Production of photopolymer printing forms. Production of letterpress forms based on photopolymer compositions

Anxiety is a child of evolution

Anxiety is a feeling familiar to absolutely everyone. Anxiety is based on the instinct of self-preservation, which we inherited from distant ancestors and which manifests itself in the form of a defensive reaction “Flight or fight”. In other words, anxiety does not arise on empty place, but has an evolutionary basis. If at a time when a person was constantly in danger in the form of an attack by a saber-toothed tiger or an invasion of a hostile tribe, anxiety really helped to survive, then today we live in the safest time in the history of mankind. But our instincts continue to operate at a prehistoric level, creating many problems. Therefore, it is important to understand that anxiety is not your personality flaw, but an evolutionary mechanism that is no longer relevant in modern conditions. The disturbing impulses that were once necessary for survival have now lost their purpose, turning into neurotic manifestations that significantly limit the life of anxious people.

Photopolymer molds from liquid photopolymerizable materials (LPFM) appeared in 1969 in Japan. Photopolymerizable plates from solid photopolymerizable materials (TFPM) 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. Improving the properties of TFPM has led to a simplification of the analog mold manufacturing technology letterpress, as well as to the development of water wash plates, such as Nyloprint WD, WM, and the water wash 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) produced flexographic forms on a steel substrate for newspaper printing Newsflex-60 and high-performance plate equipment.

Improving the printing and technical properties of photopolymer flexographic forms was due to the development and use of thin printing plates with high rigidity. Sleeve technology has been developed since the 90s of the XX century. Thanks to the release by Rotec of sleeves with rigid and compressible surfaces. Fastening on a sleeve of a flexographic form, made including on a thin plate, made it possible to significantly improve the quality of printing.

The development of solvent wash solutions that do not contain chloride hydrocarbons has significantly improved the environmental performance of the plate process for the production of flexographic printed forms.

The introduction in 1999 of the FAST technology (DuPont) for the 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 of creating a printing plate 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 plate material (mainly rubber) by engraving controlled by analog information carriers. The method of manufacturing rubber molds by laser engraving has been used in the form of two of the 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 an image bearing shaft. The main stages in the production of forms by laser engraving with masking are: rubberizing the plate cylinder; polishing the rubber surface; wrapping the cylinder with copper foil, the edges of which are joined end-to-end; applying a copy layer to the foil; photocopying; copper etching in areas corresponding to blank elements of the form, to obtain an engraving mask; engraving with CO2 laser; removing the mask from the surface of the form.

Digital technologies for the manufacture of flexo printing plates have been widely developed since 1995 as a result of the creation of photopolymerizable masked plates by DuPont.

In 2000, at the Drupa exhibition, BASF presented a machine for direct laser engraving of flexo and letterpress forms based on a 250 W CO2 laser for engraving specially designed 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 in 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 µm or less.

Flexographic photopolymer printing plates can be divided depending on the physical state of the plate material - photopolymerizable composition (FPC), into forms made from solid and liquid FPC. In digital technologies, molds from a solid composition are used.

By design, the following flexographic forms are distinguished:

• lamellar single-layer, consisting of a single elastic material, such as rubber, rubber or photopolymer;
• lamellar two- and three-layer, in which the layers are distinguished by elastic properties, which make it possible to improve the deformation characteristics of printing plates;
• 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 means of laser action on the receiving layer of the form material, followed by processing, and forms obtained by direct engraving of rubber or polymer forms.

Depending on the form material, flexographic forms made using digital technologies are classified into photopolymer and elastomeric (rubber) forms. Photopolymer plates, compared to elastomeric plates, are distinguished by the stability and quality of reproduction of high-line images, but are less resistant to esters and ketones present in printing inks.

The production of engraved forms can be carried out on plate plates mounted on a plate cylinder or sleeve, as well as on seamless plate materials made of rubber, polymer or photopolymer mounted on a metal rod, 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 photopolymerizable plate and the manufacturing process. Forms created on the most widely used single-layer photopolymerizable plates have printing and blanking elements from a photopolymerized layer located on a dimensionally stable substrate. Laser engraved elastomeric molds consist primarily of vulcanized rubber.

Technological scheme for the manufacture of 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;
• the main exposure of the photopolymerizable plate through the integral mask;
• washing out (or thermally removing) the non-polymerized layer;
• mold drying;
• finishing (finish - end);
• additional exposure.

Sometimes, in practice, the technological process begins with recording an image on a mask layer, and the exposure of the reverse side of the plate is carried out after the main exposure.

When using thermal development according to FAST technology, after the main exposure of the plate, thermal removal of the non-polymerized layer follows, followed by finishing and additional exposure of the form.

A feature of the production of cylindrical molds 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 form with the application of a mask layer on the surface of the photopolymerizable layer before laser writing. Further operations are performed in accordance with the outlined scheme.

Digital technology for manufacturing elastomeric printing plates by direct laser engraving contains the following stages:

• preparation of the plate cylinder, including rubberizing its surface;
• preparation of the plate cylinder surface for laser engraving, which consists in turning and grinding the rubber coating;
• direct laser engraving;
• cleaning the engraved surface of the cylinder from combustion products.

A feature of the technology when using a sleeve with a rubber coating, designed specifically for laser engraving, is the absence of the need to prepare the surface for engraving and the reduction of operations in the scheme technological process.

Formation of printing elements photopolymer forms, made by digital technology on plates or cylinders with a mask layer, takes place during the main exposure. At the same time, due to the directed light scattering of the light flux penetrating through the FPC, the 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 termination to form 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 for their formation during the main exposure. According to analog technology, when exposed through a negative, the radiation, before reaching the photopolymerizable layer, passes through several media (pressure film, photoform), scattering at their boundaries, which leads to the formation of a printing element with a larger area and with a wider base.Reducing light scattering during the main exposure of the photopolymerizable 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 by digital technology (Fig. 2.3), which is optimal for stabilizing and reducing dot gain during printing..gif" border="0" align="absmiddle" alt="(!LANG:with the relative area of ​​raster elements in a digital data array (Fig. 2.4).

When mounting a printing plate on a plate cylinder or sleeve, the height of the raster areas of the image increases due to the stretching of the form. The raster elements of printed forms obtained using analog technology protrude above the spot ones, which leads to strong dot gain in the highlights. When using digital technology, the pressure on the raster areas of the image is less than on the plate, which favorably affects the reproduction of an image of a different nature (Fig. 2.5).

An important task in the formation of printing elements of photopolymer forms is to impart properties to their surface that make it possible to ensure good perception and release of ink in the printing process and high wear resistance. In this case, the physicomechanical properties of the relief are of decisive importance, which are achieved during post-exposure and finishing due to, respectively, photopolymerization in the thickness of the FPC and surface oxidation. The result of additional exposure is the creation of a homogeneous structure of the printing plate with high printing performance.

Formation of whitespace elements methods of washing out or thermal development of photopolymer molds made using digital mask technology does not differ significantly from the processes for creating photopolymer molds using analog technology.

