PRINTING UNIT WITH SYNCHRONISED CONTROL FOR ROTARY OR FLAT SILK-SCREEN PRINTING MACHINES

Abstract: A printing unit for rotary, frame or cylinder (20) silk-screen printing machines, which are used for printing of substrates, and in particular textiles. Two spindles (50) for each printing unit drive a ring (30) of the printing cylinder (20), whereas each spindle (50) is controlled by means of a brushless-type electric motor (60), an epicycloidal reduction gear (70) with reduced play, a toothed drive belt (10), and ball bearings which have a small cross section, such that the digital drives of the electric motors (60) control the speed of the motors (60) in a closed loop. The digital drives are in turn controlled by axis control cards which are integrated in a PLC, in order to obtain synchronisation of the motion of the cylinders (20) and of the conveyor belt (40) for the substrate to be printed.

Description: PRINTING UNIT WITH SYNCHRONISED CONTROL FOR ROTARY OR FLAT SILK-SCREEN PRINΗNG MACHINES The present invention relates to a printing unit 5 with synchronised control, for rotary, frame or cylinder silk-screen printing machines, for printing of substrates, in particular of the textile type. The rotary printing machines which are known at 10 present comprise a plurality of printing units, which each support a silk-screen printing frame or cylinder; for each printing unit there are provided two spindles, which drive a ring of the cylinder. 15 In order to synchronise the transmission of the motion between the cylinders and the conveyor belt for the substrate to be printed, a unit is generally provided to control the rotation of each cylinder by means of a single shaft. The two spindles, which transmit the motion 20 to the cylinder, can be independent from one another, or can be connected integrally by means of shafts and gears; in the first case, since it is possible to control a single spindle at a time, the friction between the cylinder and the substrate to be printed gives rise to 25 radial torsion on the cylinder, which detracts from the printing result. In particular, if, for requirements of reproduction of the pattern, it is necessary to make the pressures of 30 the doctor blades, which carry out the task of pressing the dye from the interior of the cylinders, different, the friction differs, and thus the torsion is not equivalent between successive cylinders, thus giving rise to obvious defects of ratio, and of printing blurs. Even when the spindles are connected integrally, various problems can occur, such as the lack of orthogonality of the cylinder, misaligned construction of the rings, and friction, which can arise between the cylinder and the substrate, and can give rise to connection play between the two spindles, which can be passed on to the transmission gears, and lead to radial torsion on the cylinder; it will be appreciated that these problems are gradually accentuated, owing to the normal mechanical wear of the components. In order to correct these defects, displacement is known of the printing unit relative to the conveyor belt, but this solution is also a compromise, since, although the apparent orthogonality is corrected, and thus corrects the centring of the motifs etched on a printing cylinder relative to the remainder of the pattern which is contained on other cylinders, this gives rise to secondary effects, which have a highly significant effect on the functioning of the machine. In fact, the forces which are generated on the conveyor belt as a result of the lack of orthogonality of the cylinder relative to the belt itself, tend to deflect the latter from its straight path. The final result is that it is impossible in practice to obtain perfect centring between the various cylinders, since the displacements of the conveyor belt are random and unforeseeable. The object of the present invention is thus to provide a printing unit with synchronised control, for rotary, cylinder machines, which eliminates the above- described disadvantages, i.e. to provide a substrate printing unit for rotary, cylinder machines in which there is radial synchronism between the various cylinders, and which does not give rise to undesirable secondary effects, in order to improve the definition of the printed product in comparison with the known art. A further object of the present invention is to provide a printing unit with synchronised control, for rotary, cylinder machines, in a manner which is sufficiently simple and with substantially low costs, as a result of the advantages obtained. These objects, according to the present invention, are achieved by a printing unit with synchronised control, for rotary, frame silk-screen printing machines according to claim 1, to which reference is made for the sake of brevity. Control of the rotation of the cylinder is undoubtedly the most important and most difficult aspect to be dealt with during functioning of a rotary machine, partly also owing to the various inaccuracies in construction of the cylinder, such as the lack of orthogonality of the etchings and/or the imperfect securing of the attachments or rings which are necessary for transmission of the rotary motion. Incorrect synchronism gives rise to a poor printing result on the substrate, with obvious problems of centring of the various cylinders which form the pattern; in fact, unintentional displacements give rise to blurs of colour on the substrate, since the printing of the colours carried out by the previous cylinders does not coincide with that carried out by the successive cylinders. All these defects are then amplified during the changes of printing speed, accelerations and decelerations, which are always present during the processing. According