EP0590164A1 - Method and apparatus for producing stencil printing masters - Google Patents

Abstract

The invention relates to the production of printing stencils which are used, in particular, in the textile industry sector. These printing stencils consist of a fine-mesh screen (1) which bears a pattern. According to the invention, this pattern is produced in that a viscous covering liquid is applied to the screen (1) only at those points at which the screen (1) has to be covered in accordance with the pattern, and those points on the screen (1) at which it is to remain permeable remain uncovered. <IMAGE>

Description

The invention relates to a method for producing printing stencils according to the preamble of patent claim 1 and a device for carrying out the method according to the preamble of patent claim 16.

The printing stencils mentioned are usually thin sieves which are flat or cylindrical and have a pattern which is mostly carried out in printing ink-resistant lacquer. It is known to produce such stencils by coating the screens with light-sensitive varnishes, exposing these so-called photoresists with a pattern and then developing them. A distinction is made between negative and positive varnishes, depending on whether the exposed areas have stopped after development or have been removed by the development process. The exposure can take place in a conventional manner via a large film or can be projected on using optics. It is also known to coat the screens with an easily evaporable lacquer and then to remove this lacquer thermally or photolytically with a focused laser beam.

However, all of these processes have technological or cost disadvantages. The photochemical processes are very polluting because not only does the paint have to be washed out and thus water is contaminated, but there is also chemically contaminated waste water during the production of the large films. In contrast, the laser technology processes pollute the exhaust air due to the evaporation of parts of the lacquer layer, and in both cases complex equipment is either necessary for the production of the large films or a cost-intensive power laser is required.

The invention has for its object to overcome the disadvantages mentioned above and to provide a method for the production of stencils in which no more polluting substances fall off. The process steps required to produce the printing stencils and the amount of material required to form the cover layer should also be reduced to a minimum.

The aim of the invention is also to provide a device suitable for carrying out this method.

The procedural solution to the problem is found in the characterizing part of claim 1, while the characterizing part of claim 16 contains the device-side solution to the problem.

Advantageous embodiments of the invention can be found in the subordinate claims.

A method for producing printing stencils, in which a cover layer is applied to a fine-meshed screen, is characterized according to the invention in that the cover layer using at least one nozzle and in accordance with a desired one Print pattern is only sprayed onto the screen in certain areas.

In other words, the covering liquid required to form the covering layer is only applied to the screen at those points where the printing was not required. The areas of the screen required for printing remain free, so that here the hydraulic fluid can emerge freely from the inside of the screen. The method manages with a minimum of covering liquid, so that it can be carried out relatively inexpensively. It is no longer necessary, as in the conventional case, to remove the covering layer in regions after it has been applied, so that the method according to the invention is also extremely environmentally friendly and time-saving.

The sieves mentioned can be flat sieves or cylindrical sieves. In the case of flat screens, these can e.g. B. be covered across the country in the manner of a meander to apply the cover layer at the desired locations on the screen. For this purpose, flat sieves can also be converted into cylindrical sieves by wrapping them around a circular cylindrical die.

Sieves in cylindrical form are preferably used, the covering layer being sprayed on when the sieve is rotated about its cylinder axis and with the nozzle being displaced parallel to the cylinder axis. Here, the rotational position of the printing stencil and the position of the nozzle in the axial direction are constantly registered in order to be able to determine positions on the cylinder surface at which material is to be sprayed on in accordance with a desired printing pattern in order to form the cover layer or not.

According to an advantageous development of the invention, the cover layer is sprayed onto the screen in a direction that is inclined with respect to the normal to the screen surface at the point of impact. The covering material is thereby prevented from passing through pores of the sieve, since there is always a material wall in the direction of the trajectory of the covering material. Thus less covering material gets inside the sieve, which leads to a further reduction in the amount of covering material required to form the covering layer. The direction of rotation of the screen can be chosen so that the relative speed between the screen surface and the cover material when spraying is as small as possible to prevent the drops of cover material from being sprayed upon impact to prevent the screen surface.

According to another advantageous development of the invention, the material of the cover layer is sprayed out of the nozzle in a direction which is at least approximately parallel to the force of gravity.

