EP1025994A1

EP1025994A1 - Machine for rotary silk-screen printing of flat sheets with control system for synchronising rotation of the printing screen and feeding of the sheets to be printed

Info

Publication number: EP1025994A1
Application number: EP00200235A
Authority: EP
Inventors: Gian Piero Barozzi; Giampaolo Bianchi
Original assignee: Aisa SpA
Current assignee: Aisa SpA
Priority date: 1999-02-01
Filing date: 2000-01-21
Publication date: 2000-08-09
Also published as: IT1307658B1; ITMI990182A1; JP2000225682A; BR0000541A

Abstract

Rotary machine for the silk-screen printing of flat sheets (10a) comprising a rotating printing screen (200), a fixed doctor blade (300) arranged inside it, a conveyor belt (600) endlessly mounted on a pair of rollers, one of which is a drive roller (1600), for supplying flat sheets (10) to the printing screen (200) and an actuating motor (20,20a), there being provided devices (205,1600,605,1000,30) for controlling and adjusting the feed speed of the conveyor belt (600) so as to synchronise feeding of the flat sheet (10,Ov) to be printed, fed by means the said conveyor belt, with rotation of the printing screen (200,Or).

Description

The present invention relates to a rotary machine for the silk-screen printing of flat sheets, comprising a rotating printing screen, a fixed doctor blade arranged inside it, and a conveyor belt which is provided with devices for controlling and adjusting the feed speed of the conveyor belt, so as to synchronise feeding of the flat sheet to be printed, fed by means of the said conveyor belt, with rotation of the printing screen.
It is known in the technical sector relating to the production of glass, for example for motor cars and/or furnishings, that there exists the need to print thereon decorations and the like which are applied by means of silk-screen printing, performed using rectilinear type machines, in which the doctor blade for printing is movable on guides and, passing over the fixed printing screen, causes deposition of the ink onto the predefined zone of the glass to be printed. Although performing their intended function, these machines are unable to achieve the printing speeds which are required by modern industry since the cycle speed is limited by the idle time for the downward and upward movement of the doctor blade towards/away from the printing surface and the return stroke of the doctor blade parallel to the printing surface itself. Silk-screen printing machines of the rotary type are also known, i.e. of the type where the doctor blade is stationary inside a cylindrical screen which rotates, said machines being used in the building sector for printing tiles; these machines, however, although being able to achieve high hourly production outputs, since the cycle speed is dependent solely on the speed of rotation of the screen, nevertheless result in imprecision during printing which is unacceptable for printing on glass where tolerances of not more than a few hundredths are required.
The technical problem which is posed, therefore, is that of providing a silk-screen printing machine of the rotary type which is able to achieve very high printing precision for any form or size of sheet to be printed, and allow rapid and low-cost routine maintenance operations such as adjustment of the tension of the printing screen, cleaning thereof following each printing rotation, removal of the excess ink in the case of printing along the edge of the glass sheets for which an open area of the screen greater than the area of the glass is required, with consequent formation of an ink residue which is not deposited during rotation and which would hinder subsequent printing, adjustment of the distance between doctor blade and screen, and rapid changing of the doctor blade for the usual corrective grinding of the edge thereof.
Within the scope of this problem a further need is to be able to change rapidly the various operating parts when there is a variation in the shape of the glass and/or the impression to be printed.
These technical problems are solved according to the present invention by a rotary machine for the silk-screen printing of flat sheets comprising a rotating printing screen, a fixed doctor blade arranged inside it, a conveyor belt endlessly mounted on a pair of rollers, one of which is a drive roller, for supplying the flat sheets to the printing screen and an actuating motor, there being provided devices for controlling and adjusting the feed speed of the conveyor belt, so as to synchronise feeding of the flat sheet to be printed, fed by means of the said conveyor belt, with rotation of the printing screen.
Further details may be obtained from the following description of a non-limiting example of embodiment of the invention, provided with reference to the accompanying drawings in which:

Figure 1 shows a schematic perspective view of a machine according to the present invention;
Figures 2a,2b show a schematic front view of the machine according to Fig. 1 during two different moments of feeding of the glass towards the printing screen;
Figure 3 shows a schematic top plan view of the machine according to Fig. 1, illustrating the actuating systems thereof;
Figure 4 shows a cross-section along the plane indicated by IV-IV in Fig. 3;
Figure 5 shows a partially sectioned view, from above, of the device for actuating the screen during working conditions;
Figure 6 shows a view similar to that of Fig. 5 in the rest condition for removal of the screen;
Figures 7,8 show a front view of the screen with the device for disengagement of the doctor blade in the rest position and printing position, respectively;
Figure 9 shows a front view of a detail of the doctor-blade group with the associated device for adjusting the relative distance between doctor blade/printing screen;
Figures 10a,10b show partial cross-sectional views of the free end of the printing screen with the device for tensioning thereof in the unloading position and working position, respectively;
Figure 11 shows a partial perspective view of the screen with the associated cleaning device;
Figure 12 shows a schematic cross-section along an axial plane of the screen with the cleaning device according to Fig. 11;
Figures 13,14 show, respectively, a top plan view and a cross-section along the plane indicated by XIV-XIV in Fig. 13, illustrating the device for adjusting the relative distance between conveyor belt and printing screen;
Figure 15 shows a partially sectioned front view of the belt for conveying the workpieces to be printed;
Figure 16 shows a cross-section along the plane indicated by XVI-XVI in Fig. 15, illustrating a detail of the devices for adjusting the speed of the conveyor belt;
Figure 17 shows a view, from above, of a conveyor belt portion having perimetral profiles for containing the workpiece and devices for retaining the latter;
Figures 18, 19 show cross-sections along the plane indicated by XIX-XIX in Fig. 18 during gripping of the workpiece and positioning of the workpiece on the conveyor belt by the retaining devices, respectively.

As shown in Fig. 1, the rotary silk-screen printing machine according to the invention is composed of a structure 100 for supporting and housing the actuation, transmission and control members; a cylindrical printing screen 200 which has arranged inside it the doctor blade 300 and the ink spreader 400 and outside it the device 500 for cleaning the said screen; and a conveyor belt 600 on which the sheets 10 to be fed to the screen 200 for printing are arranged.
The printing screen 200 has a reference generatrix Or and, depending on the size of the sheet 10 to be printed, printing areas contained within surface areas angularly lying between the generatrices A,Or and Or,B, while the sector lying within the generatrices A and B must remain free so as to allow accelerations/decelerations of the speed of translation of the belt 600 - necessary for achieving synchronisation between feeding of the workpiece to be printed and rotation of the printing screen - as well as supplying of the ink during interruptions in rotation of the printing screen envisaged for this purpose.
More particularly there must be the possibility of synchronising the speed of the screen 200 and the conveyor belt 600 so as to compensate for any advance/delay of the sheet 10 (reference point Ov) with respect to the screen 200 (reference point Or) and the speed of the screen and the speed of the conveyor belt must be such that their relative speed during the printing phase is zero so that no smudging of ink occurs during printing and the precision of the latter increases.
For this purpose it is envisaged that the means for operating the machine comprise a motor 20 (Fig. 3), the shaft 20a is connected to the screen 200 by suitable means which are described in detail below and by means of which the programmed speed of rotation is obtained. The said shaft 20a of the motor 20 also has, coaxially mounted on it, in a cascade arrangement, a differential 30, the output shaft 31 of which has, mounted on it, a pulley 601 connected by means of a belt 601a to a further pulley 603 for actuating the shaft driving the belt 600.
The two planetary gears 32 of the differential 30 (Fig. 4) are mounted on a toothed wheel 33 which is idle with respect to the casing 34 of the differential and connect the two crown wheels 36a, 36b of the drive shaft 20a and the driven shaft 31.
The toothed wheel 33 in turn meshes with an endless screw 35a of a motor 35 of the differential, which motor 35 is actuated and controlled by a programming device 1000 which is activated upon receiving a signal from a sensor 605 (Fig. 2b) for detecting Ov and which is in turn connected to a sensor 205 for detecting Or. The operating principle is therefore as follows:

the sensor 205 cyclically reads Or and sends a corresponding signal to the sensor 605;
the sensor 605 detects Ov which, depending on the glass loading operation, may be in time with or in advance/delayed with respect to the reading of Or: the sensor 605 will correspondingly send a zero or negative/positive signal to the programming device 1000 which in turn
sends a negative/positive signal to the motor 35 of the differential so as to slow down/accelerate the driven shaft 31 actuating the belt 600 which, by varying its speed, ensures that the glass sheet 10 is in time with the printing screen, thereby ensuring the perfect synchronisation of the two
feeding and printing - movements and therefore the printing precision.

