Flexographic forms play an important role in obtaining high quality products. Even with the most modern equipment it is impossible to obtain a good result without using printing forms with the appropriate capabilities.
Production of flexographic printing plates
Flexographic forms play an important role in obtaining high quality products, since even with the most modern equipment it is impossible to obtain a good result without the use of printing forms with the appropriate capabilities.
The following types of printing forms are currently used in flexography:
1) elastic (rubber), made by pressing;
2) elastomeric (rubber), made by direct laser engraving;
3) photopolymer;
4) new species produced by other methods.
Making rubber printing plates using the pressing method is a long process. In addition, such forms have low visual capabilities and therefore are used very rarely today.
Rubber printing forms, produced by direct laser engraving, have a number of advantages compared to elastic ones. They do not shrink during manufacturing, reproduce the image more accurately, have an endless (jointless) pattern and therefore are used only in the production of wallpaper.
New types of printing forms will be discussed in the next article.
Production of molds
Photopolymer molds are made using photopolymerizable compositions and materials including an elastomeric binder (most often rubber), an unsaturated monomer and a photoinitiator. When such materials are exposed to ultraviolet radiation (about 360 nm), the photoinitiator molecules are split into radicals, which join the monomer molecules and form new radicals. Many such radicals polymerize and form chains of molecules, which are connected into a spatial structure through cross-links.
During the polymerization process, the original physical properties of the composition or material are changed so that if the original product was liquid, it solidifies, and if solid, it becomes insoluble in certain solvents.
Today there are two known methods for producing photopolymer forms - based on liquid compositions and solid materials (plates).
Photopolymer forms based on liquid compositions are used mainly in newspaper production and therefore are not discussed in this article.
In flexographic printing in the production of packaging, the most common are forms made on photopolymerizing plates.
Types of plates
The printing and technical properties of photopolymerizing forms largely depend on the type of plates on which they are made. When choosing a photopolymerizing plate for making a printing plate, the following factors must be taken into account.
1. Photopolymerizing plates can be single-layer (Figure 1) or multi-layer (Figure 2). Multilayer plates are designed for high-quality reproduction of raster images, as well as images with fine details. In such plates, the photopolymerizing layer is more rigid than the supporting layer. In the future, we will tell you how complex images can be reproduced on single-layer plates. Currently, multilayer plates are used only in 5-7% of cases, and in other cases - single-layer ones.
2. The plates are available in thicknesses from 0.76 to 6.35 mm. The choice of plate thickness depends on the nature of the material being printed. Plates up to 3.0 mm thick are used for sealing smooth packaging materials with a relief depth on the printing plate of 0.58-0.8 mm. Plates with a thickness of more than 3.0 mm are used for sealing rough packaging materials and corrugated cardboard with a relief depth on the printing plate of 1.0-3.5 mm. The choice of plate thickness also depends on the gap between the plate and impression cylinders of the printing machine.
3. Photopolymerizing plates can have a hardness of 25 to 75 Shore units. The choice of plate hardness and, therefore, the printing plate depends on the nature of the printed material and the reproduced image. In particular, plates of medium and high hardness are used for sealing smooth materials.
4. The plates can have a format from 30 x 40 cm to 125 x 180 cm. When choosing a plate format, it is desirable that it matches the format of the negative or is placed multiple times on its surface.
5. Photopolymerizing plates may not be ozone-resistant or ozone-resistant. Ozone-resistant plates are used in cases where the printing machine is equipped with a corona discharge treatment unit for the printed material or ultraviolet dryers, during which ozone is released.
6. Plates and, accordingly, printing forms may have different resistance to paints and solvents, which must also be taken into account when choosing plates.
7. Photopolymerizing plates can be washed out with solutions based on organic alcohols or can be water-washable.
The presence of a protective film on the surface of the plates provides protection from mechanical damage and exposure to oxygen. Plates are sensitive to heat, daylight, UV radiation and short-wave radiation from light sources, so the room where the printing plates are made must be free of actinic rays, i.e., UV rays must be removed.
Requirements for design and negatives
To obtain printing plates that are of good quality and allow you to print competitive products, it is necessary that the product design and negatives for subsequent production of printing plates meet certain requirements.
1. The process involves the use of a direct (readable) line or raster negative made on photographic film with a matte emulsion layer. Only a matte emulsion layer allows for good contact of the negative with the surface of the plate, especially when reproducing images with fine details.
2. The minimum optical density of white space elements on the negative must be no lower than 4.0, and the maximum density of the veil should not be higher than 0.06. Deviation from these parameters may cause problems during the plate making process.
3. Design and negative must take into account the visual capabilities of the process:
1) the minimum thickness of free-standing lines is 0.1 mm;
2) the minimum diameter of free-standing points is 0.2 mm;
3) raster dots in high lights of at least 3% with a raster lineature on the form of 48-54 lines/cm.
The given quantitative parameters of this item are average for the current state of flexographic printing. In real conditions of competitive production, these requirements must be clarified and correspond to the technological capabilities of the process.
4. The negative must take into account the elongation of the image on the printing plate when it is bent to mount on the plate cylinder. The printing plate is made flat, and when the plate is attached to the plate cylinder, it bends and the image is elongated. To eliminate such elongation, the negative must have a shortened image in the direction of movement of the printed material in the printing press.
5. The design and set of negatives must have a trap (overlap) of 0.1-0.5 mm. Otherwise, during the printing process, unsealed gaps may appear in places where different inks come into contact. To eliminate gaps, one of the paints must be “wider,” i.e., partially overlap the contacting paint. The amount of this overlap depends on the technological capabilities of a particular production.
6. The angles of inclination of the screen on the negative should take into account the angle of inclination of the cells of the anilox roll on the printing machine. With an inclination angle of the cells of the shaft 60, the inclination angles of the screen for inks are: magenta - 45, yellow - 90, cyan - 15, black - 75. With an inclination angle of the cells shaft 45 raster inclination angles for paints: purple - 38, yellow - 83, blue - 8, black - 68. If this requirement is not met, moire may appear on the print.
7. The design and negative must take into account dot gain during the printing process. Figure 4 shows a print printed without taking into account the dot gain of the image. Figure 5 shows a print printed with dot gain compensation. Comparing the prints in Figures 4 and 5 shows that dot gain compensation significantly reduces image squashing and improves image reproduction quality.
Making molds
Before manufacturing a printing plate, a plate is selected taking into account the requirements for the printing plate and the conditions of the printing process. A piece is cut out of the selected plate in accordance with the format of the negative, taking into account technological allowances and the possibility of attaching it during processing (the design of the processing processor). When cutting the plate, it is placed with the substrate facing up. Cutting plates can be carried out using three types of devices.
When using the cheapest device - a knife - it is difficult to ensure even cutting lines; It is also possible for the protective film to peel off, which can subsequently cause problems with the quality of the printed plates produced.
When using a cutter that performs a “reciprocating” cut, an even cut line is ensured, but the possibility of peeling off the protective film remains.
When using a circular knife, a smooth cutting line is ensured, and the possibility of peeling off the protective film is minimal. In addition, the circular knife can also cut at an angle, which is especially important when cutting a joint on a finished form to reduce the amount of clearance when printing an “endless” pattern.
The first operation of the process is exposure of the reverse side. The plate is placed in the exposure device with the substrate facing up and exposed for several seconds without vacuum or negative. This operation creates the base of the form and controls the depth of the relief on the future printing form, ensures good adhesion of the polyester substrate to the photopolymerizing layer and a stable structure of the side faces through a strong connection of the printing elements and the base of the printing form. The optimal exposure time for the reverse side is determined by testing based on step exposures. Testing is carried out when starting a process for the first time, when using a new batch of plates, and also when any conditions of the production process change, including aging of lamps in the copier.
Main exposure, the second step in the photopolymer printing plate manufacturing process, should occur immediately after back side exposure. The protective film is removed from the plate, a negative is placed on the front side and, using a vacuum, the negative is in close contact with the surface of the plate. No dust or lint is allowed. After laying the negative, its edges and the edges of the plate are covered with a special relief film. The plate and negative are then covered with vacuum film, and then a vacuum is created. Next, the air from the middle of the plate is forced out to the edges, smoothing the vacuum film with the palm of your hand or an antistatic cloth. After this, exposure is performed lasting from several minutes to several tens of minutes.
The task of the main exposure is to form the relief of the printing elements on the future form. The optimal main exposure time is also determined by testing based on stepwise exposure of a special test negative. The test negative contains 4-8 identical images. Each image consists of a combination of various positive and negative elements in the form of lines, dots and raster structures. Testing must be repeated whenever any factors in the production process change. The optimal time is considered to be when individual lines and dots, as well as raster dots in high light, will be well reproduced on the form. Equipment for exposure must ensure control of the vacuum level; it is desirable that it be equipped with a cooling device for the surface of the table on which the plates are placed during exposure. It is also desirable to have a counter for the total operating time of the lamps.
The next operation in the mold making process is washing out the white space elements. In this case, the non-polymerized material swells and is removed from the mold, leaving a polymerized relief image of the printing elements.
To wash out, the exposed plate can be placed in a washout solution (in this case, the washout time is set) or carried out in a washout installation in a horizontal plane using a conveyor (in this case, the conveyor speed is set). Washing out the plate can be done either immediately after the main exposure or several hours later, unless light is shining on the plate at this time. The duration of washing depends on the composition and temperature of the washing solution, the design and pressure of the brushes of the washing device, as well as the required depth of the relief.
As a washout solution, you can use a composition based on a mixture of perchlorethylene and butanol, as well as solutions supplied by manufacturers of photopolymerizing plates. For each type of washing solution, its own processing temperature is recommended. Therefore, the washing installation must ensure operation at a given temperature; it is desirable that it maintains this temperature constant.
To ensure high-quality processing of forms and obtain a given depth of relief, it is necessary to regulate the pressure of the brushes, changing the size of the gap between the brushes and the supporting surface depending on the thickness of the plate being processed. It is advisable to know this value accurately and set it from the installation control panel.
During the washing process, the polymer removed from the blank elements of the mold enters the wash solution and saturates it. As the polymer concentration in the washout solution increases, its washout ability decreases. Therefore, the concentration of polymer in solution must be limited. The saturation of the solution with polymer depends on the format of the processed forms, the number of space elements and the depth of their relief. The polymer concentration in the wash solution should not exceed 5.5%, while practice has established that 10-15 liters of wash solution are required per 1 m2 of treated plate with a relief depth of 1 mm. Depending on the type of washing installation, the polymer concentration in the washing solution can be maintained within the specified parameters manually or automatically.
At the end of the washing process, droplets of the washing solution with the polymer dissolved in them remain on the surface of the mold. After drying, the polymer remains on the surface of the plate and can cause problems with uniformity of the image on the print. Therefore, it is recommended to rinse the mold with a clean solution after washing.
The optimal washout time is determined by testing, and we always strive to keep it to the minimum necessary.
The spent, polymer-saturated wash solution is subjected to regeneration and distillation. In this case, 85-90% of the solution can be returned for further use.
During the washing process, the mold swells, absorbing the washing solution. The amount of absorption of the washing solution depends on the degree of polymerization of the mold, the time of washing, the type and temperature of the washing solution. Therefore, after washing, the mold is dried in a drying device by blowing air heated to 60-65 C.
Drying has a significant impact on the quality of printing plates and bringing them to the original plate thickness. The duration of drying depends primarily on the thickness of the mold and the type of washing solution and is 1.5-3.5 hours. It is necessary to monitor the uniformity of air flow to the molds during drying and compliance with the temperature regime.
After drying (if time permits), it is advisable to keep the mold in the mold department for several hours. This operation allows you to completely stabilize the thickness of the printing form and makes it possible to slightly increase its circulation resistance.
At the same time, after drying and even after aging, the photopolymer form retains the stickiness of the surface. And for this reason, it is susceptible to contamination and changes due to pressure and air. To eliminate this situation, the form is subjected to finishing processing. It consists of treatment with short-wave UV radiation with a wavelength of about 250 nm.