If electrostatic nozzles are used, a partially electrostatically charged drop mist is generated, the electrically charged drops being deflected from their trajectory and separated from the non-charged drops in this way. The drops are subject to the effects of gravity and air resistance and therefore describe parabolic trajectories as they move through the nozzle. If the initial velocity of the liquid jet from which the drops originate can always be kept at a constant and the same value, then a parabolic trajectory is not disadvantageous. But must consider z. If, for example, a different initial speed is specified for rheological parameters of the covering liquid or the perfect covering of the sieve, this also results in a different path parabola and the selection of the differently charged drops becomes more difficult. These relationships can be made more uniform if the path of the drops is selected vertically, that is, the nozzle axis is vertical and the nozzles z. B. apply the cover liquid to the sieve from top to bottom. In this way it is achieved that the path of the drops initially remains unaffected by gravity and the drops can therefore pass exactly through the center of a ring electrode in front of the ejection channel for electrostatic charging before they are finally deflected by a subsequent deflection electrode. In contrast, in the case of horizontal nozzles, the drop path is already influenced by gravity immediately behind the ejection channel, so that it is more difficult to hit the center of the ring electrode.

According to the invention, a plurality of nozzles can be provided in order to spray cover material onto one and the same point on the screen surface. The respective point can be sprayed through all nozzles at the same time or at different times.

With delayed operation, a thicker cover layer can thus be produced in a simple manner, since one and the same point on the screen surface is coated several times, according to the number of nozzles. In contrast, in the case of operation similar to a tent, a plurality of liquids can be applied simultaneously to the screen surface to form the cover layer, and can then react with one another to produce the cover layer.

In the latter case, it can e.g. B. are epoxy resin components that are only converted into a gel state after they have been mixed together and the crosslinking reaction has begun. The individual epoxy resin components are relatively thin, so that rapid droplet separation takes place within the nozzles and the nozzles can therefore have a relatively short jet length. However, since these components are converted into a gel state as a result of the crosslinking reaction after striking the screen surface, there is no danger that they will be flung off the surface again. The use of the multiple components is also advantageous in that a better edge sharpness of the pattern is obtained. In particular in the case of long stencils and thus long processing tents, flow movements often occur which can lead to a deterioration in the edge structure if the cover layer is produced from only a one-component material which has a relatively long drying time. On the other hand, the hardening time can be shortened considerably if several suitable components are selected, which leads to an improved contour definition.

The cover layer can also be formed by spraying on a viscous liquid which is, for example, an aqueous emulsion of a synthetic resin varnish or can be an aqueous suspension of pigments. It has proven to be advantageous to carry out the spraying on of the liquid accompanied by a laminar gas stream surrounding it, for example using an air or inert gas stream, in order to accelerate the drying process of the sprayed-out liquid. The gas flow also prevents small secondary droplets from accumulating inside the nozzles and otherwise contaminating them. The speed of the laminar gas flow can also be selected so that liquid drops once formed can no longer approach each other on their way to the screen surface, as a result of which the formation of larger drops can be avoided. Furthermore, the laminar gas stream can also have an elevated temperature compared to the ambient temperature, as a result of which the sprayed out can dry Fluid can accelerate even further. Last but not least, the screen can also be heated at least at the point of impact of the liquid, for example by means of a heat radiator, in order to obtain a solid covering layer on the screen surface as quickly as possible. Warm air can also be blown axially into the interior of the sieve. It is also possible to irradiate the liquid drops sprayed onto the sieve with ultraviolet (UV) radiation in order to start or accelerate the crosslinking reaction earlier, which leads to an even better edge sharpness of the pattern (UV curing). The short phase of lowering the viscosity, which occurs during heating, is therefore avoided with pure UV curing.

A device for carrying out the method, with a processing station known per se, the at least one bearing device for the front-side mounting of a hollow cylindrical screen, a drive device for rotating the screen about its cylinder axis, a processing table movable parallel to the cylinder axis and a control device for controlling the drive device, the Transporting the processing table and for controlling a tool station arranged on the processing table is characterized according to the invention in that the tool station consists of at least one nozzle suitable for ejecting liquid.

This nozzle receives electrical ejection signals from the control device, in accordance with a predetermined pattern and in dependence on the rotational position of the screen cylinder and the position of the processing table. The pattern or print pattern can be pre-stored in electronic form in an electronic memory of the control device. Each saved sample point is assigned a pair of values that contains the rotary position of the screen cylinder (angular position) and the axial position of the machining table. As soon as this pair of values is supplied to the control device by sensors, the assigned value of the pressure pattern is read out of the named electronic memory and used to form an ejection signal which is transmitted to the nozzle.