This adjustment must however be performed during rotation of the screen corresponding to the angular surface-area portion thereof lying between the generatrices B and A or the non-printing portion.
For correct synchronisation between screen 200 and belt 600 which allows a zero relative speed thereof to be obtained during printing, the peripheral speed of the screen must coincide with the speed of rotation of the belt and it is therefore necessary to provide specific devices for compensating for any tolerances, play, wear and the like which produce a variation in the said speed of the conveyor belt 600.
As illustrated in Figs. 15 and 16, said adjusting devices 1600 consist of means for adjusting the diameter of the roller 601b for actuating the belt 600. More particularly, said drive roller 601 consists of a first drum 1601 keyed onto a shaft 1603 supported by a frame 1602 and made to rotate by the pulley 603 connected to the differential 30 (Fig. 3).
The external surface 1601a of the first drum is conical with an inclination towards the inside (axis of rotation).
Coaxially with the first drum 1601 there is arranged a second drum 1604 which has a conical internal surface 1604a, with a conicity complementing that of the first drum to which it is joined in the axial direction.
The second external drum is fixed in position by means of a flange 1605 and an associated ring 1605a which are mounted on the shaft 1603 at the opposite axial ends of the drive roller.
With this configuration it is therefore possible to lengthen telescopically in the axial direction the drive roller 601, thus causing a corresponding increase in the external diameter which may be adjusted so as to obtain the programmed speed of rotation.
It is also envisaged that the external surface 1605b of the drum 1604 is provided with longitudinal grooves 1605c which do not pass through the thickness of the drum and which allow an actual increase in the diameter of the roller without causing structural damage thereto as a result of the action of the radial forces produced by the interaction between the conical surfaces of the two drums.
As shown in Figs. 3 and 5, the screen 200 is wound onto the external edge of two opposing discs 201a and 201b which are made to rotate by associated independent, but synchronised actuating means, so as to avoid any twisting of the screen during rotation, which twisting would be the cause of printing errors.
More particularly, the external disc 201a is mounted on the free end 202a of a shaft 202, the other end 202b of which is supported in a cantilevered manner by a toothed wheel 203 actuated by a pinion 204a mounted on the output shaft of a reducer 204 which is in turn mounted on a tubular element 205 which is concentric with the shaft 202 with respect to which it is rotationally immobile via bearings 205a.
A flange 206 is also mounted on the tubular element 205 by means of associated bearings 206a, said flange 206 being made to rotate by means of a toothed wheel 207 which is in turn actuated by a pinion 204b mounted on the input shaft of the said reducer 204.
Said input shaft also has, mounted on it, a pulley 208 connected by means of a belt 208a to a second pulley 208b (Fig. 3) mounted on the shaft 20a of the motor 20. In addition to the already mentioned absence of twisting of the screen 200, this configuration also avoids external volumes (present, however, in the known machines) and the need to disassemble mechanical parts when the screen is replaced, which can be achieved (Fig. 6) simply by: inserting into the special seats 211 of the discs 201a,201b associated support bars 212, slackening the device 220 tensioning the screen in the axial direction and rotating the said discs so as to disengage from the flange 206 a bayonet-type coupling of the disc 201b which is known per se and therefore not described in detail, but only indicated by 213 in Fig. 6.
