DE9011273U1 - Screen printing machine - Google Patents

Description

^{ . DIPL-ING. PETER OTTE PATENT ATTORNEY D-7250 Leonberg

Representative at the European Patent Office / European Patent Attorney Tiroler Straße

2367/ot/mü July 23, 1990

Company E,M. Müller KG, Keuchelbergstr. 3, 7107 «Sordheim 2

Screen printing machine state of the art

The invention relates to a screen printing machine according to the preamble of claim 1.

Screen printing machines are known in many different forms - some embodiments are described, for example, in the publication "Screen printing machines EMM" as a special edition from 1S+L, 8th year, No. 6, 1987. The present invention is particularly suitable for so -called belt printing systems, in which the material to be printed is transported from the feed position to the printing position by means of a circulating conveyor belt, usually a vacuum belt, and is printed at this point when the belt is at a standstill. Such belt printing systems are high-quality precision machines that are able to work at high speed, whereby a large number of precision conditions must be met, including, above all, that the material to be printed is transported to the printing position with very high precision.

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is transferred, of course, so that with today's common multi-colour printing techniques a highly precise coverage is ensured in the individual printing processes.

The general structure of such precision ribbon printing systems does not need to be discussed in detail, as this is generally known - however, when transferring the material to be printed in the area of the printing unit, it has been found that despite the

■y The use of high-precision components in conjunction with high-precision control and regulation technology means that moving into the final printing position remains a problem, even if high-precision pulse-controlled stepper motors are used to drive the transport rollers, which in turn transport the vacuum belt carrying the material to be printed, with the pulse numbers specified in the counting process of the control being an order of magnitude higher than the desired accuracy range, - and this is due to slip factors between the driven transport roller (and thus the motor) and the transport belt, which cannot be predicted and therefore cannot be

) are not predictable, but occur essentially erratically.

The invention deals with this problem and is based on the knowledge that the residual slip that occurs, giving rise to the slightest inaccuracies in the border area, and which cannot be overcome even with highly precise detection of the incoming material leading edge by means of a light barrier and input of the material length via pulse numbers, has its main cause in the butt joint of the endless vacuum belt, additionally or essentially influenced by such variables as intermittent operation of the

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Belt, different speeds, the problem that the belt joint can be located at basically any point when the belt is at the start-up speed, so that in conjunction with other smaller influencing factors a slip occurs which is largely impossible to calculate in advance and yet is in a range of magnitude that the desired accuracy of, for example, at least a tenth of a millimeter when the printing position is reached fluctuates and can also exceed these values if one introduces some numerical data here which are useful for understanding the invention, but with the express reference that the invention is of course not restricted to such data.

In other words, regardless of which side the belt impact is on at the moment of start-up, this leads to a different slip relative to the normal slip that already exists, which can therefore be calculated and then included in the control process, whereby the intermittent operation with different piece sizes and the change in speed during start-up, which also result from this, are additional negative influencing factors. The desired precision in the final positioning of the material in the printing position cannot therefore be achieved even if, as is well known, limit switches or similar systems are used.

The invention "is therefore based on the task of ensuring that the material to be printed on a screen printing machine is printed on with the elimination of erratic

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Slippage influences can be transferred to the final position for printing with the highest accuracy.

Advantages of the invention

The invention solves this problem with the characterizing features of claim 1 and has the advantage that influences on the precision positioning caused by the belt and its characteristics, but also by the drive, the control, the normal slip values, etc. are completely eliminated because the invention also carries out the transfer to the end position by means of appropriate high-precision control of the respective transport roller drive motor, but from and based on the query of the belt run itself, in that the movement size of the belt and thus also of the material carried by it is recorded by a sensor which, as the now essential actual value sensor, is directly responsible for the current respective belt position and communicates its output values directly to the control.

The entire process is digital - a predefined number of pulses, which corresponds, for example, to ten or a hundred times the required material path in millimeters to reach the end of the print position, is entered into the control system and the motion sensor driven by the belt, which therefore travels the respective infeed path of the material itself, then delivers a current number of pulses, which is constantly compared with the predefined number of pulses corresponding to the material length, until, if the two are equal, the drive process is stopped and, if necessary, reversed.

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and precision movement into the final position, which is located with high precision.

