjustments in normal printing.

In the slow running of the flexo printing machine, starting from the first color group pressure, first observe the anilox roller to the plate roller image surface ink transfer situation. Fine adjustments can be made from both ends to achieve the best ink transfer effect. The anilox roller's ink transfer pressure to the printing plate is better, which is conducive to the correct transfer of ink, ensuring the quality of graphic imprints and protecting the printing plate from damage.

Second, observe the imprinting transfer. The clarity of the imprint on the surface of the printing material is proof of the correctness of the transfer pressure. The platen roller fine adjustment screw is used to adjust the embossing force, and the two ends are gradually increased from light to heavy until the imprint is completely clear.

Flexo printing is a light-pressure printing process, which is far less than the embossing force of lithography and gravure printing. The ink transfer force of the anilox roller to the printing plate is still the embossing force of the printing plate to the printing material. We all require a small size, that is, "point to end". Only in this way can the quality of the imprint be guaranteed, especially the dot quality of the screen printing. Excessive pressure on flexographic printing is a drawback and must be overcome.

⑶Tension control

The printing material is subjected to external tension and resistance during the printing process can be called printing tension. Tension control is a very important mechanism of any web printing machine. The accurate tension control determines the overprint quality of printed products to a large extent. In the same way, in addition to the precise production of flexographic printing plates and the precise mechanical positioning of each printing plate cylinder, the tension control device to ensure the constant printing tension during the printing process is the key to ensuring accurate overprinting. .

The tension control device is mainly installed in the unwinding part, printing part and winding part. The output of the coil material requires tension, the coil material rotates with inertia, and the coil material itself has tension inside the coil, plus the pulling force of the printing speed. It seems very important.

The size of the tension control value should be determined by the thickness and texture of the printing material. The thicker the printing material, the greater the tension value, and the harder the texture, the greater the tension value. Conversely, the thinner the printing material, the smaller the tension value. Such as ultra-thin materials, the tension control requirements are higher, because it must take into account the wrinkling and stretching of the printing material.

The suitable point of the printing tension control is the multi-color printing overprint "cross line" all registration, and will not "move" back and forth as the standard. If it is found that the "cross-hair" overprint is unstable during the printing process, the tension of the unwinding or rewinding part can be adjusted appropriately to stabilize the registration of the "cross-hair" of each color. Of course, printing speed is also a factor that affects the stability of tension. Tension control at low speed, medium speed, and high speed is not exactly the same. It is recommended to adjust the tension control under the premise of normal printing speed.

In the printing process, the phenomenon of inaccurate overprinting or frequent "out of position" often occurs. People are used to adjusting the plate roller, or looking for equipment, accessories, materials and other reasons, this is essential. In fact, if you start with tension control and adjust it carefully, the problem of overprinting will be solved.

â‘·pH value and viscosity

Effectively controlling the pH and viscosity of the aqueous ink during the printing process is the main operating sequence to ensure the quality of the blot.

The pH value of the water-based ink is determined by a special test table, and the pH value is generally required to be around 8.5, within which the water-based ink is relatively stable. As the temperature rises during use, and the ammonia volatilizes in the ink, the pH value will change, affecting the printability of the water-based ink. A small amount of stabilizer can be added to control the pH value. In normal printing, it is usually required to add 5ml of stabilizer every half an hour and stir it evenly. The water-based ink can maintain a more stable normal printing suitability. However, you must not add stabilizers at will, otherwise it will affect the quality of the imprint and various printing defects will follow.

Viscosity is determined as good by No. 2 Tsaien Cup. Different brands of water-based inks have slightly different viscosities, usually around 25 to 30 seconds, and the viscosity can be slightly higher when printing dots. When the viscosity is too high, a small amount of purified water can be used for adjustment. Of course, it can also reduce the viscosity when adding stabilizers. However, as the temperature rises and the printing speed changes during the printing process, the viscosity of the aqueous ink will change. The operator should always check the viscosity of the aqueous ink in the ink fountain and make appropriate adjustments.

⑸ Die cutting

Three die-cutting stations are arranged on the narrow-width flexographic printing press, and the indentation is performed online after printing, and the desired printed product is obtained by die-cutting. It can be used for finished products processing of stickers, computer forms, tags, medicine boxes, cigarette packs, etc. The die-cutting cutter adopts the round indentation and round indentation roller, and the combination of the contour knife-roller is used for indentation die-cutting and forming. It is different from flat flattening and circular flattening die-cutting and creasing tools. The latter is composed of blades and cutting lines embedded in the same stencil sawing seam to form a mold. The former is made of integral metal rollers. The indentation and die cutting of die-cutting tools are divided into two types: integral type; split type is a pair of indentation rollers (concave die roller and convex die roller), and one profile knife roller Combined. The integral type is that the indentation line and the knife line are made on the same pair of die cutting rollers to form a die cutting tool.

