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I. Strict review of product design drafts
With the development of computer science and technology, most designers now use computers as packaging and decorating design tools. Professional advertising designers design products based on their basic requirements or market intentions. However, many packaging designers lack understanding of the printing process and the printing process, cannot master the characteristics of the printing well, and the designed samples do not meet the corresponding printing requirements; or there is a lack of communication between the designer and the printer, often resulting in the design Thought cannot be fully expressed through printing. Therefore, these designers must design the product before the printing, the printing factory should strictly review the design draft, and reject the unqualified ones.
In order to design the product to meet the printing requirements, the flexo product packaging designer should pay attention to the following points when designing:
1. In the use of color, should be as simple, bright, bright colors
If you design a product with too many overprint colors, it will cause the color to be dark. Because the amount of flexographic dots is larger, the loss of small dots is more serious, and the pattern is overprinted in a certain area. There are problems with dot enlargement or loss in each color version. The phenomenon of dot expansion or net blocking will appear many times. , making the screen dark, lack of highlights and midtones, dot transition uneven, very ugly. Therefore, under the premise that the design drawings can be fully expressed, the number of printing colors should be minimized as much as possible, which is very beneficial for mass production of printing operations, quality control, and cost control.
2. Make full use of spot colors to make up for the lack of four-color printing
Many customers of packaged products always require that designers design their products to make manuscripts capable of overprinting more than ten colors in four colors of yellow, magenta, cyan, and black, but sometimes it is not possible to pass only three primary colors or conventional ones. Color can simply represent the entire color world. Extra use of a spot color is a very necessary and very effective method. One-sided emphasis on overprinting is not desirable.
Flexo press is very suitable for printing large-area field and spot colors. Most of the flexo presses are six or eight colors. Using only four colors will cause the unit to idle, and the four-color printing effect will be inferior to offset or gravure printing. Flexo product quality.
3 should not use too thin lines and small text
As the flexographic printing plate will be deformed in the printing process, the lines and characters that are too thin can easily cause this phenomenon, and the fine edge decoration is easily submerged by the main lines. Small white or anti-white lines, due to the shallow relief, it is easy to produce paste death. So try to use a thicker line. The contrast between the decorative line and the main line should be larger.
Second, the use of digital direct platemaking process
The plate 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 dot enlargement, the traditional platemaking also has optical enlargement when the plate-making film and the plate material are closely exposed. Due to the diffuse reflection effect of UV light, all graphic and network points will expand, which will reduce the resolution of print reproduction. Therefore, it is difficult to obtain an ideal flexographic quality by printing using a flexographic plate obtained by conventional plate making.
Digital direct platemaking is a platemaking technique that uses lasers to directly expose flexographic photosensitive polymer plates. Its advantages are as follows:
1 The process is simple, greatly reducing the preparation time.
2 The exposure time has nothing to do with pattern type and dot density.
(3) Replacement of the dichroic sheet with a light-shielding black material eliminates the dot distortion caused by the treatment of the dichroic sheet.
4 In the main exposure, there is no diffuse reflection of light that causes dot gain and clear dots can be obtained.
5 to achieve no film directly plate, layout will not appear dirty, incomplete.
It is not difficult to see from the characteristics of digital direct platemaking that digital direct platemaking can obtain fine, clear, high-quality flexographic plates. Therefore, in order to improve the quality of flexo printing, digital direct plate making is imperative.
So, what is the process of flexo digital direct plate making? The flexo digital direct platemaking process flow is: digital imaging → backside pre-exposure → main exposure → rinsing corrosion → drying → post-processing → post-exposure.
1 digital imaging
The imaged plate is a dedicated photopolymer plate that has a surface layered with a black material that is fully light-blocking. The plate to be used is first installed on the plate of a digital platesetter.
During exposure, the infrared laser head is controlled by a computer to move in the direction of the drum to ablate the black material that needs to be imaged to expose the photopolymerization photosensitive layer of the following plate. Since the photopolymerizable photosensitive layer is insensitive to infrared rays, the photosensitive material that has been ablated by the laser is not affected, and the black material remains on the portion that is not exposed by the laser, thereby constituting an image forming template.
2 back pre-exposure
The purpose of the back-side pre-exposure is to increase the thickness of the substrate and ensure the required print resistance. The thickness of the substrate is directly proportional to the length of the pre-exposure time on the back side. That is, the longer the pre-exposure time, the thicker the substrate; otherwise, it is thin.