to the invention, there is advantageously provided a system for transmission and corresponding control of the motion between two spindles, which synchronise the movement of each cylinder with the conveyor belt of the printing machine. The motion is transmitted to the two spindles in a manner which is completely independent and free from all mechanical constraints; this is achieved by means of direct connection of an electric motor for each spindle. The two spindles thus remain free from one another, and can rotate with phases which are displaced relative to one another by an appropriate angular value, so as to compensate for the errors of connection of the rings to the cylinder, which inevitably occur when the cylinder itself is fitted on the machine. During functioning of the rotary machine, the two spindles rotate perfectly synchronously, since during fitting, the inaccuracies of construction of the cylinder are compensated for. An adaptive control, which is allocated to an electronic processor applicative programme designed specifically for the system in question, monitors the difference in torque between the two motors, and provides the possibility in real time of obtaining variations of settings, for correct functioning of the machine, and the lack of orthogonality between the cylinder and conveyor belt is revealed by the increase in the value of the torque which is generated between the two motors. This therefore permits control of all the forces which are imparted to the conveyor belt, and by means of the control provided by the applicative programme, it is possible to optimise the positions of all the cylinders, in order to minimise these forces, and thus allow the belt to advance without deflections. Further objects and advantages of the present invention will become apparent from the following description and the attached drawings, provided purely by way of explanatory, non-limiting example, in which: figure 1 shows schematically a lateral view of a printing unit for rotary, frame silk-screen printing machines, according to the present invention; and - figure 2 is a view in cross-section along line II-II of figure 1, according to the present invention. The aforementioned figures show one of the printing units mounted conventionally on a rotary, cylinder machine, used for printing of substrates of the textile type, or more generally for other uses. In fact, rotary, frame or cylinder silk-screen printing machines comprise a plurality of printing units, each of which supports a silk-screen printing cylinder, which fixes colour printing in specific areas of the substrate, according to movement in a direction which is perpendicular relative to the direction of translation of a conveyor belt for the aforementioned substrate to be printed. The various printing units, which act at successive moments on the various predetermined areas of the substrate, compose the required pattern on the substrate at the output from the rotary machine. Each printing unit supports a silk-screen printing cylinder 20, which in its interior contains blade or rod- type doctors, which print the dye onto pre-determined areas of a substrate, which is moved on a conveyor line or belt 40, positioned in a direction which is perpendicular relative to a longitudinal plane of symmetry of each cylinder 20, which passes through the axis of the belt. Each silk-screen printing cylinder 20 is rotated by means of two spindles 50, which are provided in the position of the two bases of the cylinder 20, at the sides of the conveyor belt 40. Each spindle 50 drives a ring 30 of the cylinder 20, by means of a toothed drive belt 10, whereas each spindle 50 is controlled by means of direct connection of a brushless-type electric motor 60, which is connected to a reduction gear 70 of the reduced play, epicycloidal type. The motor 60 rotates a pulley 80, which, by rotating in the direction of the arrow F in figure 1, drives the drive belt 10, which is connected by means of ball bearings with a small cross-section, to the ring 30 of the cylinder 20. In this case, the drive thus obtained guarantees that the spindles 50 are controlled without mechanical play. In addition, the motion is transmitted to the two spindles 50 of each cylinder 20 in a manner which is completely independent, and is free from any mechanical connection, such that the two spindles 50 are completely independent from one another, and can rotate according to angular directions with phases which are displaced as required, in order to compensate for any errors of connection of the rings 30 to the cylinders 20, during fitting of the cylinders 20 themselves on the printing machine . During functioning of the machine, the two spindles 50, which are provided for each silk-screen printing cylinder 20, rotate in a manner which is perfectly synchronised with one another, compensation having been obtained for the inaccuracies relative to the construction of the cylinder 20 itself. This synchronism is obtained by piloting each brushless electric motor 60 associated with the respective spindle 50, by digital drives, which are actuated by an angular transducer integrated in each motor 60. The digital drives control the speed of the motors 60 in a closed loop, and are in turn controlled by microprocessors, which provide the axial control of each cylinder 20. The aforementioned microprocessors are contained in axis control cards which are integrated in an electronic processing system of the PLC type. The axis control cards comprise corresponding interfaces, which execute a control algorithm of the PID type; in particular, the axis control cards, which are actuated by means of a signal obtained from each angular transducer integrated