In flexo printing, the printing plate experiences elastic deformations during the printing process. These deformations, which depend in particular on the material to be printed, the thickness and structure of the printing plates, must be taken into account when choosing the minimum allowable depth of the relief of the printing plate. When choosing the depth of the relief, the nature of the image (line or raster), printing conditions, and the thickness of the plate are taken into account. If there is a high-line image on the form, a shallower relief depth is recommended in order to avoid loss of small raster elements. In the case of using rough and dusty printed materials, a large depth of gap elements is required.

The formation of gap elements of photopolymer forms occurs in the process of washing out under the action of a wash solution (when using a water wash FPC, water is used). The rinsing process is influenced by hydrodynamic factors such as the pressure of the rinsing brushes and the way the rinsing solution is supplied, as well as its composition and temperature.

The process of creating gap elements begins with solvation with a gradual transition of the PPC into a gel-like layer, followed by unlimited swelling of the polymer, and ends with the complete removal of the PPC from unexposed areas.

Under the action of the wash solution 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.

Formation of blank elements of flexographic forms can occur when the non-polymerized FPC is removed using a thermal process. The process is implemented due to the presence of thermoplastic properties of unexposed PPC, which are lost under the action of UV-A radiation. In the process of exposure, a spatial network is formed in the polymer, and the FPC loses the ability to pass into a viscous-flow state.

Removal of FPC from the gap elements of the forms is carried out by local heating of the surface of the form by infrared radiation. In this case, the non-polymerized part of the FPC passes into a viscous state. The absorption of the molten polymer occurs due to capillary absorption and is carried out using a 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 plate thickness. The mask layer is removed from the gap elements by washing out or by thermal development along with the uncured layer.

With direct laser engraving, a flexographic form is made in one technological step on one equipment. Form 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, while combustion products are formed. Under the action of a laser that provides 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 1 mm in diameter.

The embossing occurs as a result of the physical removal of the elastomer from the gap elements of the form. To create the desired profile of the printing element in direct laser engraving, special modulation modes of laser radiation or a method of processing plate material in several passes are used. The whitespace elements deepen to the 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 the printing elements obtained under the action of UV radiation (Fig. 2.7). The side face of the printing element of the laser engraved form is directed perpendicular to the plane of the printing element, which gives certain advantages in the printing process, providing a lower degree of dragging 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 print stability and form stability in the printing process.

Varieties of form plates. Flexographic printing plates differ in structure, development method, composition of the FPC, the nature of the wash solution, the thickness and hardness of the plate, and other features. According to the method of developing the image, they are divided into plates for thermal development and washout plates. The latter, manifested by leaching, depending on the nature of the leachable solution, are divided into solvent and water-washable.

In the digital technology for the manufacture of flexographic forms, plates are used that, in addition to the photopolymerizable layer (FPS), have an additional recording mask layer (Fig. 2.8, a). It serves to create a primary image formed with a laser, and is a mask for subsequent exposure of the photopolymerizable plate to UV radiation. The mask layer, which is not sensitive to UV radiation and thermally sensitive in the IR range of the spectrum, has a thickness of 3-5 μm 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; film caching with FPC applied to the substrate using an extruder with constant control of the layer thickness; smoothing the ribbon of shaped material with a calender; preliminary exposure from the side of the substrate; 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 printing plates, a layer based on aluminum with a thickness of 1-2 microns is used, which makes it possible to eliminate radiation scattering inside the mask layer.

The main characteristics of form 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, ordinary - from 1.70 to 2.84 mm and thick - from 3.18 to 6.5 mm. The thickness of the substrate of thin plates is 0.18 mm, thick - 0.13 mm.

If several printing plates are to be placed on the surface of the plate cylinder, special attention must be paid to the control of plate thicknesses, since thickness differences can adversely affect the pressure distributions during printing. The thickness tolerance of one plate is + 0.013 mm, different plates is ± 0.025 mm.

Hardness is the most important characteristic of the plate, which makes it possible to indirectly judge the wear resistance of the future printing plate and its reproduction and graphic characteristics. It is customary to indicate the hardness of a photopolymerizable plate in units of hardness (in degrees Shore> defined "> The choice of printing 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.

Reproduction of an image containing small elements requires the use of thin shaped plates with high hardness. The necessary deformations during printing are achieved due to the elastic material located on the plate cylinder or sleeve. To reproduce a raster image, plates with a higher hardness are used than for printing a die. This is due to the fact that raster elements are more sensitive to pressure during printing. When the mold comes into contact with the anilox roll, with a strong deformation of small raster elements, the ink may transfer to the slope of the raster dot. Insufficient plate hardness can lead to increased drag.

For printing on rough, dusty papers, thick plates are chosen that provide a deeper relief on the printing plate; when using corrugated cardboard, thick plates with low hardness are used. If the printing press has a built-in device in which the corona treatment of the film is carried out, the printing plates for printing on polymer films are selected taking into account ozone resistance. These characteristics are specified, as well as the resistance of the plates to certain organic solvents (eg ethyl acetate) and the recommended types of printing inks. When choosing a printing plate, its compatibility with printing ink (based on water, organic solvents, UV curable) is taken into account.

Form plates are selected taking into account the format of the printing press and the gap (distance) in the printing pair.

The plates used should provide the possibility of obtaining the necessary printing and operational characteristics of future forms, as well as compliance with environmental requirements in their manufacture.

Image data is stored as PostScript, TIFF, or PCX files and is used to display information on the plate. The Raster Processor (RIP) converts the tonal values ​​for each color into larger or smaller bitmap dots. Modern raster processors have a built-in feature that allows you to save special calibration curves so that when they are written, they are superimposed on the output data.

At the prepress stage, the size of the minimum printed dot must be known so that there are no dots on the form below the minimum value. This is done to prevent gradation distortion on the print in the highlights of the image. The size of the minimum dot depends on the printing press, plate thickness and rigidity, and substrate properties. Thin forms with shallow relief are able to reproduce a smaller dot than thick ones. Forms made on more rigid plates also reproduce a smaller dot area. The minimum point size setting is set in the drag compensation program.

RIP controls the ratio minimum size printing element and cell size of the anilox roll. The need for control is caused by the phenomenon of abnormal ink transfer, where smaller print elements can pick up more ink when they enter the anilox roll cell.

The size of the minimum print element in a one-bit raster image file obtained after rasterization with RIP differs significantly from the size of the print element on a printing plate.

Tonal distortion compensation for digital technology includes compensation for plate and printing processes. In the manufacture of printing plates, due to the inhibitory effect of oxygen, gradation distortions occur during exposure. Their compensation is carried out using flexographic RIPs and makes it possible to make up for the reduction in the size of printing elements at the stage of generating a TIFF file transmitted when writing a mask (Fig. 2.10). To do this, to form a printing element of the desired size, from the relative area of ​​the raster dot in the file. RIP recalculates the raster dot sizes 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, angle of rotation of the raster structure and the selected compensation curve.

As a rule, in the software or hardware of devices (most often in RIP), compensation for image elongation or compression is provided. Such distortion of the image 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 press and the thickness of the printing plate, is taken into account in the RIP during the screening stage. So, for example, in the 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 accurate positioning of images presented as separate files. In this way it is possible to mount, for example, repetitive small images typical for label printing.

The image is recorded on a form plate with a mask layer using various types of lasers. For this purpose, a fiber laser, a YAG laser, as well as laser diodes are used.

