ITUD20090152A1 - RACLA FOR SERIGRAPHIC PRINTING ON A SUBSTRATE OR A PRINTING SUPPORT AND ITS PRINTING PROCEDURE - Google Patents

Description

Description of the invention having as title:

"SCREEN FOR SCREEN PRINTING ON A SUBSTRATE OR PRINT MEDIA AND RELATED PRINTING PROCEDURE"

FIELD OF APPLICATION

The present invention relates to a doctor blade, also called a spatula, for the screen printing of one or more print traces on a substrate or a printing support, for example for the screen printing of conductive tracks on plate elements for electronics. , or similar elements, such as a wafer, a substrate, or a foil, based on silicon, for making photovoltaic cells. However, nothing excludes the possibility that a further specific print support typical of other sectors in which a printing operation is envisaged can be understood as printing support.

STATE OF THE TECHNIQUE

It is known that a technique for making conductive traces or tracks on silicon-based wafers, in particular, but not only, for photovoltaic cells, is screen printing, using suitable printing materials, such as for example conductive pastes or inks, and a special screen printing screen, normally consisting of a warp and weft mesh, having on the lower side an emulsion suitably engraved according to the desired pattern of the traces to be made.

Screen printing involves the use of one or more squeegees, or spatulas, by means of which the printing material is deposited on the screen printing screen with a determined and constant pressure.

Normally, the printing doctor blade acts on the screen according to a desired fixed angular orientation with respect to the theoretical horizontal plane of the screen. This squeegee orientation can also affect the quantity and speed at which the print material is printed and thus the final quality of the print process.

The printing screen is normally supported by a peripheral frame, or frame. Furthermore, the material of which this screen is made typically has a certain flexibility.

The pressure of the printing doctor blade, in particular in the proximity of the external support frame, can cause a deflection of the screen, even marked, with respect to the hypothetical horizontal plane of the same printing screen.

The resulting different deflection, combined with the fact that the printing doctor blade has a fixed angular orientation with respect to the theoretical plane of the screen, causes the actual inclination of the screen surface to vary dynamically during the printing pass of the screen. doctor blade.

Therefore, also the effective angle that is created between the squeegee and the screen varies, even in a marked way in the vicinity of the peripheral frame.

Consequently, the same delivery conditions with which the print material is printed through the screen engravings are not always guaranteed, to the detriment of the quality and reproducibility of the print.

Furthermore, variations in height may occur relating to the presence or absence of emulsion. Furthermore, the screen, at the warp and weft level, still presents depth differences between the longitudinal and transversal trends, due to the local overlapping and overlapping of the warp and weft, thus not guaranteeing the constancy of the angle of incidence of the doctor blade on the net.

Ultimately, the mechanical constraint that fixes the doctor blade angularly with respect to the theoretical plane of the screen does not guarantee, due to the phenomena illustrated above, constant conditions for the delivery of the printing material through the screen. This can limit the repeatability and quality of the print.

An object of the present invention is to provide a doctor blade for screen printing, and to develop a relative printing process, which ensures that the angular orientation of the doctor blade is maintained with respect to the actual surface of the screen during the printing action. , even in the event of deflections of the screen during the printing operation or variations in depth due to the warp and weft structure of the screen.

In order to obviate the drawbacks of the known art and to obtain this and further objects and advantages, the Applicant has studied, tested and implemented the present invention.

EXPOSURE OF THE FOUND

The present invention is expressed and characterized in the independent claims.

The related dependent claims disclose other characteristics of the present invention or variants of the main solution idea.

In accordance with the aforementioned purpose, a doctor blade according to the present invention can be used for the screen printing of one or more print traces on substrates, or print supports, by means of a printing material which is deposited and printed through a screen formed by a sheet, normally made of flexible material which defines, in the rest condition, a first lying plane.

According to an aspect of the present invention, the aforementioned doctor blade can be rotated so as to be able to selectively vary its inclination with respect to the aforementioned first lying plane, advantageously during printing, to define desired angles with respect to a portion of the upper surface of the sheet on which it is progressively dispensed the print material.

