WO2017182144A1

WO2017182144A1 - A doctor blade assembly with support in a rotogravure printing unit

Info

Publication number: WO2017182144A1
Application number: PCT/EP2017/025095
Authority: WO
Inventors: Renzo Melotti
Original assignee: Bobst Italia S.P.A.
Priority date: 2016-04-22
Filing date: 2017-04-21
Publication date: 2017-10-26
Also published as: EP3445586A1; EP3445586B1

Abstract

The present invention refers to a novel support for a doctor blade (31) in a rotogravure printing machines. The doctor blade (31) performs a smooth lateral oscillation to remove ink that tends to dry behind the doctor blade (31), and thanks to the lack of gearing for conducting the motion, the system is extremely precise and low in maintenance. The novel support is made of two elastic laminar members (41, 42) on each side of the doctor blade (31).

Description

A DOCTOR BLADE ASSEMBLY WITH ELASTIC SUPPORT IN A ROTOGRAVURE PRINTING UNIT

DESCRIPTION

The present invention refers to rotogravure printing machines and in particular a printing unit with a novel support system for the doctor blade assembly.

Considering a generic rotogravure printing unit, this comprises a printing roller in tangential contact with a second roller, normally a rubber roller. The printing roller has on its outer surface a distribution of cells the pattern of which establishes the motif to be printed and that to this end are filled with ink. These ink filling the cells is transferred to a printing support inserted with a certain pressure between the rollers. However, the inked printing support has to be accurately shaved, so that only the strictly necessary amount of ink is transferred to the printing support, and us a high quality, clear print. To this purpose, a so-called doctor blade is provided, that is essentially a blade member arranged in a longitudinal and tangential fashion with respect to the roller. The blade then, as the printing roller rotates, removes the exceeding amount of ink from its outer surface, while the ink within the cells remain and, during the printing step, is transferred to the support to obtain the desired image and/or types.

When the printing roller surface is shaved, between such surface and the edge of the blade some ink clogs sometimes may occur, hindering a perfect shaving and causing print defects as a result of the unsatisfactory clearing of the printing roller from the exceeding ink.

Considering this problem, the blade is provided with a reciprocating drive that is a "to and fro" movement in a direction parallel to the axis of the roller. This motion achieves indeed a brushing effect capable of dislodging the clogs and ensures a higher effectiveness if conducted at high frequency, i.e. with a succession of fast runs having a reduced amplitude. At present, in order to ensure this drive, the whole doctor blade assembly (that is, the blade with its support frame and the relative calibration/adjustment systems) is held by linear bearings, which however due to the high frequency of the motion, considering also the remarkable inertia of the assembly (it can even reach a weight of 200 kg) are subject to a rapid wear with consequent vibrations and plays the necessarily affect the mutual positioning of the various printing elements, eventually causing problems to the same print and quality decay. It is, therefore, an object of the present invention to solve the above-mentioned problem, providing a movable support system for the blade ensuring improved precision, quality, reliability with respect to the known systems.

A particular object of the present invention is also to provide a system of the above- mentioned type, which permits an easy and safe control by an operator from the outside of the machine, namely via safe commands at a distanced position from the potentially hazardous region of the machine.

These objects are achieved with the doctor blade assembly with an elastic support in a rotogravure printing unit according to the invention, the essential features of which are defined by the first of the appended claims.

The characteristics and advantages of the doctor blade assembly with an elastic support in a rotogravure printing unit according to the invention will become apparent from the following description of an embodiment thereof, provided by way of examples, with reference to the attached drawings wherein: - figure 1 is an axonometric view of a printing unit, with some elements like the printing roller and its counter-roller that have been omitted for the sake of clarity, in which a device for supporting and adjusting the position of the doctor blade according to the invention is visible; figures 2 and 3 show the device of figure 1 in isolation, with two axonometric views from different angles; figures 4 and 5 are respectively a bottom view and a front view (with some parts represented according to an axial cross section) of the device of the previous figures, again shown in isolation; and figure 6 is a sectional view taken along the plane indicated by the arrows VI of figure 5.

With reference to the above figures, a rotogravure printing unit comprises a printing roller and a rubber roller, not shown here, held by a frame 1 . The rotation axis of the printing roller is sketched in figure 1 and indicated at X.

The present invention specifically concerns a doctor blade assembly comprising a shaft 2, typically with a cylindrical shape, extending along an axis/direction X' parallel with the roller axis X and supporting, as explained hereafter, the actual blade member 31 via a pair of clamping bars 3.

According to the invention, the shaft 2 is held at its ends, corresponding to respective axial ends of the printing rollers, by two elastically flexible laminar members 41 , 42 lying over respective diametrical planes orthogonal to axis X'. More precisely, the laminar members 41 , 42 are attached to the frame 1 at two mutually spaced regions 41 a, 41 b, 42a, 42b, that for instance and preferably are approximately aligned with each other (in each member), according to a vertical direction or in any case a direction approaching the vertical. The support of the shaft 2 occurs at an intermediate position between the two attachment regions, so that the same shaft is compliant to a reciprocating motion along its own axis X', by the effect of the deformability of the member when subject to a bending stress. This elastic deformability is obtained from the use of a suitable material such as spring steel along with the laminar nature of the member.

