EP2070698A2 - Device and method for measuring the screen tension of the screen fabric in a silk screen frame - Google Patents

Abstract

The apparatus has a holder for holding a screen frame (1). The holder includes bars (3a-3d), where each bar is divided into a set of segments (3a.1-3c1, 3a.2-3c.2, 3a.3-3c.3, 3a.4-3c.4). Each segment is provided with a respective sensor device for measuring printing screen tension of a screen fabric (2). Each segment is attached to a controllable device (40) for exerting printing forces and/or tensile forces. A coating (2a) of the fabric is distant for printing at an area (2b) such that a printing ink between meshes of the fabric is printed by a spreading knife in a screen-printing unit. An independent claim is also included for a method for measuring printing screen tension of a screen fabric for a screen-printing machine.

Description

The invention relates to a device, in particular for a screen printing machine, for measuring the wire tension of the screen fabric in a screen frame. The invention further relates to a method, in particular for a screen printing machine, for measuring the wire tension of the screen fabric in a screen frame.

Printing machines, which operate on the principle of screen printing and in particular screen printing machines, which operate by means of a flat screen; have long been known and are used industrially for printing a wide variety of products. Thus, for example, optical data carriers such as CDs or DVDs are screen-printed, but also garments, bottles, cans, or in particular in electronics production soldering pastes or etch-resistant masks are applied to board material by means of screen printing equipment.

Mostly this flat-screen printing machines are used, which always work on the same basic principle, by means of a clamped in a frame flat screen, in particular screen fabric through which the ink is painted on the surface to be printed. In this screen, the information to be transmitted or the printed image is present in such a way that those areas that do not contribute to the printed image, for example, covered by a paint, whereas those areas over which the Printed image is to be transferred to the surface to be printed, is not covered, so that at these points a printing ink between the meshes of the screen can be pushed through. The printing takes place in that the screen is positioned with its underside at a small distance from the surface to be printed and a printing ink, which is located on the top of the screen is painted over the surface of the screen by means of a working doctor blade with a certain contact pressure, so that the screen fabric along the doctor edge is pressed onto the surface to be printed.

As a result, at the same time, an ink located on the upper side of the printing screen is pressed along the doctor edge at the open points of the screen through the meshes of the screen and is transferred to the underside of the screen on the surface to be printed. The contact pressure of the working doctor blade is chosen so that the underside of the screen always comes into contact with the surface to be printed only at a point which substantially corresponds to the shape of the front edge of the working doctor blade.

By an appropriate up and down movement and a corresponding reciprocation of the working doctor blade and another flood doctor for more or less uniform distribution of the ink, the ink is cyclically evenly distributed on the screen surface, so that in a clocked operation each other subsequent surfaces to be printed in the same quality can be printed. Due to the repeated use of the squeegee by the squeegee and possibly existing edges on the object to be printed, however, it may happen that the wire breaks at one or more points, so that either the printing at these points becomes faulty and / or uncontrolled Print down on the products or in the underlying machine elements.

If this is not noticed by an operator in time and the damaged screen is replaced with an intact screen, a larger number of products can be used be printed incorrectly and / or the printing machine are dirty, which usually requires extensive cleaning, during which the printing press and thus the production stands still on this press.

Deterioration of print quality can also occur if the mechanical tension with which the screen fabric is stretched on the screen frame decreases, for example due to prolonged use or overstretching of the fabric. The resulting poorer print quality is often recognized too late, so that afterwards a number of products must be sorted out whose printing does not meet the requirements.

To control the contact pressure of the working doctor blade on the substrate surface is in the DE 3805363 proposed to measure by means of several pressure sensors in the corners of the printing screen the summary pressure, which is composed of the contact pressure of the working doctor blade, the pressure absorbed by the substrate and the wire tension. According to a change, the contact pressure of the doctor blade can thereby be controlled.