The required finishing time is determined by the amount of wash solution residue that is in the mold after drying and depends on the type of photopolymerizing material, the type of wash solution and the drying time. The optimal processing time is determined by testing and is 70-90% of the main exposure time. The processed forms should not be sticky, have cracks or have a matte surface.
Additional exposure is necessary to ensure complete polymerization of the possibly unpolymerized monomer located in the body of the form. In the presence of an incompletely polymerized monomer, sufficient print stability of the form cannot be ensured, and loss of fine details and high image highlights during the printing process is also possible. Additional exposure increases the resistance of the form to paint solvents and removers and provides the form with final hardness.
Additional exposure is carried out by irradiation with UV radiation with a wavelength of about 360 nm in an exposure device from the front side without a negative and vacuum. Its duration is approximately equal to or slightly less than the main exposure time. Additional exposure can be carried out simultaneously with finishing processing if the installation design allows it. However, at high air temperatures in the workshop (more than 28° C), additional exposure is carried out separately after finishing. This is due to the possibility of overheating of the processed forms and for this reason the formation of cracks on their surface.
The room in which the production of photopolymer printing forms is carried out must have non-actinic lighting and be equipped with general ventilation. Due to the fact that wash solutions are usually heavier than air, they must be suctioned from the lower part of the room. Additionally, the entire installation or sections of one installation must be equipped with local suction.
The article describes, with specific technological examples, the prepress process in flexography, i.e., how the file (original) is prepared for the printing process (formation of a colorful image on a particular printed material).
Pre-press process
Processing of the original.
The pre-press process begins with processing the original. It can be either physical (made on paper or film) or electronic (computer file). When processing an original, it is necessary to know the maximum reproducible image elements using the flexographic method, which depends primarily on the capabilities of the forming material itself (rubber or photopolymer; currently photopolymer material is most widely used) and printing equipment. The following restrictions are usually used: maximum lineature of raster images -
60-65 lines/cm; relative areas of raster dots – from 2-3 to 95%; minimum diameter of points – 0.20-0.25 mm; line thickness – from 0.1 mm; text size – at least 4 points.
It is worth noting that the above factors are standardized “flexographic printing production data”, guaranteeing their reproduction stability. Thanks to modern plate production technologies, it is quite possible to reproduce a much larger line size (for example, 80 lines/cm) with a gradation range of 1 - 99%, text 2 points, etc., however, due to the characteristics of printing production, this is not always stable reproduced directly on the print.
We emphasize that all of the above parameters largely depend on the characteristics of the raster anilox rolls, the properties of the printing ink and the form photopolymer plate. Recently, photopolymer plates made by digital laser technology (Computer-to-Plate) have been widely used, the resolution of which is much higher than that of the so-called analog (“traditional”) plates. One of the main disadvantages of the flexographic printing process is high dot gain. This is due to the use of liquid printing inks and highly elastic soft printing forms), i.e. an increase in the size of raster elements (dots) on the print relative to the size of these elements on the photographic form and on the printing form, respectively, by an average of 15–25 (20)% in halftones (thus, instead of 2–3% dots on the print, 10–15% raster dots are reproduced points). Dot gain ultimately leads to a decrease in the overall contrast of prints, as well as to the failure to reproduce shadow areas of images. To compensate for dot gain, it is necessary to make adjustments at the design development stage and use deliberately underestimated values of the relative areas of raster elements on the photo form (printing form). In this case, it is necessary to control the printing process using a reflected light densitometer. When printing plates for flexographic printing, round raster dots of a regular structure are usually used. It is worth noting that the properties of the plate material and printing inks also influence the reduction of dot gain in printing. It is also advisable to separate spot and raster elements of the same color into different printed media (printing sections) due to the different pressure in the printing area of spot (higher for saturation) and raster (minimum for lower dot gain) images.
When working with raster images, it is necessary to take into account that the ink is supplied to the form in the printing section using a special screened anilox roller (the outer surface of this roller has many cells of a certain shape and number per unit length) and the choice of raster angles when developing a design depends on its parameters . When using anilox rolls with quadrangular diamond-shaped cells located at an angle of 45° to the generatrix of the cylinder, it is necessary to use the following raster angles (for cyan, magenta, yellow and black process inks): 7.5°, 37.5°, 67, 5° and 82.5° (compared to traditional offset angles - +7.5° difference). Currently, many leading manufacturing companies (e.g. Apex, Simex, Zecher) produce anilox rolls with hexagonal-shaped cells and an angle of 60° to the cylinder generatrix, which ensures a more stable and efficient transfer of ink to the printing form (to elevated printing elements ) – traditional (offset) screen angles of 0°, 45°, 15° and 75° are suitable for these anilox rolls.
Naturally, in the case of multicolor works, the image must contain registration crosses (sleepers) in certain places (usually along the edges of the image). Most often, for a tighter fit of the form to the form cylinder during the installation process, solid sleepers with crosses are used as registration crosses.
Photoform
After processing the original and creating the design, the information is sent to the so-called. “raster-image processor” (RIP), where rasterization takes place with certain parameters (rotation angles and raster dot shape) and color separation of the image. Then the information is sent to a photo output device, in which an image is formed using laser radiation on a photographic film material (or on a photopolymer plate material in the case of CTP systems). The image (photo film) is developed in a developing device using conventional chemical solutions - the result is a finished photographic form (direct negative, i.e. with a direct image on the emulsion side of the film). It is recommended to use photographic films from Agfa, Kodak, Fujifilm, which have a high contrast of the working layer; or modern photographic films produced on specialized Jet (Epson) printers.
There are two types of plate material for making flexographic plates - rubber and photopolymer. Initially, the molds were made on the basis of rubber material (at the same time, rather low quality was achieved). In 1975, a photopolymerizing plate for flexographic printing was first introduced. This form material made it possible to reproduce images with lineatures up to 60 lines/cm and higher, as well as lines with a thickness of 0.1 mm, dots with a diameter of 0.25 mm, text, both positive and negative, from 5 points, and raster dots with an area from 3-5 to 95-98%. And, naturally, photopolymerizing plates quickly took a leading position in the market of plate materials for flexography. Note that at that time we were talking only about analog forms made by copying from photographic forms (negatives).
Rubber (elastomer) printing forms can be produced by pressing and engraving.
The production of elastomeric (rubber) printing forms by pressing is preceded by the production of the original original form - a set or cliche. Typographic typesetting forms, made by hand or machine, can be used as original ones for the subsequent production of matrices, and then flexographic printing forms.
Cliché making is a photomechanical process of transferring an image from a negative to the surface of a metal plate, which can be made of copper, magnesium and zinc. During subsequent development, the untanned copy layer is removed from the white space areas. The tanned copy layer remains on the areas of the printing elements and is additionally tanned chemically and thermally to ensure sufficient acid resistance during subsequent etching.
When etching metals, various surfactants are introduced into acids to reduce lateral etching.
Depending on the nature of the image, the clichés are either raster or lined; the depth of etching and the hardness of the elastomer subsequently used for pressing the mold depend on this. After etching, the clichés are thoroughly washed off and finished.
Next, the matrices are made; Moreover, two methods are used to produce flexographic forms: from cardboard impregnated with phenolic resin and from bakelite powder. After pressing, it takes about 20 minutes for the matrix to cure. at a temperature of 145°C. After which the matrix is separated from the original form and cooled.
A variety of rubber compounds that meet established requirements are used as material for printing plates. Three main types of rubber are most widely used - those based on natural rubber, actylnitrile rubber and butyl rubber. Rubber intended for the manufacture of molds must be characterized by resistance to solvents, ability to deform, abrasion resistance, stable properties during storage, optimal vulcanization time, viscosity, shrinkage, etc.
The need of some types of industry for seamless flexographic printing forms stimulated the development of methods for their production by engraving on a pre-rubberized and vulcanized shaft (in Russia, enterprises producing wallpaper using the flexographic printing method use rubber seamless forms; the use of rubber is determined primarily by economic considerations). First, the shaft is manufactured and prepared. Engraving can occur in two ways: using a masking system (direct method) and a scanning system (indirect method). In the first method, the engraving process is “controlled” by a metal mask formed on the surface of the rubber. The method is available for control at all stages of production. Engraving can be done at increased speed without the risk of blurry images. With the indirect method, the engraving process is controlled by a shaft with an image. In the same way as on a “helioclischograph” (for the production of intaglio printing plate metal cylinders), originals with a repeating pattern can be processed. A mask is not required here, but it is necessary to make a scanned roller (a roller with an image). An electronic device reads this roller and controls the laser beam through pulses. Compared to the direct method, the disadvantage here is that the edges of the image are not sharp.
As mentioned above, due to low productivity (this is due to the need to remove from 0.5 to several mm of the rubber layer with a laser), low technological capabilities (lineature no more than 34 lines/cm - this is due to the characteristics of the most powerful (in “nature”) CO2 laser used with the spot size of 30 - 50 microns), the labor intensity of the forming process and the economic factor (high cost) of rubber molds, this forming material is not widely used at the moment, especially in Europe and Russia. But it also has undeniable advantages - very high circulation resistance and wear resistance, tens of times higher than the characteristics of photopolymer mold materials, especially in the case of EPDM material.
Recently, photopolymerizing flexographic plates have become most widespread, which in turn determines the choice of a photoform with the necessary characteristics (with an analogue, “traditional” method of making plates). Photopolymerizing layers are layers of negative development (i.e., where light acts, the solubility in the developing solution decreases), therefore it is necessary to use a negative as a photoform. In this case, it is recommended to use matted photographic film, which ensures the tightest contact of the photoform with the photopolymerizing layer of the plate during exposure in order to avoid the formation of the so-called. optical “Newton’s rings”, according to everyday life, spots.
A direct image is formed on the negative emulsion, a mirror image on the form, and a direct image on the print.
Photopolymer forms are made by traditional (analog, using photo forms) and digital (CtP) methods (as mentioned above).
Typically, for economic reasons, flexographic photopolymer plates are still also produced using the analog (“traditional”) method, by exposing the photopolymer layer through a negative.
The manufacturing process of flexographic photopolymer printing plates includes the following steps:
1. Preliminary exposure - exposure to UV radiation of the “A” range (this wavelength range includes the interval from 200 to 400 nm) on the reverse side of the plate (from the side of the polyester substrate) to form the base of future printing elements and to increase adhesion (adhesion) between the photopolymer layer and the polyester substrate, as well as for sensing the photopolymerizing layer. This operation also has a significant impact on securing small printing elements, especially thin raster elements; and mainly still determines the height of the printing element.
2. Main exposure (“copying”) - exposure to UV radiation of the “A” range on the photopolymerizing layer through a negative, which is placed on the plate with the emulsion side under vacuum, resulting in a photopolymerization reaction on future printing elements. It is worth noting the fact that exposure occurs through a vacuum film, and not through glass, as in offset copy frames, since only this film fully transmits the necessary UV radiation of a certain wavelength.
3. Washing (“developing”) – removal of unpolymerized material from the space elements of the future form under the influence of a special washing solution (based on aromatic hydrocarbons and organic alcohols in the case of solvent solutions or an aqueous solution) and using brushes. In this case, raised printing and recessed whitespace elements are formed on the surface of the form.
4. Drying with hot air (60–65 °C) to evaporate the wash solution from the surface and from the depths of the mold.
5. Treatment with short-wave UV radiation of the “C” range (254 nm) using special lamps in a special exposure section, the so-called. "finishing". Necessary to eliminate the stickiness of the surface layer of molds that appears during the washing and drying process*.
6. Additional exposure (“hardening”) with UV radiation of the “A” range (as in the first and second operations) of the entire surface of the form from the side of the printing elements for their complete polymerization and increasing the circulation resistance and wear resistance of the finished printing forms.
* - the last operations can be performed either in a different sequence or simultaneously, depending on the type and conditions of production.
To perform preliminary, main and additional exposures, special equipment is required, which must be equipped with UV lamps with “A” radiation at a wavelength (maximum radiation) of about 360 nm. The plate is placed on a horizontal metal plate. Basic exposure requires a vacuum film, a vacuum pump, and holes in this metal plate to remove air. One or more devices may be used.