Each deformable element comprise two attachment points to the fixed frame. The objective of the attachment configuration is to block the movement of the blade along any direction perpendicular the direction of the reciprocating motion (called the spurious motion here). There are several possible configuration that can achieve this goal; these configuration are characterized by the absolute angle between the line connecting the first point of connection of the laminar member (for example 41 a) to the axis X' and the line connecting the second connecting point of the laminar member (for example 41 b) to the axis X'. This angle should be larger than 30 degrees (for example, a value of 45 degrees puts the two attachment points 41 a and 41 b on the same side compared to the axis X').To reduce the displacement of the doctor blade along the direction of the laminar member (perpendicular to axis X'), the attachment points are preferably located at opposite sides of axis X', for example with said absolute angle between 170 and 180 degrees (or at least larger than 135 degrees to obtain this reduction). Also, the length of the laminar element (between the connection point and the axis X') must be set large enough compared to the amplitude of the motion. This is because, when the attachments points are not perfectly symmetric around axis X' (i.e. an angle of 180 degrees when using two attachment points), the blade tends to move along a circle segment, whose radius (which is linked to said length) must be large enough so that the circle segment does not depart from the tangent of said circle by more than the tolerance about the spurious motion maximum amplitude. A typical tolerance is a maximum spurious motion of 0.1 mm for web width comprised between 60cm and 220cm, and an angular tolerance smaller than 1 arc min. For example, the length is chosen to be of the order of 1 m for a 5mm doctor blade motion. Nevertheless, the length cannot be chosen arbitrarily large to ensure enough stiffness.

One of the attachment regions further provides for a compliant system adapted to permit some adjustment of the position of the axis of the shaft, and thus the correction of possible parallelism errors of the blade with respect to the axis of the printing roller. To this purpose, at least one of the laminar member provides for a device with a control knob 94 that can be actuated from the outside of the frame and operates with the opposing force of a spring 95 on the connection between the laminar member (in this case the laminar member 42) and the frame, to the end of modifying the slant of the same laminar member.

A motor 5 is anchored to the frame 1 on the side of a first laminar member 41 (externally to the space delimited by the two laminar blades) and via an obviously designed eccentric system obviously transmits a reciprocating drive to an arm 6, in turn extending along the axis X' or parallel therewith. The arm 6, on an opposite side with respect to the linking side to the motor 5, is connected to the laminar member 41 at a region close to the support point of the shaft. Also, the arm has a flexible structure, to absorb the stresses and allow for the parallelism corrections. The shaking thus transmitted to the assembly of the arm and laminar members, permitted by the elastic flexion of the members, actuates the reciprocating motion along the axis X' with an amplitude of some mm - typically between 2 and 8 mm - that permits to provide the blade with the desired "to and fro" brushing movement. The amplitude of the motion (between 2 and 8 mm) is large enough to be able to dislodging the clogs. For example the amplitude can be set to 4 mm. Externally to the second laminar element 42, and therefore at an axially opposite side with respect to the motor 5, the movement of the shaft (in a direction away from the same motor) is hindered by an adjustable preload device 7, controlled as explained hereafter, that in an embodiment comprises a helical spring 71 having the function to exert a thrust as a result of its compression between the second laminar member 42 and frame 1 . Considering again the blade support system by the shaft 2, the already mentioned clamping bars 3, between which the blade 31 is clamped with screw means 32 of a known type, extend between the free ends of stems 81 that project in a substantially tangential arrangement from the shaft 2. The stems 81 are in fact slidable along their own axes Y (indeed, tangential or in any case hitting the side surface of the shaft over a plane normal to the shaft axis) inside respective guides 82 connected to the shaft 2 (as more precisely described shortly), the sliding being hindered/cushioned by respective pneumatic dampers 83. This mechanism serves to provide the blade with the cushioned pressing function that is necessary to "squeeze" the ink over the printing roller.

The guides 82 are in turn supported by the shaft 2 in such a way to ensure a displacement thereof (and with it the displacement of all the assembly of stems, dampers, bars and blade) to accomplish an adjustment depending on the specific geometry of the working circumstances. This displacement occurs linearly according to a tangential direction with respect to the shaft (indeed coinciding with the axis Y) within seatings 21 a formed for this purpose by the shaft 2, or more properly by a sleeve 21 of the shaft. The latter has, in fact, a structure in two pieces, with the mentioned sleeve that coaxially houses in a rotatable fashion, via bearings 23, a core 22. As visible in particular in figures 5 and 6, the core 22 has two geared pinion portions 22a that mesh with respective rack 82a integral with the guides 82 so that from a rotation of the core the above cited adjustable displacement of the guides ensues.

