The invention relates to a doctor blade holder for a printing machine.

In printing machines, inks are applied on a printing cylinder from which the ink is applied in a subsequent step to a substrate. E.g., in gravure printing machines, the printing cylinder comprises engravings that are filled with ink separated by separating strips. Doctor blades are used to scrape off from the surface of the printing cylinder any ink which is present outside the engravings.

As the printing cylinder is constantly turning during operation of the printing machine, due to friction between the doctor blades and the rotating printing cylinder, the doctor blades wear down over time. Additionally, the ink scraped off by the doctor blades can at least partially adhere on the surface of the doctor blades. Therefore, the doctor blades have to be exchanged and/or cleaned after some time. For this reason, doctor blade holders include mechanisms for locking and releasing the doctor blades. The used mechanisms should allow for a quick exchange of the doctor blades to minimize downtime of the printing machine. E.g., quick exchange doctor blade holders known in the art comprise a flexible steel plate clamping the doctor blade. However, to allow for an easy assembly and disassembly, these steel plates are clamped against a fixed rod such that they interact with the doctor blade only along a contact area limited in size, approximately only along a contact line.

For optimal printing quality, the doctor blade must be securely locked in its position during the operation of the printing machine. Less stable doctor blades result in a reduced capacity for wiping dried ink away from the printing cylinder, resulting in hazing artifacts.

The object of the invention is to provide a doctor blade holder which allows for a quick exchange and a secure fixation of a doctor blade.

The object of the invention is solved by a doctor blade holder for a printing machine, comprising a blade support, a clamping element, and a tensioning element with a cam. The blade support comprises a clamping surface against which a doctor blade can be pressed, an abutment for the clamping element, and a bearing structure for rotatably receiving the tensioning element. The clamping element has a flat pressing surface for pressing the doctor blade against the clamping surface, a counter abutment for cooperating with the abutment, and an engagement surface cooperating with the cam. The clamping element is formed as a two-armed lever having the pressing surface at one end and the engagement surface at the other end, with the counter abutment being arranged between the pressing surface and the engagement surface.

The doctor blade holder, according to the invention, allows for a secure fastening of a doctor blade in the doctor blade holder due to the interplay between the clamping surface of the blade support and the pressing surface of the clamping element.

Further, the clamping element is reliably fastened by the tensioning element, ensuring a constant and uniform pressure applied by the pressing surface on the doctor blade. This becomes possible as the clamping element is pressed at one of its ends by the tensioning element in a first direction and with the other end pressed in a second direction opposite to the first direction against the clamping surface due to the ability of the clamping element to rotate relative to the blade support around a rotation point formed by the abutment and the counter abutment.

At the same time, the doctor blade can easily be exchanged if the tensioning element releases the engagement surface, and the clamping element is rotated around the rotation point defined by the abutment and the counter abutment. Accordingly, the doctor blade holder, according to the invention, provides a quick exchange functionality.

In a variant, the abutment is a first profile of the blade support, and the counter abutment is a geometrically matching second profile of the clamping element. This variant of the doctor blade holder allows to tailor the profiles of the blade support and the clamping element to the desired range of rotational movement and provides a secure connection between blade support and clamping element.

In another variant, the abutment is a sleeve or bushing of the blade support, and the counter abutment is a pin of the clamping element.

The clamping element especially has a sufficient degree of stiffness to prevent the deformation of the clamping element at or close to the engagement surface. If the clamping element is not stiff enough, the cam could deform the clamping element at the engagement surface instead of initiating the rotation of the clamping element when the cam interacts with the engagement surface. The clamping element can be an aluminum profile to provide a sufficient degree of stiffness.

The clamping element especially has a thickness in the range of from 2 to 3 mm between the counter abutment and the engagement surface to provide sufficient stiffness of the clamping element while minimizing the cost in the production of the doctor blade holder.

The pressing surface can have a height or length of 10 mm or more to further increase the area of the pressing surface acting on the doctor blade. In other words: there is a surface of contact between the clamping element and the doctor blade and not only a line contact. The width of the pressing surface can be adapted to the requirements. For a wide doctor blade, more than one clamping element can be used, or several clamping elements can be arranged in even intervals spaced along the width of the doctor blade.

Further, on a gravure machine, whose nominal web width typically range from 600mm to 1700mm, the pressing surface can have a size of 6000mm ² to 18Ό00 mm ² . For example, the surface may be a rectangle of 10mm x 1300mm (= 13Ό00 mm ² ) in a machine with a nominal web width of 1200mm.

The clamping surface and the pressing surface especially are parallel to each other when being spaced from each other by a distance which corresponds to the thickness of a doctor blade clamped between them. This allows to provide uniform pressure on a doctor blade fixed in the doctor blade holder.

To amplify the force which can be exerted by the pressure surface of the clamping element, the ratio of a first distance between the counter abutment and the engagement surface and a second distance between the counter abutment and the pressing surface can be 2:1 or more, preferably 2.5:1 or more.

To further increase the quick exchange capability, the doctor blade holder can comprise a control element connected to the tensioning element, the tensioning element being rotatable by the control element. The control element can be a lever or a handle. In a further variant, the cam of the tensioning element has a cross-section in the form of a circular segment wherein the rotation center of the cam is closer to the secant of the circular segment than to the arc of the circular segment. In other words, the cam can be a cut round profile having a D-shaped cross-section. This allows the cam to cooperate with the engagement surface by means of a corner of the cam.