YAG and fiber lasers differ from diode radiation sources in greater stability and lower beam divergence. As a result, points of stable dimensions and the required round shape are created on the mask layer of the plate. Systems for exposing flexographic forms provide image recording with lineature up to 200 lpi. Resolution can be changed within 1800-4000 dpi. The exposure speed is up to 4 m2/h with a spot size of 15 µm.

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 a fiber and YAG laser, which leads to a shallow depth of field of the laser beam in the material processing zone (Fig. 2.12). Lasers operating in the single-mode mode have the greatest depth of field, in which the best radiation parameters are achieved. In the powerful multi-mode mode, which allows high-speed image recording to be realized, the parameters are reduced and the depth of field is reduced. With an insufficient depth of field, deviations in the plate thickness can lead to a change in the diameter of the laser exposure spot and recording defects.

The choice of optimal modes for making 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 an integral mask has been obtained on its surface.

The test object is used to determine the exposure time. Its content is considered on 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 a relative area of ​​raster dots 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 applied lineature does not meet the needs of the user, then 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 do not change during the conversion process. To determine the optimal exposure time, several copies of the test object, usually at least ten, are successively recorded on one photopolymerizable plate with a mask layer. To avoid discrepancy, a single RIP-rendered copy is replicated using the interface of an appropriate platemaker.

Testing of subsequent operations of the technological process is carried out in the same way as in the manufacture of photopolymer molds using analog technology.

The reverse side of the plate is exposed in order to form the base of the printing plate. By increasing the photosensitivity of the FPS as a result of exposure of the reverse side of the plate, the conditions for the formation of printing elements during the main exposure and their adhesion to the substrate are improved. Exposure is carried out through the plate substrate (see Fig. 2.8, b). The radiation, penetrating into the depth of the PPC, 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 form. The thickness of the base is the difference between the thickness of the form and the maximum depth of the space elements. The photopolymerized base limits the penetration of the wash solution and therefore 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. Exposure too short may result in washout of small print elements of the form due to insufficient base polymerization and, as a result, insufficient resistance to washout solution. Excessive exposure time can create an overly thick base plate and make it difficult to form gaps of the required depth. Determining the exposure time of the reverse side of the plate is carried out by testing. Separate sections of the form plate on the reverse side are subjected to dosed exposure, given by different exposure times. It depends on the thickness of the plate and can be, for example, 10, 20, 30 seconds or more. Usually exposed 8 steps. The necessary exposure time for the back side of the plates is determined by a graph relating the time to the depth of gaps obtained after exposure and washout.

The installation of laser image recording includes: optical device; carbon fiber exposure cylinder or sleeve cylinder; a workstation with a service unit and a program for controlling the exposure unit; a vacuum device that secures the form plate during recording; waste extraction system that occurs when removing the mask layer. The quality of the recording depends on the addressing - the ability of the laser to be controlled in the totality of its design features, scanning and focusing of the laser spot.

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, it is ablated. An integral mask is formed on the surface of the photopolymerizable layer, which carries 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 multipath technology improves system performance.

The form plate is fixed on the drum and held on it with the help of vacuum. When exposing thick plates, their mass reduces the number of revolutions of the drum.

Obtaining a clear image on the integral mask depends on the structure and specifications mask layer (homogeneity, high optical density, good adhesion to the photopolymerizable layer), as well as the correct setting of the depth of exposure to the laser beam. The system is adjusted to this parameter by preliminary testing. The built-in dynamic focusing device allows you to compensate for changes in the thickness of the photopolymerizable plate layers and improve recording parameters.

Carrying out subsequent operations of the technological process has no fundamental differences from their implementation in the manufacture of flexographic photopolymer printing plates using analog technology. The difference lies in the fact that the main exposure is carried out without 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 from 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 PPC, and the high sharpness of the mask edges and the inhibitory effect of oxygen make it possible to achieve the required value of the steepness of the profile of the printing elements (see Fig. 2.8, d).

If the mold manufacturing process begins with laser image recording on a plate, then to ensure the safety of the digital integral mask, the sequence of operations for 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, first the main exposure is carried out, and then the reverse side of the plate is exposed. The main exposure time is set using the 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 a subsequent increase in exposure. In this case, at the lowest exposure, the largest gradation interval on the printed form is provided.

With insufficient exposure, thin lines on the form become waviness, and an “orange peel” effect appears on the surface of the plate, leading to premature wear of the form. With excessive main exposure, the image on the form loses its clear contours, the contrast of the image in the shadows decreases, the depth of the white space elements is insufficient.

Removal of unpolymerized composition. The polymer solvents are subject to 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 should be characterized by a low degree of volatility, have low cost, fire safety and non-toxicity. Solvent wash solutions are a mixture of aliphatic or aromatic hydrocarbon and alcohol. Chlorine-containing solutions are of limited use due to toxicity. Wash solutions containing organic solvents are regenerated in special units (evaporators) that can be connected to wash machines. This allows you to organize a closed cycle of the leaching process, which reduces environmental pollution.

The purpose of washing out is to reveal the latent relief image obtained during exposure, and the formation of blank elements of the form. The essence of the process lies in the fact that the rate of diffusion of developing solutions into the non-polymerized areas of the plate is several times higher than into the photopolymerized ones. To increase the selectivity of development, substances (for example, butanol or isopropanol) are introduced into developing solutions that reduce the swelling of irradiated film-forming photopolymers.

Excessive washout time causes swelling of the relief, which, together with insufficient main exposure, can lead to a violation of the surface structure (“orange peel”).

As the solution is saturated with the reagents that are part of the FPC, the leaching capacity of the solution decreases. The mode of regeneration of the wash solution depends on the size of the plate and the depth of the gaps. It is determined from the calculation of approximately 10-15 liters of washable solvent solution per 1 m2 of the 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 washout processor brushes 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 plates subjected to the same exposure (with part of the plate surface protected by a template) are washed out for different times, selected taking into account the thickness of the plate. After drying and measuring the thicknesses of the washed out and unwashed areas, a dependence is obtained, which determines the washout time required to achieve the required relief depth. In this case, the required relief depth plus 0.2-0.3 mm corresponds to the optimal time. 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 the form in the processor (Fig. 2.14). In automatic processors of continuous action, the corresponding washout time value is entered into the program.

During the thermal development of a relief image using the FAST technology, the exposed plate is fixed on the drum of a thermal processor and is fed to an infrared radiation source. The required relief depth, which depends, in particular, on the thickness of the plate used, is achieved with 10-12 cycles of contact of the form, locally heated to t = 160 ° C, with absorbent non-woven material (see Fig. 2.6).

Form drying. The purpose of drying is to remove liquid from the photopolymerized mold layer using heat. When washed out, this layer is impregnated with a wash solution, the image relief swells and softens. The relative content of the 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 swelling capacity of the material, the washout time, the degree of crosslinking of the polymer, the nature and temperature of the solvent. The swelling of the relief of the form occurs unevenly, its degree depends on the nature of the image. Screened areas absorb more solvent than plates. The effect of the nature of the wash solution on the drying time is related to the degree of swelling of the photopolymer layer and the volatility of the solvent included in the solution.