If there are several nozzles lying next to each other in the longitudinal direction of the cylinder on the processing table, and if these nozzles act on different areas of the screen cylinder, the control device issues the electrical ejection signals in the transport direction of the processing table nozzle located further to the rear with a time delay, in such a way that one and the same point on the screen surface is sprayed in succession through the respective nozzles.

In the event that all nozzles act on the same area on the surface of the screen cylinder, that is to say are inclined accordingly, the electrical ejection signals are transmitted to all nozzles simultaneously.

• Figur 1 eine Vorrichtung zum Beschichten eines feinmaschigen Rundsiebes mit einer Abdeckschicht nach einem ersten Ausführungsbeispiel der Erfindung,
• Figur 2 eine Vorrichtung zum Beschichten eines feinmaschigen Rundsiebes mit einer Abdeckschicht nach einem zweiten Ausführungsbeispiel der Erfindung,
• Figur 3 Aufbau und Anordnung einer ersten Düse zum Beschichten des Rundsiebes mit Abdeckmaterial,
• Figur 4 den Gesamtaufbau der ersten Düse,
• Figur 5 Aufbau und Anordnung einer zweiten Düse zum Beschichten des Rundsiebes mit Abdeckmaterial, und
• Figuren 6 - 8 den Gesamtaufbau der zweiten Düse.

The invention is described in more detail below with reference to the drawing. Show it:

• 1 shows a device for coating a fine-mesh circular screen with a cover layer according to a first embodiment of the invention,
• FIG. 2 shows a device for coating a fine-mesh circular screen with a cover layer according to a second exemplary embodiment of the invention,
• FIG. 3 construction and arrangement of a first nozzle for coating the circular screen with covering material,
• FIG. 4 shows the overall structure of the first nozzle,
• Figure 5 Structure and arrangement of a second nozzle for coating the circular screen with cover material, and
• Figures 6-8 the overall structure of the second nozzle.

Various exemplary embodiments of the invention are described in more detail below with reference to the drawing.

A first exemplary embodiment of a device according to the invention is shown in FIG. 1. Reference number 1 denotes a rotating sieve in the shape of a cylinder, to which paint or varnish is applied as a covering liquid through one or more nozzles. Here, a jet 3 of the covering liquid sprayed out of the nozzles 2 is controlled by means of a computer 4 in such a way that the covering liquid is applied to the sieve 1 only at those points at which the sieve 1 is covered due to the pattern must be and those parts of the sieve 1 remain uncovered where it should remain permeable. For this purpose, the sieve 1 is received between two synchronously driven end heads 5 and set in rotating motion (direction of rotation D). In order to be able to accommodate different template lengths or screen lengths between the end heads 5, the right end head 5 can, for example, be displaced in the direction of the cylinder axis of the circular screen 1. The sieve 1 is placed between the right and left end heads 5 and the right end head 5 is brought up to the sieve 1. The screen 1, which is usually very thin and light, can possibly be set in rotation by the axially acting clamping force and the friction between the screen 1 and the left driven end head 5. Also, the rigidity of the screen 1 is always sufficient to notify the right end head 5 of the rotational movement via the acting frictional forces, if only the speed of the screen 1 is increased so slowly that the required acceleration torque does not overwhelm the transmission capacity of the circular screen 1. Both end heads 5 are rotatably mounted on pedestals 6, the pedestals 6 being arranged on a machine bed 7. To guide the right pedestal 6 in Figure 1 guide rods 8 are available, the z. B. can be attached to the machine bed 7.

The left end head 5 is driven by a motor 9 and a belt 10. This belt 10 spans a drive wheel 11 which is fixed on an axis 12 which carries the left end head 5. At the other end of the axis 12 there is an incremental pulse generator 13 which determines the rotational position of the axis 12 or the sieve 1 and outputs corresponding signals S _D to the computer 4. At the same time, the nozzles 2, which are attached to a processing table 14, are slowly advanced in the direction of the cylinder axis 1b of the sieve 1, so that a thin jet of droplet and covering liquid which emerges from the nozzles 2, along a helical line, is very low Slope hits the sieve 1. The feed table is impressed on the processing table 14 via a spindle 15, this spindle 15 being driven for this purpose via a stepper motor 16, which also receives its step signals S _T from the computer 4. These step signals S _T are converted into power pulses P _T by a driver stage 17. The rotation of the motor axis of the stepping motor 16 is transmitted to the spindle 15 via a belt 18 and a pulley 19. This extends through the processing table 14, which in turn is guided on guide rails 20 on the machine bed 7.