For washing, a special installation is required, which has a metal reservoir of sufficient volume for the washing solution, a system for heating the solution, and a system of brushes for removing the washed polymer. Installation can be either horizontal or vertical. The plate can be mounted on both flat and cylindrical rotary (“drums”) surfaces. In this case, the solution heating system must maintain the temperature at a given level.
Washing occurs, as mentioned above, either with the help of a special “solvent”, or with water (for JET plates, Japan) or an aqueous soap solution (for TOYOBO plates (Japan), for example). in the latter case there is no need for an exhaust device and a regeneration unit. From an environmental and economic point of view, it is advisable to use water-washable plates as forming materials, however, solvent-based plates are “traditional” and, as a rule, cheaper. The reproduction and resolution capabilities of modern water-washing and solvent-based form materials are similar.
For drying, devices are used that contain horizontal metal trays (from one to several), as well as heaters and fans to supply hot air at a certain temperature.
To carry out UVC (finishing) treatment (anti-stickiness), an exposure section is required, equipped with UV lamps of the “C” range with short-wave radiation of 254 nm (radiation of the “A” range does not eliminate the stickiness of the top layer of photopolymer printing forms due to the physico-chemistry of the process photoinitiated polymerization). This section can have both horizontal and vertical construction.
All listed devices must contain electronic timers to regulate time and other parameters of technological processes, as well as a system for removing harmful fumes (ozone, heat).
Rice. 1. Washing out the photopolymer printing plate in a tower water processor
For the production of molds, both modular and combined devices of various formats are produced. In modular (“in-line”, horizontal flow type) processors, the format of the processed plates can reach a meter or more, and in principle have no restrictions.
From an economic point of view and from a convenience point of view, it is most advisable to use a combined processor, which includes all of the above devices with one electronically programmable control. The maximum format of the processed plates in this case is 80 (90) x 100 (110) cm.
High-quality, compact and economical combined equipment of tower and flow types is produced under the Jet brand (Holland). It is intended for processing both Jet plates and other brands and manufacturers. In Fig. Figure 2 shows the Waterpress combined tower-type water-washing processor.
Recently, the use of digital Computer-to-Plate (CtP) plate making has been increasing. This technology appeared back in the 90s of the last century. With this method, using laser radiation (LEDs, fiber optic, Nd:Yag laser, with a wavelength of 800-1100 nm), a kind of negative mask is formed on the photopolymer layer. For the laser method of making molds, special plates are used with a black (so-called “mask”) carbon-based layer (5-10 microns thick) applied to a photopolymerizing layer. It is on this black layer, sensitive to radiation of more than 1640 nm, that information is applied with laser radiation, which carries out the so-called. "laser ablation". After laser exposure, the same operations are performed as when making forms using the traditional method. However, the main exposure is performed without a vacuum (without vacuum film and negative).
Digital plates can be either solvent-washed or water-washed. Also on the market there are so-called. “thermally developed” plates, which are not widely used. Also, the CtP variety of direct laser engraving technology has not yet become widely widespread, but has good prospects, when a laser (CO2, YAG, diodes) directly forms raised printing elements, removing polymer or rubber from the surface of the space elements. This is a relatively new and little widespread technology, which currently finds its main application in the manufacture of seamless sleeves - printing forms (round for endless printing, without edges in printing); however, it can be used for the production of both photopolymer and elastomeric (rubber) printing forms and has significant advantages in the form of the absence of exposure/washing/drying/regeneration of solutions, etc. However, this technology requires more practical experience in its use in various enterprises by users.
Note that “solvents” include various organic aromatic alcohols and hydrocarbons (usually having unpleasant suffocating odors with aggressive evaporation), for example, perchlorethylene with butanol. As the solvent solution becomes contaminated, it is subject to a regeneration process on special regeneration devices through the sublimation of volatile solvents and the formation of a precipitate of contaminated solution to be disposed of. As a rule, about 80-90% of the original volume of the solution can be restored. An example of a Reclaim regeneration unit is shown in Fig. 4
To process water-washing plates, ordinary water is used, to which softening (washing) surfactants can be added, depending on the type of development of the plates.
When using digital (CtP) plates (laser masking technology, LAMS, Fig. 5), better print quality is achieved, since a more regular “column-shaped” (even almost rectangular) profile shape of the printing elements is formed, which leads to less dot gain during the printing process, i.e. to higher print quality. This occurs due to the fact that, due to the inhibitory effect of oxygen during the exposure process, the reproducible point on the surface of the plate has a size smaller than required (Fig. 6). The advantages of this technology also include the absence of a negative (photoform), which significantly simplifies and optimizes the process of manufacturing flexographic photopolymer forms, primarily from the point of view of its “transparency” and control.
The printing form largely determines the quality of the flexographic print. In particular, the ability to reproduce images identical in contrast to offset and gravure printing, and without a “step” in the area of light gradation transitions, directly depends on the characteristics of the printing plate. Increased dot gain (for example, compared to traditional offset) due to the soft plate material and design of the flexographic printing press makes it very difficult to obtain high-contrast images.
Rice. 2 Formal water-wash processor (top) and Interflex solvent-wash processor (bottom)
Rice. 3. Photopolymer test water-washable mold based on the Jet plate (Japan) – below; solvent-washed test form based on a plate, also produced by JET (Japan) - above
Fig.4 Regeneration unit for recovery of solvent solution from Reclaim
Rice. 5 Digital CtP plate with a black mask after laser processing on a CtP (Computer-to-Plate, laser masking LAMS) device.
Rice. 6 Profiles of printing elements on analog (left) and digital form.
One of the ways to solve this problem was the development of molded materials that make it possible to reproduce the so-called. “flat” tops of the printing elements. Due to the inhibitory effect of oxygen during the main exposure process (photoinitiated polymerization on future printing elements), the edges of the printing elements on flexographic printing forms always turn out to be slightly rounded, which leads to excessive dot gain during the printing process, i.e. to the loss of any details and deterioration of image reproduction in flexography, especially illustrative ones.
Some manufacturers of form materials have proposed the use of special so-called. lamination films, which are printed with a laser image, are rolled onto the photopolymer plate itself, and thus the inhibitory effect of oxygen on the formation of the printing element with the reproduction of flat tops is eliminated, along with the possibility of the so-called. “microscreening” of the surface of the printing elements, which in turn determines higher ink transfer of the printing elements. The Kodak company was the pioneer and developer of this technology and similar systems. Next, it’s worth taking a closer look at some aspects of this technology:
Flat tops of points.
Unlike traditional flexo plates, where oxygen is inhibited during UV exposure and creates a rounded dot profile, especially in light colors, the Kodak Flexcel NX system eliminates exposure to oxygen during exposure to produce an all-over flat, strong dot with crisp edges. This dot structure is critical to high printing productivity, giving consistent, repeatable plate quality that is insensitive to pressure changes, print wear and cleaning. A micrograph of flat tops is shown in Fig. 7.
High resolution form output
An essential component of the technology is also the increased resolution of the output forms, which gives an increase in the range of reproduced tones and excellent image reproduction.
Flexcel NX output devices use 10 micron square dot technology at 10,000 dpi resolution, allowing the finest halftone detail to be reproduced, stretching to zero across all available grayscale levels. And because one-to-one image reproduction is achieved and the plates have a flat point, no detail is lost throughout the entire run.
Increased paint transfer
Ink transfer efficiency contributes to both print quality and production efficiency. Flexcel NX plates, with their flat point and pressure insensitivity, allow you to print with higher densities and even fills. Significant improvements in ink transfer in previously challenging applications can also be achieved using Kodak DigiCap technology, which employs “micro-texturing” on the surface of Flexcel NX plates
Rasterization DigiCap NX
DigiCap NX screening is a software option of the Flexcel NX system that can significantly improve ink transfer due to the “microtexturing” of the surface of the printed elements of the Flexcel NX plate. Jobs that have traditionally been challenging can now be completed with ease with high densities and uniformity of fills and increased color gamut. The innovative solution takes advantage of the Flexcel NX system's one-to-one reproduction capability (raster print elements of any gradation range) to create microtexture across the entire plane of the printed elements of the plate. Elements measuring 5x10 microns are evenly distributed over the entire surface of the printed elements of the plate on both fills and tone elements (with the exception of very light elements). The micro-grain size and uniformity of the resulting structure are significant. It is this structure that increases the ink transfer of the photopolymer surface of the printing elements of the flexographic form. The result of this microscreening is presented in Fig. 8.
According to the practical domestic experience of many of the largest manufacturers of flexible packaging (Edas Pak, Delta Pak, Danaflex, Tom Ltd., etc.), the best printing results from these forms are achieved when using alcohol-based printing inks. sealing various film products. However, there are also positive and impressive printing results in the case of narrow-web label flexographic enterprises using UV-curable and water-based inks (for example, “Ninth Wave”, “Neo-print”, etc.).
JET has released digital water-wash CtP plates with an integrated “anti-inhibition” layer, which also ensures the reproduction of print elements with flat tops and, as a result, reduces dot gain during the printing process. From the experience of domestic label enterprises (PC Alliance, Verger, etc.), in this case, the best results are achieved when printing various self-adhesive label products with UV-curable inks.
Thanks to these and other developments, flexography is moving ever closer in terms of image quality and contrast to flat offset and gravure printing methods, primarily from a consumer point of view, which in turn is a determining factor in a market economy and in conditions of competition between printing houses. At the same time, flexography is characterized by greater efficiency and versatility, allowing you to print various (including minimum) runs on a wide range of materials.
Preparation for the printing process begins with mounting a printing form cut to the format of the print on a plate cylinder using double-sided adhesive tape (each manufacturer, as a rule, has several types in terms of hardness, degree of adhesion and colors). In this case, the exact choice of the type of double-sided adhesive tape depends on the nature of the image and the type of material being printed, and the choice of its thickness (as well as the thickness of the printing plate) depends on the gap (distance) between the plate and printing cylinder. An example image of Biesse double-sided adhesive mounting tape is shown in Fig. 9.
For complex raster multicolor work, especially on wide-web printing machines, it is recommended to use electronic mounting devices that allow you to control this process using video monitors and positionometers (lenses). In this case, the greatest accuracy of installation of printing forms relative to each other and maximum accuracy of ink registration on the print are achieved. An example of a modern mounting device manufactured by J. M. Heaford is shown in Fig. 10.
Test print.
To obtain a spot-type test print, a special two-roller proof printing device can be used - a “color tester” (Fig. 11) with a corresponding anilox screened roller (see below for a detailed description of aniloxes) and rubber (form) rollers. This device allows you to get an objective idea of the pigmentation and adhesion of paint to a certain type of printed material before printing, and also to select the required color shade with some error.
Fig.7. Photomicrographs of various halftone dots (highlights and shadows) with perfectly shaped "flat" tops on Kodak Flexcel NX plates
Rice. 8 Microscreening of the surface of Kodak DigiCap NX printing elements
Rice. 9 Various Biesse double-sided adhesive mounting tapes"
Rice. 10 J.M. Heaford mounting device (narrow web)
Rice. 11 Proof printing device “color tester”
Rice. 12 Axcyl mold sleeve
Installation of forms
Forms can be mounted not only on a cylinder, but also on a special sleeve (using which the best print quality and ease of installation are achieved). In general, the use of sleeves ensures greater efficiency in switching from one print run to another at different print lengths. This is especially useful when there are frequent changes in orders with different print lengths. There are sleeves with an adhesive layer that do not require the use of mounting double-sided adhesive tape. The printing sections of the machine must be equipped to accommodate sleeves, which significantly increases their cost.
The feasibility of using sleeves instead of plate cylinders is determined by the format of the printing machine, so, with a printing width of over 600 mm, the use of sleeves is simply necessary due to the bulkiness of conventional plate cylinders.
An example of an Axcyl sleeve is shown in Fig. 12
When installing the form, it elongates by a certain amount, which is calculated by the formula:
D = K / R x 100%, where K = 2 π t, where t is the thickness of the mold minus the thickness of the polyester backing (approximately 0.125 mm).