During the drying process, the solvent molecules move from the inner layers of the material to the outer ones and the subsequent migration from the mold surface into the heat carrier medium. When drying with warm air heated to a temperature of 65 ° C, the solvent is removed from the mold surface due to convective diffusion. To increase the rate of internal diffusion of the solvent, 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 material of the form, the size and condition of its surface, the temperature of the coolant, its saturation with solvent vapors and the speed of movement relative to the form.

Drying is the longest operation in the manufacture of a flexo 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 UV-C treatment, the mold must be cooled down, because premature processing may fix the 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 in order to eliminate stickiness, which is formed due to the presence of a thin layer of highly viscous liquid on the surface. It is macromolecules of thermoplastic elastomer or other polymer dissolved or mixed with molecules of unpolymerized monomers or oligomers. The components that did not enter into the photopolymerization reaction during exposure diffuse to the surface during the washing process, causing it to stick.

Elimination of stickiness can be achieved in two ways: surface treatment with chemical reagents, in particular bromide-bromate solution, or UV-C irradiation of the surface (see Fig. 2.8, e). In the first method, bromine, entering into an addition reaction, reduces the concentration of unsaturated double bonds and contributes to the conversion of unsaturated monomers with a low boiling point into saturated bromo derivatives, which, due to more high temperature boiling are solid compounds. However, chemical finishing using solutions of reactive compounds is environmentally unsafe.

The most widely used is finishing by UV irradiation of the form in a gaseous medium. In the process of such treatment with radiation having high energy and low penetrating power, the stickiness of the surface layer of the printing plate is eliminated. For finishing, installations equipped with tubular UV lamps with a maximum radiation in zone C with a wavelength of 253.7 nm are used. Too long processing makes the surface of the mold brittle and reduces its ink susceptibility. The duration of UV-C treatment is affected by the type of plate, the nature of the wash solution and the duration of the preceding 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 may be less than or equal to the main exposure time.

Form control. The quality indicators of flexographic plates include the presence of printing elements of the required size, shape and surface structure, a certain relief height corresponding to the nature of the image on the printing plate, as well as the necessary adhesion to the substrate.

Possible defects in forms made using digital technology include the appearance on the form (and possibly subsequently in printing) of a single-color moiré due to the cyclic variety of forms of printing elements corresponding to the same gray level, i.e., raster dots in areas of constant tone have the same area, but different shape. 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 washout processor. It can be avoided by optimizing the screening algorithms and eliminating the difference in the shape of the printing elements.

Digital technology for manufacturing 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 the 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 plates. To obtain seamless photopolymer printing plates, the plate is exposed from the reverse side, then mounted around the sleeve, the edges of the plate are tightly pressed together and the photopolymer is melted to hold the edges of the plate together. After that, it is polished to the required thickness in a special machine, and a registering 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 shaped process. Forms made by technology computer - printed sleeve(CTS) do not require compensation for distortion associated with shape stretching.

Cylinder seamless (sleeve) forms (digisleeve) are made on a polymeric form material in the form of a flexible hollow cylinder, which is pulled over a sleeve, and then it is processed on equipment designed for cylindrical forms. Depending on the properties of the photopolymerizable 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 the unpolymerized PPC.

Compression sleeves are used when printing from thin plates. The surface of the sleeve has high compression properties, due to which, under pressure during printing, small printing elements are partially pressed into the compression layer of polyurethane elastomer. As a result, the plate is pressed in less and it accounts for more specific pressure (Fig. 2.15). This allows you to print different images from one form without strong dragging.

The advantages of seamless forms are high print quality, accurate registration, high speed printing, the ability to control the placement of repeating images (rapports) on the form. For the formation of seamless (endless) images, appropriate software and screening algorithms. 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 length of the focusing lens. The pairing of the laser recording device with the equipment for subsequent processing makes it possible to create a single automated technological line for the manufacture of sleeve molds.

Plate cylinders or sleeves coated with elastomer are used to make printing plates by laser engraving. The composition of rubber coatings includes polymers (for example, ethylene propylene rubber, acrylonitrile butadione rubber, natural and silicone rubber), fillers (carbon black, chalk), initiators and accelerators (sulfur, amides and peroxides), pigments, dyes, plasticizers and other components. Form cylinders have a length along the generatrix of up to several meters and a diameter of up to 0.5 m.

The preparation of the plate cylinder begins with mechanical cleaning of the old coating and sandblasting the surface of the rod. 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 cured 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 polished. Dimensional parameters and hardness of the plate cylinder are controlled.

Elastomeric forms, engraved by a gas laser, are made for printing line and raster images with a relatively low lineature (up to 36 lines/cm). This is due to the fact that the removal of the elastomer is carried out using laser radiation with a spot size of an elementary point of about 50 μm. The large divergence of the CO2-laser beam does not allow recording an image with a high lineature. At right choice engraving mode, if the spot size is 1.5 times the theoretical dot size, no raw material remains between adjacent lines of the recorded image. To obtain an elementary dot with a size of 10–12 μm, which is necessary for reproducing an image of high lineature (60 lines/cm), a spot of laser radiation with a diameter of 15–20 μm is required. This can be achieved by using an Nd:YAG laser using special shaped 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 solvents of printing inks, hardness, runtime) and make it possible to ensure high productivity of the direct laser engraving process.

Forms are engraved in a laser engraving machine. During the rotation of the plate cylinder, the laser beam moves along the axis of the cylinder, forming an image in a spiral. The spiral stroke is typically 50 µm. Synchronization of the movement of the plate cylinder and the laser, as well as the control of laser radiation, are carried out using a computer.

The radiation emitted by the laser with the help of a system of mirrors is directed to the 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. Under the influence of laser radiation, the elastomer is burned out and evaporated in a process similar to sublimation, and the resulting gaseous waste and material particles are sucked off and filtered. The printed plate engraved by the laser is cleared of the combustion products which have remained on a surface and is exposed to control.

We display forms for flexographic printing

Dr. tech. sciences, prof. MGUP im. Ivan Fedorov

A type of letterpress that is widely used for printing labels and packaging products from paper, foil, plastic films, as well as for printing newspapers, is flexography. Flexographic printing is carried out with elastic rubber or highly elastic photopolymer printing plates with fluid fast-setting inks.


In the printing apparatus of a flexographic printing machine, rather liquid ink is applied to a printing plate fixed on a plate cylinder, not directly, but through an intermediate rolling (anilox) roller. The roller is made from steel pipe, which can be covered with a layer of copper. A raster grid is applied to this surface by etching or engraving, the deep cells of which are made in the form of pyramids with a sharp top. The raster surface of the anilox roller is usually chrome-plated. The transfer of ink from the ink box to the printing plate is carried out by a rubber (ductor) roller to the anilox roller, and from it to the printing elements of the form.

The use of resilient printing plates and low-viscosity fast-setting inks makes it possible to print almost any roll material at high speed, to reproduce not only line elements, but also single- and multi-color images (with screening lineature up to 60 lines/cm). Slight typing pressure ensures b about Greater circulation stability of printed forms.

Flexography is a direct printing method in which ink is transferred from a plate directly onto the printed material. In this regard, the image on the printing elements of the form must be mirrored in relation to the readable image on paper (Fig. 1).

In modern flexographic printing, photopolymer printing plates (FPF) are used, which are not inferior to offset ones in terms of printing and technical and reproduction and graphic properties, and, as a rule, surpass them in run resistance.