The nozzles 2 must be supplied with a covering liquid suitable for the subsequent printing process. For this purpose, they are connected to small pressure vessels 21 via supply lines 22. In the pressure vessels 21, the covering liquid is under a slight excess pressure of approximately 1 to 5 bar. Expediently, a separate pressure container 21 will be provided for each nozzle 2, since differences in the line resistances and the need to be able to regulate the application quantity separately per nozzle 2 cause different outlet pressures of the covering liquid. Each nozzle 2 also has a not inconsiderable amount of unused covering liquid which has to be continuously sucked off and conveyed back. For this purpose, vacuum tanks 23 are provided, into which the unused covering liquid is returned via return lines 24 due to the negative pressure prevailing in these tanks. The recirculated cover liquid, which has lost diluent as a result of the process which has passed through, can in turn be supplied to the application process as a cover liquid after preparation. In order to achieve a corresponding thickness of the covering layer 1a on the screen 1, the nozzles 2 are arranged several times, in the present case twice. They are spaced apart from one another in the direction of the cylinder axis 1b or template axis in order to give the covering liquid time to dry at least slightly before the second application. This drying can be supported by blowing warm air, or by generating appropriate heat radiation. For this purpose, a correspondingly designed heating device H can be mounted on the processing table 14. The liquid can also be cured alone or additionally by UV radiation, as already mentioned, so that in this case there is also a UV light source (for example a mercury vapor lamp) on the processing table 14.

In principle, the nozzles 2 can also be displaced in the circumferential direction of the cylinder 1 or sieve, but this leads to a more difficult handling of the coating process if successive circular sieves 1 of different diameters are to be coated.

The nozzles 2 are preferably designed as electrostatic nozzles, each of which is supplied with a control signal S 1, S 2 from the computer 4 in order to inject the covering liquid when a control signal is received.

FIG. 2 shows a device that is basically the same as in FIG. 1, the same elements being provided with the same reference numerals. Deviating of Figure 1, the processing table 14 is mounted on a rear support wall 25 on guide rails 26 in the axial direction of the cylinder 1. The spindle 15 and the stepping motor 16 with spindle drive 18 and 19 are also fastened to this rear guide wall 25. On the front side of the processing table 14 facing the screen 1 there is a holding device 27 which serves to clamp two nozzles 2, which are now vertical with their nozzle axes, that is to say perpendicular to the flat surface of the machine bed 7. The nozzle openings 28 point downward.

In this way, it is possible to first spray the drops of the covering material parallel to and in the direction of gravity before they hit the surface of the screen 1.

In the method according to the invention, the covering liquid is applied in very fine drops in order to achieve a sufficiently high resolving power when generating the print pattern on the surface of the screen 1. The liquid can have a high viscosity in order to be able to carry a sufficient proportion of solid substance with a relatively small droplet size. However, several liquid components can also be sprayed on separately through different nozzles, which are combined at one point on the surface of the sieve 1. These can be different epoxy resin components which are only converted into a gel state when a crosslinking reaction has started after their meeting. In addition, the method only makes sense if a very high drop frequency can be achieved.

All of this is possible through the use of so-called electrostatically acting nozzles, in which a liquid jet is regularly broken down into drops by a very high-frequency vibration, for example a pipe wall, and in which the drops are then electrically charged and deflected in an electrostatic field, depending on the state of charge not be distracted. However, conventional nozzles of this type are not suitable for processing the highly viscous covering liquids required for coating screens. While in the case of low-viscosity liquids, even minor initial disturbances are sufficient to quickly cause the liquid jet to disintegrate into individual drops immediately behind the nozzle outlet due to the effect of the surface tension of the liquid, in the case of those necessary for masking high viscosities jet lengths of 0.5 - 1.0 m are created before the first drop is created by jet constriction. An annular charging electrode with a very small diameter must be arranged at the location of the first drop formation. Due to the inevitable air swirls, at such intervals neither the location of the first drop formation can be determined precisely nor the course of the jet, so that it can no longer be passed through such a small annular charging electrode. The invention therefore uses electrostatic nozzles with a modified design.

FIG. 3 shows the structure of such an electrostatic nozzle 2.

