Discussion on Flexographic Production

At present, flexo printing has developed rapidly and has become a mature process. The reason for this is due to the improvement in the quality of flexo printing, and the improvement in printing quality largely depends on the photosensitive resin plate, ink, anilox roller, and printing. Machine development. This article only introduces some aspects of flexographic printing.

First, flexo plate and type

The so-called flexographic plate refers to a flexible and elastic relief plate having a hardness of HS30-HS60. According to the nature of the plates and processing methods, they can be divided into:







The hand-engraved rubber plate is a printing plate used in the early stage of printing. Because of its low quality, it can only print low-grade rough lines, color blocks, and text-based printing products. Currently, it is commonly used in corrugated box printing. The molded rubber plate adopts the molding process. The plate making process is complicated and the resolution is low. Laser engraving process not only simplifies the process, but also improves the resolution of the printing plate. It is an important scientific and technological achievement obtained in recent years. Photosensitive resin plate is the mainstream of current flexible plate making.

In the field of modern flexographic technology, the development of photosensitive resin plates has important significance. Its basic structure is as follows:

1. Polyester film protection film: This layer is used to protect the photosensitive layer from loss and the oxygen in the air, in order to maintain the best performance of the photosensitive layer;

2. Smoothing layer (also known as release layer), generally 10-20μm in thickness, transparent and non-sticky. Prevent the adhesion of the photosensitive layer and the negative film during plate making;

3. Photosensitive polymer layer: composed of a high-elastic polymer, a photoinitiator, a sensitizer, a cross-linking agent, and a thermal polymerization inhibitor;

4. Adhesive layer (agent): to prevent the release of the photosensitive layer and other substrates, but also can withstand the impact of various solvents;

5. Film base layer: It has a certain tensile strength, good dimensional stability, and good bonding performance; the flexo plate composed of these five parts is composed of a general flexo plate. Another type of pad is added to the substrate.

Second, the plate making process

1. Rubber version: Due to the complexity of hand-engraved rubber plates, the quality of the rubber plate and the process of stamping the rubber plates have been gradually eliminated. Although laser engraving and plate making technology is the latest achievement in recent years, due to the widespread use of photosensitive resin plates, carton manufacturers have gradually adopted resin plates instead of rubber plates. So the following only briefly introduces the laser engraving plate making technology.

The laser engraving method uses electronic scanning, recording, and laser heads for operation. The pattern, thickness text, lines, and continuous layering on the manuscript or on the separation shade can all be carved into different depths of the rubber plate.

Laser engraving products mainly include laser-engraved rubber plates, rubber plate cylinders, and rubber sleeve plates. As one of its specialty products, the laser-engraved sleeve plate can realize seamless continuous printing of webs. Widely used in packaging paper, candy paper, wallpaper, form paper, decorative paper, ticket shading printing.

2. Photosensitive resin plate: The former needs to cut the flexible sensitized plate before cutting it. When cutting, the protective film should be directed downwards. It is best to cut with a copper-zinc plate cutter and put the flexible sensitized plate on the glass plate to protect it. Membrane down, according to the required size, with a knife cut, the four sides of the plate is slightly larger than the film.

The process of solid photosensitive resin plate is:

Original → Photograph negative → Open → Back pre-exposure → Vacuum → Main exposure → Flush → Plate drying → Post-processing → Post-exposure → Plate refurbishment

The process of liquid photosensitive resin plate: original → photo negative film → frame preparation → coating photosensitive solution → mounting negative film → vacuum → exposure on both sides → recovering photosensitive solution → rinse → drying → post-processing → post-exposure → printing Edition refurbishment

1 The manuscript is one of the key links in flexible plate making. It not only determines the ease of plate making, but also relates to the printing process and the development of printing. In general, there are some requirements for the manuscript: the number of colors should not be too much, no more than five colors at most, superimposition as little as possible, especially in the carton printing should not reproduce small details; the number of screen lines should not be too high, usually carton printing is 30-40l /in. Paper and film are printed at 80-133 l/in.

For a black and white manuscript, ie, a draft, the first step is to ensure that the blackness of the ink, the paper is not reflective, and the drawn text, line blackness is consistent; second, the size and size of the black and white manuscript must meet the proportion of the original, the general flexographic ratio is the most For 1:2, the carton can be drawn in the original size; third, the draft in the black and white draft should be taken into account in the circumferential direction of the roller. The formula for elongation is:

Elongation = Plate thickness (plate cylinder radius + double-sided tape thickness) × 100%

When drawing a black and white manuscript, shrink it by a certain percentage in the direction of printing.

Shrinkage = 100% - Elongation

Fourth, when choosing the font of the text in the black and white manuscript, for the positive character, it is required to select the font with the same font and the fine stroke, and for the vulgarity, select the slightly thick font; the fifth, the thickness of the color line at the junction of the two colors should be appropriate. . Too rough printing may overly affect the appearance of the overlay, too thin and easy to break, the phenomenon of bottom exposed during printing, generally can be left in 0.1-0.2mm; sixth, the edge of the graphic should be smooth, can not Glitches.