As mentioned above, the screen requires, in order to operate correctly, tensioning in the axial direction which ensures the planarity of the generatrices of the said screen during printing so that the latter operation is precise.
This axial tensioning is obtained by means of a corresponding device 220, the detail of which is illustrated in Figs. 10a and 10b and which essentially consists of a crank 221 provided with a female thread 221c designed to engage with a corresponding threading 201c formed on an axial extension 201d of the disc 201a. The crank 221 and the disc 201a have, arranged between them, the end 223a of levers 223 which are angularly arranged at a distance of 120° and pivotably mounted by means of a pin 224 on the disc 201a, the other end 223b of which is inserted into a corresponding seat 201e in the disc 201a.
In this way the rotation (clockwise in the Figures) of the crank 221 produces the forward movement thereof in an axial direction with a consequent inward axial thrust on the end 223a of the lever 223 which, rotating about the fulcrum 224, produces a forward movement of the opposite end 223b with a consequent outward axial thrust on the disc 201a which thus tensions the printing screen 200.
Rotation of the crank in the opposite direction slackens the screen 200 and allows the subsequent operations of replacement thereof to be performed.
As illustrated in Figs. 1 to 5, with said configuration it is also possible to mount both the doctor blade 300 and the ink spreader 400, both stationary with respect to the rotating screen 200, on the tubular element 205. Said doctor blade 300 must in turn be mounted on devices which allow it to be installed/disassembled, independently of the screen 200, for the associated maintenance operations and allow adjustment of its distance from the screen so as to compensate for any wear in the edge as well as disengagement thereof from the screen for removal of the latter.
For these purposes it is envisaged (Figs. 7,8) that the device supporting the doctor blade 300 comprises a pair of flanges 301 which are mounted axially spaced on the tubular element 205 and provided with two arms 301a and 301b extending towards the screen 200.
The free end of the first arm 301a has, mounted on it, via a pin 321b, the support 320 for the doctor blade 300 which is also pivotably mounted on one end of a first lever 331, the other end of which is pivotably mounted on the second end of a second lever 332, the other end of which is pivotably mounted on the second arm 301b of the support bracket 301.
The pin 331a for connecting the two levers 331,332 also has, connected to it, the end of a strut 341 which is hinged by means of a pin 341a with the arm 301a supporting the doctor blade. The length of the operative portion of the strut is adjustable by means of screw/nut-screw 343 joining system so as to allow variation of the stable-equilibrium position of the pin 331a during printing.
The strut 341 also has, coaxially mounted on it, a spring 342 which is arranged between the pin 341a and the pin 331a and which exerts a thrusting action on the pin 331a in the raised position of the doctor blade which is thus kept stably in position, being prevented from rotating by means of gravity towards the screen. With this lever system it is possible, by means of suitable - manual or servo-assisted - movements (not shown) to cause the bracket 301 supporting the doctor blade 300 to pass from a position substantially secant with respect to the screen 200 (Fig. 7), allowing extraction of the screen, to a working position, substantially radial with respect to the screen 200 (Fig. 8).