R is the length of the print (rapport) or the diameter of the plate cylinder. π = 3.14.
As a result, the percentage of the required print length is calculated by which the image must be reduced before making a printing form (either an electronic image or a negative photographic form).
When using cylindrical seamless flexographic printing plates, there is no stretching. However, special expensive mold equipment is needed for the production (processing) of sleeve round molds (they were also mentioned above).
Thus, the process of pre-press preparation in flexography of the future printed impression is fully described.
Rice. 11.14. Formation of a flexographic form by laser engraving: 1 - focused laser beam; 2 - printing form
Methods for producing flexographic plates using element-by-element recording of information on plate material were known back in the late 60s. last century. EMG was used from analog originals to produce printing forms on rubberized shafts according to the EMG cliché principle. This method made it possible to produce seamless (joint-free) forms for printing “endless” (like wallpaper) images. Due to low reproductive-graphic indicators and other disadvantages, EMG was later replaced by laser engraving on the same material.
This technology for manufacturing rubber molds was used in two versions:
Engraving using a metal mask previously created on the surface of a rubberized plate cylinder;
direct engraving, which was controlled using an electronic device that reads information from the shaft carrying the image.
According to the first option, the mold manufacturing process consisted of the following steps:
The technology considered is very complex and labor-intensive. It was modernized, the copper mask began to be made by laser engraving. To do this, a thin layer of copper was applied to the surface of a rubberized plate cylinder, which was burned with an argon laser, forming a mask. Then the laser burned the bare rubber to the required depth of the space elements. After this, the mask was removed and the form was ready for printing. The lineage of the resulting image ranged from 24 to 40 lines/cm, the print life of the forms reached 2 million copies. This technology was later replaced by direct engraving technology, which was improved and survived to this day as a digital technology.
In 1995, DuPont (USA) developed flexographic FPPs with a mask layer. Using digital technology LAMS (from English - Laser Ablatable Mask), laser radiation creates a mask that performs the function of a negative. Further operations for the manufacture of FPPFs are, in principle, no different from the manufacture of molds using analog technology. The same digital technology for the production of weldless plate molds on sleeves was proposed by BASF (Germany).
In 2000, at the Drupa exhibition, BASF presented a plant for direct laser engraving forms of flexographic and printing printing based on laser for engraving using digital technology of specially created polymer form material. Some companies have proposed using FPPs for the same purposes after their preliminary UV irradiation. Other digital technology options were also proposed. Thus, for direct recording of printed forms on the FPP without a mask layer, Global Graphics developed a device that uses not a laser as a radiation source, but 500 W UV lamps controlled by a computer. However, these developments have not been widely used.
Currently used flexographic printing forms made using digital technologies can be classified according to various criteria, for example, (Fig. 11.1 
):
Variant of mold manufacturing technology: made by laser engraving and mask technology;
Type of mold material: elastomeric (vulcanized rubber), polymer and photopolymer;
Geometric shape: cylindrical and lamellar.
The classification can be continued according to a number of other characteristics: the thickness of the forms, the height of the relief, the resistance of the forms to printing ink solvents, etc.
The structure of photopolymer forms, in principle, does not differ from the structure of forms made using analog technology (see § 8.1.1), since the formation of printing and space elements is also carried out in the thickness of the FPC under the influence of the same processes (see Fig. 8.2, c 
). The difference lies in the different configuration of the printing elements (Fig. 11.2 
).
They have steeper side edges. This ensures less dot gain of the printing elements during the printing process (highlight">Photopolymer cylindrical forms. The manufacturing scheme of these forms is characterized by a number of distinctive features. Cylindrical forms (sleeve, less often jointless - plate with soldered edges) are made on a photopolymerizable material with a mask layer. This material is placed on the sleeve and, as a rule, is preliminarily exposed on the reverse side (this operation is carried out during its manufacture). The process of manufacturing forms is carried out, as for plate ones, first, information is recorded on the mask layer on the LEU. Further operations, starting with the main exposure, are carried out similarly to the scheme outlined above on equipment that provides the possibility of circular exposure and processing.
Elastomeric cylindrical shapes. The production of elastomeric printing forms using digital technology is carried out by direct laser engraving and includes operations for the manufacture of a plate cylinder, which is a rubber-coated rod, and the preparation of its surface for laser engraving, which consists of turning and grinding the rubber coating. Subsequently, direct laser engraving is carried out on it, the engraved surface of the cylinder is cleaned from residues of rubber combustion products, and shape control is carried out.
When using sleeves with a rubber coating specifically designed for laser engraving, surface preparation is not required and, therefore, the number of steps in the molding process is reduced.
Polymer cylindrical shapes. Cylindrical shapes can be obtained from polymer materials (cylindrical seamless sleeves, less commonly weldless plate sleeves). They are manufactured in one stage on one piece of equipment. After monitoring the EVPF and selecting engraving modes, laser engraving is directly carried out.
The formation of printing elements of lamellar and cylindrical FPPFs made using digital mask technology occurs in the same way, during the main exposure of the FPSF of the form material. Since the main exposure to UV-A radiation is carried out through a mask (as opposed to exposure through a photoform in analogue technology) and occurs in an air environment, due to the contact of the FPS with atmospheric oxygen, the polymerization process is inhibited, causing a decrease in the size of the forming printing elements. They turn out to be somewhat smaller in area than their images on the mask (Fig. 11.4 
).
This happens because the FPS is open to the effects of atmospheric oxygen (or, as a number of researchers believe, due to the ozone formed during exposure, which has greater chemical activity and can accelerate the oxidation process). Air oxygen molecules react faster through open bonds than monomers with each other, which leads to inhibition or partial cessation of the polymerization process.
The result of exposure to oxygen is not only a slight decrease in the size of the printing elements (this affects small raster dots to a greater extent), but also a decrease in their height (Fig. 11.5, a 
).
formula" src="http://hi-edu.ru/e-books/xbook609/files/208.gif" border="0" align="absmiddle" alt="c - die
In Fig. 11.6 shows the differences in the height of the printing elements with the formula" src="http://hi-edu.ru/e-books/xbook609/files/204.gif" border="0" align="absmiddle" alt="(! LANG:, the smaller their height (transition" href="part-008.htm#i1615">§ 8.3.3) when it is placed on the plate cylinder, there is a slight alignment of the height of the printing elements on the raster image 1 and on the plate 2 (Fig. 11.7 
).
However, the raster dots have a smaller height (Fig. 11.7, a), while on a form made using analog technology (Fig. 11.7, b), on the contrary, they exceed the height of the die. Thus, the dimensions and height of the printing elements on a form made using digital mask technology differ from the printing elements formed using analog technology (see Fig. 11.5).
Certain differences are also characteristic of the profile of the printing elements. Thus, printing elements on forms made using digital technology have steeper side edges than printing elements on forms produced using analog technology (Fig. 11.8 
).
This is explained by the fact that during the main exposure through a photoform, the radiation, before reaching the FPS, passes through several media and layers (air, pressure film, photoform), successively refracting at the boundaries and scattering in each of the layers. This leads to the formation of a printing element with flatter edges (see Fig. 11.8, a) on forms made by analogue methods. The almost complete absence of light scattering during the main exposure through the mask, which is an integral part of the plate, makes it possible to obtain printing elements with steeper edges. Such features of the printing elements of forms made using mask technology affect the reduction of dot gain during the printing process (Fig. 11.9 
), and the expansion at the base characteristic of printing elements (see Fig. 11.8, b) gives the forms greater stability in the printing process.
Formation of whitespace elements, as in analog technology, occurs during washing out or heat treatment of exposed FPPs, so the process of their formation does not differ significantly (see § 8.2.2). The presence of a mask layer in unexposed areas does not affect the process of formation of whitespace elements. In the case of washing and heat treatment, this layer is removed along with the non-polymerized layer.
When making molds by engraving, elastomers (rubber) are exposed to laser radiation. A laser, as a heat source, creates a temperature of several thousand degrees (for example, a laser at - 1300°C). Thermal destruction of the material occurs and as a result, depressions are formed - whitespace elements. Printing elements Such forms are made from original material that has not been exposed to laser radiation.
General characteristics of devices. To perform the entire complex of operations for the manufacture of flexographic photopolymer forms using mask technology, a set of equipment is required, including LEU, as well as equipment used in analog technologies for exposing the FPS of the plate and subsequent processing of the form (see § 11.1.2).
LEDs for obtaining an image on the FPP mask layer (i.e., recording a mask) are built according to a scheme with an external drum (see Fig. 10.11, c 
). Their design and technological capabilities are in many ways similar to devices for STP offset technologies, but taking into account the specific requirements for devices for the production of flexographic plates. The LEU includes a carbon fiber drum or “air” cylinder for cartridges, a workstation for recording management, a vacuum system that secures the plate on the drum, and an exhaust system (suction of waste at the sites of its generation) to eliminate contamination of the plate.
Different models are equipped with various types of systems that provide fastening of plates to polymer and metal (for example, steel) substrates. Fastening can be carried out by vacuum clamping, magnetically using permanent magnets, including with installed registration pins, or a combined method using vacuum and magnetic clamping. On such devices it is possible to record at a speed of 1.5-8 example">dpi, which allows you to record images with a lineature of up to 220 lpi.
Depending on the type of optical system in various types of LEU, both single-beam recording and recording with several (8, 15, 25, 48) beams (for small and medium format models) and more than 200 beams (for large format models) are possible. Higher productivity is achieved by parallel exposure of multiple beams. This allows the rotation speed of the drum to be reduced compared to devices with a single-beam recording system of the same performance, and this significantly reduces the force that causes beating and separation of the plates from the drum. As a result, in this design it is possible to implement automatic balancing regardless of the plate format and its thickness.
Various models of LEU can be automated and equipped with magazines for FPPs of various formats. The list of built-in capabilities of a number of devices also includes recording information on cylindrical materials, re-equipping them with more powerful lasers, converting them to direct engraving and other capabilities, for example, using a special table on an air cushion for loading and unloading plates.
Features of laser sources. The following types have found practical application for recording images on the FPP mask layer in various devices: laser sources(see § 9.2.2):
Transition" href="part-009.htm#i1817">§ 9.2.2) provides the ability to record image elements without distortion due to defocusing on the FPS, the thickness of the FPS of which can reach 20-25 microns.
Features of exposure devices. Each power unit comes with its own software, which makes it possible to compensate for distortions that arise at the stages of the forming and printing processes, these are also distortions (gradation and graphic) associated, for example, with the inhibitory effect of oxygen during exposure to FPS. The software also allows you to take into account:
Features of the images formed on the mask;
Compression and elongation of the image along the axis of the plate cylinder and along its circumference (see § 8.3.3) when placing (mounting) a plate form on the cylindrical surface of the plate cylinder in a printing machine;
The effect of the interaction of two raster structures (the image on the form and the rasterized anilox roll);
Type and thickness of the plate;
Type of printing machine;
Type of printed material, paint, etc.
Thus, unlike devices for making offset plates, the dot gain of which is standardized, when making flexographic printing plates it is necessary to maintain an entire database of dot gain with all sorts of variations, including those listed above. This is due to a process specific to flexographic printing that compensates for image distortions during the plate manufacturing process.
Test objects for monitoring the manufacturing process of flexographic forms. To control the manufacturing process of flexographic forms and assess their quality, digital test objects are used. They consist of fragments containing line (including text) and raster elements of various sizes, made in both negative and positive designs. The dimensions of the elements, as in analog test objects, are set taking into account the technological capabilities of the plates to reproduce elements of certain sizes on them. Raster test scales on test objects, consisting of fields with different selections">Fig. 11.10 
a test object from DuPont is shown.
Test objects of this type make it possible to determine mold manufacturing modes, including main exposure modes, which, as in analog technologies, are assessed by testing. In the image of such a test object on a printed form, its quality can be determined by the reproduction of strokes, individual dots, raster and text images.
