Inkjet coating method

Abstract

In an inkjet coating method that applies a coating material (2) to a target coating surface (3) using an inkjet coating device (1) that includes a nozzle (11) for discharging the coating material, the coating material is applied so that the film thickness (t) of a peripheral edge part (25b) of a wet coating film (25) applied to the coating surface decreases toward a peripheral edge (25c).

Description

Inkjet coating method

[0001] The present invention relates to an inkjet coating method using an inkjet coating device.

[0002] Conventionally, in an inkjet coating method using an inkjet coater, it is known that the occurrence of the coffee ring phenomenon can be suppressed by drying the fluid coating film formed on the substrate after the inkjet coater has finished discharging the film liquid in the coating film drying step under reduced pressure in an atmosphere with a temperature of 10°C to 40°C (Patent Document 1).

[0003] JP 2015-160176 A

[0004] However, in the automobile painting process, there is a certain period of time, known as the setting time, between the completion of paint application and the start of baking and drying. Therefore, even if the above-mentioned conventional technology is applied during baking and drying, the occurrence of the coffee ring phenomenon cannot be suppressed because the solvent evaporates from the wet paint film during the setting time.

[0005] The problem to be solved by the present invention is to provide an inkjet coating method that can suppress the occurrence of coffee ring phenomenon in the automobile painting process.

[0006] The present invention solves the above problem by applying paint in an inkjet coating method using an inkjet coating device so that the thickness of the wet coating film applied to the coating surface at the periphery becomes thinner as it approaches the periphery.

[0007] According to the present invention, it is possible to suppress the occurrence of the coffee ring phenomenon in the automobile painting process.

[0008] 3A is a structural diagram showing an embodiment of an inkjet coating apparatus used in the inkjet coating method according to the present invention. 3B is a perspective view showing an embodiment of the inkjet coating method according to the present invention. 3C is a plan view, a cross-sectional view along line X-X, and a cross-sectional view along line Y-Y showing a wet coating film coated by the inkjet coating method according to the present invention. 3D is a plan view, a cross-sectional view along line X-X, and a cross-sectional view along line Y-Y showing a nozzle movement trajectory for forming a first layer of the wet coating film of FIG. 3A. 3E is a plan view, a cross-sectional view along line X-X, and a cross-sectional view along line Y-Y showing a nozzle movement trajectory for forming a second layer of the wet coating film of FIG. 3A. 3F is a plan view, a cross-sectional view along line X-X, and a cross-sectional view along line Y-Y showing a nozzle movement trajectory for forming a third layer of the wet coating film of FIG. 3A. 3G is a plan view, a cross-sectional view along line X-X, and a cross-sectional view along line Y-Y showing a nozzle movement trajectory for forming a fourth layer of the wet coating film of FIG. 3A. 3H is a cross-sectional view of a coating film for explaining the mechanism of occurrence of the coffee ring phenomenon. 7 is a cross-sectional view of a coating film for explaining the mechanism for suppressing the occurrence of coffee ring in the present invention. FIG. 8 is a diagram showing examples of a wet coating film and a dried coating film coated by the inkjet coating method according to the present invention, and a comparative example thereof. FIG. 9 is a cross-sectional view showing a wet coating film coated by an inkjet coating method according to another embodiment of the present invention. FIG. 10 is a cross-sectional view showing a wet coating film coated by an inkjet coating method according to yet another embodiment of the present invention. FIG. 11 is a cross-sectional view showing a wet coating film coated by an inkjet coating method according to yet another embodiment of the present invention. FIG. 12 is a plan view and a cross-sectional view showing an example of a method for coating an adjacent coating surface after completing coating of one coating surface, using an inkjet coating method according to yet another embodiment of the present invention. FIG. 13 is a cross-sectional view (first layer) showing a coating method for a wet coating film formed on the second coating surface of FIG. 7. FIG. 14 is a cross-sectional view (second layer) showing a coating method for a wet coating film formed on the second coating surface of FIG. 7. FIG. 15 is a cross-sectional view (third layer) showing a coating method for a wet coating film formed on the second coating surface of FIG. 7. FIG. 16 is a cross-sectional view (fourth layer) showing a coating method for a wet coating film formed on the second coating surface of FIG. 7.

[0009] Hereinafter, with reference to the drawings, an embodiment of the present invention will be described. The inkjet coating method of this embodiment is not particularly limited, but can be applied to, for example, processes for painting automobile parts such as the outer and inner panels of automobile bodies and bumpers using automobile paint (a thermosetting paint with a base resin such as acrylic resin, alkyd resin, or polyester resin, which may be either a water-based paint or an organic solvent-based paint and may contain color pigments or luster pigments as necessary), such as an undercoat process or a topcoat process. When painting automobile parts such as the outer and inner panels of automobile bodies and bumpers, the surfaces to be painted, such as side doors and fenders, are nearly vertical, so a paint with a relatively high viscosity must be used. In this sense, the application of resist liquid to silicon wafers and the recording heads of commercially available inkjet recording devices used for printing on paper media, as mentioned in the background art, are premised on the use of low-viscosity resist liquids or inks, and are therefore not suitable for painting automobile bodies.