The doctor-blade disengaging device is completed by means 350 for collecting any droplets of residual ink remaining attached to the doctor blade 300, consisting of an arm 351 which is fixed to the doctor-blade support bracket 301 and the free end of which has, hinged to it, a right-angled piece 352 extending in the axial direction over the whole length of the doctor blade 200.
The two sides 352a and 352b of the right-angled piece are arranged so that the rotation of the support 320 towards the rest position (Fig. 7) causes interference of the doctor blade with the side 352b of the right-angled piece 352, which causes anti-clockwise rotation of the said right-angled piece and positioning of its second side 352a underneath the edge of the doctor blade.
The rotation, in the opposite direction, of the doctor-blade support bracket into the printing position causes, on the other hand, interference of the doctor blade with the side 352a of the right-angled piece and the consequent clockwise rotation of the latter which releases, as a result of gravity, any ink accumulated. The stable fixing of the two end-of-travel positions is ensured by a spring 353 which is arranged between one of the two arms and the bracket supporting the right-angled piece.
As illustrated in Fig. 9, it is also envisaged that the doctor-blade support is formed by a first clamp 325 which is formed in the support body 320 and which acts against a second clamp 1325 for retaining the doctor blade 300.
The second clamp 1325 is also provided with means for adjusting the relative distance between the doctor blade 300 and the printing screen 200 so as to allow compensation of the wear affecting the said doctor blade, said wear causing an increase in the said relative distance, resulting in printing errors.
More particularly the first clamp 325 has fixed jaws 325a formed by one of the two sections of a seat 326 which is substantially in the form of an inverted U and formed in the body of the support 320 of the doctor blade 300 and movable jaws which are formed by a cam 329 pivotably mounted, by means of an associated spindle 329a, on the end of the other section 326b of the seat 326; said cam can be externally operated so as to rotate, by means of an extension 329b or the like.
The internal surface of the fixed jaws 325a is provided with a fixed pin 325b arranged at the free end of the said fixed jaws and a seat 327 arranged in the vicinity of the bottom of the U, which are designed to co-operate with a corresponding seat and extension of the fixed arm of the second clamp 1325 as will emerge more clearly below.
The second clamp 1325 is in turn formed by two arms 1325a and 1326a forming a seat in the form of an inverted U 1326, the closed part of which extends, forming a beak-like extension 1327 which is designed to co-operate with said seat 327 of the first clamp 325. The external surface of the arm 1325a is provided with a seat 1325b which is designed to co-operate with said pin 325b of the first clamp 325.
The two arms 1325a and 1325b of the second clamp are connected together in the transverse direction by suitable screw means 1328 which allow tightening/slackening of the two arms onto/away from the doctor blade 300.
The said means for adjusting the relative distance between the doctor blade 300 and the printing screen 200 are provided inside the second clamp 1325 and are substantially formed by a thin metal plate 1329 which can be inserted in the longitudinal direction onto respective parallel seats 1330 which are formed on the internal opposite surfaces of the arms 1325a and 1325b and are designed to form a transverse end-of-travel abutment for insertion of the doctor blade 300 into the seat 1326 of the second clamp.
By suitably dimensioning the height of the seats 1330 it is possible to make the interval of the said seats suitably smaller and hence ensure a corresponding fine positioning precision of the doctor blade in the radial direction.
The operating principle of the two clamps is as follows:

the thin metal plate 1329 is inserted into the associated seats 1330 corresponding to the predefined position, repeating if necessary the operation until the most suitable position is reached;
the screw means 1328 are tightened so as to fix the clamp on the doctor blade 300;
the cam 329 of the first clamp is rotated into the open position of the first clamp;
the second clamp is inserted into the first clamp so that the beak 1327 coincides with the seat 327 and the seat 1325b with the pin 325b;
the cam 329 is rotated into the locked position.

As shown in Figs. 11 and 12, the machine according to the invention envisages also means 500 for cleaning the screen 200 which are particularly important for the already mentioned operation of printing along the edge of the glass, for which an open area of the screen greater than the area of the glass is required, with the consequent formation of an ink residue which is not deposited during rotation and would adversely affect subsequent printing.
Said devices 500 are essentially formed by a device for emitting compressed air conveyed in a substantially radial direction towards the external surface of the said screen 200.
More particularly, the device 500 comprises a first tubular element 501 which is provided with a longitudinal slit 501a and supplied at the opposite end sections by means of an air supply pipe 502.
Both the tubular element 501 and the pipe 502 are supported by an arm 503 which is attached to a plate 101 connected to the frame 100 of the machine.
The tubular element 501 is connected to the support arm 503 by means 504 for adjusting the angular position thereof so as to allow precise positioning of said tubular element at a correct distance from the screen 200.
An element 505 for opposing the camber affecting the screen during tensioning thereof is also envisaged, said camber causing in fact the movement of the screen away from the air emission device, resulting in imperfect cleaning of the screen. Said opposition element 505 may for example be formed in a manner entirely similar to the doctor-blade support with its associated movements.
As illustrated in Figures 13 and 14, it is envisaged that the printing screen 200 may vary its position relative to the plane of the conveyor belt 600 so as to be able to adjust the interference of the said screen with the surface of the workpiece to be printed in accordance with the variation in thickness of the latter.
This adjustment is obtained by means of a device 1200 which comprises a screw 1201 supported by the frame 100 and coupled with a female thread 101a, inserted in the plate 101 supporting the screen 200 and oscillating inside it about an associated pin 101b.
Said plate 101, in addition to being coupled with the screw 1201, is also pivotably mounted on a pin 1202 which is arranged in a direction perpendicular to that of the screw 1201 and about which it is able rotate.
In this way, operation of the means for actuating the screw 1201, which in Fig. 14 are represented by way of example as a crank 1203, causes rotation of the plate 101 about the pin 1202 with consequent displacement of the surface of the screen with respect to the belt 600; the degree of displacement may be detected by indicator means, for example consisting of a fixed graduated scale 1205 associated with a reference notch 1205a formed on the surface of the plate 101.
As a result of the particular dimensions of the machine component parts it is possible to use simpler rotational devices in order to obtain a variation in the relative position of the screen and belt, which is comparable to a translatory movement, although more complicated actuating means would be required in order to perform this movement.
As illustrated in Figs. 15, 17, 18 and 19, the machine according to the invention comprises vacuum devices 1110 such as vacuum suction pads or the like for gripping workpieces 10 during loading and for retaining them during feeding thereof by the belt 600.
These gripping devices consist of vacuum elements 1111 which are mounted on the free end of associated rods 1112a of cylinders 1112 which are attached to the support surface 606 of the belt 600, in a fixed position corresponding to the glass loading zone.
The belt 600 is consequently provided with openings 1113 which, once the loading zone has been reached, are aligned with the said devices 1110, allowing them to emerge without interference, during the time interval where the belt is stopped in order to allow loading of the workpieces.
Said cylinders are operated by means of the cycle control devices so that, when the workpiece 10 arrives on the belt 600, they are projecting perpendicularly beyond the said belt and are able to adhere to the sheet following activation of the vacuum; subsequently the rod 1112a is recalled downwards by the cylinder so that the glass is arranged in contact with the belt and inside perimetral profiles 1210 for containing the sheet which are described below, the relative centring being performed by the loading gripper.
Once the glass 10 has been seated on the belt inside the perimetral profiles, the glass is kept fixed to the belt by means of a suction produced by associated means, not shown, and acting on the glass through holes 610 in the belt.
Once printing has been performed and the unloading position reached, the ambient pressure is restored so that the sheets may be removed by unloading devices, not shown.
As illustrated in the said figures, said perimetral profiles 1210 are arranged on the conveyor belt and have the edge of the front side relative to the direction of feeding which is chamfered at a suitable angle of inclination which is designed to allow easy upward movement of the doctor blade 300 during printing, thus preventing, during this upward movement stage, the edge of the doctor blade from striking a sharp edge which would result in a drastic reduction in the working life of the screen, said devices also being able to be arranged on the external part of the screen so as to simplify the construction of the conveyor belt.
It is also envisaged that at least one protective rim may, by way of an alternative, be arranged on the printing screen so as to come into contact with the sheet during the synchronised passing movement of the latter for printing.