Test object required to select a compensation curve(Fig. 11.11 
), in contrast to that discussed in Fig. 11.10, has an additional fragment, which is a continuous element 1, designated by letters from A to U, containing raster dots with a given screening lineature (from field A to field U, the size of raster dots increases). Raster fields on this test object with example ">Cgeo are used to optimize image recording modes on the FPP mask layer. They serve to calibrate the device and allow you to set focus, drum rotation speed, laser power, movement of the optical head along the drum, recording resolution and etc.
In digital mask technology, molds can be used to test the modes of subsequent (after recording the mask) stages of manufacturing test negatives(see Fig. 8.5 
), or specially modeled test negatives containing fragments with test elements of the required size.
Forming a mask. The mask is created as a result of the thermal effect of laser radiation on the mask layer of the FPS and is formed on the surface of the FPS. In this case, the IR laser does not affect the FPS, which is sensitive to UV radiation. Treatment in chemical solutions after recording is not required. Performing the same functions as a negative photoform, the mask is characterized by a number of features. Thus, the image elements obtained on the mask are sharper compared to the image on the photoform, since they are formed on a heat-sensitive mask layer (see § 10.3.1).
In addition, there is no requirement to obtain elements of a minimum size corresponding to the size of the raster dot with the transition" href="part-011.htm#i2498">§ 11.2.1), which after removing the mask layer from the surface of the FPS (see Fig. 11.3 
) inhibits the photopolymerization reaction. This simplifies the recording process, since in order to obtain minimally sized printing elements on the form, it is necessary to record large sized elements on the mask. For example, to obtain a raster dot on a printed form with selection">Fig. 11.13 
shows the nature of the dependence formula" src="http://hi-edu.ru/e-books/xbook609/files/204.gif" border="0" align="absmiddle" alt="raster element in the digital file created to record the mask. It can be seen from the graph that in the area of light where small-sized raster dots are formed (it is they that are most affected by oxygen inhibition), the decrease in printing elements is nonlinear. Such dependences for different types of PPP may differ, since the effect of the inhibitor is related to which monomers and oligomers enter into the photopolymerization reaction, i.e. determined in particular by the composition of the FPS.
Distinctive features of operations. The subsequent operations of the printing plate manufacturing process (see § 11.1.2) are not fundamentally different from their implementation in the manufacture of PPPF using analog technology. The only difference is that main exposure carried out through a mask without vacuum. The use of a mask, which is an integral part of the plate, eliminates light scattering during exposure, and the properties of the mask layer (homogeneity, uniform thickness, high optical density) guarantee a higher quality image formed on the FPS.
Determining the main exposure time. Main exposure time selection">Fig. 11.10). For this purpose, a test object with preset settings for recording resolution, screening lineature, rotation angle of the raster structure is recorded on the mask layer. Then the main exposure of the FPS is carried out for various times, which depends on FPS sensitivity.
After all other operations of manufacturing the mold (under modes pre-selected as a result of testing - see §§ 8.3.2 -8.3.6), the results of reproducing the gradation scale 4 are evaluated on it ..gif" border="0" align="absmiddle" alt="reproduction of small elements improves and the length of the gradation scale 4 increases, i.e. increasingly smaller image elements are reproduced.
Starting from a certain selection ">4 stops changing and a further increase in time does not affect the size of the reproduced elements, but the angle of inclination of the side edges of the printing elements decreases - they become flatter. Therefore, the selection">4 is considered optimal and stops changing and small ones are steadily reproduced on the form image elements..gif" border="0" align="absmiddle" alt="It is difficult, for example, in the case of removing an unpolymerized layer by heat treatment, increasing the length of the gradation scale 4.
Cylindrical photopolymer forms obtained using mask technology expand the scope of flexographic printing, creating opportunities for printing products with an “infinite” image, for example, packaging, etc. Thanks to mask technology using cylindrical photopolymer forms, it is possible to achieve higher print quality, including due to better registration. In addition, when producing such printing forms, there is no need to compensate for distortions due to stretching of the form, since the image is applied to a cylindrical surface.
The implementation of the technology for manufacturing cylindrical forms, known as the “computer-to-sleeve” technology (from English - computer-to-sleeve), is ensured by using “sleeve” structures consisting of a sleeve with a wall thickness of 0.7 mm, a FPS and an upper mask layer. Such structures are manufactured at specialized enterprises from plate-like FPPs, which are pre-exposed on the reverse side. After cutting to size, the plates are mounted end-to-end, the edges of the joints are fused, ground, and then a mask layer is applied to the surface of the “sleeve” material. Different types of “sleeve” structures differ in the thickness of the FPS. The use of sleeves with compression (from Latin - compression - compression) properties allows printing without much dot gain. This is due to the fact that elements of different sizes (small printing elements and a die), placed on the same form, create different specific pressures and provide different compression of sections of the sleeve.
The technological process for manufacturing printing plates follows the scheme for producing flexographic plates on plates with a mask layer (see § 11.1.2), but exposure of the reverse side is not required. Features of the process, as in analog technologies, include the use of equipment for circular processing of cylindrical mold materials for the production of molds. To implement the technology, there is also the possibility of creating a single automated line for the production of flexographic forms on the sleeve by pairing a device for recording an image on the mask layer and equipment for further processing of the exposed material. Printing forms made using this technology have a hardness of up to 65 Shore i2668 ">rubber coatings include polymers (ethylene propylene, acrylonitrile butadione, natural or silicone rubbers), fillers (carbon black) and targeted additives (accelerators, fillers, dyes, etc.).
The preparation of the rod and its rubber coating is carried out as follows: an adhesive layer is applied to its surface, which is necessary to ensure adhesion of the rubber to the material of the rod. If the rod was previously covered with rubber, then it is removed, and its bare surface is processed using a sandblasting device. Subsequently, a covering of raw rubber in the form of strips is wound onto the rod and covered with a bandage (from the French - bandage - bandage) tape, then the rubber is vulcanized in an atmosphere of steam or hot air. After vulcanization, a homogeneous, smooth coating without seams is formed, which, after cooling, is released from the bandage. This is followed by turning and grinding the cylinder coating. The finished coating is subjected to control in terms of size, surface quality and hardness; the latter can be 40-80 Shore units example">LEP (from English - Laser Engraved Plate) is a technology for manufacturing polymer flexographic (cylindrical and plate) forms by direct laser engraving. This technology successfully combines the capabilities of polymer materials and economical and high-speed laser engraving method This method can be considered as a one-step non-contact process, providing a fairly high repeatability, which is less than 1% around the circumference.
The relief image on a flexographic printing plate is obtained as a result of material removal under the influence of laser radiation. The resulting exposure products in the form of dust, aerosol and other volatile components are captured by the ventilation system and purified as a result of a two-stage process: absorption of solid particles, coarse aerosols and subsequent removal of volatile components. The finished printing form undergoes a cleaning procedure to remove residual polymer decomposition products.
The main disadvantage of the technology is the relatively low engraving speed, equal to 0.06 formula" src="http://hi-edu.ru/e-books/xbook609/files/m2.gif" border="0" align=" absmiddle" alt="/hour (with a depth of space elements of 0.6 mm). However, multibeam engraving increases the cost of the device.
Polymer mold materials. To ensure acceptable characteristics of forms, direct engraving technology requires the use of polymers or mixtures thereof that have sufficient sensitivity in the IR wavelength range and meet the requirements of the printing process in terms of printing performance indicators (circulation resistance, hardness, resistance to solvents of printing inks). This may be a material based on ethylene-propylene-diene monomers (EPDM), which has a high heat capacity, is incapable of spatial polymerization and is characterized by greater hardness compared to those used in analog technologies. Such a polymer must contain black particles that absorb IR radiation when used for engraving lasers in the IR wavelength range (solid-state and fiber).
Engraving devices. The main feature of these devices is that they use a stationary laser source and a moving drum, which ensures the movement of the plate material in front of the laser beam. They are equipped with one or more laser sources with a power of 250-300 W each. Practical applications in these devices are lasers, as well as solid-state and fiber lasers. Thanks to the use of acousto-optical modulators, it is possible to focus the laser beam to a size of 20-25 microns in diameter. Accordingly, raster dots with example "> dpi are obtained. In such devices, the engraving depth can be set, as well as other parameters that allow you to change the steepness of the profile of the engraved cell. In addition to three-dimensional controlled engraving, there is also the possibility of lowering the height of some raster elements on the form (Fig. 11.15 
). This leads to a reduction in their dot gain during the printing process and allows simultaneous reproduction of spot, raster and line elements on one form.
Engraving devices of various types are equipped in such a way that they can be converted from engraving with one beam to working with several beams with different powers. They engrave the material to varying depths, ensuring the formation of steep side edges of the printing elements. The use of two lasers, one of which operates at the top of the future printing element (cuts it), and the other engraves the base of the printing element, makes it possible to obtain printing elements of various heights well fixed to the base. This ensures a print run life of up to 4 million copies. The combination of two types of lasers in engraving devices, for example, a laser for preliminary shaping of the profile of printing elements and a solid-state laser that forms the side edges of a predetermined shape, expands the capabilities of direct laser engraving technology.
When producing photopolymer forms for flexographic printing based on TFPC (Fig. 4), the following basic operations are performed:
preliminary exposure of the reverse side of the photopolymerizable flexographic plate (analog) in an exposure installation;
main exposure installation of the photo form (negative) and the photopolymerized plate in the exposure installation;
processing of a photopolymer (flexographic) copy in a solvent (washout) or thermal (dry heat treatment) processor;
drying the photopolymer form (solvent-wash) in a drying device;
additional exposure of the photopolymer form in the exposure installation;
additional processing (finishing) of the photopolymer mold to eliminate the stickiness of its surface.
Exposing the reverse side of the plate is the first step in making the mold. It represents an even illumination of the reverse side of the plate through a polyester base without the use of vacuum and negative. This is an important technological operation that increases the photosensitivity of the polymer and forms the base of the relief of the required height. Proper exposure of the reverse side of the plate does not affect the printing elements.
The main exposure of the photopolymerized plate is carried out by contact copying from a negative photoform. On a photographic plate intended for making molds, the text must be mirrored.
Photo forms must be made on one sheet of photographic film, since composite mounts glued with adhesive tape, as a rule, do not ensure reliable adherence of the photo form to the surface of the photopolymerized layers and can cause distortion of the printing elements.
Before exposure, the photoform is placed on the photopolymerized plate with the emulsion layer down. Otherwise, a gap equal to the thickness of the film base will form between the plate and the image on the photo form. As a result of the refraction of light in the base of the photographic film, severe distortion of the printing elements and copying of raster areas can occur.
To ensure tight contact of the photo form with the photopolymerized material, the photographic film is matted. Microroughness on the surface of the photoform allows air to be completely and quickly removed from underneath it, which creates tight contact of the photoform with the surface of the photopolymerized plate. For this purpose, special powders are used, which are applied with a cotton gauze swab with light circular movements.
As a result of processing photopolymer copies based on solvent-washed plates, the monomer that has not been exposed and polymerized is washed out - it dissolves and is washed off the plate. Only the areas that have undergone polymerization and form the relief of the image remain.
Insufficient washout time, low temperature, improper brush pressure (low pressure - the bristles do not touch the surface of the plate; high pressure - the bristles bend, reducing the washout time), low level of solution in the washout tank lead to too shallow relief.
Excessive washout time, elevated temperature and insufficient solution concentration lead to too deep a relief. The correct washout time is determined experimentally depending on the thickness of the plate.
When washed, the plate is soaked in the solution. The polymerized image relief swells and softens. After removing the washing solution from the surface with non-woven napkins or a special towel, the plate must be dried in a drying section at a temperature not exceeding 60 °C. At temperatures exceeding 60 °C, difficulties may arise in registering, since the polyester base, which under normal conditions remains stable in size, begins to shrink.
Swelling of the plates when washed leads to an increase in the thickness of the plates, which, even after drying in a drying device, do not immediately return to their normal thickness and must be left in the open air for another 12 hours.