A disadvantage of the type described is that at a relaxation of the wire tension despite a readjustment of the contact pressure of the working doctor blade there is a danger that increases the contact zone between Siebunterseite and substrate surface or in particular at an uneven change in the wire tension, the contact zone inhomogeneously changed, creating a uneven printing results.

Another disadvantage is that the DE3805363 no possibility to detect a sieve.

It is therefore desirable even during normal operation, the average current mechanical wire tension reliably with a certain Accuracy to measure, whereby a decrease in the mean wire tension are detected in good time and thus the mean wire tension can be readjusted accordingly, either by the operator or by means of a controlled device.

It is also desirable to determine the wire tension reliably and with a certain accuracy in a spatially resolved manner even during normal operation, in order to detect local strain variations, such as a local overstress or a sieve crack, in order to be able to respond to disturbances occurring in the printing process in a timely manner.

The object of the invention is therefore to provide a method and an apparatus with which the mentioned disadvantages of existing devices and systems are eliminated and beyond a higher reliability of a screen printing machine is ensured. The object of the invention is furthermore to provide a method and a device, whereby it is possible to continuously measure the mechanical tension of a printing screen, in particular during operation in a printing machine and spatially resolved, and optionally readjust automatically. The object of the invention is also to provide an apparatus and a method, whereby it is possible to detect a sieve break or the beginning of a sieve unambiguous and timely during the normal production process.

The object is achieved in that the device according to the invention has a receiving device for receiving the screen frame, the receiving strips are each divided into several segments and each segment is associated with at least one sensor device for Zugspannungsmessung. According to the method, the object is achieved in that a sieve frame provided with a screen fabric is mounted in a receiving device / whose receiving strips are each subdivided into several segments and with sensor devices, of which at least one is assigned to a segment between a segment and the Screen frame acting force is detected.

Thus, the mechanical tension of the mesh fabric can be measured by means of a number of force sensors, being acted upon by the respective sides of the screen frame to respective corresponding force sensors. For example, a printing unit on such a recording device, which may be formed as a holding frame, which does not necessarily have to be circumferentially closed and in which an example rectangular, covered with a mesh screen frame is inserted and fixed there by means of appropriate fasteners.

From the acquisition of the measured data, additional information about the condition of the sieve can be obtained or, for example, a sieve break can be detected. Furthermore, it can also be provided to readjust the wire tension continuously in zones.

The receiving device or the holding frame can be designed so that it / he has corresponding receiving strips for the screen frame, which are divided along their extension into individual segments. These receiving strips are arranged substantially parallel to the respective frame members of a screen frame. Of the segments may preferably be two opposite each other. Furthermore, according to the invention, at least one force sensor is associated with each segment, in particular within each segment at least one force sensor is provided and mounted in such a way that the tensile force acting in this segment through the clamped screen frame via the receiving strip can be measured.

In order to set an initial tensile force and, if appropriate, the intended readjustment of the zonal mechanical stresses that are defined over the segments, it may be advantageous to equip each of the mentioned segments with an individually controllable traction device, whereby on the one hand a mechanical pretension is applied to the sieve can be, which acts in addition to the specified during the manufacture of the screen mechanical tension and on the other hand, a possibly imprinted imbalance of zonewise detectable mechanical stresses in the sieve can be compensated.

By using a plurality of sensors and their preferred pairwise complementary arrangement and a symmetrical arrangement of the pairs to each other and a symmetrical arrangement of the pairs based on the printing screen, it is also possible by means of a corresponding arithmetic unit, at least in zones, the locally acting in the printing screen screen stresses spatially resolved and continuous determine during operation, whereby it is possible to detect occurring imbalances of the tensile stresses with respect to possibly recorded as reference values in a control initial voltage values and thus to clearly detect a beginning Siebriss.