According to a variant, the doctor blade comprises, or is associated with, movement means suitable for determining, advantageously automatically, its selective rotation.

Favorably, the handling means are autonomous with respect to further movements provided for the doctor blade or in general in the print head.

According to a variant, the aforementioned handling means comprise a cylindrical actuator.

According to another variant, the aforesaid moving means comprise motor means rotatably constrained, by means of a joint member, to said doctor blade.

According to a further variant, the doctor blade comprises a printing end adapted to act under pressure on an upper surface of the aforementioned sheet to cause the printing of the printing material through the screen and a frame suitable to support said printing end and lying on a second plane which defines a certain first angle with respect to said first plane. According to a variant solution of the invention, the doctor blade can be configured in rotation to maintain constant, advantageously during the same printing operation, a second angle between the second plane of the frame and the aforementioned portion of the upper surface of the sheet, which portion is normally inclined with respect to the aforesaid first angle as a result of the deformation of the sheet due to the action of the printing end.

In accordance with this variant, the handling means are configured to keep said second angle constant.

In an embodiment of the invention, the doctor blade comprises, or is associated with, a system controller, adapted to control and command the rotation of the blade itself on the basis of received signals relating to the reciprocal angular position of the doctor blade and of the portion of the surface of the sheet. .

Another aspect of the present invention relates to a print head for silk-screen printing of one or more print traces on printing supports comprising a doctor blade according to the present invention as expressed above.

A further aspect of the present invention relates to a process for the silk-screen printing of one or more print traces on printing supports by means of a printing material deposited on a screen formed by a sheet which defines, in the rest condition, a first lying plane .

According to an aspect of the invention, the method provides for rotating the aforementioned doctor blade so as to be able to selectively vary its inclination with respect to said first lying plane to define desired angles with respect to a portion of the upper surface of the sheet on which the material is progressively dispensed. of printing. According to an advantageous variant of the process, the selective inclination of the doctor blade is performed dynamically during printing.

The doctor blade according to the present invention, therefore, has the possibility of modifying its angle of inclination with respect to the theoretical plane of positioning of the screen.

This makes it possible to compensate for the angular variations between the plane of the doctor blade and the theoretical plane of the screen due to the different deflection of the screen during printing and thus keep the angle between the plane of the blade and the actual inclined surface of the screen constant.

In this way, constant conditions of delivery of the print material through the screen are obtained and therefore the repeatability and quality of the print is guaranteed.

The variable inclination of the doctor blade according to the present invention can be determined independently with respect to the further operation of the print head, allowing:

adjusting the angle of inclination of the squeegee at the start of printing based on the type of screen and / or the type of printing material and / or the type of printing desired;

- the adjustment of the angle of inclination of the doctor blade also during the printing phase in order to compensate for the different flexion of the screen surface and / or different height of the screen between weft and warp of the screen itself and / or the relative height variations the presence or absence of the emulsion, and / or the more or less marked resistance to printing by the printing material, due to the density of the printing material itself;

- a local acceleration of the doctor blade stroke, with respect to the alternative printing movement typical of the print doctor blade block mounted on the print head, by suitably varying the angle of incidence of the doctor blade and instead keeping the overall advancement speed of the blade constant same block as the print squeegee.

Advantageously, the possibility of locally accelerating or decelerating the doctor blade by varying its inclination allows, for example, to have a lower printing speed at the edges of the wafer to be printed, allowing better control of printing in this peripheral region, while it is possible to have a higher speed in the central region of the wafer to be printed.

With the inclination of the doctor blade according to the invention it is also possible to vary the pressure applied on the screen, being able to apply different operating conditions of delivery during the printing phase.

In some embodiments, with the doctor blade in accordance with the present invention, it is possible to rotate the printing end, so as to switch from a position control mode, for example in correspondence with the perimeter of the screen in which the position of the doctor blade it is adjusted with respect to the screen and the substrate, to a pressure control mode, during printing operations.