When using heat-sensitive photopolymerizable plates, the development of the relief image occurs by melting the non-polymerized areas of the forms when they are processed in a thermal processor. The melted photopolymerizable composition is adsorbed, absorbed and removed by a special cloth, which is then sent for disposal. This technological process does not require the use of solvents, and therefore, drying of the developed forms is eliminated. Both analog and digital shapes can be produced in this way. The main advantage of technology using heat-sensitive plates is a significant reduction in mold production time, which is due to the absence of a drying stage.
Making a photo form:
- exposure
- manifestation in alkaline solution
- spin
- fixation in an acidic environment
- washing with water
- drying
3. Making a printing plate:
- incoming inspection of equipment and materials
- illumination of the reverse side
- main exposure
- manifestation
- drying atto 40-60 o C
- additional exposure
Photopolymer molds made from liquid photopolymerizable materials (LPPM) appeared in 1969 in Japan. Photopolymerizable plates made of solid photopolymerizable materials (SPPM) have been used for the manufacture of printing forms since the mid-70s of the last century. In 1975, flexographic photopolymerizable materials (FPM) Cyrel (DuPont, USA) appeared on the world market. Improvement of the properties of TFPM led to the simplification of analog technology for the production of letterpress forms, as well as to the development of water-washing plates, such as Nyloprint WD, WM, and the water-washing unit Nylomat W60 (BASF, Germany), which appeared in the early 80s. In 1985, the widespread industrial introduction of Nyloflex plates began. In 1986, Letterflex (USA) released flexographic forms on a steel substrate for newspaper printing Newsflex-60 and high-performance form equipment.
The improvement of the printing and technical properties of photopolymer flexographic forms occurred due to the development and use of thin plates with high rigidity. Sleeve technology has been developed since the 90s of the 20th century. Thanks to the release by Rotec of sleeves with rigid and compressible surfaces. Mounting a flexographic form on a sleeve, also made on a thin plate, made it possible to significantly improve the quality of printing.
The development of solvent wash solutions that do not contain hydrocarbon chloride has significantly improved the environmental performance of the plate process for the production of flexographic printing plates.
The introduction in 1999 of FAST technology (DuPont) for thermal development of a relief image on flexographic photopolymer forms, due to the absence of solvents and the drying stage, made it possible to reduce the time for creating a printing form by 3-4 times.
The use of digital technologies for flexographic printing plates was preceded by technologies known since the 70s of the last century, using element-by-element recording of information on the plate material (mainly rubber) by engraving controlled by analog storage media. The method of making rubber molds by laser engraving has been used in the form of two most common technologies: engraving under the control of a metal mask created on the surface of a rubberized plate cylinder, and engraving under the control of an electronic device that reads information from the shaft carrying the image. The main stages of production of forms by laser engraving with masking are: rubber coating of the form cylinder; grinding the rubber surface; covering the cylinder with copper foil, the edges of which are butt-joined; applying a copy layer to the foil; copying photo forms; etching of copper in areas corresponding to the blank elements of the form, obtaining an engraving mask; CO2 laser engraving; removing the mask from the surface of the mold.
Digital technologies for the production of flexographic printing plates have been widely developed since 1995 as a result of the creation of photopolymerizable plates with a mask layer by DuPont.
In 2000, at the Drupa exhibition, BASF presented an installation for direct laser engraving of flexographic and letterpress forms based on a 250 W CO2 laser for engraving specially created polymer plate material.
Digital technology in the production of printing plates for printing seamless images was proposed by BASF in 1997 and was called computer - printed sleeve (Computer to Sleeve).
Among the latest developments is the Flexdirect direct laser engraving process, which consists of a single-stage engraving of polymer or elastomeric materials with the formation of a shape relief. To increase the lineature of the engraved image in Flexposedirect direct engraving devices (ZED, England; Luesher, Switzerland), the spot size was reduced due to signal modulation, which made it possible to reproduce printing elements with a size of 20-25 microns or less.
Flexographic photopolymer printing forms can be divided, depending on the physical state of the plate material - the photopolymerizable composition (FPC), into forms made from solid and liquid PPC. Digital technologies use forms made from a solid composition.
By design, the following flexographic forms are distinguished:
- plate single-layer, consisting of a single elastic material, such as rubber, caoutchouc or photopolymer;
- plate two- and three-layer, in which the layers are distinguished by elastic properties, which make it possible to improve the deformation characteristics of printed forms;
- cylindrical in the form of hollow replaceable cylinders (or sleeves) with an elastic coating.
Forms made using digital technologies are divided into flexographic forms, obtained by laser, exposure to the receiving layer of the form material with subsequent processing, and forms, obtained by direct engraving of rubber or polymer forms.
Depending on the plate material, flexographic plates made using digital technologies are classified into photopolymer and elastomeric (rubber). Photopolymer forms, compared to elastomeric forms, are distinguished by stability and quality of reproduction of high-lineature images, but are less resistant to esters and ketones present in printing inks.
The production of engraved plates can be carried out on plates mounted on a plate cylinder or sleeve, or on seamless rubber, polymer or photopolymer plate materials mounted on a metal core, plate cylinder or sleeve. Seamless molds from FPM are made on plates or on sleeves, most often placed on sleeves.
The structure of the photopolymer mold is determined by the structure of the photopolymerized plate and the manufacturing process. Forms created on the most widely used single-layer photopolymerized plates have print and space elements from a photopolymerized layer located on a dimensionally stable substrate. Laser-etched elastomeric molds are composed primarily of vulcanized rubber.
Technological scheme for manufacturing flexographic forms on photopolymerizable plates with a mask layer includes the following operations:
- exposure of the reverse side of the plate;
- recording an image on the mask layer using laser radiation;
- main exposure of the photopolymerized plate through an integral mask;
- washing out (or thermal removal) of the non-polymerized layer;
- drying the mold;
- finishing (finish - ending);
- additional exposure.
Sometimes in practice, the technological process begins with recording an image on the mask layer, and exposure of the back side of the plate is carried out after the main exposure.
When using thermal development using FAST technology, after the main exposure of the plate, thermal removal of the uncured layer follows, followed by finishing and additional exposure of the form.
The peculiarity of the production of cylindrical forms is that a plate with a mask layer, previously exposed on the reverse side, is glued to the sleeve, and then the image is recorded on the mask layer in a laser device. There is a technology for obtaining a seamless shape by applying a mask layer to the surface of the photopolymerized layer before laser recording. Further operations are performed in accordance with the outlined scheme.
Digital technology for the production of elastomeric printing forms by direct laser engraving contains the following stages:
- preparing the plate cylinder, including rubberizing its surface;
- preparing the surface of the plate cylinder for laser engraving, which consists of turning and grinding the rubber coating;
- direct laser engraving;
- cleaning the engraved surface of the cylinder from combustion products.
A special feature of the technology when using a rubber-coated sleeve designed specifically for laser engraving is the absence of the need to prepare the surface for engraving and the reduction of operations in the technological process diagram.
Formation of printing elements photopolymer forms made using digital technology on plates or cylinders with a mask layer occurs during the main exposure process. In this case, due to the directional scattering of the light flux penetrating through the FPC, a profile of the printing element is formed (Fig. 2.1).
Photoinitiated radical polymerization occurs according to the following scheme:
excitation of photoinitiator molecules
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chain break with the formation of the final product
selection">Fig. 2.2). The difference in the steepness of the edges of the printing elements of the forms is associated with the conditions of their formation during the main exposure process. According to analog technology, when exposed through a negative, radiation, before reaching the photopolymerized layer, passes through several media (pressure film, photo form), scattering at their boundaries, which leads to the formation of a printing element with a larger area and a wider base.Reducing light scattering during the main exposure of the photopolymerized layer through an integral mask makes it possible to form printing elements that provide image reproduction in a wide range of gradations.
A relief is formed on the form obtained using digital technology (Fig. 2.3), which is optimal for stabilizing and reducing dot gain during the printing process..gif" border="0" align="absmiddle" alt="with the relative area of raster elements in the digital data array (Fig. 2.4).
When mounting a printing plate on a plate cylinder or sleeve, due to stretching of the plate, the height of the raster areas of the image increases. Raster elements of printed forms produced using analog technology protrude above the spot elements, which leads to strong dot gain in highlights. When using digital technology, the pressure on the raster areas of the image is less than on the solid, which has a beneficial effect on the reproduction of images of different nature (Fig. 2.5).
An important task when forming printing elements of photopolymer forms is to impart to their surface properties that ensure good perception and return of ink in the printing process and high wear resistance. In this case, the physical and mechanical properties of the relief are of decisive importance, which are achieved during additional exposure and finishing due to photopolymerization in the FPC thickness and surface oxidation, respectively. The result of additional exposure is the creation of a homogeneous structure of the printing form with high printing and performance characteristics.
Formation of whitespace elements methods of washing out or thermal development of photopolymer forms made using digital mask technology do not differ significantly from the processes of creating photopolymer forms using analogue technology.
In flexographic printing, the printing plate experiences elastic deformation during the printing process. These deformations, depending, in particular, on the material being printed, the thickness and structure of the plates, must be taken into account when choosing the minimum permissible depth of relief of the printing form. When choosing the relief depth, the nature of the image (line or raster), printing conditions, and the thickness of the plate are taken into account. If there is a highly linear image on the form, a smaller relief depth is recommended to avoid loss of small raster elements. In the case of using rough and dusty printed materials, a greater depth of white space elements is required.
The formation of space elements of photopolymer forms occurs during the process of washing under the action of a washing solution (when using a water-washing FPC, water is used). The washing process is influenced by hydrodynamic factors, such as the pressure of the washing brushes and the method of supplying the washing solution, as well as its composition and temperature.
The process of creating gap elements begins with solvation with a gradual transition of the FPC into a gel-like layer, followed by unlimited swelling of the polymer and ends with the complete removal of the FPC from unexposed areas.
When the washing solution acts on the exposed areas, the process of interaction of the solvent with the polymer stops at the stage of limited swelling of the photopolymerized layer. This is due to the presence of a spatial network in the polymer subjected to irradiation.
The formation of blank elements of flexographic forms can occur when uncured FPC is removed using a thermal process. The process is implemented due to the presence of thermoplastic properties of unexposed FPC, which are lost under the influence of UV-A radiation. During exposure, a spatial network is formed in the polymer and FPC loses the ability to transform into a viscous-flow state.
Removal of FPC from blank elements of forms is carried out by local heating of the surface of the form with infrared radiation. In this case, the non-polymerized part of the FPC passes into a viscous flow state. Absorption of the molten polymer occurs due to capillary absorption and is carried out using non-woven material with repeated close contact of the form with the absorbate (Fig. 2.6). This process depends on the heating temperature, the thixotropic properties of the FPC and the thickness of the plate. The mask layer is removed from the space elements by washing or thermal development along with the uncured layer.
With direct laser engraving, a flexographic form is produced in one technological step on one equipment. The forming material is rubber or special polymers. The formation of gap elements is carried out by laser radiation due to the transfer of a large amount of energy to the material, and combustion products are formed. Under the influence of a laser, providing a temperature of several thousand degrees, the rubber is burned out. For example, a CO2 laser creates a temperature of 1300 °C in a spot with a diameter of 1 mm.
The formation of the relief occurs as a result of the physical removal of the elastomer from the space elements of the form. To create the desired profile of the printing element during direct laser engraving, special laser radiation modulation modes or a method of processing the form material in several passes are used. The whitespace elements are deepened to a set depth, while the printing elements remain in the same plane. The profile of the printing elements is set by the engraving mode and has distinctive features compared to printing elements obtained under the influence of UV radiation (Fig. 2.7). The side edge of the printing element of the laser-engraved form is directed perpendicular to the plane of the printing element, which provides certain advantages during the printing process, providing a lower degree of pulling out and good ink transfer. In addition, when the form is abraded during the printing process, there is no increase in the optical density of the print, since the relative area of the printing elements does not change. The expansion of the base of the printing element gives greater circulation resistance and shape stability in the printing process.
Types of plates. Flexographic plates differ in structure, development method, FPC composition, nature of the wash solution, plate thickness and hardness, and other characteristics. Based on the method of image development, they are divided into thermal development plates and washout plates. The latter, manifested by leaching, depending on the nature of the leaching solution, are divided into solvent and water-washing.