3 main exposure
The main exposure can be done in a normal exposure machine. Because the black material is fully integrated with the photosensitive resin and acts as a negative film, no vacuum is required for exposure, and the exposure time is short and uniform, and the dots are not deformed.
In the main exposure, the exposed photosensitive resin layer undergoes a hardening reaction upon light exposure to form a latent image of the image, and the photosensitive resin layer covered with the black material does not see light, and no photochemical reaction takes place.
4 flushing corrosion
The black material that has not been ablated by the laser and the uncured photopolymer below it are dissolved to form a concave blank portion; see photohardenable photopolymer where the photopolymer does not dissolve, forming an embossed graphic portion.
5 drying
After washing and etching, the flexographic plate is slightly swollen due to the erosion of the solvent and is soft and sticky. The original line appears to be a wavy line, and the text will also be distorted. The printing plate is dried in order to completely evaporate the solvent remaining in the plate, so that the printing plate restores its original characteristics.
6 post-processing
The post-treatment is to completely wipe the plate with a clean cloth and a solvent to remove the residual monomer material on the surface. Finally, chlorination treatment is also needed to eliminate the stickiness of the printing plate and increase the hardness of the printing plate.
7 after exposure
After the exposure is also called stereotyped exposure, refers to the bottom of the printing plate and the front of the graphics are carried out for a certain period of time, "naked exposure", the purpose is to make the prepared plate completely hardened, is conducive to improve the folding plate flex degree, Hardness and resistance to printing, but also can reduce the impact of ink and solvent on the printing plate.
Third, a reasonable choice of anilox roller
The anilox roller is the heart of a flexographic printer. Its role is to quantitatively and evenly transfer the desired ink to the graphics part on the printing plate. The quality of the anilox roller determines the quality of the ink, which directly affects the quality of the flexo product.
In order to improve the quality of flexographic products, an anilox roll must be used reasonably.
1. Anilox roller transfer ink performance
The quality of the anilox roller in the ink transfer process is the key to determining the quality of ink transfer, so it must have a certain performance:
1 The number of screen lines of the anilox roller
It 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 of ink transfer by the anilox roller and the amount of ink carried.
2 Anilox roll ink coating amount
It refers to the ability to store ink in the ink hole per unit area on the surface of the anilox roller, usually expressed as BCM.
In general, the higher the number of screen lines of the anilox roller, the smaller the volume of the individual ink injecting holes and the less the amount of ink applied to the anilox roller. Therefore, the number of screen lines of the anilox roller is not as high as possible, and 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 screen lines of the anilox roller is relatively high, and the ink transmission is even.
3 The life of the anilox roller
The life of the anilox roll is determined by the wear caused by the contact friction it receives during use. The anilox roller is subject to friction from the ink fountain roller, printing plate, or doctor blade during the ink transfer process. Under these frictional forces, the anilox roller's ink hole will be worn to a certain degree, causing an anilox roller. Changes in ink storage. Once the anilox roller is unable to provide the amount of ink required for the plate, the anilox roller should be replaced.
In general, the life of a pyramidal anilox roller is shorter than that of a prismatic anilox roller under the same printing conditions. Even if the two are worn to the same degree, the decrease in ink storage of the pyramidal anilox roller is faster.
2. Reasonable selection of anilox roller should comprehensively consider the following aspects
1 according to the type of inking method
With different inking methods, the anilox roller wears differently during use.
Reverse blade type due to the application of a certain degree of pressure, an increase of the anilox roller wear, the use of pyramidal anilox roller; and two-roller and forward scraper ink transfer method, due to the presence of ink deposit area, the anilox roller wear less Pyramidal anilox rollers can be used.
2 according to the number of screen printing plate
In order to ensure the accurate reproduction of the outlets, every dot on the printing plate must be accurately inked. It is often the case that one or more ink holes on one anilox roller are used to ink a specific dot, rather than one or several dot dots falling into an ink hole. Therefore, the number of screen lines of the printing plate must be taken into consideration when selecting the number of screen lines of the anilox roller. In general, the principle of choice is that the number of screen lines of the anilox roller is 3 to 4 times the number of screen lines.
3 according to printing requirements
For the printing of general products, an anilox roller with a low number of lines can be selected; for the printing of a fine product, an anilox roller with a relatively higher number of screen lines is required.
4 according to the nature of the substrate
For corrugated paper, which is generally more rough and absorbent, anilox rollers with a lower number of screen lines are often used for printing; for relatively smooth, poorly absorbing metals and plastic films, an anilox roller with a higher number of screen lines is required. Print.