Furthermore, the invention allows to vary the angle of the doctor blade, to change the print quality, for example by tilting the doctor blade more forward in correspondence with certain regions of the screen, in order to have a greater quantity of printing material that is pressed and printed. on the underlying substrate.

Another advantage of the solution according to the present invention is that of providing data, for example relating to the inclination of the doctor blade as a function of the properties of the screen and / or of the printing material used, which can possibly be stored in the system controller, in order to control and regulate, on the basis of such data, the subsequent operating phases and / or subsequent operating cycles.

ILLUSTRATION OF DRAWINGS

These and other characteristics of the present invention will become clear from the following description of a preferential embodiment, given by way of non-limiting example, with reference to the attached drawings in which:

Figure 1 is a schematic representation of a doctor blade according to the present invention, in a first operating condition;

Figure 2 is a schematic representation of the doctor blade of Figure 1, in a second operating condition;

Figure 3 is a schematic representation of the doctor blade of Figure 1, in a third operating condition;

Figure 4 is a schematic representation of the doctor blade of Figure 1, in a fourth operating condition;

Figure 5 represents an embodiment of a doctor blade according to the present invention.

For ease of understanding, identical reference numbers have been used where possible to identify identical common elements in the figures. It should be understood that elements and features of one embodiment can be conveniently incorporated into other embodiments without further specification.

DESCRIPTION OF A PREFERENTIAL FORM OF

REALIZATION

With reference to the attached Figures, a doctor blade 10 according to the present invention is used for screen printing in a suitable printing head 102 of a substrate processing system, or system, 100 for printing a screen printing material 11, in this case conductive paste, on a substrate 150 by means of a screen printing screen 12, in order to reproduce desired traces or conductive tracks on substrates, or printing supports, 150 in this case plate elements for electronics, or similar, placed under the screen 12, for example silicon-based wafers for making photovoltaic cells.

Figure 1 is an isometric schematic view of the system 100, used with the doctor blade 10 according to an embodiment of the present invention. In one embodiment, the system 100 generally comprises two inlet conveyors 111, an actuator assembly 140, a plurality of processing nests 131, a plurality of processing heads 102, two output conveyors 112, and a system controller 101 . The inlet conveyors 111 are configured in a parallel processing configuration so that each can receive unprocessed substrates 150 from an input device, such as a feed conveyor 113, and transfer each unprocessed substrate 150 to a coupled processing nest 131 to the actuator assembly 140. In addition, the outlet conveyors 112 are configured in parallel so that each can receive a processed substrate 150 from a processing nest 131 and transfer each processed substrate 150 to a substrate remover, such as a feed conveyor. evacuation 114. In one embodiment, each evacuation conveyor 114 is adapted to transport the processed substrates 150 through an oven 199 to treat the material deposited on the substrate 150 by the processing heads 102.

In one embodiment, the substrates 150 are microcrystalline silicon substrates used for processing solar cells thereon. In another embodiment, the substrates 150 are green-tape or similar ceramic substrates.

In one embodiment of the present invention, the system 100 is a screen printing processing system and the processing heads 102 include screen printing components, which are configured to screen print a layer of material in a pattern onto a substrate 150. In another embodiment, the system 100 is a processing system that includes components for removing material in the processing head 102, such as a laser for ablation or etching of one or more regions on a substrate 150. In other embodiments, the system 100 may comprise other substrate processing modules that require precise handling and positioning of the substrates for processing.

Figure 2 is a schematic plan view of the system 100 illustrated in Figure 1. Figures 1 and 2 illustrate the system 100 which has two processing nests 131 (in positions "1" and "3") each positioned for both transfer a substrate 150 worked towards the outlet conveyor 112 and to receive an unworked substrate 150 from the inlet conveyor 111. Thus, in the system 100, the movement of the substrate generally follows the path "A" illustrated in Figures 1 and 2. In this configuration, each of the other two processing nests 131 (in positions "2" and "4") is positioned under a processing head 102, so that a processing (for example silk-screen printing) can be carried out on the unprocessed substrates 150 placed on the respective processing nests 131. Such a parallel processing configuration allows an increase in production capacity with a minimized overall dimensions of the processing system. Although the system 100 is illustrated with two processing heads 102 and four processing nests 131, the system 100 may comprise additional processing heads 102 and / or processing nests 131 without departing from the scope of the present invention.