It may be expedient to make the frame of the screen mechanically less stable compared to conventional screen frame, so as to exercise a force on the receiving strip of the holding frame easier. It may also be expedient not to connect the frame elements of the screen frame in the respective corners firmly but, for example, pluggable or flexible, so that in a carried out by means of the holding frame retightening of the mesh a more homogeneous distribution of mechanical stresses in the individual zones is possible ,

After clamping the screen frame in the receiving strips of the receiving device or the holding frame, in a possible embodiment, a first tensile force exerted by each of the segments on the respective associated portion of the screen frame on the respective pulling device, so that sets a predetermined mechanical stress in the mesh, which is at least locally in the active region of the screen substantially the same. The active area of the screen is defined as the area in which the image to be printed is introduced into the screen fabric.

This first tensile force can be chosen such that when the wire tension decreases, it can always be reliably measured via the mentioned force sensors. At the same time, the tensile forces of the traction devices can be set so that the tensile forces measured by the opposing force sensors are equal and / or at least electronically compensated by their measured values being subtracted from one another in an evaluation circuit, for example.

Since during normal operation both work squeegee and flood squeegee exert on the screen mesh a substantially defined force by alternately or simultaneously drawn during their movement over the screen surface acts on the screen surface locally an additional tensile force against the frame clamped in the holding frame, which can be measured by means of the force sensors. The fact that this additional tensile force is also distributed symmetrically on the screen frame due to the generally symmetrical arrangement of the doctor blade to the screen surface, a symmetrical mechanical voltage change is also measured by the force sensors, which also compensates in the normal case.

It is also possible to detect an initial asymmetry of the measured forces, for example, in an additional initial step and to deposit it as a reference force image in a controller. All subsequent measurements and changes can then be checked against this reference force image.

It is thus also possible to continuously detect an initial, usually undesirable, asymmetry of the additional forces exerted via the doctor blade and, if appropriate, to compensate automatically by means of corresponding devices on the doctor blade holders, so that a defined starting state can be established at the beginning of a printing process.

Occurs in the subsequent normal operation, for example, the case that the screen mesh tears at one point, the ratios of the measured forces are disturbed, at least in the associated complementary pairs of sensors, which is recognized, for example, above a certain amount of interference by a corresponding control as an error condition, which, for example a stop of the printing press can be triggered with a corresponding warning message.

Depending on the size of the crack, the measured differences in force can be so great that they can already be clearly identified via the force sensors without additional force exerted by the doctor blades, or at least when crossing the affected area with one of the doctor blades, the force difference is detected via the corresponding complementary sensor pairs can. This is particularly relatively easy and reliable to recognize, because due to the multiple force sensors on each side additional comparison of the measured forces of adjacent pairs of sensors can be done to reliably exclude other external influences.

Moreover, if an above-mentioned reference force image is used, then even the smallest changes, and in particular asymmetries, can be recognized particularly easily. In order to increase the sensitivity further, it may also be possible to buffer a continuous history, at least within a running time window, of the forces measured, for example, with each sweep of the doctor blades across the screen surface, so as to provide each force change for each complementary pair of sensors compared to its respective immediate past.

Such intermediate storage can take place, for example, in a corresponding control according to the FIFO principle (First In First Out).

Figur 1:

einen typischen Siebdruckrahmen mit einem bebilderten Siebgewebe zur Bedruckung in einer Druckmaschine

Figur 2:

eine erste erfindungsgemäße Ausführung zur Messung und Nachregelung der Siebspannung

Figur 3:

eine zweite erfindungsgemäße Ausführung zur Messung und Nachregelung der Siebspannung

Embodiments of the invention are illustrated in the following figures. Show it:

FIG. 1:

a typical screen frame with an imaged screen fabric for printing in a printing press

FIG. 2:

a first embodiment of the invention for measuring and readjusting the wire tension

FIG. 3:

a second embodiment of the invention for measuring and readjusting the wire tension

The representation of an imaged screen frame, as it is commonly used in industrial screen printing is schematically in FIG. 1 shown. On a screen printing frame 1, which is for example rectangular and the 4 side frame members 1a, 1b, 1c, 1d comprises, a coated screen fabric 2 is stretched so that it has a certain predetermined during manufacture basic tension.

