Application system for coating components and coating device

Abstract

The invention describes a coating device for coating components with a coating agent, comprising an application system which includes the following assemblies: - an application device that applies the coating agent, wherein the application device is a printhead (1) that ejects the coating agent from several coating agent nozzles (2, 19), wherein a nozzle valve (20) is attached to each individual coating agent nozzle (2) which opens for a valve opening time when a coating agent droplet is to leave the respective nozzle (2), - a coating agent supply line (22) to which the coating agent nozzles (19) of the printhead (1) are jointly connected, - a valve control (25) for controlling the valve opening times and valve closing times of each individual valve (20), wherein the coating device includes a robot on which at least the application device is mounted, characterized in thatthat the application system further comprises a pump (21) by means of which the coating material to be applied is supplied to the coating material nozzles (19) via the coating material supply line (22), and that the application system is configured so that the pump (21) operates with a constant flow rate of the coating material during coating, and that the pressure at each nozzle (19) when the valve (20) is opened is exactly the same as when this valve (20) was previously opened.

Description

[0001] The invention relates to an application system for coating components according to the preamble of claim 1 and a coating device according to the preamble of claim 2.

[0002] Such an application system comprises the following assemblies: An application device for applying the coating material, wherein the application device is a printhead that ejects the coating material from several coating material nozzles, each individual coating material nozzle having a nozzle valve which opens for a valve opening time when a coating material droplet is to leave the respective coating material nozzle, a coating material supply line to which the coating material nozzles of the printhead are connected, a coating material return line in which a return valve is installed, a valve control for controlling the valve opening times and valve closing times of each individual nozzle valve and the return valve, and a pump by means of which the coating material to be applied is supplied to the coating material nozzles via the coating material supply line.

[0003] A coating system comprises a robot to which at least the application device is attached. The application device is typically mounted on the robot at the so-called Tool Center Point, or TCP for short.

[0004] DE102014013158 A1 shows a free jet device for the contactless delivery of liquids onto the surface of a body.

[0005] DE 10 2008 053 ​​178 A1 discloses a coating system for coating, in particular painting, automotive body components. In such a painting system, the components to be painted are transported through a paint booth, where they are then painted by painting robots. The painting robots have one or more swiveling robot arms and each guides an application device via a multi-axis robot wrist axis on its TCP (traction control point). The application device is designed as a printhead that ejects the coating material from several nozzles. The nozzles of the printhead are connected to a supply line through which the coating material is fed.Painting with such a printhead is advantageous, for example, when a vehicle body is to be painted in multiple colors, or when different colors are to be applied to different areas of the body. Painting with a printhead applicator allows for the precise application of paint to different zones on the workpiece, such as the car body, without any additional preparations, especially without masking off differently colored areas.

[0006] When painting with a printhead applicator, the coating material is typically dispensed using a pressure regulator. This pressure-regulated dispensing method has several disadvantages, particularly when painting car bodies or body parts in an automotive paint line. The flow rate of a paint used as a coating material in automotive painting depends on both its viscosity and the pressure. The viscosity of the coating material can vary considerably between different paint materials. Some paint materials are thixotropic, meaning their viscosity is pressure-dependent. This significant dependence of the coating material's viscosity on the material type and pressure often leads to inconsistent droplet size during coating, making it very difficult to achieve a homogeneous coating.It is therefore the object underlying the present invention to improve an application system of the type mentioned above in such a way that a constant droplet size is ensured during the coating process. Furthermore, it is an object of the present invention to further improve a coating device.

[0007] This problem is solved with respect to the application system by an application system having the features of claim 1. With respect to the coating device, the problem is solved with a coating device having the features of claim 2.

[0008] According to the invention, the valve control is arranged such that the pump operates with a constant flow rate of the coating agent during coating, and that the return valve is open when all nozzle valves are closed, and the return valve is completely closed when all nozzle valves are open, so that the pressure at each coating agent nozzle when the nozzle valve is opened is exactly the same as when this nozzle valve was previously opened.

[0009] The invention, as well as further embodiments and further advantages of the invention, will now be explained and described in connection with the following description of the figures.