in each electric motor 60, carry out position control of the motors 60 in a closed loop. Regulation of the PID type of the control cards is managed by means of an applicative programme for electronic processors, to be implemented on each printing machine. In particular, the software control unit of the machine monitors a difference in torque between the two motors 60 of each cylinder 20, and makes it possible to vary in real time the torque values of these motors 60, such that the said settings by a user guarantee correct functioning of the machine. The lack of orthogonality between each silk-screen printing cylinder 20 and the conveyor belt 40 is revealed by the increase in the torque value, which is generated between the two electric motors 60 of each cylinder 20 during functioning of the machine. The use of two independent motors 60 on each cylinder 20 thus makes it possible to control all the forces which are discharged onto the conveyor belt 40 during printing of the dye on the pre-determined areas of the work substrate, whereas by means of the software control, it is possible to optimise the positions of all the cylinders 20 present on a rotary machine, in order to minimise these forces, simultaneously synchronising the rotary motion of all the cylinders 20 with one another, and the motion of each cylinder 20 relative to the translation motion of the conveyor belt 40, such as to allow the belt, which supports the substrate, to be printed to advance without being deflected during the processing. In fact, the applicative software of the printing unit according to the present invention manages the regulation of the axis control cards according to the operative methods of the printing machine, so that the regulation of the PID type of the axis control cards takes place respectively according to tracking or positioning modes. During the printing step, i.e. when the cylinders 20 are rotating in a manner which is synchronised with the conveyor belt 40, the tracking mode consists of tracking by the motors 60 of a position reference which is obtained from an incremental transducer device fitted to a control motor for the conveyor belt 40. In this step, there are active the commands for manual correction of the timing between the two motors 60 of each cylinder 20, which in fact operate by adding or subtracting a fixed, pre-determined speed value to or from the momentary speed value of each motor 60, throughout the duration of the command. The correction commands can be imparted to both the spindles 50, or to only one of them, in this case obtaining a reciprocal phase displacement value. During the printing operation, the torque values applied to the two spindles 50 are acquired by the applicative programme, and displayed on appropriate screens; in addition, the differences between the torque values planned, and the abnormal values which can be encountered, are revealed. When the machine is set correctly, it is possible to store the angular position values of each spindle 50 relative to each printing unit. During the steps of preparation of the machine, and of preparation and fitting of the cylinders 20, the microprocessors for the axis control cards are configured in the positioning mode. In this step, it is possible to execute an operation of resetting each parameter relative to the machine, which consists of searching for an unambiguous initial angular position for each spindle 50 of each printing unit; this position is detected by an optoelectronic transducer device which is integrated in each spindle 50, and the operation of resetting the machine parameters puts the two spindles 50 into phase with one another, relative to each cylinder 20, and relative to every other printing unit installed on the machine. Subsequently, it is possible to carry out automatic positioning of the rotary machine, i.e. to carry out a search for angular phase positions which are displaced relative to each pair of spindles 50 of a single printing unit, and of the two spindles 50 of one printing unit relative to the other units. The positioning is carried out on the basis of angular position values which are stored in a preceding repetition of the processing, or of theoretical angular position values obtained from the geometric characteristics and from the nominal development of the cylinders 20 used for all the printing units which are in use on a single machine. When the cylinders 20 are fitted on the machine, it is also possible to carry out self-alignment of the rings 30, by means of automatic learning of a natural position of the cylinder 20; this operation is carried out with the cylinder 20 mounted on the machine, and tensioned according to a pre-determined speed of translation of the conveyor belt 40, by disabling the control of the angular position of one of the two spindles 50, and re-enabling it a few seconds later, with the basic consequence that the spindle 50, which is free to move, assumes a position which corresponds to the natural deployment of the cylinder 20 fitted. The description provided makes apparent the characteristics and advantages of the printing unit according to the invention, with synchronised control, for frame silk-screen printing machines. Finally, it is apparent that numerous other variants can be applied to the printing unit with synchronised control which is the subject of the present invention, without departing from the principles of novelty which are inherent in the inventive concept, and it is also apparent that in the practical implementation of the invention, any materials, forms and dimensions can be used for the details illustrated, according to requirements, and can be replaced by others which are technically equivalent.