In digital technology for the manufacture of flexographic forms, plates are used that, in addition to the photopolymerizable layer (PPL), have an additional recording mask layer (Fig. 2.8a). It serves to create the primary image formed using a laser, and serves as a mask for subsequent exposure of the photopolymerized plate to UV radiation. The mask layer, insensitive to UV radiation and thermosensitive in the IR range of the spectrum, has a thickness of 3-5 microns and is a soot filler in an oligomer solution. The FPS of the plate is sensitive to UV radiation in the range of 330-360 nm and is similar in composition and properties to the layer used in analog technology. The stages of manufacturing a photopolymer plate with a mask layer are: applying a mask layer to a protective film, including the processes of varnishing, caching and sputtering; caching of films with the application of FPC onto the substrate using an extruder with constant control of the layer thickness; smoothing the strip of form material using a calender; preliminary exposure from the substrate side; cutting the tape according to the plate format (Fig. 2.9). To acquire the necessary properties, the plates are aged for several weeks.
As a layer sensitive to laser radiation, on some plates an aluminum-based layer with a thickness of 1-2 microns is used, which makes it possible to eliminate the scattering of radiation inside the mask layer.
Main characteristics of plates. The thickness of a photopolymer flexographic plate is in most cases specified in thousandths of an inch (from 30 to 250) or in millimeters. There are thin plates - 0.76 or 1.14 mm, regular ones - from 1.70 to 2.84 mm and thick ones - from 3.18 to 6.5 mm. The thickness of the substrate for thin plates is 0.18 mm, for thick ones - 0.13 mm.
If several printing forms must be located on the surface of the plate cylinder, then special attention should be paid to controlling the thickness of the plate plates, since thickness differences can adversely affect the pressure distribution during the printing process. The tolerance for the thickness of one plate is + 0.013 mm, for different plates ± 0.025 mm.
Hardness is the most important characteristic of the plate, which allows us to indirectly judge the wear resistance of the future printing plate and its reproduction and graphic characteristics. The hardness of a photopolymerized plate is usually indicated in hardness units (in degrees Shore >definition)> The choice of plates for specific conditions is carried out taking into account the nature of the image, the type of printed material, the type of printing ink, and also depends on the printing machine and printing conditions.
Reproducing an image containing small elements requires the use of thin plates with high hardness. The necessary deformations during printing are achieved due to the elastic material placed on the plate cylinder or sleeve. To reproduce a raster image, plates with a higher hardness are used than for printing a solid. This is due to the fact that raster elements react more strongly to pressure during the printing process. When the mold comes into contact with the anilox roller and there is severe deformation of small raster elements, paint may transfer to the slope of the raster dot. Insufficient plate hardness can lead to increased pull-out.
For printing on rough, dusty papers, thick plates are chosen that provide deeper relief on the printing plate; When using corrugated cardboard, thick plates with low hardness are used. If the printing machine has a built-in device in which the film is processed with a corona discharge, the plates for printing on polymer films are selected taking into account ozone resistance. These characteristics are indicated, as well as the resistance of the plates to certain organic solvents (for example, ethyl acetate) and the recommended types of printing inks. When choosing a printing ink, its compatibility with printing ink (water-based, organic solvent-based, UV-curable) is taken into account.
The plates are selected taking into account the format of the printing machine and the gap (distance) in the printing pair.
The plates used must ensure the possibility of obtaining the necessary printing and operational characteristics of future forms, as well as compliance with environmental requirements during their manufacture.
Image data is stored as PostScript, TIFF, or PCX files and is used to output information to the platen. In a raster processor (RIP), the tonal values for each color are converted into larger or smaller raster dots. Modern raster processors have a built-in function that allows you to save special calibration curves so that when recording they are superimposed on the output data.
At the prepress stage, the size of the minimum printable dot must be known so that there are no dots on the form with an area below the minimum value. This is done in order to prevent disruption of the gradation transfer on the print in image highlights. The size of the minimum dot depends on the printing machine, the thickness and rigidity of the platen and the properties of the printed material. Thin forms with shallow relief are able to reproduce a smaller point than thick ones. Forms made on stiffer plates also produce a smaller dot area. The minimum point size is set in the drag compensation program.
RIP controls the relationship between the minimum print element size and the mesh size of the anilox roll. The need for control is caused by the phenomenon of abnormal ink transfer, when smaller printing elements can pick up more ink, falling inside the cell of the anilox roll.
The size of the minimum print element in a one-bit raster image file obtained after rasterization using RIP is significantly different from the size of the print element on the printing plate.
Gradation compensation for digital technology includes compensation for plate and printing processes. When producing printing plates, due to the inhibitory effect of oxygen during exposure, gradation distortions occur. Their compensation is carried out using flexographic RIPs and makes it possible to compensate for the reduction in the size of the printing elements at the stage of generating a TIFF file transmitted when recording a mask (Fig. 2.10). To do this, to form a printing element of the required size, based on the relative area of the raster dot in the file. RIP recalculates the sizes of raster points of the original PostScript file and writes the required window size on the integral mask to the TIFF file. Before sending the file to RIP, the necessary parameters are set: recording resolution, lineature, rotation angle of the raster structure and the selected compensation curve.
As a rule, the software or hardware of the devices (most often RIP) provides compensation for image elongation or compression. Such image distortion occurs both along the axis of the plate cylinder and along its circumference. Stretching the printing elements around the circumference of the cylinder leads to a difference in their sizes on the print from the sizes on a flat form - distortion (Fig. 2.11). This value, related to the printing machine and the thickness of the printing plate, is taken into account in the RIP during the screening phase. For example, in RIP FlexWorks of the Laser Graver system, compensation for image elongation or compression is performed in the form of setting the appropriate coefficients.
The electronic editing module should allow geometrically precise placement of images presented as separate files. In this way, it is possible to mount, for example, repeating small images typical for label printing.
An image is recorded on a plate with a mask layer using various types of lasers. Fiber laser, YAG laser, and laser diodes are used for this purpose.
YAG and fiber lasers differ from diode radiation sources in their greater stability and lower divergence of the light beam. Due to this, dots of stable sizes and the required round shape are created on the mask layer of the plate. Systems for exposing flexographic plates provide image recording with a line size of up to 200 lpi. Resolution can vary within 1800-4000 dpi. The exposure speed is up to 4 m2/h with a spot size of 15 microns.
It is believed that a depth of field of 100 μm is sufficient to record an image on a photopolymerizable plate with a mask layer. In devices using laser diode arrays, the divergence and focusing range of the laser beam is worse than that of fiber and YAG lasers, which leads to a shallow depth of field of the laser beam in the material processing area (Fig. 2.12). Lasers operating in single-mode mode have the greatest depth of field, in which the best radiation parameters are achieved. In the powerful multimode mode, which allows for high-speed image recording, the parameters are reduced and the depth of field is reduced. If the depth of field is insufficient, deviations in the thickness of the plate can lead to changes in the diameter of the laser exposure spot and recording defects.
The selection of optimal modes for manufacturing molds on photopolymerizable plates with a mask layer is carried out using testing. Determining the increase in the size of a raster element during laser image recording is inextricably linked with the selection of processing modes for the plate after obtaining an integral mask on its surface.
A test object is used to determine the exposure time. Its content is discussed using the example of a DuPont test object (Fig. 2.13). Testing is carried out by element-by-element recording of the test object on a photopolymerizable plate with a mask layer. The digital basic test object includes stepless gradation elements, raster scales with relative dot areas from 2 to 100%, positive and negative strokes and dots of various sizes. The file for the test object was created using Macromedia FreeHand 8.0. If the lineature used does not meet the user’s needs, it can be replaced using this program. When a file needs to be converted to another format or used with another program, care must be taken to ensure that the control elements are not changed during the conversion process. To determine the optimal exposure time, several copies of the test object, usually at least ten, are sequentially recorded on one photopolymerized plate with a mask layer. To avoid differences, one copy screened in RIP is reproduced using the interface of the appropriate plate maker.
Testing of subsequent operations of the technological process is carried out in the same way as in the manufacture of photopolymer forms using analog technology.
The reverse side of the plate is exposed to form the base of the printing plate. By increasing the photosensitivity of the FPS as a result of exposing the reverse side of the plate, the conditions for the formation of printing elements during the main exposure and their adhesion to the base are improved. Exposure is carried out through the plate substrate (see Fig. 2.8, b). Radiation, penetrating deep into the FPC, leads to layer-by-layer polymerization, the degree of which gradually decreases. With increasing exposure, the thickness of the photopolymerized layer increases, reducing the possible depth of the relief of the future shape. The thickness of the base is the difference between the thickness of the form and the maximum depth of the white space elements. The photopolymerized base limits the penetration of the washing solution and, consequently, the depth of the relief.
The amount of exposure when exposing the reverse side of the plate depends on its thickness and the nature of the image on the printing plate. Too short an exposure can lead to washing out of small printing elements of the form due to insufficient polymerization of the base and, as a result, insufficient resistance to the action of the washing solution. Excessive exposure time can create a mold base that is too thick and make it difficult to form white space elements of the required depth. Determination of the exposure time of the reverse side of the plate is carried out using testing. Separate sections of the plate on the reverse side are subjected to dosed exposure, set by different exposure times. It depends on the thickness of the plate and can be, for example, 10, 20, 30 s or more. Typically the exposure is 8 stops. The required exposure time for the reverse side of the plates is determined by a graph relating the time to the depth of the gaps obtained after exposure and washout.
The installation for laser image recording includes: an optical device; carbon fiber exposure cylinder or case cylinder; a workstation with a service unit and a program for controlling the exposure unit; a vacuum device that secures the plate during recording; system for extracting waste generated when removing the mask layer. The quality of recording depends on addressing - the ability of the laser to be controlled in the entirety of its design features, scanning and focusing of the laser spot.
The creation of the primary image on the recording mask layer is carried out using a laser beam of high energy density. Due to the active absorption of IR radiation by the black mask layer, its ablation occurs. An integral mask is formed on the surface of the photopolymerized layer, carrying a negative image of the original, which has a high optical density (see Fig. 2.8, c). In this case, the laser emitting in the infrared range does not affect the photopolymerizable layer, which is sensitive to UV radiation. The required power can be generated by a single laser beam or multiple beams; This multi-beam technology improves system performance.
The plate is attached to the drum and held there by vacuum. When exposing thick plates, their mass reduces the rotation speed of the drum.
Obtaining a clear image on an integral mask depends on the structure and technical characteristics of the mask layer (uniformity, high optical density, good adhesion to the photopolymerized layer), as well as on the correct setting of the laser beam exposure depth. The system is adjusted to this parameter through preliminary testing. The built-in dynamic focusing device allows you to compensate for changes in the thickness of the layers of the photopolymerized plate and improve recording parameters.
The subsequent operations of the technological process are not fundamentally different from their implementation in the manufacture of flexographic photopolymer printing forms using analog technology. The difference is that the main exposure is carried out without a vacuum, and the image is transferred by exposing the photopolymerizable layer of the plate through an integral mask.
Main exposure. The purpose of the main exposure is the formation of printing elements. During this process, through a negative integral mask in areas free of the mask layer, photopolymerization of the FPC occurs with the formation of a profile of the printing elements. Due to the absence of a photoform, there is no weakening of the light flux acting on the FPC, and the high sharpness of the edges of the mask and the inhibitory effect of oxygen make it possible to achieve the required steepness of the profile of the printing elements (see Fig. 2.8, d).
If the process of making a plate begins with laser recording of an image on a plate, then to ensure the safety of the digital integral mask, the sequence of operations of the main exposure and exposure of the reverse side of the plates is selected depending on the characteristics of the exposure device. Then, in order not to damage the mask, the main exposure is carried out first, and then the back side of the plate is exposed. The main exposure time is set using a stepless gradation element of the test object (see Fig. 2.13). The optimal time is considered to be the time from which the stepless gradation elements reproduced on the form have approximately the same length and cease to lengthen with the subsequent increase in exposure. In this case, the smallest exposure provides the largest range of gradations on the printing plate.
