Nowadays, the characteristics of flexographic printing in terms of production process and product environmental protection have been adopted by more and more manufacturers of food packaging and pharmaceutical packaging. Many manufacturers ask this question: how to improve the quality of flexographic printing? However, there are indeed many factors that affect flexographic printing. In order to obtain good printing quality for flexographic printing, it must be considered in many ways. This article mainly discusses how to improve the quality of flexo printing from four aspects: strict review of product design drafts, the use of digital or direct plate-making processes, the reasonable selection of anilox rollers, and the use of a reverse scraper-type inking system.
1. Strictly review the product design draft
The development of computer science and technology has allowed most designers to use computers as packaging and decoration design tools. Professional advertising designers design products based on customers' basic requirements or market intentions. However, many packaging designers lack understanding of the printing process and the printing process, and cannot well grasp the characteristics of printing. The designed samples do not meet the corresponding printing requirements; or there is a lack of communication between the designer and the printer. As a result, the design often Ideas cannot be fully reflected by printing. Therefore, before the products designed by these designers are put into printing, the printing house should strictly review the design draft, and reject the unqualified products.
In order to make the designed products meet the printing requirements, flexographic product packaging designers should pay attention to the following points when designing:
1. In the use of color, it should be as simple, bright and bright as possible. If the number of overprinted colors of the designed product is too large, it will cause the color to be gray. Because the expansion of flexographic dots is large, the loss of small dots is serious, and the pattern is overprinted with color plates in a certain area. Each color plate has the problem of dot expansion or loss. The phenomenon of dot expansion or extinction will appear many times. , Making the picture dark, lack of highlights and midtones, the transition of the dots is uneven, very ugly. Therefore, on the premise of fully expressing the design drawings, the number of printing colors should be minimized as much as possible, which is very beneficial to the printing operations, quality control and cost control of mass production.
2. Try to use spot colors to make up for the shortcomings of four-color printing. Many customers of packaging products always require designers to design their products to overprint more than ten colors in four colors: yellow, magenta, cyan, and black. Manuscript, but sometimes can not simply use the three primary colors or conventional four colors to simply express the entire color world, the additional use of a spot color is very necessary and a very effective method. One-sided emphasis on overprinting is not desirable.
The flexo printing machine is very suitable for printing large areas of spot and spot colors. Most flexo printing machines are six-color or eight-color. Using only four colors will cause the unit to be idle, and the four-color printing effect is not comparable to offset printing or gravure printing. Flexo product quality.
3. Do not use too thin lines and too small text. Because the flexographic printing plate will be deformed during the printing process, too thin lines and text are very prone to this phenomenon, and too thin edge decoration is easily overwhelmed by the main line. Small white letters or reverse lines, due to the shallow relief, are very prone to stagnation. Therefore, try to use thicker lines, and the contrast between the decorative lines and the main line should be larger. [next]
Second, the use of digital direct plate-making process
The plate-making quality of the flexographic plate directly affects the quality of the flexographic product.
At present, the flexographic plate-making method is mainly divided into traditional plate-making and digital direct plate-making. In addition to the physical network expansion of the traditional plate-making, there is also an optical expansion when the plate-making film and the plate material are in close contact and exposed to light. Due to the diffuse reflection effect of UV light, all graphics, dots will be expanded, which will reduce the resolution of printing and copying. Therefore, it is difficult to obtain ideal flexographic quality for printing using the flexographic plate obtained by traditional plate-making.
Digital direct plate-making is a plate-making technology that directly exposes flexographic photopolymer plates with a laser. Its advantages are as follows:
â‘ The process is simple, which greatly shortens the preparation time.
â‘¡The exposure time has nothing to do with the pattern type and dot density.
â‘¢ Replace the dichroic film with shading black material, eliminating the dot deformation caused by the dichroic film processing.
â‘£ During the main exposure, there is no dot reflection increase caused by the diffuse reflection of light, and a clear dot can be obtained.
⑤ Realize the direct plate-making without film, the layout will not appear dirty, incomplete.
It is not difficult to see from the characteristics of digital direct plate-making that digital direct plate-making can obtain fine, clear and high-quality flexible plates. Therefore, in order to improve the quality of flexo printing, it is imperative to use digital direct plate-making.
So, what is the process of flexo digital direct platemaking? The process flow of flexo digital direct plate-making is: digital imaging → backside pre-exposure → main exposure → rinse corrosion → drying → post-processing → post-exposure.
â‘ Digital imaging
The imaged plate material is a special photopolymerized plate material, and its surface is compounded with a layer of black material with complete shading performance. First install the plate to be used on the cylinder of the digital plate-making machine.