In a small pressure chamber 29, the covering liquid, which is supplied from the pressure containers 21 shown in FIG. 1, is under excess pressure. From there it exits continuously through a bore 30. In the bore 30, a thin needle 31, which is excited by ultrasound to produce high-frequency vibration in the longitudinal direction of the needle, provides regular disturbances in the annular flow channel formed by the needle 31 and the bore 30. In addition, the oscillatory movement of the needle 31 also prevents the bore 30 from becoming blocked, for example. B. by small particles. With each oscillating movement of the needle 31 in the direction of the exit, to which the covering liquid flows due to the pressure gradient, the covering liquid is entrained with the needle wall due to its viscosity and is thus additionally accelerated, and is decelerated in the same way with each oppositely directed oscillating movement. The movement of the end face 32 of the needle 31 also produces an effect which has the same effect. This end face movement of the needle 31 is of particular advantage in the viscous liquids present here, since the solid bodies are thrown off at correspondingly high acceleration values of the end face 32, which leads to a particularly strong support of the constriction process. By dimensioning the diameter of the needle 31 and the bore 30 it is in hand to make the acceleration or deceleration effects sufficiently large. The larger the diameter of the needle 31 and the smaller the diameter of the bore 30, the stronger the acceleration and thus the disturbance effects. The strong disturbances produced in this way result in pronounced constrictions of the liquid jet leaving the bore 30 which correspond to the oscillation frequency and which are further developed outside the bore 30 due to the surface tension of the liquid and thus lead to rapid drop formation. So that the resulting drops can be charged electrostatically, a ring electrode 33 is provided, which is kept small in diameter, because sufficient charging of the drops can then be achieved even at low voltages. The aim is to be able to work with a voltage of 100 - 200 V. This voltage must be present at the ring electrode 33 at the moment the drop breaks off. Voltages of this size can still be conveniently switched at high frequencies using transistors. At the time the drop is torn off from the still connected jet, it must be kept at a zero voltage potential with respect to the ring electrode 33 so that a negative charge remains on the tearing drop, and the tear must also take place in the area of the ring electrode 33. The internal conductivity of the covering liquid must ensure the electrical connection to the pressure chamber 29, which is constantly kept at earth potential (= 0 V). It is therefore extremely expedient to choose an aqueous emulsion of synthetic resin paints or an aqueous suspension of pigments for the covering liquid. The ring electrode 33 is kept small in diameter, whereby high field strengths are achieved even at lower switching voltages. So that the liquid jet emerging from the bore 30 hits the center of the ring electrode 33 with the greatest possible certainty, this ring electrode 33 is brought as close as possible to the exit of the bore 30. At this point the beam must just begin to decay into drops. The accuracy of the ring electrode center is significantly increased by vertical beam guidance, as already mentioned in connection with FIG. 2, the correspondingly pronounced initial constrictions of the liquid jet emerging from the bore 30, which is caused by a correspondingly strong vibration of the needle, ensuring the necessary rapid decay of the beam 31 be forced.

The charged liquid drops, which here have the reference numeral 34, are then guided into a catcher 37 by the action of a direct voltage field applied via an electrode 35 on a curved path 36. From there, they arrive via the return lines 24 mentioned in FIG. 1 into the vacuum tanks 23, which are also shown there. The uncharged liquid drops 38 are not deflected by this DC voltage field and accordingly continue their path almost linearly along the line 39 to finally reach the sieve 1 hold true. The screen 1 here has one to the web 39 of this impinging, uncharged drops 38 vertical position. However, it may well be expedient to incline this sieve 1 in relation to such a position, which is shown in connection with the next FIG. 4. The covering liquid must transport solids to a sufficient extent in order to form a well covering film after drying on the sieve 1, as a result of which a high viscosity is required. The high viscosity helps, however, that after the covering liquid has been applied to the sieve 1, it remains at the impact point despite the centrifugal force acting on it and does not shoot through the perforation of the sieve due to the high impact speed or sprayed into even smaller droplets during the impact on the sieve 1.