Frequently, the screen fabric 2 is glued to the frame, on the one hand to ensure a good binding of the screen fabric 2 to the screen printing frame 1 and on the other hand produce a color-tight connection between the screen printing frame 1 and the screen fabric 2. An attachment can also be made in other ways, e.g. also by pinching the screen mesh between an upper and a lower frame part.

The coating 2a of the screen fabric 2 is removed for printing at the points 2b, so that an ink between the mesh of the screen fabric 2 can be pressed by means of a doctor blade 30 in a screen printing unit and the ink can be transferred to a substrate. During the printing process, the screen cloth 2 is repeatedly loaded and stretched by the action of the doctor blade 30 in the direction of the arrow 100, so that the original screen tension decreases over time.

FIG. 2 shows a first embodiment according to the invention for measuring the wire tension. For this purpose, the screen frame 1 covered with an illustrated screen cloth 2 is clamped in a holding frame 3 which forms the receiving device and its respective receiving strips 3a, 3b, 3c, 3d assigned to the frame elements of the screen frame into individual segments 3a.1, 3a.2, 3a. 3, ... or 3b.1, 3b.2, 3b.3, ... or 3c.1, 3c.2, 3c.3, ... or 3d.1, 3d.2, 3d.3, ... are divided. The holding frame shown here does not form a closed holding frame, since no segments are provided directly in the corner regions. However, this may also be provided in an alternative embodiment, in particular wherein then also a diagonal tensile force or force measurement in diagonal direction may be possible.

The arrangement of the respective segments relative to each other is such that on the one hand the opposing receiving strips 3a and 3c and 3b and 3d each have the same number of segments and on the other the size of all segments is equal and continue two segments of opposite receiving strips each other lie opposite and thus each form a corresponding pair of segments. For example, the segments 3a form. 1 and 3c. 1 and the segments 3a.2 and 3c.2, etc. each one pair.

It is further provided according to the invention that each of the segments 3a.1, 3a.2, ..., 3b.1, 3b.2, ..., 3c.1, 3c.2, ..., 3d, 1, 3d.2, ... at least one force sensor 6 is associated, which is integrated in a possible embodiment, for example, in the respective segment and detected by which each acting on the associated segment portion of a force exerted on the mesh 2 force and to a not shown Parent control is forwarded and processed there by means of a control program.

A force 100, which, as in FIG. 1 is shown perpendicular to the screen surface 2 is applied, is thus of the different sensor elements 6 via the Sieve frame 1 and the respective segments 3a.1, 3a.2, ..., 3b.1, 3b.2, ..., 3c.1, 3c.2, ..., 3d.1, 3d.2, .., detected with different strength, depending on where it acts on the mesh 2. Thus, for example, a force acting horizontally on the screen cloth 2 at the point A is detected by the sensors 6 of the segments 3a.2 and 3c.2 of this sensor pair with the same strength, whereas the sensors 6 of the segments 3b.2 and 3d. 2 each measure different forces.

The same applies, for example, to the point B shown, in which all respective sensor pairs considered each measure different force components.

The position and the mean strength of the force action can be calculated continuously from the determined force components of each individual sensor 6 and their relationships to one another in the higher-order control. The same applies to the action of force by means of a squeegee acting essentially linearly on the screen cloth 2.

By the sensory and computational determination of the force relationships in the normal operating state, in particular when using a new and unused sieve, it is thus possible to have an initial spatially resolved image of the force distribution, for example in the higher-level control e.g. stored by storage in a memory and used in the subsequent continuous measurements as a reference image, for example, by during the first movement of the doctor over the unused surface of a new printing screen or screen mesh, the initial balance of power are detected. Of course, it is also possible to derive and store an averaged force reference image from a certain number of doctor blade movements.