There is a magnetic roller die-cutting tool, which is embedded with a high-strength permanent magnet material in the overall metal roller. A precision-processed thin metal sheet-shaped knife can be adsorbed on the surface of the magnetic roller to form a contour tool. It is mainly used for adhesive products. Die-cutting. The metal blade knife is easy to use. If you choose a magnetic roller for online die cutting of flexographic printing, you must be equipped with several specifications of magnetic rollers to form a series of specifications, which can meet the die cutting processing requirements of most different sizes of adhesive products.

Another kind of die-cutting knife is called cutting single-sheet roller. It is a knife roller embedded with a die-cutting blade. One, two, four or more knife grooves can be cut out as needed. It is mainly used for cutting single finished products. Its blades can be changed frequently. The single-cut roller can be the same size as the printing plate roller, or double the size.

The ordering of die-cutting tools requires design and layout under the guidance of professional technicians to determine the repeating circumference of the tools. Only professional technical personnel can draw the tool processing drawings and submit them to the manufacturer for proofreading before they can be put into manufacturing.

The special knife roll must be pasted with special sponge cloth before use to ensure the quality of die cutting and protect the knife edge from damage. Using the pressure to cut the printing material is the best pressure, which can extend the service life of the die cutter.

3. Post-press processing

With the development of market economy, the demand for the appearance of packaging is getting higher and higher, and the post-press decoration processing procedures of packaging and printing products are becoming increasingly prominent. The post-press processing device configured by the flexographic printing machine makes the flexo printing machine form a complete printing comprehensive processing production line, which is unmatched by other printing equipment. The main post-press processing procedures are as follows:

â‘´Gloss

The glazing of the flexo printing machine is completed by the printing unit, so it can also be called "printing". You can use water-based varnish, or install a UV drying system, use UV varnish. It is also equipped with a special glazing device for both front and back glazing. It can also be used for the application of self-adhesive glue, and the machine can be combined into a self-adhesive printed product. The glaze coating is thicker than the printing ink layer to obtain better gloss or other effects.
There are quite a variety of glazing (printing) materials, and choosing a suitable varnish is a major factor in improving the quality of flexo products.

⑵ Laminating

The online laminating method is very convenient and labor-saving. In the past, hot pre-coated roll film was often used, which must be heated to bond with the printing material. In the printing, the roll film and the printing material directly enter the hot pressing roller to form a composite, and the laminating process is completed.

Another lamination method is cold pre-coated roll film, the adhesive film is similar to self-adhesive tape, and the layer with glue on the surface is the 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 on-line laminating method currently popularized, which is easy to use and has low cost.

⑶ Bronzing

Special hot stamping device needs to be installed for online hot stamping of flexographic printing. If the continuous hot stamping is in the paper feeding direction, its structure is relatively simple. It only needs to have a constant temperature control device, a heating device, and a custom hot stamping roller. If the gap-type stamping of trademarks and text is used, in order not to waste the anodized aluminum material, it is necessary to configure a skipping stamping device. This mechanism is relatively complicated and expensive, and is rarely used in China at present.

Because the method of hot stamping is a round pressing structure, the area of ​​the anodized aluminum material heated and pressed is a wire connection, which is difficult for large area hot stamping. If you need to design a large area bronzing pattern, it is better to change the printing material and printing process for flexographic printing, which can also achieve bronzing effect.

â‘· Die cutting

The flexographic printing unit is generally equipped with three groups of die-cutting stations. The first two groups can be used for hot stamping, indentation or embossing, and indentation, and the latter group completes forming or cutting sheets. The die-cutting tool should be used with great care. First of all, it is heavy, and the tool needs to be protected, and it is installed into the die-cutting station by the lifting frame. Secondly, a pair of die-cutting rollers need to be aligned with the upper and lower rollers before they can fall into the die-cutting station, otherwise the die-cutting rollers will be easily damaged. The adjustment principle of the die cutter roll is that after the upper and lower rolls are registered, the pressure is gradually increased 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 includes punching, folding, slitting, etc., which will not be introduced here.

Conclusion

This article only briefly describes the technology of flexographic printing, I believe it can become a stepping stone for printing personnel to engage in flexographic printing. Facing the ever-changing packaging and printing products, there is no fixed model for the flexo printing technology. Therefore, I hope that printing workers will participate in practice, analyze and research, and accumulate experience in production, which will make you an excellent flexographic printing expert.

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