In one embodiment, the inlet conveyor 111 and the outlet conveyor 112 comprise at least one belt 116 for supporting and transporting the substrates 150 to a desired location in the system 100 using an actuator (not shown) that is in communication with the controller. system 101. Although Figures 1 and 2 generally illustrate a substrate transfer system of the two-belt type 116, other types of transfer mechanisms may be used to accomplish the same substrate transfer and positioning functions without departing from the basic purpose. of the invention.

In one embodiment, the system 100 also comprises an inspection system 200, which is adapted to locate and inspect the substrates 150 before and after the processing has been carried out. The inspection system 200 may comprise one or more cameras 120 which are positioned to inspect a substrate 150 positioned in the loading / unloading positions "1" and "3", as illustrated in Figures 1 and 2. The inspection system 200 comprises generally at least one camera 200 (for example a CCD camera) and other electronic components which are capable of identifying, inspecting, and communicating the results to the system controller 101. In one embodiment, the inspection system 200 locates certain features of an incoming substrate 150 and communicates the inspection results to the system controller 101 for analysis of the orientation and position of the substrate 150 to assist in precise positioning of the substrate 150 under a processing head 102 prior to processing the substrate 150. In one embodiment, the inspection system 200 inspects the substrates 150 so that damaged or poorly processed substrates can be removed from the production line . In one embodiment, each processing nest 131 may contain a lamp, or other similar optical radiation device, to illuminate the substrate 150 positioned thereon so that it can be more easily inspected by the inspection system 200.

System controller 101 facilitates control and automation of the entire system 100 and may include a central processing unit (CPU) (not shown), memory (not shown), and auxiliary (or I / O) circuits ( not illustrated). The CPU can be any type of computer processor that is used in industrial settings to control different chamber processes and hardware devices (such as conveyors, detectors, motors, fluid dispensing devices, etc.) and monitor the system and chamber processes (such as substrate position, process times, signal detectors etc.). The memory is connected to the CPU, and can be one or more of those readily available, such as random access memory (RAM), read only memory (ROM), floppy disk, hard disk, or any other form of digital storage. , local or remote. Software instructions and data can be encoded and stored in memory to drive the CPU. Auxiliary circuits are also connected to the CPU to aid the processor in a conventional manner. Auxiliary circuits may include cache circuits, power supply circuits, clock circuits, input / output circuitry, subsystems, and the like. A program (or computer instructions) readable by the system controller 101 determines which tasks can be performed on a substrate. Preferably, the program is software readable by the system controller 101, which comprises a code for generating and storing at least position information of the substrate, the movement sequence of the various components controlled, information of the substrate inspection system, and any other corresponding combination.

In one embodiment, the two processing heads 102 used in the system 100 can be conventional screen printing heads available from Applied Materials Baccini S.p.A. which are adapted to deposit the material according to a desired pattern on the surface of a substrate 150 arranged on a processing nest 131 in position "2" or "4" during a screen printing process. In one embodiment, the processing head 102 comprises a plurality of actuators, for example, actuators 105 (for example, stepper motors or servomotors) which are in communication with the system controller 101 and are used to adjust the position and / or the angular orientation of a silk-screen printing mask 12 (not shown in Figures 1 and 2) arranged in the processing head 102 with respect to the substrate 150 which is printed. In one embodiment, the silkscreen print template 12 is a metal foil or plate with a plurality of holes, slots, or other openings made therebetween to define a pattern and arrangement of the screen printed material on a surface of a substrate. 150. In one embodiment, the printing mask 12 is made of a relatively flexible material, for example consisting of a knit with a weft and warp structure. In one embodiment, the screen printed material 11 may comprise a conductive ink or paste, a dielectric ink or paste, a dopant gel, an etching gel, one or more masking materials, or other conductive or dielectric materials. In general, the silkscreen pattern to be deposited on the surface of a substrate 150 is automatically aligned with the substrate 150, orienting the silkscreen printing template 12 using the actuators 105 and the information received by the system controller 101 from the inspection system. 200. In one embodiment, the processing heads 102 are adapted to deposit a metal-containing or dielectric-containing material on a solar cell substrate which has a width between about 125 mm and 156 mm and a length between about 70 mm and 156 mm. mm.