[0010] They show: Figure 1 schematically and by way of example shows a printhead for use in an application device according to the invention, with ten coating agent nozzles arranged in a line one behind the other, the line extending approximately perpendicular to the direction of movement of the application device during coating; Figure 2 schematically and by way of example shows a printhead with thirty-two coating agent nozzles, wherein a valve is attached to each coating agent nozzle, and the coating agent nozzles are connected together to a coating agent supply line, in two embodiments: left with coating agent supply from one side, right with coating agent supply from two sides; Figure 3 schematically and by way of example shows an embodiment of an application system in which three printheads, each with thirty-two coating agent nozzles, are arranged in a row to increase the coating performance.and in which the coating agent nozzles in adjacent printheads are offset from one another, Figure 4 schematic illustration of an operating mode of the coating agent nozzles of a printhead, according to the prior art, Figure 5a schematic and exemplary application system with a printhead having thirty-two coating agent nozzles, wherein a valve is attached to and associated with each coating agent nozzle, Figure 5b the time course of the valve position during two complete switching periods T1 and T2 and half a switching period T3, Figure 5c the time course of the coating agent flow at the nozzle outlet opening of a coating agent nozzle (solid line), and the flow rate of the coating agent imposed by the pump (dashed line) Figure 5d the time course of the pressure at the coating agent nozzle of the printhead, Figure 6a an embodiment according to the one in , Figure 5aThe illustrated figure shows an additional coating material return line with a built-in return valve. Figure 6b shows the switching states of the nozzle valves according to the illustration. Figure 6a during two complete switching periods T1 and T2 and one half switching period T3, Figure 6c shows the switching states of the nozzle valves according to Figure 6b corresponding switching state of the return valve during two complete switching periods T1 and T2 and one half switching period T3. Figure 7 shows how the detrimental build-up of a continuously increasing overpressure in the system with the negative effects on a non-uniform droplet size is avoided by the measure according to the invention. .

[0011] Figure 1Figure 1 schematically and exemplarily shows a printhead 1 for use in an application device according to the invention. Here, the printhead 1 is schematically and exemplarily depicted as a cuboid structure. In this example, it has ten coating agent nozzles 2 arranged in a line one behind the other on one narrow side of the printhead, the line extending approximately perpendicular to the direction of movement of the application device during coating (see Figure 1). Figure 3 Arrow D indicates the direction of the ejection of the coating agent droplets.

[0012] Figure 2 Figure 1 schematically and exemplarily shows a printhead 3 with thirty-two coating agent nozzles 4, which are essentially identical in construction to the coating agent nozzles 2 according to the figure in Figure 2. Figure 1In the embodiment shown, a valve 5 is attached to each coating material nozzle 4, and the coating material nozzles 4 are connected to a coating material supply line 6. The valves can be switched at a switching frequency in the range of a few kHz, typically in the range of 3 kHz. (Left in) Figure 2 An embodiment is shown in which the coating material is supplied only from one side, here from above. (On the right in the) Figure 2 An embodiment is shown in which the coating material is supplied from two sides, here from above and below, wherein the coating material supply line 6 is divided into an upper and a lower partial arm 7, 8.

[0013] Figure 3Figure 1 schematically and exemplarily shows an embodiment of an application system 9 in which three printheads 10, 11, 12, each with thirty-two coating agent nozzles 13, are arranged in a row to increase the coating capacity. The direction of movement of the application system 9 during coating is indicated by arrow P; it can be seen that this direction runs approximately perpendicular to the line in which the coating agent nozzles 13 are arranged in each of the three printheads 10, 11, 12. The distance between adjacent coating agent nozzles 13 is denoted by a. For design reasons, this distance cannot be made arbitrarily small. To achieve a uniform application of the coating agent, the coating agent nozzles 13 of the three printheads 10, 11, 12 arranged in a row are offset from each other by an amount of a / 3, as shown in the right part of the figure. Figure 3The depicted section shows an enlarged view of a sub-area 14 of the coating agent nozzle field.

[0014] Figure 4Figure 4 schematically and exemplarily shows the operating mode 16 of the coating agent nozzles of a printhead, as it is typically used according to the prior art. For the purposes of this illustration, a printhead 15 with six coating agent nozzles D1–D6 is assumed. The horizontal bars, with alternating black and white fields, indicate the switching state of the respective valves when the printhead 15 is moved for coating. A dark field indicates that the corresponding valve is open during this time, and a coating agent droplet exits the respective nozzle. A light field indicates that the corresponding valve is closed during this time, and no coating agent droplet exits the corresponding nozzle. In the usual operating mode 16, as illustrated in Figure 4, all valves are opened and closed simultaneously.This is often done to achieve a sharp beginning and end line for the coating. In the . Figure 4 A total of eight switching periods T1 - T8 are shown in their chronological order.