Solid or liquid photopolymerizable compositions are used as photopolymer materials. These include solid or liquid monomeric, oligomeric or monomeric-polymer mixtures capable of changing the chemical and physical state under the action of light. These changes lead to the formation of solid or elastic insoluble polymers.

Solid photopolymerizable compositions (SFPs) retain a solid state of aggregation before and after the production of a printing plate. They are delivered to the printing company in the form of photopolymerizable shaped plates of a certain format.

The structure of photopolymerizable plates for flexographic printing is shown in fig. 2.

Liquid photopolymerizable compositions (LFP) are supplied to printing companies in containers in liquid form, or they are made directly at the enterprises by mixing the initial components.

The main technological operation in the manufacture of any FPF, during which a photopolymerization reaction occurs in the photopolymerizable composition and a latent relief image is formed, is exposure (Fig. 3 a) of the photopolymerizable layer. Photopolymerization occurs only in those parts of the layer that are exposed to UV rays and only during their exposure. Therefore, negative photoforms and their analogues in the form of a mask layer are used for exposure.

Rice. Fig. 3. Technological operations for obtaining photopolymer printing plates on solid photopolymerizable plates: a - exposure; b - washing out of gaps; c - drying of the printing plate; d - additional exposure of printing elements

The development of a relief image, as a result of which non-polymerized areas of the photopolymerizable plate are removed, is carried out by washing them out with an alcohol, alkaline solution (Fig. 3 b) or water depending on the type of plates, and for some types of plates - dry heat treatment.

In the first case, the exposed photopolymerizable plate is processed in the so-called solvent processor. As a result of the washout operation (see Fig. 3 b) of non-polymerized sections of the plate, a relief image is formed on the form with a solution. Washout is based on the fact that in the process of photopolymerization, the printing elements lose their ability to dissolve in the wash solution. After washing, drying of photopolymer forms is required. In the second case, processing is carried out in a thermal processor for processing photopolymer forms. Dry heat treatment completely eliminates the use of traditional chemicals and wash solutions, reduces the time of obtaining molds by 70%, since it does not require drying.

After drying (Fig. 3 in) the photopolymer form is subjected to additional exposure (Fig. 3 G), which increases the degree of photopolymerization of printing elements.

After additional exposure, photopolymer plates based on TFP for flexo printing have a shiny and slightly sticky surface. The stickiness of the surface is eliminated by additional processing (finishing), as a result, the form acquires the properties of stability and resistance to various solvents of printing inks.

Finishing can be done chemically (using chloride and bromine) or by exposure to ultraviolet light in the range of 250-260 nm, which has the same effect on the form. With chemical finishing, the surface becomes matte, with ultraviolet - shiny.

One of the most important parameters of photopolymer printing plates is the profile of the printing elements, which is determined by the angle at the base of the printing element and its steepness. The profile determines the resolution of photopolymer printing plates, as well as the adhesion strength of the printing elements to the substrate, which affects the runtime. The profile of the printing elements is significantly affected by the exposure modes and the conditions for washing out white space elements. Depending on the exposure mode, the print elements may have a different shape.

With overexposure, a flat profile of the printing elements is formed, which ensures their reliable fixation on the substrate, but is undesirable due to the possible decrease in the depth of gaps.

With insufficient exposure, a mushroom-shaped (barrel-shaped) profile is formed, leading to the instability of the printing elements on the substrate, up to the possible loss of individual elements.

The optimal profile has an angle at the base of 70 ± 5º, which is the most preferable, as it ensures reliable adhesion of the printing elements to the substrate and high image resolution.

The profile of printing elements is also influenced by the ratio of exposures of preliminary and main exposure, the duration of which and their ratio are selected for different types and batches of photopolymer plates for specific exposure installations.

Currently, for the manufacture of photopolymer printing plates for flexographic printing, two technologies are used: “computer-photoform” and “computer-printing plate”.

So-called analog plates are produced for the “computer-printing plate” technology, and digital plates for the “computer-printing plate” technology.

In the manufacture of photopolymer forms of flexographic printing based on TFPK (Fig. 4), the following main operations are performed:

• preliminary exposure of the reverse side of the photopolymerizable flexographic plate (analogue) in the exposure unit;
• the main exposure of mounting the photoform (negative) and the photopolymerizable plate in the exposure unit;
• processing of a photopolymer (flexographic) copy in a solvent (washout) or thermal (dry heat treatment) processor;
• drying of the photopolymer form (solvent-washout) in a drying device;
• additional exposure of the photopolymer form in the exposure unit;
• additional processing(finishing) photopolymer form to eliminate the stickiness of its surface.

Rice. Fig. 4. Scheme of the process of manufacturing photopolymer molds based on TPPC using the “computer-photoform” technology

Exposing the reverse side of the plate is the first step in the manufacture of the form. 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. Correct exposure of the reverse side of the plate does not affect the printing elements.

The main exposure of the photopolymerizable plate is carried out by contact copying from a negative photoform. On a photoform intended for making molds, the text must be mirrored.

Photoforms must be made on a single sheet of film, since composite montages glued with adhesive tape, as a rule, do not provide a reliable fit of the photoform to the surface of the photopolymerizable layers and can cause distortion of the printing elements.

Before exposure, the photoform is applied to the photopolymerizable plate with the emulsion layer down. Otherwise, a gap equal to the thickness of the base of the film is formed between the plate and the image on the photoform. As a result of the refraction of light in the basis of the film, a strong distortion of the printing elements and copying of the raster areas can occur.

To ensure close contact of the photoform with the photopolymerizable material, the film is matted. Microroughnesses on the surface of the photoform allow you to completely quickly remove air from under it, which creates a tight contact between the photoform and the surface of the photopolymerizable plate. For this, special powders are used, which are applied with a cotton-gauze swab with light circular movements.

As a result of the processing of photopolymer copies based on solvent washout plates, the monomer that has not been exposed and polymerized is washed out - it dissolves and is washed off from the plate. Only areas that have undergone polymerization and form a relief image remain.

Insufficient washout time, low temperature, improper brush pressure (low pressure - bristles do not touch the surface of the plate; high pressure - bristles arch, reduced washout time), low solution level in the wash tank leads to too fine relief.

Excessive washout time, high temperature and insufficient solution concentration lead to too deep relief. The correct washout time is determined experimentally depending on the thickness of the plate.

When washing out, the plate is impregnated with a solution. The polymerized image relief swells and softens. After removing the wash solution from the surface with non-woven napkins or a special towel, the plate must be dried in the drying section at a temperature not exceeding 60 °C. At temperatures above 60°C, register problems can occur because the polyester backing, which is dimensionally stable under normal conditions, begins to shrink.

Swelling of the plates when washed out leads to an increase in the thickness of the plates, which, even after drying in the dryer, do not immediately return to their normal thickness and must be left for another 12 hours in the open air.

When using heat-sensitive photopolymerizable plates, the manifestation of the relief image occurs by melting the non-polymerized sections of the forms during their processing in a thermal processor. The molten photopolymerizable composition is adsorbed, absorbed and removed with a special cloth, which is then sent for disposal. Such a technological process does not require the use of solvents, and therefore, drying of the developed forms is excluded. In this way, both analog and digital forms can be produced. The main advantage of the technology with the use of heat-sensitive plates is a significant reduction in mold manufacturing time, which is due to the absence of a drying stage.