With insufficient exposure, the fine lines on the plate become wavy and an “orange peel” effect appears on the surface of the plate, leading to premature wear of the plate. With excessive main exposure, the image on the form loses clear contours, the contrast of the image in the shadows decreases, and the depth of the white-space elements is insufficient.
Removal of uncured composition. Polymer solvents have a number of general requirements, including high dissolving power with minimal impact on cross-linked areas and the ability to form concentrated solutions with low viscosity. Solvents must be characterized by a low degree of volatility, low cost, fire safety and non-toxicity. Solvent wash solutions are a mixture of an aliphatic or aromatic hydrocarbon and an alcohol. Chlorine-containing solutions have limited use due to toxicity. Wash solutions containing organic solvents are regenerated in special units (evaporators), which can be connected to washing machines. This allows you to organize a closed cycle of the leaching process, reducing environmental pollution.
The purpose of washing out is to reveal the hidden relief image obtained during exposure and to form white-space elements of the form. The essence of the process is that the rate of diffusion of developing solutions into non-polymerized areas of the plate is several times higher than into photopolymerized areas. To increase the selectivity of development, substances (for example, butanol or isopropanol) that reduce the swelling of irradiated film-forming photopolymers are introduced into developing solutions.
Excessive washout time causes swelling of the relief, which, together with insufficient main exposure, can lead to disruption of the surface structure (“orange peel”).
As the solution becomes saturated with the reagents included in the FPC, the washout capacity of the solution decreases. The regeneration mode of the washout solution depends on the size of the plate and the depth of the gaps. It is determined at the rate of approximately 10-15 liters of washout solvent solution per 1 m2 of plate surface and 1 mm of gap depth. Determination of the washout time of the non-polymerized layer of the plate is carried out by testing. It is based on the assumptions that for different thicknesses of the plates, a constant pressure of the brushes of the wash processor is established, the temperature of the solution is maintained stable, and the absorption capacity of the solution does not change due to its regeneration.
To determine the optimal washout time, several identical wafers subjected to the same exposure (with part of the wafer surface protected by a template) are washed for varying times based on the thickness of the wafer. After drying and measuring the thicknesses of washed and unwashed areas, a relationship is obtained from which the washing time required to achieve the required relief depth is determined. In this case, the optimal time corresponds to the required relief depth plus 0.2-0.3 mm. The increase in washout time is explained by the fact that between the polymerized and non-polymerized parts of the layer there is a phase in which the material is partially polymerized and therefore is washed out slowly. When using a washout processor, the washout time is determined by the speed of movement of the mold in the processor (Fig. 2.14). In automatic continuous processors, the corresponding washout time is entered into the program.
When thermally developing a relief image using FAST technology, the exposed plate is fixed on the thermal processor drum and brought to an IR radiation source. The required relief depth, depending, in particular, on the thickness of the form plate used, is achieved with 10-12 cycles of contact of the form, locally heated to t = 160 ° C, with an absorbent non-woven material (see Fig. 2.6).
Drying the form. The purpose of drying is to remove liquid from the photopolymerized layer of the mold using heat. When washed, this layer is saturated with the washing solution, the image relief swells and softens. The relative content of solvent absorbed by the photopolymer after washing out usually exceeds 30%, the surface is covered with a very thin continuous film, and the capillaries are filled with solvent.
The moisture content of the photopolymer after washing out depends on the ability of the material to swell, the time of washing out, the degree of cross-linking of the polymer, and the nature and temperature of the solvent. The swelling of the shape relief occurs unevenly, its degree depends on the nature of the image. Screened areas absorb more solvent than solids. The influence of the nature of the washing solution on the drying time is associated with the degree of swelling of the photopolymer layer and with the volatility of the solvent included in the solution.
During the drying process, solvent molecules move from the inner layers of the material to the outer layers and subsequent migration from the surface of the mold into the coolant medium. When drying with warm air heated to a temperature of 65 ° C, the solvent is removed from the surface of the mold due to convective diffusion. To increase the rate of internal solvent diffusion, it is possible to use FPC based on granular polymers containing micropores.
The intensity of the drying process depends on the chemical nature and structure of the mold material, the size and condition of its surface, the temperature of the coolant, its saturation with solvent vapor and the speed of movement relative to the mold.
Drying is the most time-consuming operation in the production of a flexographic printing plate. The drying time can be 1-3 hours, after which the original thickness of the plate returns, and its surface remains slightly sticky. After drying, before additional treatment with UV-C radiation, the mold must be cooled, since premature processing may cause residual swelling of the layer and the thickness of the finished mold will be uneven.
Elimination of stickiness and additional exposure of the form. Additional processing (finishing) is carried out to eliminate stickiness, which is formed due to the presence of a thin layer of highly viscous liquid on the surface. It represents macromolecules of thermoplastic elastomer or other polymer, dissolved or mixed with molecules of unpolymerized monomers or oligomers. Components that did not enter into the photopolymerization reaction during exposure diffuse to the surface during leaching, causing it to become sticky.
Elimination of stickiness can be achieved in two ways: by treating the surface with chemical reagents, in particular bromide-bromate solution, or by UV-C irradiation of the surface (see Fig. 2.8, f). In the first method, bromine, entering into an addition reaction, reduces the concentration of unsaturated double bonds and promotes the conversion of unsaturated monomers with a low boiling point into saturated bromine derivatives, which, due to their higher boiling point, are solid compounds. However, chemical finishing using solutions of reactive compounds is environmentally unsafe.
Finishing by UV irradiation of a form in a gaseous environment is the most widely used. In the process of such radiation treatment, which has high energy and low penetrating power, the stickiness of the surface layer of the printing plate is eliminated. For finishing, installations are used that are equipped with tubular UV lamps with a maximum radiation in zone C with a wavelength of 253.7 nm. Processing for too long makes the surface of the mold brittle and reduces its paint susceptibility. The duration of UV-C treatment is influenced by the type of plate, the nature of the wash solution and the duration of previous drying. The finishing time for thin plates is usually longer than for thick ones.
Additional exposure is carried out with UV-A radiation (see Fig. 2.8, g) in order to increase the resistance of the form to printing ink solvents and to achieve the necessary physical and mechanical properties. The additional exposure time can be less than or equal to the main exposure time.
Form control. Quality indicators of flexographic forms include the presence of printing elements of the required sizes, shape and surface structure, a certain relief height corresponding to the nature of the image on the printing form, as well as the necessary adhesion to the substrate.
Possible defects of forms made using digital technology include the appearance on the form (and possibly subsequently in printing) of a single-color moire due to the cyclic variety of shapes of printing elements corresponding to the same gray level, i.e., raster dots in areas of constant tone have the same area but different shapes. The reason for this is a combination of the effect of oxygen on the photopolymer along the contour of the window on the mask and screening technology, since the decrease in the area of the printing element is proportional to the change in its perimeter, the size of the element on the printing plate will depend on its geometric shape. The occurrence of a defect is also influenced by the laser power, the sensitivity of the mask layer, and the trajectory of the brushes in the wash processor. It can be avoided by optimizing rasterization algorithms and eliminating differences in the shape of printing elements.
Digital technology for making molds on sleeves by laser exposure of photopolymerizable plates with a mask layer consists of the following steps:
- preliminary exposure of the reverse side of the plate;
- mounting the plate on the sleeve using adhesive tape;
- installation of the sleeve in the replaceable holder of the exposure device;
- laser exposure to the mask layer of a photopolymerizable plate;
- exposure of the photopolymerizable layer to UV-A radiation.
All subsequent operations: washing, drying, finishing, and additional exposure are carried out in the usual manner, but on special equipment for processing cylindrical printing forms. To produce seamless photopolymer printing plates, the plate is exposed from the reverse side, then mounted around a sleeve, the edges of the plate are pressed tightly end-to-end, and the photopolymer is melted to seal the edges of the plate. After this, it is ground to the required thickness in a special installation and a recording heat-sensitive mask layer is applied to the seamless surface. An image is recorded on it with a laser, followed by the operations of the printing process. Molds made using technology computer - printed sleeve(CTS) do not require compensation for distortions associated with mold stretching.
Cylindrical seamless (sleeve) molds (digisleeve) are made on a polymer mold material in the form of a flexible hollow cylinder, which is pulled onto a sleeve, and then it is processed on equipment designed for cylindrical molds. Depending on the properties of the photopolymerized layer, after laser recording of the image on the mask layer and exposure, processing can be carried out either by washing out or by thermal development of unpolymerized FPC.
Compression sleeves are used when printing from thin printing plates. The surface of the sleeve has high compression properties, due to which, under printing pressure, small printing elements are partially pressed into the compression layer of polyurethane elastomer. As a result, the die is pressed in less and it accounts for a greater specific pressure (Fig. 2.15). This allows you to print images of different nature from one form without much pulling apart.
The advantages of seamless forms are high print quality, accurate registration, high printing speed, and the ability to control the placement of repeating images (repeats) on the form. To generate seamless (endless) images, appropriate software and rasterization algorithms are required. The results of recording information are greatly influenced by the parameters of the sleeves (diameter range, weight characteristics) and the optical-mechanical equipment of the device, which provides the required stroke length of the focusing lens. Interfacing the laser recording device with equipment for subsequent processing makes it possible to create a single automated production line for the production of sleeve molds.
For the production of printing plates by laser engraving, plate cylinders or sleeves coated with an elastomer are used. The composition of rubber coatings includes polymers (for example, ethylene propylene rubber, acrylonitrile butadione rubber, natural and silicone rubbers), fillers (carbon black, chalk), initiators and accelerators (sulfur, amides and peroxides), pigments, dyes, plasticizers and other components. Form cylinders have a generatrix length of up to several meters and a diameter of up to 0.5 m.
Preparation of the plate cylinder begins with mechanical cleaning of the old coating and sandblasting of the surface of the core. An adhesive layer is applied to the cleaned surface, the composition of which is selected depending on the material of the rod and the composition of the elastomer. An elastomer plate with a thickness of 3 to 10 mm is applied to the adhesive layer and wrapped with bandage tape. The cylinder is placed in an autoclave, where it is vulcanized at a pressure of 4-10 bar for several hours in an atmosphere of steam or hot air. After removing the bandage tape, the surface of the cylinder is turned and ground. The dimensional parameters and hardness of the plate cylinder are controlled.
Elastomeric forms, engraved by a gas laser, are manufactured for printing line and raster images with a relatively low lineature (up to 36 lines/cm). This is due to the fact that the elastomer is removed using laser radiation with a spot size of an elementary point of about 50 microns. The large divergence of the CO2 laser beam does not allow recording images with a high lineature. When the engraving mode is correctly selected, if the spot size is 1.5 times the theoretical dot size, there will be no raw material left between adjacent lines of the recorded image. To obtain an elementary point of 10-12 microns in size, necessary to reproduce an image of high lineature (60 lines/cm), a laser radiation spot with a diameter of 15-20 microns is required. This can be achieved by using an Nd:YAG laser using special mold materials.
The widespread use of lasers with a solid active substance and laser diodes will be facilitated by the creation of shaped materials (polymers) that have the necessary printing properties (resistance to printing ink solvents, hardness, circulation resistance) and allow for high productivity of the direct laser engraving process.
Engraving of forms is carried out in a laser engraving installation. As the plate cylinder rotates, the laser beam moves along the cylinder axis, forming an image in a spiral. The spiral stroke is usually 50 µm. Synchronization of the movement of the plate cylinder and the laser, as well as control of the laser radiation is carried out using a computer.
The radiation emitted by the laser is directed using a system of mirrors onto a lens, which focuses the beam on the surface of the plate cylinder (Fig. 2.16). Depending on the radiation power and technological parameters, the engraving depth can be set from several micrometers to several millimeters. When exposed to laser light, the elastomer is burned off and vaporized in a process similar to sublimation, and the resulting gaseous waste and particulate material is sucked off and filtered. The laser-engraved printing form is cleaned of combustion products remaining on the surface and subjected to control.