During exposure, the infrared laser head is controlled by the computer to move along the direction of the roller, ablating the black material in the imaged area, and exposing the photopolymerization photosensitive layer of the plate underneath. Since the photopolymerization photosensitive layer is not sensitive to infrared rays, the photosensitive material is not affected by the laser ablation, and the black material remains in the part not exposed by the laser, thereby forming an imaging template.
â‘¡ Back exposure
The purpose of back-side pre-exposure is to increase the thickness of the plate base and ensure the durability required for printing. The thickness of the base is proportional to the length of the back exposure time, that is, the longer the pre-exposure time, the thicker the base; otherwise, the thinner.
â‘¢Main exposure
The main exposure can be performed in an ordinary exposure machine. Because the black material is fully compounded with the photosensitive resin and acts as a negative film, no vacuum is required during exposure, and the exposure time is short and uniform, and the dots are not deformed.
During the main exposure, the exposed photosensitive resin layer hardens when exposed to light, thereby forming a latent image of the image. The photosensitive resin layer covered by the black material does not see light and does not undergo photochemical reactions.
â‘£Flushing corrosion
The black material that has not been ablated by the laser and the uncured photopolymer below it are dissolved to form a recessed blank portion; the photopolymer where the photohardening is seen does not dissolve, thereby forming an embossed graphic part.
⑤ Dry
The flexographic plate is slightly swollen and sticky and soft due to the corrosion of the solvent after being rinsed and corroded. The original straight line seems to be a wavy line, and the text will also be distorted. The purpose of drying the printing plate is to completely volatilize the solvent remaining in the plate material and restore the original characteristics of the printing plate.
â‘¥Post-processing
The post-treatment is to wipe the printing plate with a clean cloth and solvent to remove the residual monomers on the surface. Finally, chlorination treatment is needed to eliminate the viscosity of the printing plate and increase the hardness of the printing plate.
⑦Post exposure
Post-exposure is also called stereotype exposure, which refers to the "bare exposure" of the bottom of the printing plate and the front of the graphic for a certain period of time. Hardness and printing resistance, but also can reduce the impact of ink and solvent on the printing plate. [next]
3. Reasonable selection of anilox roller
The anilox roller is the heart of the flexo printing machine. Its function is to quantitatively and evenly transfer the required ink to the graphic parts on the printing plate. The quality of the anilox roller determines the quality of the ink transfer, which directly affects the quality of the flexo product.
If you want to improve the quality of flexo products, you must choose anilox rollers reasonably.
1. Ink transfer performance of anilox roller
The performance of the anilox roller in the ink transfer process is the key to determining the quality of the ink transfer, so it must have certain performance:
â‘ The anilox number of the anilox roller refers to the number of ink holes per unit length in the linear direction of the anilox roller surface. The number of screen lines of the anilox roller determines the uniformity and ink carrying capacity of the anilox roller.
â‘¡ The amount of ink applied to the anilox roller refers to the ability of the anilox roller to store ink per unit area on the surface of the anilox roller, usually expressed in BCM.
In general, the higher the anilox roller's line number, the smaller the volume of its single ink hole, the less the anilox roller's ink coating amount. Therefore, if the number of screen lines of the anilox roller is not as high as possible, there must be a limit. If it is too high, the amount of ink required for the printing plate cannot be provided. Within the allowable range, the number of anilox rollers is relatively high, and the ink transfer is even.
â‘¢ Service life of the anilox roller The service life of the anilox roller is determined by the wear caused by the contact friction it receives during use. The anilox roller will be subjected to friction from the ink fountain roller, plate or doctor blade during the ink transfer process. Under these friction forces, the anilox roller's ink hole will be worn to some extent, causing the anilox roller Changes in ink storage. Once the anilox roller cannot provide the amount of ink required by the printing plate, the anilox roller should be replaced.
In general, under the same printing conditions, the life of the pyramidal anilox roller is shorter than that of the pyramidal anilox roller. Even if the two are worn to the same degree, the ink storage capacity of the pyramidal anilox roller decreases more rapidly.
2. Reasonable choice of anilox roller, should consider the following aspects
â‘ According to different types of inking method, the anilox roller will be different in abrasion during use.
The reverse scraper type applies a certain pressure to aggravate the wear of the anilox roller, and the prism-shaped anilox roller should be used; while the double-roller type and the forward scraper type ink delivery method, due to the existence of the ink accumulation area, the anilox roller wears less , Pyramid anilox roller can be used.
â‘¡ According to the number of screen lines of the printing plate, in order to ensure the accurate reproduction of the dots, each dot on the printing plate must be accurately inked. Often a specific dot has one or several inking holes on the anilox roller to give ink, so that one or several dots do not fall into an inking hole. Therefore, the number of screen lines of the printing plate must be considered when selecting the number of screen lines of the anilox roller. Generally speaking, the principle of selection is that the anilox roller has three to four times the number of screen wires.
