CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority, under 35 U.S.C. § 119, of German patent application DE 10 2019 118 762, filed Jul. 11, 2019; the prior application is herewith incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to a device for supplying liquid ink to an ink print head in an ink printing machine. The device includes an ink reservoir, a line for guiding the ink between the reservoir and the print head, a line for guiding the ink between the print head and the reservoir, and a damper for damping pressure fluctuations in the liquid ink. The damper has a movable and/or deformable membrane.

The technical field of the invention is the field of the graphics industry and in particular the field of inkjet printing on an industrial scale, i.e. high-performance inkjet printing on flat substrates, i.e. the application of liquid ink to sheet-shaped or web-shaped printing substrates, preferably made of paper, paperboard, cardboard, plastic, or composite materials.

In the known DOD (drop-on-demand) ink printing processes for creating a printed image on a flat printing substrate, the application of liquid ink is achieved by an ink print head (in short: head) with individually controlled nozzles that generate tiny ink drops, preferably in a picoliter range, and transfer them to the printing substrate in a contact-free way and in accordance with the image to be printed.

Devices for a circulatory supply of liquid ink to the ink heads are known in the art. A disadvantage of such a device may be that when the ink printing machine is in operation, i.e. when the machine is printing and ink is being consumed, ink pressure fluctuations may be too high or there may be undesired ink pressure peaks, resulting in a loss of printing quality for the printed image and thus for the printed product. This problem may in particular occur in industrial inkjet printing machines, which include a plurality of print heads with a joint ink supply and consequently require a high volume flow of ink. When the print heads start to print, the volume flow increases and pressure in the liquid ink takes a sudden dip; when the print heads stop printing, the volume flow decreases and pressure in the liquid experiences a sudden increase. Furthermore, the volume flow fluctuates as a function of the image to be printed.

It has also become known to provide pressure dampers for the liquid ink. Commonly assigned U.S. Pat. No. 9,840,083 B2 and its counterpart German published patent application DE 10 2016 212 733 A1, for instance, disclose a method for damping pressure peaks in an ink line of an inkjet printer. The method uses two dampers: one in the supply line and one in the return line. Each of the dampers includes a membrane. The document does not provide any information on the shape of the membrane.

Another problem that may occur is that despite the use of dampers, the damping effect may be insufficient.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a device for supplying a print head with liquid ink, which overcomes the above-mentioned and other disadvantages of the heretofore-known devices and methods of this general type and which provides for simple and cost-efficient means for preventing high pressure fluctuations, in particular high pressure peaks, in the liquid ink.

With the above and other objects in view there is provided, in accordance with the invention, a device for supplying liquid ink to an ink print head in an ink printing machine, the device comprising:an ink reservoir for liquid ink, a supply line for guiding the liquid ink between said ink reservoir and the print head, and a return line for guiding the liquid ink between the print head and said ink reservoir;a damper for damping pressure fluctuations in the liquid ink, the damper having a movable and/or deformable membrane;the membrane having a given thickness D and, in a position of rest, the membrane having a length L and a width W, with the length L being at least twice as long as the width W.

In other words, the objects are achieved, in accordance with the invention, with a device for supplying liquid ink to an ink print head in an ink printing machine comprising an ink reservoir, a line for conducting the ink from the reservoir to the print head, a line for conducting the ink from the print head to the reservoir, and a damper for damping pressure fluctuations in the liquid ink. The damper has a movable and/or deformable membrane with a given thickness and, when it is at rest, i.e., in a position of repose, the membrane has a length L and a width W and the length L is at least twice as long as the width W.

The invention advantageously provides a simple and cost-efficient way reliably to prevent high pressure fluctuations in the liquid ink, in particular high pressure peaks.