To give durability, the plate is placed in an exposure unit for additional illumination with UV lamps for 4-8 minutes.

To eliminate the stickiness of the plate after drying, it must be treated with UV radiation with a wavelength of 250-260 nm or chemically.

Analog solvent-washout and heat-sensitive photopolymerizable flexographic plates have a resolution that provides 2-95 percent halftone dots at a screen lineature of 150 lpi, and a print run of up to 1 million prints.

One of the features of the process of manufacturing flat photopolymer forms of flexographic printing using the “computer-photoform” technology is the need to take into account the degree of stretching of the form along the circumference of the plate cylinder when it is installed in the printing machine. The stretching of the mold surface relief (Fig. 5) leads to an elongation of the image on the print compared to the image on the photoform. In this case, the thicker the stretchable layer located on the substrate or stabilizing film (when using multilayer plates), the longer the image.

The thickness of photopolymer forms varies from 0.2 to 7 mm and above. In this regard, it is necessary to compensate for elongation by reducing the image scale on the photoform along one of its sides, oriented in the direction of movement of the paper web (tape) in the printing machine.

To calculate the scale value M photoforms, you can use the stretching constant k, which for each type of plates is equal to k = 2 hc (hc is the thickness of the relief layer).

Print length Lott corresponds to the distance that a certain point on the surface of the mold travels during a complete revolution of the forme cylinder, and is calculated as follows:

where Dfts— diameter of the plate cylinder, mm; hf— thickness of the printing plate, mm; hl— adhesive tape thickness, mm.

Based on the calculated impression length, the necessary shortening of the photoform Δ is determined d(in percent) according to the formula

.

So, the image on the photoform in one of the directions should be obtained with a scale equal to

.

Such scaling of the image on the photoform can be performed by computer processing of a digital file containing information about the imposition or individual pages of the publication.

The production of photopolymer flexographic printing plates using the “computer-printing plate” technology is based on the use of laser methods for processing plate materials: ablation (destruction and removal) of the mask layer from the surface of the plate and direct engraving of the plate material.

Rice. Fig. 5. Stretching of the surface of the printing plate when installed on the plate cylinder: a - printing plate; b - printing plate on a plate cylinder

In the case of laser ablation, the subsequent removal of the non-polymerized layer can be performed using a solvent or thermal processor. For this method, special (digital) plates are used, which differ from traditional ones only in the presence of a mask layer 3-5 μm thick on the surface of the plate. The mask layer is a soot filler in an oligomer solution that is insensitive to UV radiation and thermally sensitive to the infrared range of the spectrum. This layer is used to create the primary image formed by the laser and is a negative mask.

The negative image (mask) is necessary for the subsequent exposure of the shaped photopolymerizable plate with a UV light source. As a result of further chemical processing, a relief image of the printing elements is created on the surface.

On fig. 6 shows the sequence of operations for manufacturing a flexographic plate on a plate containing a mask layer 1 , photopolymer layer 2 and substrate 3 . After laser removal of the mask layer in places corresponding to the printing elements, a transparent substrate is exposed to create a photopolymer substrate. Exposure to obtain a relief image is carried out through a negative image created from the mask layer. Then the usual processing is carried out, consisting of washing out the unpolymerized photopolymer, washing, post-exposure with simultaneous drying and light finishing.

When recording an image using laser systems, the dot size on masked photopolymers is, as a rule, 15–25 μm, which makes it possible to obtain images with a lineature of 180 lpi and higher on the form.

In the manufacture of photopolymer plates in the "computer-printing plate" technology, plates based on solid photopolymer compositions are used, which provide high quality printing plates, the further processing of which occurs in the same way as analog flexo photopolymer plates.

On fig. 7 shows the classification of photopolymerizable plates for flexographic printing based on solid photopolymer compositions.

Depending on the structure of the plate, single-layer and multi-layer plates are distinguished.

Single-layer plates consist of a photopolymerizable (relief-forming) layer, which is located between the protective foil and the lavsan base, which serves to stabilize the plate.

Multi-layer plates designed for high-quality raster printing consist of relatively hard thin-layer plates with a compressible substrate. On both surfaces of the plate there is a protective foil, and between the photopolymerizable layer and the base there is a stabilizing layer, which ensures almost complete absence of longitudinal deformation when the printing plate is bent.

Depending on the thickness, photopolymerizable plates are divided into thick-layer and thin-layer ones.

Thin-layer plates (thickness 0.76-2.84 mm) have high hardness in order to reduce dot gain during printing. Therefore, printing plates made on such plates provide high quality finished products and are used to seal flexible packaging, plastic bags, labels and tags.

Thick-layer plates (thickness 2.84-6.35 mm) are softer than thin-layer ones and provide tighter contact with an uneven printed surface. Printing forms based on them are used for sealing corrugated cardboard and paper bags.

Recently, when printing on materials such as corrugated cardboard, plates with a thickness of 2.84-3.94 mm are more often used. This is explained by the fact that when using thicker photopolymer forms (3.94-6.35 mm) it is difficult to obtain a high-line multicolor image.

Depending on the hardness, plates of high, medium and low hardness are distinguished.

Plates of high hardness are characterized by less dot gain of raster elements and are used for printing high-line works. Plates of medium rigidity allow you to print raster, line and solid works equally well. Softer photopolymerizable plates are used for ink printing.

Depending on the method of processing photopolymer copies, plates can be divided into three types: water-soluble, alcohol-soluble, and plates processed using thermal technology. For machining inserts belonging to different types, it is necessary to use different processors.

The method of laser ablation of the mask layer of photopolymerizable plate materials produces both flat and cylindrical printing plates.

Cylindrical (sleeve) flexographic forms can be tubular, put on a plate cylinder from its end, or represent the surface of a removable plate cylinder installed in a printing machine.

The process of manufacturing flat flexographic printing plates based on solvent washout or heat-sensitive digital photopolymerizable plates with a mask layer using the “computer-printing plate” technology (Fig. 8) includes the following operations:

• preliminary exposure of the reverse side of the photopolymerizable flexographic plate (digital) in the exposure unit;
• transferring a digital file containing data on color separation images of stripes or a full-size printed sheet to a raster processor (RIP);
• digital file processing in RIP (reception, interpretation of data, rasterization of the image with a given lineature and raster type);
• writing the image on the mask layer of the plate by ablation in the forming device;
• main exposure of the photopolymerizable layer of the plate through the mask layer in the exposure unit;
• processing (washing out for solvent-washable or dry heat treatment for heat-sensitive plates) of a flexographic copy in a processor (solvent or thermal);
• drying of the photopolymer form (for solvent-washable plates) in a drying device;
• additional processing of the photopolymer form (light finishing);
• additional exposure of the photopolymer form in the exposure unit.

The process of manufacturing sleeve photopolymer flexographic printing plates by the ablation method (Fig. 9) differs from the process of manufacturing flat plates mainly in the absence of the operation of preliminary exposure of the reverse side of the plate material.