[0015] Figure 5 Figure 5a schematically and exemplarily shows an application system 17 with a printhead 18 having thirty-two coating agent nozzles 19, each coating agent nozzle 19 having a valve 20 attached to and associated with it. A pump 21 is located in the coating agent supply line 22, which pumps the coating agent at a constant flow rate through the supply line 22 to the printhead 18. The pump 21 can be, for example, a gear pump or a piston pump, both types of pumps that can generate a constant flow rate even under varying pressure. Figures 5b-5d show the time profiles of the valve position ( Fig. 5b ), of the coating material flow at the nozzle outlet opening of a coating material nozzle ( Figure 5c , solid line), the flow rate of the coating agent impressed by pump 21 ( Figure 5c , dashed line) and the pressure at the coating medium nozzle 19 of the printhead 18 ( Figure 5d ) during two complete switching periods T1 and T2 and half a switching period T3. If all valves are closed simultaneously, for example at time A during switching period T1, pump 21 continues to pump the coating agent into the application system 17 at a constant flow rate. The pressure in the application system, and thus the pressure at the coating agent nozzle of the printhead, increases, creating overpressure in the application system, see Fig. 5dBecause the hoses and other components of the application system have a certain degree of elasticity, when the valve is reopened at the beginning of the next switching cycle (see, for example, time B at the beginning of switching cycle T2), the pressure drops again and after a short time the coating fluid flow at the outlet of the coating fluid nozzle is constant.

[0016] In the present invention, it was surprisingly discovered that under certain circumstances, when the time constants of the application system are so large that a complete reduction of the overpressure is not possible before the start of the next switching period, the overpressure continues to increase from a higher starting point during the next switching period, and so on, so that the pressure in the application system continues to rise. Since the flow rate of the coating agent at the printhead's outlet nozzle depends on both viscosity and pressure, this leads to the unexpectedly discovered problem that, although the pump generates a constant flow rate, the flow rate at the outlet of the coating agent nozzle, and thus the droplet size, is not uniformly constant but changes over time. This is detrimental with regard to achieving an optimal coating result.

[0017] Figure 6Figure 6a shows the solution according to the invention for this surprisingly discovered problem. An embodiment corresponding to the one shown in Figure 6a is depicted. Figure 5a The illustration shows an additional coating material return line 23 in which a return valve 24 is installed. A valve control 25 controls the nozzle valves 20 in the printhead and the return valve 24. .

[0018] Figure 6b shows the switching states of the nozzle valves 20 and Figure 6cFigure 1 shows the corresponding switching state of the return valve 24 during two complete switching periods T1 and T2, as well as half a switching period T3. The return valve 24 is open when the nozzle valves 20 are closed, and it is closed when the nozzle valves 20 are open. When the nozzle valves 20 are closed, the pump 21 circulates the coating compound through the open return valve 24 and the coating compound return line 23. As a result, this measure ensures that the overpressure is completely reduced until the start of the next switching period following the closing of all nozzle valves 20. Thus, the overpressure during the next switching period then rises again from the pressure level that existed at the beginning of the previous opening of the nozzle valves 20, preventing the pressure in the application system from continuously increasing.This ensures that the flow rate at the coating nozzle's outlet, and therefore the droplet size, remains uniform and does not change over time. This is advantageous for achieving optimal coating results.

[0019] Figure 7Figure 7a shows how the inventive measure, as described above, avoids the detrimental build-up of a continuously increasing overpressure in the system, with its negative effects on non-uniform droplet size. Figure 7a schematically and by way of example shows an application system 17 with a printhead 18 having thirty-two coating agent nozzles 19, each of which is fitted with and associated with a valve 20. A pump 21 is located in the coating agent supply line 22, which pumps the coating agent to the printhead 18 at a constant flow rate. The pump 21 can, for example, be designed as a gear pump or a piston pump, both types of pumps that can generate a constant flow rate even under varying pressure. Not shown in Figure 7a is a valve 20. Figure 7athe coating agent return line with the return valve. Figures 7b-7d show the time courses of the valve position ( Fig. 7b ), of the coating material flow at the nozzle outlet opening of a coating material nozzle ( Figure 7c , solid line), the flow rate of the coating agent impressed by pump 21 ( Figure 7c , dashed line) and the pressure at the coating medium nozzle 19 of the printhead 18 ( Figure 7d ) during two complete switching periods T1 and T2 and one half switching period T3. In contrast to the in Figures 5b to 5dAs explained above, after implementing one of the measures according to the invention, it is achieved that a complete reduction of the overpressure has taken place by the beginning of the next switching period, and the overpressure during the next switching period is then increased again from the same initial pressure as it was at the beginning of the previous switching period, and so on, so that the pressure in the application system does not continue to rise. Additional explanations:

[0020] The individual nozzle openings for the coating material typically have a diameter of approximately 10 µm to 200 µm. Due to manufacturing tolerances, wear, and / or deposits, the individual nozzle openings of a printhead are not completely identical.

[0021] Each of the nozzle outlets controlled by a valve therefore has a different flow resistance. The amount of material flowing through nozzle outlet i V̇ i depends on the pressure in front of the outlet opening. pi This relationship is described by the function V̇ i = fi ( pi ) or the inverse function pi = gi ( V̇ i ).

[0022] When designing a printhead, care is taken to ensure that the pressure at all nozzle outlets is always the same, i.e., the pressure drop in the coating agent supply line should be negligible.

