Discussion on flexographic plate making

At present, flexographic printing has developed rapidly and has become a mature process. The reason is the improvement of flexographic printing quality, which largely depends on the development of photosensitive resin plate, ink, roller and printing machine. This article only introduces flexographic plate making

I. flexographic plates and types

the so-called flexographic plates refer to flexible and elastic relief plates with hardness of hs30-hs60. According to JTG E50 (2) 006 "experimental specification for geosynthetics in Highway Engineering", the properties and processing methods of the plates can be divided into:

among them, the hand carved rubber plate is the printing plate used in the early stage of printing. Because of its low quality, it can only print low-grade prints based on thick lines, color block patterns and words. At present, it is commonly used in corrugated box printing. The molded rubber plate adopts the molding process, which has complex plate making process and low resolution. Laser engraving plate making process not only simplifies the process, but also improves the resolution of the printing plate, which is an important scientific and technological achievement in recent years. Photosensitive resin plate is the mainstream of flexible plate making at present

in the field of modern flexographic technology, the development of photosensitive resin plate is of great significance. Its basic structure is as follows:

1. Polyester sheet protective film: this layer is used to protect the photosensitive layer from loss and isolate oxygen in the air, so as to maintain the best performance of the photosensitive layer

2. The thickness of the smooth layer (also known as the anti sticking layer) is generally 10-20 μ m. Transparent and non stick. Prevent the adhesion between the photosensitive layer and the photographic negative during plate making

3. photosensitive polymer layer: it is composed of five parts: high elastic polymer, photoinitiator, sensitizer, crosslinker and thermal inhibitor

4. Adhesive layer (agent): prevent the plate base such as photosensitive layer from falling off, and can withstand the influence of various solvents at the same time

5. sheet base: it has certain tensile strength, good dimensional stability and good bonding performance; The flexible version composed of these five parts is the composition of general flexible version. Another kind adds a cushion on the base layer

II. Plate making process

1. Rubber plate: due to the low quality of manually carved rubber plate and the complex process of molding rubber plate, it has been gradually eliminated. Although laser engraving plate making technology is the latest achievement in recent years, due to the wide application of photosensitive resin plate, carton manufacturers have gradually adopted resin plate instead of rubber plate. Therefore, the following is a brief introduction to laser engraving plate making technology

laser engraving method adopts electronic scanning, recording and laser engraving head for operation. The patterns, thick and thin words, lines and continuous layers on the original or color separation negative can be carved into rubber plates with different depths according to the original image

laser engraving products mainly include laser engraving rubber plate, rubber plate cylinder and rubber sleeve plate. As one of its characteristic products, laser engraving sleeve printing plate can realize seamless continuous printing on Web. It is widely used in the printing of packaging paper, candy paper, wallpaper, table paper, decorative paper and ticket shading

2. photosensitive resin plate: the former needs to cut the soft photosensitive plate first. When cutting, make the protective film face downward. It is best to cut with the cutter of copper zinc plate. Place the flexible photosensitive plate on the glass plate with the protective film downward. Cut with a knife according to the required size. The four sides of the plate should be slightly larger than the film

the process flow of solid photosensitive resin plate is:

original → photographic negative positive → cutting → back pre exposure → vacuumizing → main exposure → washing → plate drying → post-processing → post exposure → plate renovation

the process flow of liquid photosensitive resin plate: Original → photographic negative positive → frame preparation → coating photosensitive solution → loading negative → vacuumizing → upper and lower exposure → recycling photosensitive solution → washing → drying → post-processing → post Exposure → plate renovation

① the manuscript is one of the key links of flexible plate making, which not only determines the difficulty of plate making, but also relates to the printing process and printing development. Generally speaking, there are some requirements for the original: the number of colors should not be too many, no more than five colors at most, and the overprint should be as few as possible, especially in carton printing, small details should not be reproduced; The number of lines should not be too high. Usually, the carton printing is 30-40l/in. Paper and film printing is 80-133l/in

for black-and-white manuscript, that is, inked manuscript, the blackness of ink should be ensured first when drawing, the paper should not reflect light, and the blackness of drawn words and lines should be consistent; Second, the size and size of the black-and-white manuscript should conform to the proportion of the original. Generally, the proportion of the flexo is preferably 1:2, and the original size can be used for cartons; Third, when drawing black-and-white manuscript, its elongation in the circumferential direction of the drum should be considered. Its elongation formula is:

elongation = plate thickness (plate roller radius + double-sided adhesive thickness) × 100%

when drawing a black-and-white manuscript, it should shrink a certain proportion in the direction of printing

shrinkage = 100% - elongation

fourth, when selecting the text font in the black-and-white manuscript, it is required to select the text with the same font and thinner strokes for the Yang characters, and select the font with slightly thicker strokes for the Yin characters; Fifth, the thickness of the color separation line at the junction of two colors should be appropriate. When printing too thick, too much color may overlap, which will affect the appearance. If it is too thin, it is easy to break the line. When printing, the bottom appears, which is generally left at 0.1-0.2mm; Sixth, the edges of pictures and texts should be smooth without burrs