Claims

Rotary machine for the silk-screen printing of flat sheets (10a) comprising a rotating printing screen (200), a fixed doctor blade (300) arranged inside it, a conveyor belt (600) endlessly mounted on a pair of rollers, one of which is a drive roller (1600), for supplying flat sheets (10) to the printing screen (200) and an actuating motor (20,20a), characterized in that it comprises devices (205,1600,605,1000,30) for controlling and adjusting the feed speed of the conveyor belt (600) so as to synchronise feeding of the flat sheet (10,Ov) to be printed, fed by means the said conveyor belt, with rotation of the printing screen (200,Or).
Machine according to Claim 1, characterized in that the shaft (20a) of said motor (20) operates, via associated connection means (204,30), both the shaft (202) for rotation of the screen (200) and the shaft for driving the belt (600).
Machine according to Claim 1, characterized in that said devices for controlling and adjusting the speed of the conveyor belt (600) comprise at least one differential (30), the input shaft of which is the drive shaft (20a) of the motor and the output shaft of which is connected to the drive shaft (1603) of the conveyor belt (600).
Machine according to Claim 3, characterized in that said differential (30) has a motor (35) for actuating a toothed wheel (33) on which the planetary gears (32) of the said differential are mounted.
Machine according to Claim 1, characterized in that said devices for controlling the speed of the conveyor belt (600) comprise a programming device (1000) for operation of the motor (35) and at least one sensor (605) for detecting a reference point (Ov) of the sheet (10) to be printed, which sends corresponding signals to the programming device (1000).
Machine according to Claim 1, characterized in that said devices for controlling and adjusting the speed of the belt comprise a sensor for detecting the reference position (Or) of the screen (200).
Machine according to Claims 5 and 6, characterized in that said device for detecting the reference position of the screen (200) sends a corresponding signal to the sensor (605) for detecting the reference position (Ov) of the sheet (10).
Machine according to Claim 1, characterized in that said devices for controlling the speed of the belt comprise means for adjusting the external diameter of the drive roller (601a).
Machine according to Claim 8, characterized in that said means for adjusting the external diameter of the drive roller consist of two drums (1601,1603) with an associated external surface (1601a) and internal surface (1603a) with a complementary conicity, said drums being coaxially coupled together and telescopically movable relative to one another.
Machine according to Claim 9, characterized in that said drums have longitudinal grooves in the thickness of the said drum.
Machine according to Claim 1, characterized in that said printing screen (200) is attached to two facing discs (201a,201b) which are made to rotate separately by associated actuating means (204,204a,203;204,204b,207).
Machine according to Claim 11, characterized in that said means for actuating the screen comprise a motor reducer (204) actuated by the shaft (20a) of the motor (20) by means of associated transmission devices (208b,208a,208).
Machine according to Claim 12, characterized in that the shaft of the motor reducer (204) has a first gear (204a) for actuating a shaft (202) coaxial with the screen (200) and carrying the first disc (201a).
Machine according to Claim 12, characterized in that the shaft of the motor reducer (204) has a second gear for actuating a flange (206) mounted idle with respect to the shaft (202) and carrying the second disc (201b).
Machine according to Claim 14, characterized in that said flange (206) is mounted on a tubular element (205) coaxially mounted on the shaft (202) by means of bearings (205a).
Machine according to Claim 11, characterized in that the external disc (201a) is associated with means 220 for tensioning the screen (200) in the axial direction.
Machine according to Claim 1, characterized in that said printing screen (200) is associated with devices (220) for tensioning thereof in the axial direction.
Machine according to Claim 17, characterized in that said devices (220) for performing tensioning in the axial direction consist of levers (223) one end of which is connected to the disc (201a), on which the levers are pivotably mounted by means of associated pins (224), and the other end of which is connected to means (221) for actuating rotation of the lever about said pivot point.
Machine according to Claim 18, characterized in that said means for actuating the levers (223) consist of a crank (221) provided with a female thread (221c) suitable for coupling with a corresponding thread (201c) formed on an axial extension (201d) of the disc (201a) supporting the screen.
Machine according to Claim 18, characterized in that said levers (223) are angularly arranged at a distance of 120°.
Machine according to Claim 15, characterized in that the doctor blade (300) is mounted on said tubular element (205) by means of a pair of support brackets (301) mounted axially spaced on the tubular element (205).
Machine according to Claim 21, characterized in that the doctor blade is attached to the tubular element (205) by means for disengaging the doctor blade from the screen (200).
Machine according to Claim 22, characterized in that said means for disengaging the doctor blade are located between two arms (301a,301b) extending from the body of each support bracket (301) towards the screen (200).