2Photography of negative film: It also has certain quality requirements: the size and specification on the negative film is accurate; the graphic and text are clear; the blackness of the non-graphic part of the Yin Wen reaches 3.5 or more, and the density of the transparent part of the graphic document is about 0.05 or less; The density of the four corners is uniform and there is no white point on the layout.

3 Pre-exposure (also called back exposure): Through pre-exposure to establish a solid foundation supporting the raised relief, to enhance the printing plate base, ensure the depth of relief, and increase the need for force. The pre-exposure can determine the necessary base thickness, the exposure time is too long, and the printing is easy to get dirty; the time is too short, the depth becomes shallow, and the supporting ability for small characters decreases.

4 The main exposure: It is the key to the quality of the platemaking. It determines the sharpness of the plate image and the side slope of the word picture. The determination of the main exposure time is determined by factors such as the type of the resin plate, the strength of the light, the thickness of the image on the negative film, and the complexity. If the time is too short, the version of the relief produced will not have enough slope, the lines will be wavy and the dots will be washed away; if the time is too long, the Yin script will become contaminated and the writing will become thick and fuzzy.

5 Flushing: after exposure, the latent image of the graphic remains on the plate, the plate is still flat, and it cannot be used as a printing plate. Only after the organic solvent solution is washed, the exposed part of the resin is hardened, and the polymeric layer is left on the plate. In the above, the resin of the unlighted part is removed by brushing and rinsing, and it becomes a concave-convex printing plate.

The flushing time is proportional to the depth of relief on the layout. Therefore, the time for washing the plate must be determined according to the depth of the plate to be washed. If the time is too short, the resin will remain on the plate, which will affect the depth of the plate. If the time is too long, the plate will swell and the fine part will be deformed. At the same time, the flexographic washing time is directly related to the solution temperature, concentration, the oldness of the liquid, the pressure of the brush, and the level of the liquid.

6 Drying: Due to the large amount of solvent contained in the freshly washed plate, the plate must be thickened by heating. Drying is generally performed with hot air at 60°C. According to experience, the drying time of the plate with a thickness of 1.14 - 3.18 mm is 1 h, and the drying time of a 3.94 mm plate is 1.5 - 2 hours. If the temperature is lower than 60°C, the drying time will be too long, and the resin will be hardened. If the temperature exceeds 60°C, the plate will have brittle deformation.

7 Post-processing: The purpose is to remove the tackiness of the printing plate surface, increase the plate hardness, finish, and improve the printing rate, but also make the printing plate have a better ink transfer effect. The main methods are UV exposure, dilute acid or dilute alkali solution, dusting (magnesium carbonate, powder, etc.).

8 After the exposure: It is through ultraviolet irradiation, so that the plate produces the largest photopolymerization. Insufficiently incompletely cured part of the sensitization fully cured, so that the plate material to achieve the appropriate hardness, but also increased the version of the printing ink, solvent low resistance, increased the wear resistance of the resin plate.

Third, the latest plate making process

More and more modern packaging printing uses CD1 flexible direct plate making technology. This is a new process that uses computer and laser control technology to make plates directly. The characteristics of the flexographic plate obtained by this process are: complete dot shape, extremely high light is not easy to lose, the printing dot enlargement rate is low, the tone adjustment is long, and the large dots are not pasted. A flexo-imprinted plate (also known as a digital flexographic photosensitive layer is coated on top of it with an infrared-sensitive black UV absorbing layer.

At the time of plate making, the infrared absorption layer formed at the image is first ablated by the infrared laser generated by the plate-making machine image generator to expose the photopolymerized photosensitive layer below. Since the photopolymerizable photosensitive layer is insensitive to infrared rays, the photosensitive emulsion layer burned by the laser is not affected. After the laser is ablated, the plate can be exposed. The black coating remained in the blank space of the printing plate to block the light, and the photo-resist layer at the picture and text was protected from photo-polymerization due to the loss of the black layer, ie, printing dots were formed. After exposure, the vast majority of digital flexo plates can be developed in the usual way.

Digital flexo has its own characteristics compared to ordinary flexo:

1 At the time of exposure, the light passes through only a layer of black coating less than 0.001 inch thick, and there is only a slight scattering and diffuse reflection phenomenon; 2 digital flexo printing does not need to be exposed in vacuum, only exposure in air; 3 raised The dots are relatively small, and the slope is small. The vertices of the dots are rounded and not skewed. The edge of the dot is relatively smooth and the printing quality is good; 4 The main exposure time of flexo printing is extended by 5-10 minutes, and the raised height of the image part on the printing plate will not be affected.

With the development of technology, digital flexo will create a reform for flexo printing, flexo printing will surely get greater development.