A further advantage is that the working speed of such a high-precision screen printing machine is not affected by such a direct actual value transmitter, since means are provided to immediately return the sensor to its starting position so that it is prepared for the next feed of the next piece of material to be printed. &igr;

Another advantage is that the invention ensures that the highly precise end printing position is reached for all conceivable materials, regardless of their characteristics, thicknesses, etc., and any material size can be used. An absolutely precise coverage is achieved, which enables the perfect printing of materials that have been dried in the meantime and built up into stacks in the next run, without the risk of

* time-consuming and complicated re-adjustments

&igr; requires revision and reassessment. The invention

J manages to orient itself directly to the external conditions &igr; when transporting the materials, which means

a highly precise and, above all, reproducible accuracy that is completely free from slippage factors can be achieved.

The measures listed in the subclaims make it possible to develop advantageous further developments and improvements of the screen printing system specified in claim 1. Particularly advantageous is the design of the motion sensor in the form of a clamping element dragged along by the belt, which drives a sensor, for example a digital tachometer or the like, via a belt.

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A further advantageous embodiment consists in that the belt is a toothed belt and the pulse generator is combined with a return motor, so that immediately after the end position is reached, the associated motor returns the clamping element (as it will be referred to below) that moves with the vacuum belt or is dragged along by it.

drawing

Embodiments of the invention are shown in the drawing and are explained in more detail in the following description. They show:

Fig. 1 shows only the area of the vacuum belt with the indicated printing frame and a possible embodiment of the fine control system according to the invention for determining the position of the material to be printed and

Fig. 2 also shows a schematic side view of the clamping element of the motion sensor, greatly enlarged.

Description of the embodiments

The basic idea of the present invention is to control the drive motor for the transport mechanism in a screen printing machine, in particular in the case of a belt printing system, the drive motor for the vacuum belt, depending on the actual passage of the vacuum belt measured by measurement, up to a predetermined position and then to stop it or to move it to the

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to run into the end position, if necessary also by reversing.

The actual value of the belt passage is recorded by

a motion sensor that docks at least to the band;|

and preferably designed as a pliers clamping element so ';^;

&Aacgr; is that mechanically the belt with the &sfgr;

Material is clamped and the clamping element of the I

Belt is driven in its movement. In an advantageous embodiment, this movement is in turn transmitted to a sensor by means of an endless belt (toothed belt), which thus reproduces an exact image of the belt passage at its output, preferably in the form of a series of pulses.

It is pointed out again that the sub-elements of a tape printing system shown in Fig. 1 and Fig. 2 only represent very schematically only the sub-area that is essential for the present invention, whereby the specification and explanation of details of other peripheral components is deliberately omitted for the better understanding of the invention. The following essentially deals with the design of a special tape printing system with a vacuum belt for the slip-free transport of the material to be printed, although it is understood that the invention can also be used with all such screen printing machines or other similar arrangements and devices that have to contend with the same problems - the invention is therefore not limited to the tape printing system specified below.

in Fig. 1 the vacuum belt of the belt printing system 10

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designated 11; the two transport rollers for the vacuum belt 11 bear the reference numerals 12a and 12b. Usually one of the two transport rollers is driven, in the illustrated embodiment the transport roller 12b, by a suitable drive motor, for example stepper motor 13. The specific design of the transport belt, here as a vacuum belt, is arbitrary; the material to be printed by the printing unit (not shown) lies on the vacuum belt 111 - the printing unit is indicated in the form of a printing frame at 14.

The problem with the present invention becomes understandable when one considers that in a special embodiment, for example, to realize a multi-color print on a single substrate, several colors must be printed on top of one another, each with drying and stacking steps in between. In this case, it is absolutely essential that for each printing process in which the printing frame 14* then applies a different color in succession, the template, i.e. the material to be printed, is positioned with pinpoint accuracy by the conveyor belt 11 in the final printing position so that the colors also lie on top of one another. Variations of a tenth of a millimeter are the most that could possibly be acceptable in terms of relative offset.

As mentioned at the beginning, such accuracy requirements cannot be realized due to the basic conditions of such a tape printing system and the invention now consists in providing a motion sensor 14 which is connected to a central control unit 15, which may also contain microprocessors, for example.

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Small computers, computers etc., provide precise information about the belt run as well as the current position and finally the end position of the material to be printed.