Machine according to Claim 23, characterized in that said disengaging means comprise a support (320) for the doctor blade (300) which is hinged with the free end of the first arm (301a) and a lever mechanism (331,331a,332) located between said support (320) for the doctor blade (300) and the end of the second arm (301b) of the support bracket (301).
Machine according to Claim 24, characterized in that said lever mechanism consists of two levers (331,332) which are connected together by a pin (331b).
Machine according to Claim 25, characterized in that said pin (331b) for connecting together the two levers (331,332) also has, connected to it, the end of a strut (341) hinged by means of a pin (341a) with the arm (301b) of the bracket (301) supporting the doctor blade (300).
Machine according to Claim 26, characterized in that resilient means (342) are arranged between the pin (331a) of the levers (331,332) and the pin (341a) of the strut (341).
Machine according to Claim 26, characterized in that the operative length of the strut (341) is adjustable by means of screw/nut-screw means (343).
Machine according to Claim 1, characterized in that it comprises means for rapid changing of the doctor blade (300).
Machine according to Claim 29, characterized in that said means for rapid changing of the doctor blade (300) consist of a first clamp (325) which is formed in the support body (320) and which acts on a second clamp (1325) for retaining the doctor blade (300).
Machine according to Claim 30, characterized in that said first clamp (325) has fixed jaws (325a) formed by one of the two sections of a seat (326) which is substantially in the form of an inverted U and is formed in the body of the support (320) of the doctor blade (300) and movable jaws formed by cam (329) hingeably mounted, by means of an associated spindle (329a), on the end of the other section (326b) of the seat (326).
Machine according to Claim 31, characterized in that said cam (329) can be externally operated so as to rotate, by means of an extension (329b) or the like.
Machine according to Claim 31, characterized in that the internal surface of the fixed jaws (325a) is provided with a fixed pin (325b) arranged at the free end of the said fixed jaws and a seat (327) arranged in the vicinity of the bottom of the U-shaped seat (326).
Machine according to Claim 1, characterized in that it comprises means for adjusting the relative distance between doctor blade (300) and screen (200).
Machine according to Claim 34, characterized in that said adjusting means consist of a thin metal plate (1329) which can be inserted in the longitudinal direction onto respective parallel seats (1330) formed on the opposite internal surfaces of the arms (1325a,1325b) of the second clamp (1325).
Machine according to Claim 1, characterized in that it comprises means (500) for cleaning the screen (200) consisting of a device for emitting compressed air in a substantially radial direction towards the external surface of the said screen (200).
Machine according to Claim 36, characterized in that the cleaning device (500) comprises a tubular element (501) which is provided with a longitudinal slit (501a) and is arranged on the outside and parallel to the screen (200) and supplied by means of an air supply pipe (502).
Machine according to Claim 37, characterized in that the tubular element (501) is supported by an arm (503) attached to a plate (101) connected to the frame (100) of the machine.
Machine according to Claim 38, characterized in that the tubular element (501) is connected to the support arm (503) by means (504) for adjusting the angular position thereof relative to the surface of the screen (200).
Machine according to Claim 1, characterized in that it comprises a device (1200) for adjusting the relative distance between screen (200) and conveyor belt (600).
Machine according to Claim 40, characterized in that said adjusting device (1200) essentially consists of a screw (1201) supported by the frame (100) and coupled with a female thread (101a), oscillating inside the plate (101) supporting the screen (200).
Machine according to Claim 41, characterized in that said plate (101) is pivotably mounted on a pin (1202) which is arranged in a direction perpendicular to the direction of the screw (1201) and about which it is able to rotate.
Machine according to Claim 1, characterized in that it comprises devices (1110) for retaining the sheets (10) to be printed on the conveyor belt (600).
Machine according to Claim 43, characterized in that said retaining devices (1110) are of the vacuum type.
Machine according to Claim 44, characterized in that said devices comprise vacuum elements (1111) which are mounted on the free end of associated rods (1112a) of cylinders (1112) attached to the support surface (606) of the belt (600) in a fixed position corresponding to the zone for deposition of the sheet (10) on the belt.
Machine according to Claim 45, characterized in that the belt (600) has corresponding openings (1113) which are aligned with the said vacuum elements (1111) in the zone for loading the sheet (10).
Machine according to Claim 1, characterized in that profiles (1210) for perimetrally containing the sheet (10) to be printed are provided.
Machine according to Claim 47, characterized in that said profiles (1210) have a contact edge with the doctor blade which is chamfered at a suitable angle of inclination.
Machine according to Claim 47, characterized in that said profiles (1210) are arranged on the conveyor belt (600).
Machine according to Claim 47, characterized in that said profiles (1210) are arranged on the printing screen (200).