â‘¢ According to printing requirements, low-line anilox rolls can be used for printing general products; for fine products, anilox rolls with a relatively high line count need to be used.
â‘£According to the nature of the substrate, for the generally rough corrugated paper with strong absorption, the anilox roller with a lower screen number is often used for printing; for the smoother and less absorbent metal and plastic films, a higher screen wire is required. Several anilox rollers are used for printing.
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4. Using reverse scraper
Inking system
Stabilizing the anilox roller's ink supply during flexo printing is the key to ensuring the quality of flexo printing. Flexographic printing ink directly affects the stability of its ink delivery. Ink feeding methods of flexo printing machines are mainly divided into three categories: double roller ink feeding, forward scraper ink feeding and reverse scraper ink feeding.
1. Double roller inking
In the double-roller type ink delivery, the ink in the ink tank is first transferred to the anilox roller by the ink fountain roller, then to the printing plate by the anilox roller, and finally to the surface of the substrate. In the double-roller inking mode, the ink transmission and the control of the amount of ink are jointly achieved by the squeeze and speed difference between the ink fountain roller and the anilox roller.
Because the surface linear velocity of the ink fountain roller is lower than the surface linear velocity of the anilox roller, it plays a role of scraping ink. At the same time, through the adjustment of the speed difference, the transmission of the ink amount can be controlled, that is, the speed difference increases, and the amount of ink transmission decreases accordingly. We should also see that the existence of the double-roller speed difference can also effectively reduce the phenomenon of flying ink.
However, the speed difference between the ink fountain roller and the anilox roller not only affects the ink transfer amount, but also affects the uniformity of the ink amount distribution on the anilox roller. During high-speed printing, the accumulated ink between the ink fountain roller and the anilox roller will generate a certain hydraulic pressure, which causes the elastic ink fountain roller to bend. As a result, the transferred ink appears thin on both sides and thick in the middle, and the ink film is not distributed. Evenly.
The anilox roller in the double-roller inking is not only filled with ink in the ink hole, but also covered with a layer of ink on the mesh wall. The amount of ink covered on the mesh wall will increase with the increase of printing speed and ink viscosity A substantial increase.
It can be seen that the double-roller inking will cause the phenomenon of unstable inking volume, and the ideal flexo quality cannot be obtained.
2. Forward scraper type ink feeding
The forward doctor blade type ink transfer means that the doctor blade and the anilox roller rotate in the same direction. This type of installation of the doctor blade forms a wedge-shaped ink accumulation area between the doctor blade and the anilox roller surface. Due to the hydraulic pressure of the ink, the doctor blade tends to be lifted off the surface of the cylinder, and the printing speed increases The larger or the greater the viscosity of the ink, the greater the hydraulic pressure of the ink on the doctor blade, and the greater the degree of the doctor blade lifted off the surface of the cylinder, the more the amount of excess ink on the screen wall of the anilox roller.
It can be seen that the anilox roller in the forward angle scraper type ink supply system also has unstable ink delivery, and it will increase significantly with the increase of printing speed or ink viscosity, and it cannot obtain the ideal flexo quality.
3. Reverse squeegee inking
Reverse-squeegee type inking means that the installation direction of the doctor blade is opposite to the rotation direction of the anilox roller. This installation method of the doctor blade has a tendency to press against the surface of the anilox roller due to the pressure of the ink on the doctor blade. Therefore, as long as the squeegee has ideal accuracy, the squeegee applies very light pressure, and the ink on the anilox roller surface can also be scraped off, effectively controlling the amount of ink delivered.
In the reverse doctor-blade inking system, the amount of ink delivery mainly depends on the parameters of the anilox roller itself, but the reaction to the pressure and installation angle between the doctor blade and the anilox roller is small. Due to the pressure of the ink liquid, the ink accumulates above the doctor blade, which is equivalent to applying a pressure to the doctor blade. Therefore, after the ink is scraped by the reverse doctor blade, there is no excess ink on the anilox roller wall. Because the pressure of the ink liquid presses the doctor blade, the pressure between the doctor blade and the anilox roller can also be reduced, so that the wear of the anilox roller and the doctor blade can be reduced.
It can be seen that the ink supply volume of the reverse scraper type ink supply system is very stable, and is basically not affected by the printing speed and the viscosity of the ink.
To sum up, if you want to improve the quality of flexo printing, you should use a reverse scraper-type ink delivery system to keep the amount of ink delivery basically stable.
To improve the quality of flexographic printing, in addition to starting from the above aspects, pay attention to the skills training of employees, use excellent plate materials, better printing materials, etc., which are also some issues that cannot be ignored.
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