This adjustment as just described, i.e. the rotation of the core, is actuated manually via a first crank 91 accessible from the outside at a front side 10 of the frame 1 . The crank 91 rotatably drives a spindle 92, in turn having a tangential axis with respect to the shaft 2 over a plane orthogonal to the axis X', meshing via a gear transmission with an end of the core 22, in this case, adjacent to the motor 5.

At the other axial end of the shaft, that is the one adjacent to the second laminar member 42 and supported by this very end, the preload adjustment system is arranged, comprising a crank to be actuated from the outside. This adjustment operates through suitable gearings on the device 7, compressing to a variable extent the above-mentioned spring 71 and thus affecting the preload.

The above clearly shows that the support according to the invention permits to achieve the required brushing motion of the doctor blade, by simply exploiting an elastic deformation, and then without the wear due to friction that in the known art ensue from the sliding motion of a remarkable mass, such as that of the doctor blade assembly. The consequent reduction of vibrations and plays in general improves the printing performance and ensures an increased overall work reliability, a longer life of the machine and a reduction of the maintenance needs. Moreover, the elastic support system permits to effectively optimise the architecture of the unit finalised to greater safety, with drive/transmission systems that permit an easy control of the unit adjustments through actuation means external to the hazardous area and comfortably available to the operator. When mentioning an absolute value of an angle in this invention, we mean an angle with a positive value comprised between 0 and 180 degrees. It is understood that when measuring an angle between two arbitrary intersecting lines, it is always possible to obtain said angle value between 0 and 180 degrees.

Claims

1 . A doctor blade assembly suitable for a rotogravure printing unit, the assembly comprising a doctor blade (31 ), doctor blade support means adapted to be connected to a fixed frame (1 ) of the unit and to permit a reciprocating motion of the blade along a motion axis (Χ') parallel to the elongation direction of the blade, and drive means functionally linked with the blade to transmit said reciprocating motion thereto, wherein the support means comprise elastically deformable means arranged between respective end regions of said blade (31 ) and said fixed frame (1 ), characterized in that the elastically deformable means are arranged to allow said reciprocating motion while blocking the movement of the blade along any direction perpendicular to the motion axis.

2. The assembly according to claim 1 , wherein said deformable elements comprise a pair of elastically deformable elements (41 , 42) that extend over respective planes orthogonal to the motion axis (Χ').

3. The assembly according to claim 2, wherein said deformable elements comprise a pair of laminar members (41 , 42).

4. The assembly according to claim 3, wherein the blade support means comprise a shaft (2) centered on the motion axis (Χ') and from which the blade is spaced in a parallel fashion, wherein the shaft is connected to the laminar members at a point of connection, wherein each deformable element comprise two attachment points to the fixed frame, and wherein an angle between the line connecting the first attachment point to said point of connection and the line from said point of connection to the second attachment point, measured in the plane perpendicular to the motion axis, has an absolute value larger than 30 degrees, preferably larger than 135 degrees, preferably larger than 170 degrees, preferably equal to 180 degrees.

5. The assembly according to claim 4, wherein in each deformable element the two attachment regions to the fixed frame and the intermediate point of connection to the shaft (2) are substantially aligned.

6. The assembly according to claim 4 or 5, wherein said drive means comprise a transmission arm (6) connected between a motor (5) and a first (41 ) of said elastically deformable elements, adapted to transmit a shaking force along said motion axis (Χ') to said first element.

7. The assembly according to claim 6, comprising eccentric means arranged between said motor (5) and said arm (6) adapted to convert the rotational motion of the first into said shaking motion.

8. The assembly according to claim 6 or 7, comprising adjustable preload means (7) associated with a second (42) of said elastic elements.

9. The assembly according to claim 8, wherein said preload means (7) comprise a helical spring (71 ) urged in compression between said second elastic element (42) and said fixed frame (1 ), and preload adjustment means adapted to vary the compression force acting on the spring, provided with actuation means (93) adapted to be controlled from an outer region of said fixed frame (1 ).

10. The assembly according to any of the claims from 4 to 9, wherein at least one attachment between one of said deformable elements (42) and said fixed frame (1 ) is carried out with a compliant system and equipped with an adjustment device (94, 95) adapted to control the slant of said motion axis (Χ').

1 1 . The assembly according to any of the claims 4 to 10, wherein said shaft (2) comprises a sleeve (21 ) fixed to said elastically deformable elements, and a core (22) adapted to rotate inside said sleeve (21 ) around said motion axis (Χ'), the blade support means (31 ) comprising guide means (82) projecting in a displaceable fashion from said sleeve (21 ) of said shaft, said guide means being engaged via gears with said core so that the displacement of the guide means is responsive to a rotation of the core, actuation means (91 ) being further provided for controlling the rotation of the core, adapted to be controlled from an outer region of said fixed frame (1 ).

12. The assembly according to claim 1 1 , wherein said blade (31 ) is supported by a bar (3) sustained by at least two stems (81 ) that project in a substantially tangential fashion from said shaft (2) sliding on said guide means (82), the sliding being opposed by damper means (83).