The membrane advantageously forms a hydraulic capacity of sufficient magnitude to deal with the expected volume changes/volume flow changes of the liquid ink at least in the region of the print heads and manifolds. To achieve this aim by simple means, the invention proposes to select a length L of the membrane that equals twice the width W at the minimum, i.e. in other words, the membrane has an elongated shape. This shape is particularly advantageous if the manifolds are likewise of elongated shape. In such a case, the length of the manifolds may advantageously be used to accommodate the membrane and thus to attain a hydraulic capacity of sufficient magnitude. Even when high pressure fluctuations or high pressure peaks occur during printing, the dynamic stroke of the membrane about the operating point is advantageously small, ensuring that the membrane is not put under too much stress; for instance, the dynamic stroke is smaller than n times the thickness D, with n<5 or n<10. In contrast, the static stroke at the operating point may be greater; here, n may be approximately 50. Additional elongated elements that would waste installation space are no longer required.

The membrane is preferably integrated into a manifold or a section or a part of the manifold. Every one of the two manifolds may have a membrane.

Further, the membrane is preferably disposed close to the print heads in the ink circulation system. In particular, the membrane is preferably disposed immediately upstream and/or immediately downstream of the print heads.

The membrane is a passive-action hydraulic capacity. Therefore, the membranes do not need an electronic (potentially closed-loop) control and/or drives, and consequently costs are low. In addition, the membrane does not require any maintenance.

The membrane provides a quick balancing movement at very low forces (like a soft, pre-loaded spring).

The membrane may comprise a stop or two stops to limit its movement (in one or two directions). The stops, in particular on the side facing the ink, may be designed as grids.

There is preferably no air between the membrane and the ink. The membrane has a slight inclination to provide easy deaeration.

In accordance with a preferred further feature of the invention the membrane comprises a plate embodied as a metal or stainless steel plate. Such a material provides the required elastic properties of the membrane.

In accordance with an added feature of the invention, the plate has a built-in shape, in particular an embossed or stamped shape, in particular a (cross-sectional) wave shape. The shape advantageously acts to create sufficient reset forces. The dimensions of the waves (or corrugations) are preferably such that a required and predefined spring rate is achieved. The preferred material is an embossed metal foil of a thickness D of between 50 and 200 μm.

The spring rate is a function of the number of waves, the shape of the waves (sinusoidal, rectangular, etc.), the wave height, the material (Young's modulus), the length L, the width W, and thickness D. Up to the operating point (and a little beyond), the spring rate preferably behaves in a linear way.

A preferred further development of the invention may be characterized in that the embossed shape forms a bellows or multiple bellows in a row or in a two-dimensional arrangement.

In accordance with a further feature of the invention, the membrane is designed as a cover, bottom, or wall or as a section or element of a cover, bottom, or wall that is a part of the ink reservoir, of an ink line, or of an ink manifold. In this way, i.e. due to the proximity of the membrane and the manifold, the membrane as a hydraulic capacity is coupled to the manifold in an optimum way, i.e. no undesired hydraulic resistance is created. The membrane may also be any desired combination of cover, wall, and/or bottom. It is possible to provide multiple covers, walls, and/or bottoms as a membrane.

In accordance with a preferred further development of the invention, the damper is disposed in an ink circulation system comprising at least the reservoir, the lines, the manifolds, and a pump, with the print heads connected to the ink circulation system or integrated into the ink circulation system, and in that the damper is essentially disposed in the immediate vicinity of the print heads or on the element that is closest to the print heads in the ink circulation system.

In accordance with again a further feature of the invention, when it is in its rest position, the shape A of the membrane is not a circular or quadrangular shape, but in particular a rounded rectangular shape (rectangle with two semicircles) or an elliptic shape.

In accordance with an added feature of the invention, the thickness D is a tenth or a hundredth or a thousandth of the width W at the maximum.

In accordance with another preferred development of the invention the membrane is attached to a carrier via spring elements in a way for the membrane to be movable and/or deformable.

A preferred further development of the invention is that the carrier and the spring elements are manufactured as a single component in an additive process or in a 3D printing process.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as embodied in a device for supplying liquid ink to an ink print head in an ink printing machine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