The use of the mask layer ablation method in the manufacture of photopolymer flexo plates not only shortens the technological cycle due to the lack of photo plates, but also eliminates the causes of quality degradation that are directly related to the use of negatives in the production of traditional printing plates:

• there are no problems arising due to loose pressing of photoforms in a vacuum chamber and the formation of bubbles during exposure of photopolymer plates;
• there is no loss in the quality of forms due to dust or other inclusions;
• there is no distortion of the shape of the printing elements due to the low optical density of photoforms and the so-called soft point;
• no need to work with vacuum;
• the profile of the printing element is optimal for dot gain stabilization and accurate color reproduction.

When exposing a montage consisting of a photoform and a photopolymer plate, in traditional technology, the light passes through several layers before reaching the photopolymer: silver emulsion, frosted layer and film base, and vacuum copy frame glass. In this case, the light is scattered in each layer and at the boundaries of the layers. As a result, the halftone dots have wider bases, resulting in increased dot gain. In contrast, when laser-exposing masked flexographic plates, there is no need to create a vacuum and there is no film. The almost complete absence of light scattering means that the image with high resolution on the layer mask is exactly reproduced on the photopolymer.

When making flexographic plates using the digital technology of mask layer ablation, it must be borne in mind that the formed printing elements, in contrast to exposure through a photoform in traditional (analogue) technology, turn out to be somewhat smaller in area than their image on the mask. This is explained by the fact that the exposure takes place in an air environment and, due to the contact of the FPS with atmospheric oxygen, the polymerization process is inhibited (delayed), causing a decrease in the size of the emerging printing elements (Fig. 10).

Rice. Fig. 10. Comparison of printing elements of photopolymer forms: a — analog; b - digital

The result of exposure to oxygen is not only a slight decrease in the size of the printing elements, which affects small raster dots to a greater extent, but also a decrease in their height relative to the height of the plate. In this case, the smaller the raster dot, the smaller the height of the relief printing element.

On a form made using analog technology, the printing elements of raster dots, on the contrary, exceed the die in height. Thus, the printing elements on a plate made by digital mask technology differ in size and height from the printing elements formed by analog technology.

The profiles of the printing elements also differ. So, the printing elements on the forms made by digital technology have steeper side edges than the printing elements of the forms obtained by analog technology.

Direct laser engraving technology includes only one operation. The mold manufacturing process is as follows: the plate without any pre-treatment is mounted on a cylinder for laser engraving. The laser forms the printing elements by removing material from the space elements, that is, the space elements are burned out (Fig. 11).

Rice. Fig. 11. Scheme of direct laser engraving: D and f are the aperture and focal length of the lens; q - beam divergence

After engraving, the form does not require treatment with washable solutions and UV radiation. The form will be ready for printing after rinsing with water and drying for a short time. Dust particles can also be removed by wiping the mold with a damp soft cloth.

On fig. 12 presented structural scheme technological process of manufacturing photopolymer flexographic printing plates using direct laser engraving technology.

The first engraving machines used a 1064nm infrared high-power ND:YAG neodymium yttrium aluminum garnet laser to engrave rubber sleeves. Later, they began to use a CO2 laser, which, due to its high power (up to 250 W), has about performance, and due to its wavelength (10.6 microns) allows you to engrave a wider range of materials.

The disadvantage of CO2 lasers is that they do not provide image recording with lineatures of 133-160 lpi, necessary for the modern level of flexographic printing, due to the large beam divergence. q. For such lineatures, the image should be recorded with a resolution of 2128-2580 dpi, that is, the size of an elementary point of the image should be approximately 10-12 microns.

The spot diameter of the focused laser radiation must correspond in a certain way to the calculated size of the image dot. It is known that at proper organization During the laser engraving process, the spot of laser radiation should be much larger than the theoretical size of the dot - then there is no unprocessed material between adjacent lines of the recorded image.

Increasing the spot by 1.5 times gives the optimal diameter of the elementary point of the image: d 0 = 15-20 µm.

In the general case, the diameter of the CO2 laser radiation spot is about 50 μm. Therefore, printing plates obtained by direct CO2 laser engraving are mainly used for printing wallpaper, packaging with simple patterns, notebooks, that is, where high-line raster printing is not required.

Recently, there have been developments that allow increasing the resolution of image recording by direct laser engraving. This can be done through the skilful use of overlapping laser recording points, which make it possible to obtain elements smaller than the spot diameter on the form (Fig. 13).

Rice. 13. Obtaining small details on the form using overlapping laser spots

To do this, laser engraving devices are modified in such a way that it is possible to change from one beam to work with several beams (up to three), which, due to different power, engrave the material to different depths and thus provide better formation of slopes of raster dots. Another innovation in this field is the combination of a CO2 laser for pre-embossing, especially in deep areas, with a solid-state laser, which, due to the much smaller spot diameter, can form slopes of the printing elements of a predetermined shape. The limitations here are set by the mold material itself, since the radiation of the Nd:YAG laser is not absorbed by all materials, in contrast to the radiation of the CO2 laser.

3. Manufacture of letterpress forms based on photopolymer compositions

An essential factor in the development of flexographic printing was the introduction of photopolymer printing plates. Their use began in the 1960s, when DuPont introduced the first Dycryl letterpress plates to the market. However, in flexo they could be used to make original cliches, from which matrices were made, and then rubber molds by pressing and vulcanization. A lot has changed since then.

Today, the following manufacturers of photopolymer plates and compositions are best known on the global flexographic printing market: BASF, DUPONT, Oy Pasanen & Co and others. pressure generated by the impression cylinder). These include paper, cardboard, corrugated cardboard, various synthetic films (polypropylene, polyethylene, cellophane, polyethylene terephthalate lavsan, etc.), metallized foil, combined materials (self-adhesive paper and film). The flexographic method is used mainly in the field of packaging production, and also finds application in the manufacture of publishing products. For example, in the USA and Italy, about 40% of the total number of all newspapers are printed flexographically on special flexographic newspaper units.

There are two types of plate material for making flexographic plates: rubber and polymer. Initially, the plates were made on the basis of rubber material, and their quality was low, which, in turn, made the quality of flexo prints in general poor. In the 70s of our century, a photopolymerizable (photopolymer) plate was first introduced as a plate material for the flexographic printing method. The plate made it possible to reproduce high-line images up to 60 lip/cm and above, 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 pixels and bitmap 3-, 5-, and 95-percentage points; thus allowing flexography to compete with the "classic" methods, especially in the field of packaging printing. And, naturally, photopolymer plates have taken a leading position as a plate flexographic material, especially in Europe and in our country.

Rubber (elastomer) printing plates can be obtained by pressing and engraving. It should be noted that the molding process itself based on elastomers is laborious and not economical. The maximum reproducible lineature is about 34 lines/cm, i.e. the reproduction capabilities of these plates are at a low level and do not meet modern packaging requirements. Photopolymer forms make it possible to reproduce both complex color and transitions, various tonalities, and raster images with a lineature of up to 60 lines / cm with a rather small spreading (increase in tonal gradations). Currently, as a rule, photopolymer forms are made in two ways: analog - by exposing UV radiation through a negative and removing unpolymerized polymer from gaps using special wash solutions based on organic alcohols and hydrocarbons (for example, using a wash solution from BASF Nylosolv II ) and by means of the so-called digital method, i.e., laser exposure of a special black layer applied over a photopolymer layer, and subsequent washing out of unexposed areas. It is worth noting that recently new developments by BASF have appeared in this area, which make it possible to remove the polymer in the case of analog plates using ordinary water; or directly remove the resin from the gaps using laser engraving in the case of digital mold making.