If, after a certain time, the screen cloth 2 begins to tear at one point due to signs of fatigue, then a force pattern, which is greatly changed at least in the area of the sieve crack, is directly transmitted via the sensors 6 detected by the controller, whereby, for example, a machine stop can be triggered to counteract contamination or incorrect printing.

In order to ensure a safe working range of the sensor devices 6 and to counteract a general decrease in the screen tension, it may further be provided according to the invention, each of the segments 3a.1, 3a.2, ..., 3b.1, 3b.2 3c .1, 3c.2,... 3d.1, 3d.2,... To be provided with a respective pulling device and / or printing device 40, which acts on the respective segment via a respective connection 4, for example.

As a pulling device, for example, electric motors, pneumatic cylinders, linear motors or the like can be used. By an appropriate control of the traction devices 40, it is possible, for example, in addition to the initial already impressed on the printing screen 2 in the production screen tension to superimpose a further tensile stress, whereby it is on the one hand possible to set a respective favorable operating point of the sensors 6 and others, for example an initially determined inhomogeneity of the embossed in the printing screen 2 screen tension largely compensate, for example by the traction devices 40 of the respective segments 3a. 1, 3a.2, ... 3b.1, 3b.2, ..., 3c.1, 3c.2, ..., 3d.1, 3d.2, ... are controlled accordingly.

Furthermore, it can be inventively provided to compensate for a decrease in the screen tension occurring, for example, due to fatigue phenomena by a corresponding control of the respective pulling devices 40.

FIG. 3 shows a further embodiment of a device according to the invention for detecting the wire tension, in which case the sensors 6 are arranged in / on the respective segments that they are directly in contact with the screen fabric 2, for example, at a certain distance to the screen frame 1. For a force effect towards 100, as in FIG. 1 As shown, each sensor 6 experiences each segment a certain force whose strength depends essentially on the distance of the sensor 6 to the force.

Such sensors 6 can be embodied, for example, as a sensor cable and operate, for example, on the basis of piezoelectricity, the dielectric located in a coaxial cable having piezoelectric properties in addition to its insulating properties. A force acting on the plastic sheath of the cable thereby also deforms the piezoelectric dielectric in the interior of the coaxial cable, whereby a voltage pulse is generated at the ends of the cable. Alternatively, other sensors may be used, which for example, work as part of a resonant circuit and in which a force on the sensor causes a frequency shift, which can be evaluated accordingly.

With regard to all embodiments, it should be noted that the technical features mentioned in connection with one embodiment can be used not only in the specific embodiment, but also in the other embodiments. All disclosed technical features of this invention description are to be classified as Erfrhrung essential and arbitrarily combined or used alone.

Claims (17)