② positive image of photographic negative film: there are also certain quality requirements for it: the size and specification on the negative film are accurate; Clear graphics and texts; The blackness of the non graphic part of the Yin text is more than 3.5, and the density of the transparent part of the text is less than 0.05 due to its deformation ability; The density of the four corners is uniform, and there are no white spots on the layout

③ pre exposure (also known as back exposure): through pre exposure, a stable base is established to support the protruding relief, which plays a role in enhancing the plate base, ensuring the relief depth and improving the printing force. Pre exposure can determine the necessary plate base thickness. If the exposure time is too long, the printing is easy to get dirty; The time is too short, the depth becomes shallow, and the support ability for small characters decreases

④ main exposure: it is the key to the quality of plate making, which determines the sharpness of the plate image and the size of the side slope of the word image. The determination of the main exposure time depends on the type of resin version, the intensity of light, the thickness and complexity of the image on the negative film, and other factors. If the time is too short, the relief slope of the produced version will not be enough, the lines will be wavy and curved, and the dots will be washed away; If the time is too long, the Yin text will be pasted, and the handwriting will become thick and fuzzy

⑤ washing: after exposure, there are latent images of pictures and texts on the printing plate, and the printing plate is still flat and cannot be used as a printing plate. Only after cleaning with organic solvent solution, harden the resin of the exposed part, leave the polymer layer on the plate, and remove the resin of the unlit part by brush and washing, can it become a concave convex printing plate

the washing time is proportional to the relief depth of the layout. Therefore, the time of edition washing must be determined according to the depth of edition washing. If the time is too short, there will be resin left on the plate, which will affect the depth of plate making; If the time is too long, the plate will swell, resulting in fine part deformation. At the same time, the flexo washing time is directly related to the temperature and concentration of the solution, the new and old of the liquid medicine, the pressure of the brush, the level of the liquid water, etc

⑥ drying: because the freshly washed plate contains a lot of solvents, the plate must be thickened by heating. Generally, 60 ℃ hot air is used for drying. According to experience, the drying time of plates with a thickness of 1.14-3.18mm is 1H, and that of plates with a thickness of 3.94mm is 1.5-2h. When the temperature is lower than 60 ℃, the drying time is too long, which will age and harden the resin; When the temperature exceeds 60 ℃, the layout will be brittle and deformed

⑦ post treatment: the purpose is to remove the stickiness on the surface of the printing plate, increase the hardness, smoothness and printing resistance of the printing plate, and also make the printing plate have a better ink transmission effect. The main methods are UV exposure, dilute acid or alkali solution, powder spraying (magnesium carbonate, talcum powder, etc.)

⑧ post exposure: it refers to the maximum photopolymerization of plates through ultraviolet irradiation. It can cure all the parts that are not fully cured, so that the plate can reach the required hardness. At the same time, it also increases the low resistance of the plate to printing ink and solvent, and improves the wear resistance of the resin plate

III. The latest plate making process

modern packaging printing is increasingly using CD1 flexible direct plate making technology, which is a new process of direct plate making using computer and laser control technology. The flexo made by this process is characterized by complete dot shape, extremely high gloss is not easy to lose, low printing dot expansion rate, step length, large dots do not paste, etc. Flexo direct plate making the spirit of the Paralympic Games is worthy of our personnel in any industry to learn from the plate (also known as digital flexo printing, the top of the photosensitive layer is coated with a black ultraviolet absorption layer sensitive to infrared ray.

during plate making, the infrared laser generated by the image generator of the direct plate making machine first ablates the black absorption layer at the image, exposing the lower photopolymer photosensitive layer. Because the photopolymer photosensitive layer is not sensitive to infrared ray, the photosensitive emulsion layer at the place burned by the laser is not affected. After laser ablation To expose the printing plate. The black coating reserved in the blank of the printing plate blocks the light, and the photosensitive adhesive layer at the image and text loses the protection of the black layer, resulting in photopolymerization reaction, that is, the formation of printing points. After exposure, most of the properties of different materials can be compared. Digital flexographic plates can be developed in a common way

compared with ordinary flexo printing, digital flexo printing has its own characteristics:

① when exposed, light only passes through a black coating less than 0.001in thick, and there is only weak scattering and diffuse reflection; ② Digital flexo printing does not need to be exposed in vacuum, but only in air; ③ The convex point is relatively small, and the inclination is small. The vertex of the point is circular, but not skewed. The edge of the dot is smooth and the printing quality is good; ④ The extension of the main exposure time of flexo printing for min will not affect the bulge height of the image part on the printing plate

with the development of technology, digital flexo printing will produce a reform of flexo printing, and flexo printing will be further developed