So that the ring electrode 33 remains clean even during long operating times, a combined liquid and air or inert gas supply is carried out in the area of the ring electrode 33. Just before the start of the spraying operation, liquid is first introduced through holes 40 in order to clean the ring electrode 33. It is then blown dry through the same bores 40, for example by dry, heated air or an inert gas. The same configuration of the nozzle is additionally used to prevent the thin bore 30 from drying out during longer work breaks. In this case, the adjacent air space 41 in front of the bore 30 and inside the ring electrode 33 is filled with flushing liquid through the bores 40. This rinsing liquid is kept under a very slight excess pressure (approximately 10 to 20 mm water column), as a result of which a liquid cell meniscus 43 is formed within the nozzle channel 42, which can persist for a long time and which prevents liquid from escaping from the nozzle channel 42. This filling protects the thin bore 30 from drying out. A conical countersink 44 can be provided in order to allow the liquid as good an access to the bore 30 as possible. Through it the bore 30 opens into the nozzle channel 42 in the direction of the ring electrode 33. However, it may also be expedient not to let the rinsing liquid come into contact with the covering liquid within the bore 30 in order not to dilute the latter. In this case, the conical countersink 44 is omitted, and there is only a correspondingly small cylindrical drilling attachment at this point. The flushing liquid will then also form a meniscus in this bore, similar to meniscus 43. The covering fluid at the exit of the bore 30 also forms a meniscus. Between the two menisci there is then a small air space which, thanks to its small volume, quickly contains vaporous molecules the easily evaporable components of the topcoat and the rinsing liquid are saturated. A further evaporation of these components from the masking lacquer is then no longer possible, so that drying is prevented without the risk of the masking liquid being thinned by rinsing liquid.

Air flows through the nozzle 2 even when covering liquid is applied to the sieve 1. As a result, the dry air emerging from the bores 40 keeps small secondary droplets away from the ring electrode 33 and thus cleans them. Such secondary droplets arise simultaneously with the main droplets when the liquid jet emerging from the bore 30 decays. Because of the smallness and the low mass of these secondary droplets, they can be thrown against the ring electrode 33 by the pinching process of the main droplets. If droplet deposits would form there, then the proper functioning of the electrode could be questioned over time. A further effect results from the flow through the diffuser-like channel 42. Here, the flight speed of the drops should be delayed somewhat, but not too strongly, since they may only touch one another after they have hit the screen 1. Touching the liquid drops inside the nozzle channel 42 would lead to the immediate formation of large drops, which in turn capture further normal drops due to the specifically lower air resistance and, overall, these processes lead to a falsification of the electrically impressed pattern. This phenomenon can then be prevented if the drops are enveloped in their trajectory within the nozzle channel 42 by a laminar air flow which has a flow velocity suitable for this. Pre-drying of the individual drops can also be achieved by such an air flow. This has advantages if the drop hits sieve 1. Predrying increases the drop viscosity and also reduces the size of the drops. This prevents the drop from bursting into many small individual drops when it strikes the screen 1 and prevents the formation of a correspondingly blurred paint contour. For a sufficient predrying, however, a relatively large length of the nozzle channel 42 is required, which is possible in particular with the axis of the nozzle channel 30 running parallel to the gravitational field, that is to say with a vertical nozzle axis.

FIG. 4 shows the overall structure of the nozzle according to FIG. 3. The following apply the same reference numerals as in Figure 3. The direction of impact of the drops 38 on the screen 1 is no longer vertical here, but is at an angle 45. This helps to prevent the drops from passing through the screen 1, because then before each Drop in the direction of its trajectory is always a material wall. In addition, there is a reduced relative speed between the drops and the sieve, which also reduces the risk of the drops bursting. The needle 31 is held in a needle holder 46 which is designed as a step horn, ie the diameter of the needle holder 46 decreases towards the tip of the needle 31. This amplifies the amplitude of the high-frequency mechanical vibration introduced into the needle holder 46, so that the needle 31 vibrates in the region of the bore 30 with the maximum amplitude. The needle holder 46 is firmly held in a membrane 47 and this is excited by a piezo element 48 to the high-frequency vibration. A pressure piece 49 transmits this vibration to the membrane 47, whereby the liquid in the pressure chamber 29 is also pressurized by the membrane 47 itself. To ensure this, the supply lines to the pressure chamber 29 must be designed to be correspondingly thin. With a corresponding design of the pressure piece 49, a pre-amplification of the vibration amplitude can be achieved mechanically. The piezo element 48 is excited by supply lines, which are no longer shown, with a high-frequency sine or square wave voltage corresponding to the natural frequency of the nozzle arrangement. Since the piezo element 48 is composed of a large number of thin layers in a sandwich-like manner, even low electrical voltages are sufficient to generate violent contractions or elongations, in particular in the region of the natural frequency of the overall arrangement. The piezo element 48 is statically prestressed in its longitudinal direction by a pressure screw 50, and a counter nut 51 secures this screw setting. A housing 52 statically and dynamically closes the power flow of all individual components. The bore 30 of the nozzle 2 is made in a sapphire plate 53, which is pressed by a screw 54 into a holder 55 and is fixed there in this way. The choice of the sapphire bore material largely reduces the risk of the needle 31, which is made of a metallic material, from rubbing or welding to the bore wall due to the needle vibration.