For this purpose, the motion sensor comprises a running part 16 which is docked at least on the conveyor belt 11 and which follows the movement carried out by the conveyor belt 11 and its support to be printed on up to the final position and ultimately creates an output actual value which corresponds in principle to a value of the actual position preset, for example, on the central control unit 15, so that the drive motor 13 for the conveyor belt drive is stopped.

In detail, the motion sensor 14 is constructed in such a way that the running part 16 is formed by a pliers-like clamping element, as shown in the side view of Fig. 2; the running part 16 also has a transmission part 17 firmly connected to it, preferably in the form of a toothed belt. The toothed belt runs as an endless belt over two rollers or toothed belt pulleys 18a, 18b on both sides, the mutual distance between which is at least as large, preferably larger, than the largest length of a piece of material to be printed.

A sensor 19a is driven by one of the two toothed belt pulleys 18a, 18b, in the illustrated embodiment by the toothed belt pulley 18b, which is sensibly located on the same shaft as a return motor 19b. The sensor 19a can be a tachogenerator or another suitable device that is driven by the running part.

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The measuring device can be an electronic, mechanical or optical element that converts the rotary motion caused by the toothed belt pulley 18b into a digital pulse sequence, for example a decoder or slotted disk or the like with an associated light sensor and an optical light receiver, whereby the output pulses of the light receiver are sent to the electronic control circuit 15 via a line 20 as an actual value for the movement of the vacuum conveyor belt 11. The return motor 19b is provided to ensure that after the completion of a respective setting process the moving part 16 is immediately returned to its starting position shown in the illustration in Fig. 1 in order to then carry out a new measuring process when a new part of the material to be printed is fed in.

The basic function of the present invention can then be such that the length of the piece of material to be printed or the transport length is pre-entered into the control part by operating suitable input keys 15a, for example by setting it in a counter. As soon as the piece of material is then placed and transported towards the printing frame by the belt start, the piece of material passes with its front edge a light barrier 21 and at this point the counting process of the pulses generated by the pulse generator 19a is initiated. This can be a high-resolution system, or the pulses supplied can be multiplied electronically, so that these counting processes take place, for example, at a level that is, let's say, one order of magnitude higher than the actual measurement accuracy desired, in order to be able to work with the highest precision.

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The pulses input by the pulse generator 19a can, for example, be fed to a second counter that counts up, or they can also be used to count down the previously set counter - in any case, it is ensured that by dragging the running part 16, which clamps itself to the conveyor belt 11, encompassing this and preferably also the piece of material lying on it, held by vacuum and to be printed, a highly precise respective position indication is fed to the central control unit 15.

As soon as these two counting processes are identical, the drive motor 13 for the belt transport 11 is stopped; the piece of material is then in the printing position or is at least moved into it with incomparable precision, since the motion sensor 14 is able to indicate the actual current position of the conveyor belt and the piece of material to be printed at any moment, completely excluding any linear or non-linear slip influences, other irregularities, acceleration and braking influences, intermittent operation, different lengths of the piece of material, etc.

An advantageous embodiment of the present invention also consists in the fact that the running part 16 is guided in a highly precise manner in its movement, namely by means of a precision guide, consisting of a carrier 22, along which the clamp-like running part 16 with guide jaws 24a, 24b on both sides is mounted so that it can move linearly via a recirculating ball guide. The clamping of the running part 16 on the conveyor belt and piece of material can

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This can be done in any way; preferably an upper tong part ^16a and a lower tong part 16b are provided, both of which extend laterally, i.e. transversely to the path of the conveyor belt 11, so that the conveyor belt is located between the two tong-like parts. The upper tong part 16a can be stationary and the lower hydraulically designed as a clamping device, for which a piston-cylinder unit 25a, 25b is provided. The piston rod then carries a lower pressure plate 16c, which grasps the belt from below and presses it against the upper tong part 16a, which simultaneously results in an immovable clamping and docking of the running part 16 to the belt and pieces of material.

The toothed belt 17 preferably used here ensures that the transmission of the linear movement from the running part 16 to the toothed belt pulley 18b and from there to the pulse generator 19a is completely slip-free.

Therefore, the invention also ensures the advantage that the entry of the piece of material is detected by the light barrier 21 on the fly when the counter is zeroed at that moment, with a corresponding preset distance secured by the light barrier 21 - the running part 16 docks onto the piece of material and the conveyor belt 11 before the front edge of the piece of material passes over the light barrier 21.