In some embodiments, the screen or screen printing mask 12 (Figures 3 - 7) is normally formed by a sheet 14, which is supported by a peripheral frame 16. The sheet 14 lies, in the rest condition, on a theoretical foreground "PI" of horizontal position, illustrated in broken lines in the attached drawings.

In a known way and by means of known motorization means, such as a linear movement actuator 38 partially visible in Figure 7, the doctor blade 10 can be made to advance on the screen 12 according to an alternative linear movement, indicated by the arrow "F" in the figures, to carry out the desired printing operation.

For example, the linear movement actuator 38 can be configured like the aforesaid actuators 105 (Figure 2).

Normally, the doctor blade 10 is moved in a first direction, for example indicated by the arrow "F" in Figures 3 - 6, to perform the printing, while in the return direction printing is not performed and the recovery of the printing material is foreseen. that accumulates. Nothing excludes that, with suitable precautions, it is possible to print in both directions.

The doctor blade 10 comprises a frame 20 for supporting a printing end 18. The doctor blade 10 defines a second plane "P2" represented by the lying plane of the relative frame 20.

The first plane "Pi" of theoretical position of the screen 12 and the second plane "P2" of the doctor blade 10 form a first angle between them, indicated with the letter "al" for convenience only in Figure 3.

The printing end 18 is adapted to act on the upper surface of the central sheet 14, so as to spread, with a determined printing pressure, the printing material 11 which is progressively dispensed and thus define the conductive tracks on the substrate 150 below.

With the solution according to the present invention, any type of screen printing paste available on the market can be used, for example in metallic material (silver, aluminum or other), doping pastes and pastes for etching.

As can be seen in the attached figures, the printing material 11 is dispensed downstream of the doctor blade 10 with respect to the direction and direction of advancement of the doctor blade 10 itself, indicated by the arrow "F". or with respect to the opposite direction.

The angle formed by the surface of the sheet 14 varies dynamically as a result of the deformation of the flexible material of which the sheet is constituted by the pressure action of the printing end 18 during printing. In some embodiments, the pressure exerted by the print end 18 can be determined by the actuators 105 of the print head 102 as described above. The situation in which the sheet 14 is deformed during printing is represented in Figures 3 - 6, schematically and deliberately neglecting the real proportions between the extent of the deflection and the size of the screen for the purposes of ease of understanding. In particular, depending on where the printing action takes place, closer to the frame 16, first and fourth operating conditions of Figures 3 or 6, or in a position more similar to a central, second and third operating conditions of Figures 4 or 5 , the extent of the deformation suffered by the sheet 14 varies.

The doctor blade 10 is positioned on the screen 12 so that the second plane "P2" defines a determined second printing angle "a2" with respect to a portion 34 of the actual surface of the sheet 14 which deforms by tilting during printing. This second printing angle "a2" is defined, for example, on the basis of parameters such as the intrinsic flexibility of the screen 12 and the rheological properties of the printing material.

In any case, according to the present invention, the desired second printing angle "a2" between the second plane "P2" of the doctor blade 10 and the portion 34 of the inclined surface of the sheet 14 is calculated with an optimal amplitude value according to the operation of print to be performed and the print quality to be obtained.