Further properties and advantages of the present invention become more apparent by the following description of an exemplary embodiment and the Figure, which shows a cut through a doctor blade holder according to the invention.

In the only Figure, a cut through a doctor blade holder 10 according to the invention is shown, with a doctor blade 12 fixed in the doctor blade holder 10.

The doctor blade holder 10 comprises a blade support 14, which is attached to a (not shown) printing machine, e.g., a gravure printing machine, by a connection element 16 connected to a lower surface 18 of the blade support 14.

The doctor blade 12 is made of a blade 13, and a thin plate 15 called the counter blade. The doctor blade is placed against a blade abutment 17 used to position the doctor blade 12 parallelly to the doctor blade holder 10, along the width of the blade.

The blade support 14 is made of aluminum or an aluminum alloy.

The doctor blade 12 is pressed against a clamping surface 20 of the blade support 14 by a pressing surface 22 of a clamping element 24.

The clamping surface 20 and the pressing surface 22 are parallel to each other and spaced apart by the doctor blade 12.

As shown in the Figure, the pressing surface 22 is in contact with the doctor blade 12 over the full area of the pressing surface 22.

The pressing surface 22 has an area of 100 mm ² or more, especially of 300 mm ² .

The blade support 14 and the clamping element 24 are rotatably connected by an abutment 26 of the blade support 14 and a counter abutment 28 of the clamping element 24, i.e. , the abutment 26 and the counter abutment 28 form a rotation axis Ri _, which allows the clamping element 24 a rotation as indicated with the double-arrow A in the Figure.

The abutment 26 and the counter abutment 28 are geometrically matching profiles formed integrally with the blade support 14 and the clamping element 24, respectively.

The abutment 26 and the counter abutment 28 could also use differing mechanisms, as long as a rotation of the clamping element 24 relative to the blade support 14 is possible. E.g., the abutment 26 could be a sleeve or bushing of the blade support 14 and the counter abutment 28 could be a pin of the clamping element 24.

The blade support 14 further comprises a bearing structure 30 in which a tensioning element 32 is mounted. The tensioning element 32 comprises a cam 34, which is rotatable around a rotation axis R2 in the bearing structure 30. In other words, the cam 34 can be rotated as indicated by the double-arrow B in the Figure.

The cam 34 has a D-shaped cross-section. In other words, the cross-section of the cam 34, as shown in the Figure, has the form of a circular segment. The rotation axis R2, i.e. , the rotation center of the cam 34, is closer to the secant of the circular segment than to the arc of the circular segment.

The doctor blade holder 10 further comprises a control element 36, in the shown embodiment a lever, which interacts with the cam 34, i.e., by handling the lever, the cam 34 is rotatable clockwise and counter-clockwise within the bearing structure 30.

The clamping element 24 comprises an engagement surface 38, which is located close to the cam 34.

Accordingly, the clamping element 24 is a two-armed lever, with the pressing surface 22 at one end 40 and the engagement surface 38 at the other end 42 of the clamping element 24.

The ratio of a first distance di between the counter abutment 28 and the engagement surface 38 and a second distance d2 between the counter abutment 28 and the pressing surface 22 is 2:1 or more, preferably 2.5:1 or more. The distances di and d2 are determined based on the rotation point Ri defined by the counter abutment 28 and the part of the engagement surface 38 and the pressing surface 22, respectively, which is closest to the counter abutment 28.

The clamping element 24 is made of aluminum and has a thickness of from 2 to 3 mm along the part described by the distance di.

In the following, the mode of action of the doctor blade holder 10 will be described.

The terms “clockwise” and “counter-clockwise” are in reference to the orientation of the doctor blade holder 10, as shown in the Figure.

In the Figure, the doctor blade holder 10 is shown in a locked state, i.e. , the doctor blade 12 is firmly held in place between the clamping surface 20 and the pressing surface 22.

The pressure applied at the end 40 of the clamping element 24 by the pressing surface 22 is created due to the cam 34 cooperating with the engagement surface 38 at the opposite other end 42 of the clamping element 24. More precisely, the cam 34 is pressing against the engagement surface 38, causing a counter-clockwise rotation of the clamping element 24 around the rotation axis Ri.

The alignment of the cam 34 is controlled by the lever and fixed to the state shown in the Figure as long as the doctor blade 12 shall be securely held in place.

If the doctor blade 12 needs to be exchanged, e.g. because the doctor blade 12 has been worn down during operation of the (not shown) printing machine, the cam 34 can be rotated counter-clockwise around the rotation axis R2 with the lever.

Due to the eccentric shape of the cam 34, the pressure on the engagement surface 38 of the clamping element 24 is released when the cam 34 is rotated counter-clockwise, causing the clamping element 24 to rotate clockwise around the rotation axis Ri .

Accordingly, the pressure surface 22 of the clamping element releases the doctor blade 12, which can easily be taken from the clamping surface 20 and, therefore, from the doctor blade holder 10. After a new doctor blade 12 has been placed on the pressure surface 22, the cam 34 can be rotated clockwise by means of the lever, causing a counter clockwise rotation of the clamping element 24 until the pressure surface 22 is again in the position shown in the Figure, thereby securing the new doctor blade 12. The doctor blade holder 10, according to the invention, therefore allows to easily and quickly change the doctor blade 12 while at the same time providing a controlled and secure fastening of the doctor blade 12.