It should also be pointed out that the ring electrode 33 is connected to a supply line 56 in order to transmit an electrical potential to the former to be able to supply the supply line 56.

A further embodiment of an electrostatic nozzle for carrying out the method according to the invention is shown in FIG. 5. Here, too, the same elements as in FIGS. 3 and 4 are provided with the same reference symbols and are not explained again. The bore 30 is so small in this embodiment, for example in the final diameter 17 microns that it can no longer be penetrated by the needle 31 in its entire length. The needle 31 therefore only extends to the vicinity of the narrowest bore point. However, the action of the needle 31 is similar to the action that was described earlier. An oscillating movement of the needle 31 in the direction of the nozzle outlet increases the pressure in the nozzle interior 57 both because of the wall thrust forces and because of the displacement effect of the needle end face 32. The corresponding return movement of the needle 31 causes a pressure reduction. As a result, strong disturbances are in turn imprinted on the emerging liquid jet and this shows a strong tendency to regulated and rapid decay. The formation of the individual drops takes place in the area of the ring electrode 33, which here too is provided with a suitable supply line for applying an electrostatic potential. The nozzle interior 57, in which the needle 31 moves, is obtained by a nozzle body 58 which is made of hard metal or ceramic. This nozzle body 58 is inserted into a bore 59 of the holder 55, the needle holder 46 can still partially protrude into the bore 59.

FIGS. 6, 7 and 8 show the overall structure of the nozzle according to FIG. 5. FIG. 6 shows a section through an elevation of the nozzle, FIG. 7 shows a cross-section and FIG. 8 shows a cross section through the nozzle. The same elements as in FIGS. 3 to 5 are again provided with the same reference symbols and are not described again.

A holder 60 presses a mouthpiece 61, into which the deflection electrode 35 is cast, against the nozzle base body 62. The nozzle channel 42 runs through the mouthpiece 61 and is surrounded on the input side by the ring electrode 33. It is also carried by the mouthpiece 61. The oscillating membrane 47 is located between the housing 52 and the nozzle body 62. The oscillating membrane 47 is clamped between the housing 52 and the nozzle body 62, wherein it is formed by an approximately 0.5 to 1.0 mm thick steel plate, which Because of the special nature of the clamping 31 can only perform bending vibrations in a surrounding area of the needle. In the protruding area, this membrane 47 is used as a clamping element for a microsieve 63. The relatively large thickness of the membrane causes natural frequencies that are between 200 and 300 kHz. The microsieve 63 is clamped between the membrane 47 and the nozzle base body 62 and prevents particles which are larger than 5 μm and which are inadvertently carried along with the covering liquid from entering the channel system leading to the nozzle. Here, the membrane 47 guided over the microsieve 63 in the inlet area of the liquid and the ultrasound oscillation introduced into it help to avoid a blockage of the microsieve 63 by interlocking pigments. In order to utilize the largest possible filter area of the microsieve 63, this is held by a system of very small, finely milled support channels 64. The covering liquid is fed through the supply line 22 to the nozzle 2. This supply line 22 is placed tightly on a clamping piece 66 by means of a union nut 65.

Via an air-water supply line 67, the liquid required for the cleaning and drying of the nozzle 2 or the necessary air is fed to the nozzle 2 if necessary. This line 67 is also pressed with a union nut 68 against a screw-in clamping piece 69. The line 67 leads to a changeover valve 70, which is shown symbolically here and is located at a greater distance from the nozzle 2.

In Figure 8 it can be seen that the piezo element 48 is held within the housing 52 by two short grub screws 71 in a symmetrical position relative to the housing 52.

The electrostatic nozzles described in FIGS. 3 to 8 are particularly suitable for carrying out the method according to the invention, since they can also be used to spray a highly viscous or viscous covering liquid dropwise onto the sieve without the length of the nozzle and thus the dimensions the device for performing the method must assume extremely large values. The cover liquid is resistant to abrasion and chemical influences from the printing chemicals.