In the present description, the second printing angle "oc2" is referred to and determined in particular with respect to a portion 34 of the surface of the sheet 14 on which the printing material 11 is progressively dispensed, downstream of the doctor blade 10 with respect to the movement indicated by the arrow "F", and similarly also with respect to the movement of direction and opposite.

Under operating conditions, the portion 34 of the surface of the sheet 14, inclined due to the flexibility of the material of which it is made during printing, does not coincide with the theoretical foreground "Pi" on which the sheet 14 itself lies at rest. As is known, this would cause an undesirable and involuntary dynamic variation of the second printing angle "a2".

To compensate for the dynamic angular variation between the surface portion 34 of the sheet 14 on which the printing end 18 acts and the second plane "P2" on which the doctor blade 10 lies and to keep the relative second printing angle "a2" constant, the doctor blade 10 itself is configured to be selectively rotated, automatically by means of the system controller, as indicated by the arrow G, in a manner consistent with the necessary angular compensation and thus define constant dispensing conditions and a uniform and desired printing speed of the printing material 11. Maintaining a constant value of the second printing angle "a2" between the doctor blade 10 and portion 34 of the surface of the sheet 14 during printing requires the variation, instead, of the first angle "a" formed by the doctor blade 10 and from the theoretical first plane "Pi" of horizontal position of sheet 14 of screen 12.

To this end, according to an embodiment, the doctor blade 10 is provided with handling means 25, for example of the mechanical, electromechanical type (for example stepping motors or servomotors) or other, adapted to cause the rotation of the doctor blade 10 in a manner that it can, during printing, follow, and therefore compensate, the variation in inclination of the portion 34 of the upper surface of the sheet 14, keeping the second printing angle "a2" at a fixed value during the printing operations. Advantageously, the actuation of the handling means 25 is completely autonomous with respect to the actuation which allows the alternating linear movement of the doctor blade 10 on the screen 12 indicated by the arrow "F".

In this case, the aforementioned handling means 25 comprise a motor 22 (for example a stepping motor or a servomotor) and a linear actuator 24 rotatably connected, by means of a joint member, or other angular release element, 26, to the frame 20 of the doctor blade 10. The motor 22 determines an alternative movement of the linear actuator 24 which, thanks to the articulation member 26, is transformed into the desired rotation movement of the doctor blade 10.

The handling means 25 can be activated by the system controller 101 at the beginning of the printing operation to define a desired angular position of the doctor blade 10, for example different from time to time according to the type of screen 12 and / or the type of print material 11.

In this case, at least one further articulation member 26 of the doctor blade 10 can be provided, for example between frame 20 and printing end 18, to allow self-adaptation of the angular position of the doctor blade 10 with respect to the screen according to the more or less pronounced stiffness of the sheet 14 which meets the printing end 18 itself passing from the periphery towards the central area of the sheet 14 and vice versa in the movement indicated by the arrow "F".

In other words, the further articulation member 26 would allow a reciprocal inclination between said printing end 18 and said frame 20 to achieve a self-adaptation of the angular position of the second plane "P2" as a function of the progressive inclination of the portion 34 of the upper surface of the sheet 14.

Alternatively, or in combination with the above, the handling means 25 can be selectively activated by the system controller 101 during the printing operation to dynamically vary the angular position of the doctor blade 10. This can be done, for example, depending on the different depths at which the doctor blade 10 is found when it is moved along the mesh of the screen 12, due to the bending deformation of the sheet 14 and / or the angular variations that would occur due to the variation in height due to presence or absence of emulsion and / or of the different weft and warp depths / heights.

In particular, the means 25 for moving the doctor blade 10 can receive command and control signals provided by sensor means, for example a sensitive element or pressure or inclination detector, not shown in the drawings, which detects desired data relating to the variation of the position. reciprocal angular between the doctor blade 10 and the deformed surface of the sheet 14, due to both the deformation in macroscopic bending of the screen 12, and the variation in microscopic depth due to the differences in height between the weft and the warp.