As soon as the end position is reached, the clamping exerted by the running part 16 is released again, preferably by the central control circuit 15, the running part is released from the conveyor belt 11 and is returned to the starting position by appropriate control of the return motor 19b, where an immediate

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Docking to the next piece of material and the conveyor belt 11 can take place.

It is understood that the special type of clamping or docking of the running part 16 to the conveyor belt can also be designed in a way other than hydraulically; SO it is possible to clamp the running part to the conveyor belt 11 using a lifting magnet designed as a hydraulic lifting device, and other means can also be provided here to fasten the running part 16 to the conveyor belt 11.

Finally, it should be noted that the claims and in particular the main claim are attempts to formulate the invention without comprehensive knowledge of the state of the art and therefore without restrictive prejudice. Therefore, we reserve the right to regard all features presented in the description, the claims and the drawings as essential to the invention, both individually and in any combination with one another, and to set them down in the claims.

Claims (8)

DIPL-ING. PETER OTTE PATENT ATTORNEY 0-7250Leonberg Representative at the European Patent Office / European Patent Attorney Tiroler Straße 2367/ot/mu
23.07.1990 Company EM Müller KG,
Heuchelberastr. 8. 7107 Nordheim 2 protection claims; . Screen printing machine, in particular a belt printing system with a printing and application station and a conveyor belt, on which the material to be printed is conveyed in intermittent operation, characterized by an (additional) motion sensor (14) which is driven by the conveyor belt (11) and stops the conveyor belt drive motor (13) in the printing position. 2. Screen printing machine according to claim 1, characterized in that the conveyor belt (11) is a circulating vacuum belt and is moved by a drive motor (13) actuated by a central control circuit (15) which drives one of the transport rollers (12a, 12b) of the conveyor belt (11). 3. Screen printing machine according to claim 1 or 2, characterized in that the motion sensor (14) comprises a running part (16) which intermittently attaches (docks) to the conveyor belt (11) with the piece of material to be printed placed thereon and is /2 2367/ot/mu 23.07.1990 - 2 - belt is dragged along, whereby this linear movement of the running part (16) generates an actual value which directly indicates the respective throughput size of the conveyor belt. 4. Screen printing machine according to claim 3, characterized in that the running part (16) encompasses the conveyor belt (11) and the piece of material to be printed placed on it in a clamp-like manner and is guided linearly in a precision guide with recirculating ball guide (23) and at the same time drives an (endless) toothed belt (17), which in turn transmits its rotary movement to a pulse generator (19a) via one of its two end toothed belt pulleys (18a, 18b). 5. Screen printing machine according to claim 4, characterized in that the pulse generator (19a) is a coding or slotted disk and generates an output pulse sequence corresponding to the respective passage position of the conveyor belt (11) and feeds it to the central control circuit (15), in which the pulses are added up and compared with a preset pulse number corresponding to the length of the piece of material to be printed and, if they are equal, the conveyor belt (11) is stopped in the printing position or transferred to it by appropriately controlling its drive motor (13). 6. Screen printing machine according to one of claims 1 to 5, characterized in that the docking position of the running parts (16) is at a predetermined distance in front of a light barrier (21) detecting the front edge of the piece of material to be printed, in order to form a preliminary section. At the moment of the front /3 2367/ot/mu 23.07.1990 - 3 - edge passage of the piece of material to be produced a zero point setting of a counter controlled by the pulse generator (19a) of the movement sensor {14} takes place and the counter reading reached in each case is compared with a pre-set value (counter value) related to the length of the piece of material to be printed. 7. Screen printing machine according to one of claims 1 to 6, characterized in that the running part (16) has an upper and a lower clamp part (16a, 16b) which grasp and clamp the conveyor belt (11) from above and below at the moment of control by the central control circuit (15), such that the running part (16) is dragged along by the conveyor belt (11) without slipping, while simultaneously generating output pulses corresponding to the total distance covered in each case via the pulse generator (19a) for forming the actual value. 8. Screen printing machine according to claim 7, characterized in that the fixing of the running part (16) takes place by means of a hydraulic or pneumatic piston/cylinder unit, by means of control of lifting magnets or the like.