Device, in particular for a screen printing machine, for measuring the wire tension of a screen fabric in a screen frame, characterized in that it comprises a receiving device (3) for receiving the screen frame (1), the receiving strips (3a, 3b, 3c, 3d) each in a plurality Segments (3a1, 3a2, ..., 3b1, 3b2, ..., 3c1, 3c2, ... 3d1, 3d2, ...) are divided and each segment (3a1, 3a2, ..., 3b1, 3b2 , ..., 3c1, 3c2, ... 3d1, 3d2, ...) at least one sensor device (6) is associated with the Zugspannungsmessung. Apparatus according to claim 1, characterized in that each segment (3a1, 3a2, ..., 3b1, 3b2, ..., 3c1, 3c2, ... 3d, 3d2, ...) a controllable device (40) for Exercise of compressive forces and / or tensile forces is assigned. Device according to one of the preceding claims, characterized in that each receiving strip (3a, 3c) of a longitudinal side of the receiving device (3) in M segments (3a1, 3a2, ..., ..., 3c1, 3c2, ...) divided and each receiving strip (3b, 3d) of a transverse side of the receiving device (3) is subdivided into N segments (3b1, 3b2, ... 3d1, 3d2,...), in particular whereby M x N surface areas in the screen fabric (2a) result, in which the wire tension can be determined. Device according to one of the preceding claims, characterized in that with each sensor device (6) in each case an associated portion of the wire tension of a in the receiving device (3) is clamped and provided with a screen fabric (2a) screen frame (1) is measurable, in particular each Sensor device (6) is connected to a higher-level control. Device according to one of the preceding claims, characterized characterized in that the segments (3a1, 3a2 ".., 3b1, 3b2, ..., 3c1, 3c2, ... 3d, 3d2, ...) of the receiving strips (3a, 3b, 3c, 3d) are at least in pairs similar are executed and arranged in pairs opposite each other. Device according to one of the preceding claims, characterized in that via the sensor means (6) respective measured forces in a higher-level control are computationally converted to an at least spatially resolved force distribution image, in particular wherein the spatial resolution corresponds at least to the number and size of the measuring surfaces. Method, in particular for a screen printing machine, for measuring the wire tension of a screen fabric in a screen frame, characterized in that a sieve frame (2) provided with a sieve frame (1) is in a receiving device (3) is attached / whose receiving strips (3a, 3b , 3c, 3d) are each subdivided into a plurality of segments (3a1, 3a2, ..., 3b1, 3b2, ..., 3c1, 3c2, ... 3d1, 3d2, ...) and are equipped with sensor devices (6), of which at least one in each case is assigned to a segment (3a1, 3a2,..., 3b1, 3b2,..., 3c1, 3c2... 3d, 3d2,...) which is arranged between a segment (3a1, 3a2 , ..., 3b1, 3b2, ..., 3c1, 3c2, ... 3d1, 3d2, ...) and the screen frame (1) acting force is detected. A method according to claim 7, characterized in that the respective sensor devices (6) of the segments (3a1, 3a2, ..., 3b1, 3b2, ..., 3c1, 3c2, ... 3d, 3d2, ...) at a force acting on the screen fabric (2a) measure a respective, depending on the point of application of the force on the printing screen (2, 2a) different force. Method according to claim 7 or 8, characterized in that the forces respectively measured via the sensor devices (6) are converted by computation into an at least spatially resolved force distribution image, in particular wherein the spatial resolution is at least the number and size corresponds to the measuring surfaces. Method according to one of the preceding claims 7 to 9, characterized in that a continuous measurement of the wire tension during a printing process with all sensor devices (6) takes place and each determined therefrom force distribution image is compared with a reference force distribution image. A method according to claim 10, characterized in that a reference force distribution image at the beginning of a printing process from the measurement of at least one printing operation is determined, wherein the at least one doctor blade (30) of the printing device on the surface of the screen fabric (2a) sweeps. A method according to claim 10, characterized in that a reference force distribution image is determined continuously by averaging out of a plurality of printing processes in which the at least one doctor blade (30) of the printing device cyclically sweeps over the surface of the printing screen (2, 2a). Method according to one of the preceding claims 7 to 12, characterized in that the forces are measured by opposing force sensors (6) and compared with each other. A method according to claim 13, characterized in that tensile forces of the traction devices (40) are adjusted so that the tensile forces measured by the opposing force sensors (6) are equal and / or at least electronically compensated, in particular by their measured values in one Evaluation circuit, for example, be subtracted from each other. Method according to one of the preceding claims 7 to 14, characterized in that at least within a predetermined / predetermined time window, preferably continuously the history which is intermediately stored at each sweep of the doctor blades (30) over the mesh surface (2a). A method according to claim 15, characterized in that for each complementary sensor pair (6) the currently recorded forces are compared with the forces measured during the previous sweeping of the doctor blade (30), in particular wherein in a deviation, a readjustment of the forces by driving the traction devices (40 ) he follows. Method according to one of the preceding claims 7 to 16, characterized in that by means of the tension / compression devices (40) a desired between segment (3a1, 3a2, ..., 3b1, 3b2, ..., 3c1, 3c2, .. .3d1, 3d2, ...) and Siebrahmen (1) acting force is adjusted, in particular depending on the measured forces.

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