For example, these control signals provided by said sensor means can be indicative of the depth variations of the blade 10 along the screen 12 with respect to the first theoretical "PI" plane, or of the relative angular variations between the blade 10 and the inclined portion 34 of the surface. of the sheet 14, or of the dynamic slope variations of the same deformed portion 34 of the sheet 14, or the aforementioned signals provided by said sensor means can be indicative of the position of the doctor blade 10 inside the screen 12 or of the variation in deflection of the sheet 14, for example more or less near the frame 16 or the central region of the sheet 14.

In any case, these control signals provided by said sensor means, advantageously processed by said system controller 101 which manages everything, determine the selective and automatic actuation of the handling means 25 to coherently rotate the doctor blade 10 for the purposes of angular compensation. above, or in any case for the purpose of dynamically defining a desired angle of incidence of the doctor blade 10. In the following, the system controller 101 maintains the selected second printing angle "a2" at a constant value during the printing operations. Figure 7 represents a further embodiment of the doctor blade 10 according to the invention, alternative to the embodiments illustrated in Figures 3 - 6. In this Figure 7 part of the aforementioned linear movement actuator 38 is visible which allows the movement of the doctor blade 10 along the direction indicated by the arrow "F" in Figures 3 - 6. Furthermore, in this embodiment, the aforementioned handling means 25 comprise a cylindrical actuator 27 equipped with a gearmotor, which acts both as a means of motorization for rotation and as a cylindrical articulation element between the frame 20 of the doctor blade 10 and the linear movement actuator 38. Also in this solution, the movement means 25 are advantageously controlled by the system controller 101.

It is clear that modifications and / or additions of parts and / or phases may be made to the doctor blade 10 for screen printing on a substrate or a printing support and to the relative printing process described up to now, without thereby departing from the scope of the present document. found.

It is also clear that, although the invention has been described with reference to specific examples, a person skilled in the art will be able to produce other equivalent forms of doctor blade for screen printing on a substrate and of the relative printing process, having the characteristics expressed in the claims and therefore all falling within the scope of protection defined by them.

Claims (9)

CLAIMS 1. Squeegee for silk-screen printing of one or more print traces on substrates, or print media, by means of a printing material capable of being deposited on a screen comprising a sheet which defines, in the rest condition, a foreground lying plane , characterized in that it can be rotated so as to be able to selectively vary its inclination with respect to said first lying plane, to define one or more desired angles with respect to a portion of the upper surface of the sheet on which the printing material is progressively dispensed. 2. Squeegee as in claim 1, characterized in that it comprises movement means adapted to determine the selective rotation of said squeegee. 3. Squeegee as in claim 2, characterized in that said handling means comprise a cylindrical actuator. 4. Squeegee as in claim 2, characterized in that said handling means comprise motor means rotatably constrained, by means of a joint member, to said squeegee. 5. Squeegee as in any one of the preceding claims comprising a printing end adapted to act under pressure on an upper surface of said sheet to cause printing of the printing material through the screen and a frame adapted to support said printing end and lying on a second plane which defines a certain first angle with respect to said first plane, characterized by the fact that it can be configured in rotation to keep a second angle constant between the second plane of the frame and said portion of the upper surface of the sheet, which portion is inclined with respect to said first angle as a result of the deformation of the sheet due to the action of the printing end. 6. Squeegee as in claims 2 and 5, characterized in that said movement means are configured to keep said second angle constant. 7. Squeegee as in any one of the preceding claims, characterized in that it comprises a system controller adapted to control and command the rotation of said squeegee on the basis of received signals relating to the mutual angular position of said squeegee and said portion of the surface of the sheet. 8. Print head for silk-screen printing of one or more print traces on print supports comprising a doctor blade as in any one of the preceding claims. 9. Process for the silk-screen printing of one or more print traces on print media by means of a printing material placed on a screen formed by a sheet which defines, in the rest condition, a first lying plane, characterized by the fact of rotating said doctor blade so as to be able to selectively vary its inclination with respect to said first lying plane to define desired angles with respect to a portion of the upper surface of the sheet on which the printing material is progressively dispensed,