Claims: CLAIMS 1. Printing unit with synchronised control for rotary, frame or cylinder (20) silk-screen printing machines, comprising a cylinder (20) , which can print imprints of colour onto pre-determined areas of a substrate which is advanced on a conveyor line or belt (40) along a direction which is substantially perpendicular to an axis of rotation of the said cylinder (20) , the said cylinder (20) being rotated by at least one electric motor (60), by means of at least one spindle (50) , which actuates a drive belt (10) connected to the said cylinder (20), characterised in that the said printing unit comprises two spindles (50) , each of which is connected directly to an electric motor (60), such that the said spindles (50) remain free from one another, and can rotate with phase displacement relative to one another by a specific variable angular value. 2. Printing unit according to claim 1, characterised in that each spindle (50) drives a ring (30) of the said cylinder (20), by means of the said drive belt (10) . 3. Printing unit according to claim 1, characterised in that each spindle (50) is controlled by means of a brushless-type electric motor (60), which is connected to an epicycloidal reduction gear (70) with reduced play, which transmits the motion to the said cylinder (20) , by means of pulleys (80) and ball bearings with a small cross-section. 4. Printing unit according to claims 1 and 3, characterised in that the said electric motor (60) is controlled by digital drive means, which is actuated by an angular transducer device which is integrated in each motor (60) . 5. Printing unit according to claims 1 and 4, characterised in that the said digital drive means control the speed of the said electric motors (60) in a closed loop, and are in turn controlled by cards, which are integrated in a PLC, for controlling the axis of rotation of the said cylinders (20), which belong to a plurality of printing units present on a rotary machine. 6. Printing unit according to claims 4 and 5, characterised in that the said control cards, which are actuated by a signal obtained from the said transducer device, control the position of the electric motors (60) in a closed loop. 7. Printing unit according to claims 4 and 5, characterised in that the said control cards consist of microprocessors, which perform a control algorithm of the PID type, by means of an applicative programme for electronic processors. 8. Printing unit according to claim 7, characterised in that, according to the operative methods of the said rotary machine, the said microprocessors function in a tracking mode or in a positioning mode. 9. Printing unit according to claims 1 and 8, characterised in that, during the printing step, the said tracking mode consists of the fact that the said electric motors (60) track a reference in a position which is obtained from an incremental transducer device fitted to a control motor of the said conveyor belt (40) . 10. Printing unit according to claim 9, characterised in that commands for manual correction of the phase displacement are active, such as to add or subtract a fixed angular speed value to or from the momentary angular speed value of the electric motor (60), for the duration of the said commands. 11. Printing unit according to claims 1 and 8, characterised in that, during the step of preparation of the said rotary machine, the said positioning method consists of an operation of searching for an initial, unambiguous reference position for each of the said spindles (50), in order to put the said two spindles (60) into phase with one another, the said initial position being detected by an optoelectronic transducer device, which is integrated in the said spindle (60). 12. Printing unit according to claims 1 and 11, characterised in that the said operation of searching for an initial reference position is followed by an operation of automatic positioning of the said rotary machine, consisting of searching for positions of phase displacement of one of the said spindles (60) relative to the other, and of the said two spindles (60) relative to other printing units, the said positioning taking place on the basis of positions stored in a previous repetition of a single processing operation, or of theoretical positions obtained from geometric characteristics of the said cylinders (20) used for the processing.

Inventors: NISOLI FABIO, NISOLI, FABIO
Applicants: ALTAIR S R L, NISOLI FABIO, ALTAIR S.R.L., NISOLI, FABIO
Application year: 1999
Main classification: B41F15
IPC codes: B41F13, B41F15