Fourth, the use of reverse scraper
Ink system
The stability of the flexographic quality is the key to stabilizing the anilox roller during the flexo printing process. Flexographic ink transfer directly affects the stability of the amount of ink it delivers. The ink transfer methods of flexographic printing machines can be divided into three major categories: two-roller ink infusion, forward scraper ink inversion and reverse scraper ink infusion.
1. Double roll ink
Double-roller ink transfer, the ink in the ink tank is first transferred to the roller by the ink fountain roller, and then transferred to the printing plate by the anilox roller, and finally transferred to the surface of the substrate. The transfer of the ink and the control of the amount of ink in the double roll ink transfer method are achieved by the extrusion and speed difference between the ink fountain roller and the anilox roller.
Since the surface linear velocity of the ink fountain roller is smaller than the surface linear velocity of the anilox roller, the ink squeegee effect is achieved, and at the same time, the ink amount can be controlled by adjusting the speed difference, that is, the speed difference is increased, and the ink transfer amount is accordingly reduced. We should also see that the existence of a double roller speed difference can also effectively reduce the flying ink phenomenon.
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 ink accumulated between the ink fountain roller and the anilox roller will generate a certain hydraulic pressure, which will cause the elastic ink fountain roller to bend. As a result, the transmitted ink will appear thin and thick in the middle, and the ink film will not be distributed. Uniform.
The anilox roll in double-roller type ink 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 printing speed and ink viscosity. A substantial increase.
It can be seen that the two-roller type ink infusion will cause the phenomenon of instable ink volume, and the ideal flexographic quality cannot be obtained.
2. Forward scraper ink
The forward scraper type inking refers to the same direction of rotation of the doctor blade and the anilox roller. The installation of the squeegee blade forms a wedge-shaped ink storage area between the squeegee and the surface of the anilox roller. Due to the hydraulic pressure of the ink, the squeegee has a tendency to be lifted off the surface of the roller, and the printing speed is increased. The greater the viscosity of the ink or ink, the greater the hydraulic pressure of the ink on the squeegee, and the greater the degree to which the squeegee is lifted off the surface of the roller, the greater the amount of excess ink on the screen wall of the anilox roller.
It can be seen that the amount of ink transferred by the anilox roller in the forward angle scraper type inking system is also unstable, and it will increase substantially with the increase of the printing speed or the viscosity of the ink, and the ideal flexographic quality cannot be obtained.
3. Reverse scraper ink
Inverse scraper type inking means that the direction of the squeegee is opposite to that of the anilox roller. This type of mounting of the doctor blade tends to press against the surface of the anilox roller due to the pressure of the ink against the doctor blade. Therefore, as long as the squeegee blade has the desired accuracy, the squeegee blade exerts a very light pressure, so that the ink on the surface of the anilox roller can be scraped off, and the amount of ink can be effectively controlled.
In the reverse-squeegee type inking system, the amount of ink to be transferred is mainly determined by the parameters of the anilox roller itself, and the response of the pressure between the squeegee and the anilox roller and the installation angle is less. Due to the ink liquid pressure, the ink accumulates above the doctor blade, which is equivalent to applying a pressure to the doctor blade. Therefore, after the reverse squeegee is scraped, there is no excess ink on the anilox roll's web wall. Because the pressure of the ink liquid presses the squeegee blade, the pressure between the squeegee blade and the anilox roller can also be reduced. This reduces the wear of the anilox roller and the squeegee blade.
It can be seen that the amount of ink delivered by the reversed blade-type ink inking system is very stable, and is basically independent of the printing speed and the viscosity of the ink.
To sum up, in order to improve the quality of flexographic printing, a reverse-squeegee-type ink infusion system should be used to keep the amount of ink delivered to a stable level.
To improve the quality of the flexographic printing, in addition to starting from the above aspects, we must pay attention to the skill training of employees, adopting excellent plate materials, better printing materials, and so on, which are also some problems that cannot be ignored.
Today's flexographic printing features in the production process and product environmental protection are increasingly used by food packaging, pharmaceutical packaging and other manufacturers. Many manufacturers ask such questions: How to improve the quality of flexographic printing? However, there are many factors that affect flexographic printing. To obtain good print quality, flexographic printing must be considered in many ways. This article mainly discusses how to improve the quality of flexo printing from four aspects: strictly review the product design draft, use digital or direct plate making process, use an anilox roller rationally, and use reverse doctor blade ink system.