Claims (25)

Process for the production of printing stencils, in which a cover layer is applied to a fine-mesh screen, characterized in that the cover layer (1a) is sprayed onto the screen (1) only in regions using at least one nozzle (2) and in accordance with a desired print pattern becomes. Method according to claim 1, characterized in that the sieve (1) is cylindrical and the spraying of the cover layer (1a) when the sieve (1) rotates about its cylinder axis (1b) and with displacement of the nozzle (2) parallel to the cylinder axis ( 1b) takes place. Method according to claim 1 or 2, characterized in that the cover layer (1a) is sprayed onto the screen (1) in a direction which is inclined at the point of impact with respect to the normal to the screen surface. Method according to one of claims 1 to 3, characterized in that the material of the covering layer (1a) is sprayed out of the nozzle (2) in a direction which is at least approximately parallel in or to the force of gravity. Method according to one of claims 1 to 4, characterized in that a plurality of nozzles (2,2) are provided in order to spray one and the same point on the screen surface. Method according to claim 5, characterized in that the nozzles (2, 2) spray the position at different times. Method according to one of claims 1 to 6, characterized in that the cover layer (1a) is formed by spraying on a viscous liquid. A method according to claim 7, characterized in that an aqueous emulsion of a synthetic resin varnish is used as the viscous liquid. A method according to claim 7, characterized in that an aqueous suspension of pigments is used as the viscous liquid becomes. Method according to claim 5, characterized in that the cover layer (1a) is formed by spraying on different liquids. Method according to one of claims 1 to 10, characterized in that an electrostatic nozzle is used as the nozzle (2). Method according to one of claims 7 to 12, characterized in that the liquid is sprayed on accompanied by a laminar gas stream which bypasses it. A method according to claim 12, characterized in that the gas stream is an air or inert gas stream. A method according to claim 12 or 13, characterized in that the laminar gas flow has a temperature which is higher than the ambient temperature. Method according to one of claims 7 to 14, characterized in that the sieve (1) is heated at least at the point of impact of the liquid and / or is irradiated with UV radiation. Device for carrying out the method according to one of claims 1 to 15, with a processing station known per se, the at least one bearing device (5, 6) for the front-sided mounting of a hollow cylindrical sieve (1), a drive device (9, 10, 11) for rotation the sieve (1) about its cylinder axis (1b), a processing table (14) movable parallel to the cylinder axis (1b) and a control device (4) for controlling the drive device (9, 10, 11), the transport of the processing table (14) and for controlling a tool station arranged on the processing table (14), characterized in that the tool station consists of at least one nozzle (2) suitable for ejecting liquid. Apparatus according to claim 16, characterized in that the tool station has a plurality of nozzles (2, 2) arranged side by side in the longitudinal direction of the hollow cylindrical sieve (1). Apparatus according to claim 16 or 17, characterized in that each nozzle (2) is connected to an overpressure container (21) for liquid to be sprayed on and with a vacuum container (23) into which unused liquid sprayed out by the nozzle (2) is returned . Device according to one of claims 16 to 18, characterized in that a longitudinal axis (39) of the nozzle lies essentially parallel and outside of a horizontal plane receiving the cylinder axis (1b). Device according to one of claims 16 to 18, characterized in that the longitudinal axis of the nozzle (39) extends essentially in the vertical direction. Device according to one of claims 16 to 20, characterized in that the nozzle (2) electrical ejection signals (S₁, S₂) from the control device (4) in accordance with a predetermined pattern and in dependence on the rotational position of the screen cylinder (1) and the position the processing table (14) receives. Apparatus according to claim 17 and 21, characterized in that the nozzles (2, 2) act on different areas of the screen cylinder (1) and the control device (4) the electrical ejection signals (S₁, S₂) to the respective in the transport direction of the processing table (14) outputs the rear nozzle (2) with a time delay. Device according to claim 17 and 21, characterized . that all nozzles (2, 2) act on the same area of the screen cylinder (1) and the control device (4) transmits the electrical ejection signals (S₁, S₂) to all nozzles simultaneously. Device according to one of claims 16 to 23, characterized in that, viewed in the direction of rotation of the screen cylinder (1), behind the respective liquid impingement point there is a heating device (H) and / or radiation device for heating and / or UV radiation of those on the screen cylinder (1). sprayed liquid. Apparatus according to claim 24, characterized in that the heating device (H) and / or the UV radiation device on the Processing table (14) are mounted.

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