The basis of a photopolymer plate of any type (both analog and digital) is a photopolymer, or the so-called relief layer, due to which the formation of raised printing and recessed blank elements, i.e. relief, occurs. The basis of the photopolymer layer is a photopolymerizable composition (FPC). The main components of FPC, which have a significant impact on the printing and technical characteristics and quality of photopolymer printing plates, are the following substances.

1) Monomer - a compound of relatively low molecular weight and low viscosity, containing double bonds and, therefore, capable of polymerization. The monomer is a solvent or diluent for the remaining components of the composition. By changing the monomer content, the viscosity of the system is usually controlled.

2) Oligomer - capable of polymerization and copolymerization with a monomer, an unsaturated compound of a molecular weight greater than the monomer. These are viscous liquids or solids. The condition for their compatibility with the monomer is solubility in the latter. It is believed that the properties of cured coatings (eg photopolymer printing plates) are determined mainly by the nature of the oligomer.

As oligomers and monomers, oligoether- and oligourethane acrylates, as well as various unsaturated polyesters, are most widely used.

3) Photoinitiator. The polymerization of vinyl monomers under the action of UV radiation can, in principle, proceed without the participation of any other compounds. This process is simply called polymerization and is rather slow. To speed up the reaction, small amounts of substances (from fractions of a percent to percent) are introduced into the composition, capable of generating free radicals and/or ions under the action of light, initiating a polymerization chain reaction. This type of polymerization is called photoinitiated polymerization. Despite the insignificant content of the photoinitiator in the composition, it plays an extremely important role, which determines both many characteristics of the curing process (photopolymerization rate, exposure latitude) and the properties of the obtained coatings. Derivatives of benzophenone, anthraquinone, thioxanthone, ascilphosphine oxides, peroxy derivatives, etc. are used as photoinitiators.

The nyloflex ACE plate is designed for high quality flexo screen printing in areas such as:

Flexible film and paper packaging;

Beverage packaging;

Labels;

Pre-sealing of corrugated cardboard surface.

It has the highest hardness among all nyloflex plates - 62 ° Shore A (Shore A scales). Main advantages:

Plate color change during exposure - the difference between exposed / unexposed areas of the plate is immediately visible;

Large exposure latitude ensures good fixation of halftone dots and clean indentations on reverses, masking is not required;

Short processing time (exposure, washout, post-processing) saves work time;

A wide range of tone gradations on the printed form allows you to simultaneously print raster and line elements;

Good contrast of printed elements facilitates installation;

High-quality ink transfer (especially when using water-based inks) allows you to evenly reproduce the raster and solid, and reducing the required amount of transferred ink makes it possible to print smooth raster transitions;

High hardness with good stability, transfer of high-line raster transitions when using the technology of "thin printing plates" in combination with compression substrates;

Wear resistance, high circulation-resistance;

Ozone resistance prevents cracking.

The plate shows excellent ink transfer, especially when using water-based inks. In addition, it is well suited for printing on rough materials.

Nyloflex ACE can be supplied in the following thicknesses:

ACE 114-1.14mm ACE 254-2.54mm

ACE 170-1.70 mm ACE 284-2.84 mm

The plate has a low hardness (33° Shore A), which ensures its good contact with the rough and uneven surface of the corrugated board and minimizes the effect of the "washboard". One of the main advantages of FAC-X is its excellent ink transfer, especially for water-based inks used in printing on corrugated board. Uniform printing of plates without high printing pressure helps to reduce the increase in gradations (dot gain) during raster printing and increase the contrast of the image as a whole. In addition, the plate has a number of other distinctive features:

The violet shade of the polymer and the high transparency of the substrate make it easier to control images and mount forms, using adhesive tapes, on a plate cylinder; - high bending strength of the plate eliminates peeling of the polyester substrate and protective film;

The form is well cleaned both before and after printing.

The nyloflex FAC-X plate is single layer. It consists of a photosensitive photopolymer layer deposited on a polyester substrate for dimensional stability.

Nyloflex FAC-X is available in 2.84mm, 3.18mm, 3.94mm, 4.32mm, 4.70mm, 5.00mm, 5.50mm, 6.00mm, 6.35mm .

The relief depth of nyloflex FAC-X plates is set by pre-exposing the reverse side of the plate by 1 mm for plates with a thickness of 2.84 mm and 3.18 mm and in the range from 2 to 3.5 mm (depending on each specific case) for plates with a thickness of from 3.94mm to 6.35mm.

With nyloflex FAC-X plates, it is possible to obtain screen lineature up to 48 lines / cm and a gradation interval of 2-95% (for plates with a thickness of 2.84 mm and 3.18 mm) and a screen lineature of up to 40 lines / cm and a gradation interval of 3-90% (for inserts with a thickness of 3.94 mm to 6.35 mm). The choice of plate thickness is guided both by the type of printing machine and the specifics of the printed material and the reproduced image.

The digiflex II photopolymer plate was developed from the first generation of digiflex plates and combines all the advantages of digital communication with even simpler and easier processing. Benefits of the digiflex II plate:

1) No photographic film, which enables direct transfer of data to the printing plate, protecting the environment and saving time. After removing the protective film, a black layer becomes visible on the surface of the plate, which is sensitive to infrared laser radiation. Image and text information can be written directly on this layer using a laser. In places that are affected by the laser beam, the black layer is destroyed. After that, the printing plate is exposed to UV rays over the entire area, washed, dried, and the final illumination occurs.

2) optimal transfer of gradations, allowing to recreate the slightest shades of the image and providing high quality printing;

3) low installation costs;

4) the highest quality of the press. The basis of laser-exposed photopolymer printing plates are nyloflex FAH printing plates for highly artistic raster flexographic printing, which are covered with a black layer. The laser and subsequent conventional exposures are chosen such that significantly lower gradation increments are achieved. Get print results exclusively High Quality.

5) reduced load on environment. No film processing not used chemical compositions for photo processing, closed exposure and washing units with closed regeneration devices lead to a reduction in the harmful impact on nature.

The scope of plates for digital transmission of information is wide. These are paper and film bags, corrugated cardboard, films for automatic machines, flexible packaging, aluminium foil, film bags, labels, envelopes, napkins, beverage packaging, cardboard products.

Nyloflex Sprint - new for Russian market plate from the nyloflex series. At the moment, it is being tested at a number of production printing enterprises in Russia. This is a special water washable plate for printing with UV inks. Washing with ordinary water makes sense not only from a nature protection point of view, it also significantly reduces the processing time compared to the technology using an organic wash solution. The nyloflex sprint plate requires only 35-40 minutes for the entire deprinting process. Due to the fact that only clean water is needed for flushing, nyloflex sprint also saves on additional operations, because the used water can be poured directly into the sewer without filtration or additional treatment. And those who already work with nyloprint water-washable plates and letterpress processors do not even need to purchase additional equipment.