[0023] It should be noted that the functions fi or gi depend on the viscosity and therefore on the applied coating material.

[0024] As a first approximation, one can assume that the functions described above are linear in the region of interest. The equation for the nozzle outlet is V ˙ i = k i ν p where ki a characteristic value of the outlet opening is and v the viscosity of the material.

[0025] The printhead is moved at a constant speed across the surface to be coated. Based on the material properties, the desired layer thickness, and the distance between the nozzle openings, an average varnish flow rate is calculated, or, if varnish is the coating medium, an average coating medium flow rate. V ˙ m = μ ⋅ d D v a f V With V̇ m average paint flow rate per nozzle d D Distance between nozzles perpendicular to the direction of movement (resulting distance when several applicators are arranged in a row) µ Layer thickness (dry film) f VVolume solids content of the applied material; applicator speed (TCP speed)

[0026] The switching frequency or the time of one period (from opening the valve to the next opening) T p and time T v The time the valve is open is based on experience.

[0027] The paint flow rate through a nozzle outlet is approximately described by the following relationship: V ˙ D = T D T ν V ˙ m

[0028] The volume of coating material ejected when a valve opens (droplet volume) is described by the following relationship: V T = V ˙ m ⋅ T D = μ ⋅ d D v a f V ⋅ T D

[0029] The goal for a homogeneous coating is to ensure that, on average, the same amount of coating material flows through all nozzle outlets over time. This can be achieved by keeping the individual valves open for different durations.

[0030] The procedure to follow if the pressure drop in the distribution line is negligible is as follows.

[0031] In the first step, the characteristic curve is calculated for each outlet opening. V̇ i = fi ( p ) or pi = g ( V̇ i determined.

[0032] In the case of the linearity described above, the characteristic values ​​are obtained. ki ... kn with k ¯ = 1 n ∑ i = 1 n k i

[0033] The following applies to the variant of coating material supply with a material pressure regulator: Default value: Paint quantity per valve opening, average valve opening time

[0034] The material pressure to be set on the material pressure regulator and the valve opening time are calculated as follows: p = ν k ¯ V ˙ D T i T ν = k ¯ k i The valve opening time is inversely proportional to the characteristic value.

[0035] In the case of coating material supply via, for example, a metering pump, the following applies: The advantage of using a metering pump in a coating system is that the fluidic conditions in the entire application system are independent of the viscosity of the coating material, and therefore no adjustment of parameter values ​​to the respective coating material used is necessary.

[0036] Since the pump operates at the same coating agent flow rate the entire time (i.e., a constant paint flow rate if paint is the coating agent), the coating agent pressure, or paint pressure, depends on how many valves are open. The fewer valves are open, the higher the pressure. Therefore, the time differences between individual valve switching times are smaller than when operating at constant pressure. Reference symbol list 1 printhead 2 Coating agent nozzle 3 printhead 4 Coating agent nozzle 5 valve 6 Coating agent supply line 7 partial arm 8 partial arm 9 Application system 10 printhead 11 printhead 12 printhead 13 Coating agent nozzle 14 sub-area 15 printhead 16 Schematic of an operating mode 17 Application system 18 printhead 19 Coating agent nozzle 20 valve 21 pump 22 Coating agent supply line 23 Coating material return 24 Return valve 25 Valve control 26 Operating mode 27 Operating mode

Claims

1. An application system for coating components with a coating means, which comprises following assemblies: - an application device, which applies the coating means, wherein the application device is a print head (1, 3) which ejects the coating means from a plurality of coating means nozzles (2, 4, 19), wherein a nozzle valve (5, 20) is attached to each individual coating means nozzle (2), which valve opens for a valve opening time when a drop of coating means is to leave the respective coating means nozzle (4), - a coating means supply line (6, 22), to which the coating means nozzles (4) of the print head (3) are jointly connected, - a coating means return line (23) in which a return valve (24) is installed, and - a valve control (25) for controlling the valve opening times and valve closing times of each individual nozzle valve (20) and the return valve (24), wherein the application system further comprises a pump (21), by means of which the coating means to be applied is supplied to the coating means nozzles (19) via the coating means supply line (22), the valve control (25) is configured so that the pump (21) operates at a constant flow rate of the coating means during coating, characterized in that the valve control (25) is configured such that the recirculation valve (24) is open when all nozzle valves (20) are closed and the recirculation valve (24) is completely closed when all nozzle valves (20) are open, so that the pressure at each coating means nozzle (19) is exactly the same when the nozzle valve (20) is opened as when this nozzle valve (20) was previously opened.

2. A coating device for coating components with a coating means, with an application system according to claim 1, wherein the coating device comprises a robot on which at least the application device is mounted.

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