[0010] <Embodiment of Inkjet Coating Apparatus> Figure 1 is a structural diagram showing one embodiment of an inkjet coating apparatus 1 used in an inkjet coating method according to the present invention. The inkjet coating method according to the present invention is a coating method using a so-called inkjet coating apparatus (a machine that prints by ejecting minute droplets of ink from a thin nozzle toward paper), and is widely known as this "inkjet coating apparatus." Therefore, in this specification, the term "ink" will be used to refer to the "inkjet coating method" according to the present invention and the "inkjet coating apparatus" used in this method. However, since the object to be coated according to the present invention is an automobile part such as an outer panel / interior panel of an automobile body or a bumper, this ink actually means "automotive paint."

[0011] The inkjet coating device 1 of this embodiment comprises a nozzle 11 having an inlet 111 for the paint 2, a paint chamber 112 and an outlet 113 for the paint 2, a needle 12 having at least a tip 121 arranged in the paint chamber 112 and capable of moving back and forth in the axial direction Y toward the outlet 113, an actuator 13 that moves the needle 12 back and forth in the axial direction Y so that the tip 121 approaches the outlet 113 when the needle 12 advances and moves away from the outlet 113 when the needle 12 retreats, a pressure sensor 14 that detects the pressure of the paint 2 in the paint chamber 112, and a control unit 15 that controls the actuator 13.

[0012] The nozzle 11 has a hollow housing 114 made of a metal, resin, or ceramic material, with an inlet 111 formed on one side and an outlet 113 at the tip, and a paint chamber 112 formed inside. Paint 2 is introduced from the inlet 111 into the paint chamber 112 and is ejected (dribbled) from the outlet 113 to the outside by being pushed by the needle 12. The interior of the housing 114 is partitioned liquid-tightly into the paint chamber 112 and an actuator chamber 115 by a seal member 123.

[0013] The needle 12 is a needle-shaped rod made of metal, resin, or ceramic material, with a tip end 121 disposed in the paint chamber 112 and a base end 122 disposed in the actuator chamber 115, with a seal member 123 provided therebetween. The actuator 13 is fixed to the base end 122 of the needle 12. The needle 12 is provided within the housing 114 so as to be movable back and forth in the axial direction Y.

[0014] The actuator 13 is, for example, a stack of multiple piezoelectric elements, and has the property of expanding and contracting in the axial direction Y in response to a voltage applied to the electrodes. The application of voltage to the actuator 13 is executed by the control unit 15, and by applying a voltage to the actuator 13 in response to a command signal from the control unit 15, the needle 12 can be moved back and forth in the axial direction Y. Note that the stroke start position of the needle 12 may also be controlled based on the pressure of the paint 2 in the paint chamber 112 detected by the pressure sensor 14.

[0015] The paint 2 in this embodiment is contained in a paint tank 21 and supplied by a paint pump 23 via a paint pipe 22. When the inkjet coating apparatus 1 of this embodiment is used to paint automobile parts such as automobile bodies and bumpers, a thermosetting paint having an acrylic resin, alkyd resin, polyester resin, or the like as a base resin and containing color pigments and luster pigments as necessary can be used as the automotive paint. Either a water-based paint or an organic solvent-based paint may be used, and the paint may be adjusted to the desired viscosity using a solvent. Note that a paint pipe may be provided to return the paint 2 introduced into the paint chamber 112 of the nozzle 11 to the paint tank 21, allowing the paint 2 to be circulated and supplied.

[0016] <Embodiment of Inkjet Coating Method> Figure 2 is a perspective view showing one embodiment of the inkjet coating method according to the present invention. In the embodiment shown in Figure 2, two nozzles 11 are provided in the inkjet coating device 1 described above for a target coating surface 3. In Figure 2, if a plane parallel to the coating surface 3 is defined as the X-Y plane and an axis perpendicular to the X-Y plane is defined as the Z axis, the two nozzles 11 are arranged side by side along the Y axis. The two nozzles 11 drop coating material 2 onto the coating surface 3 while moving in a direction (X axis direction) perpendicular to the direction in which the nozzles 11 are arranged side by side (Y axis direction), thereby forming two rows of a wet coating film 25 on the surface of the coating surface 3. The distance between the two nozzles 11 is set to a distance roughly equivalent to one droplet 24 of the coating material 2. As shown in Figure 2, the droplets 24 of the coating material 2 dropped from the two nozzles 11 form a continuous wet coating film 25 on the coating surface 3. In this specification, for the sake of simplicity, an inkjet coating device 1 including two nozzles 11 will be used as an example, but an appropriate number of nozzles 11 can be arranged side by side depending on the size of the coating surface 3.

[0017] In this specification, the paint 2, droplets 24 of paint 2, wet coating film 25, and dry coating film 26 are used as different technical terms. That is, the liquid material from the paint tank 21 until it adheres to the coating surface 3 is collectively referred to as paint 2, and the granular liquid material from the discharge portion 113 of the nozzle 11 until it adheres to the coating surface 3 is referred to as droplets 24 of paint 2. In contrast, the liquid or semi-hardened film from the time when the droplets 24 of paint 2 adhere to the coating surface 3 until it is baked and dried is referred to as wet coating film 25, and the film obtained by baking and hardening the wet coating is referred to as dry coating film 26.

[0018] The inkjet coating method of this embodiment uses an inkjet coating device 1 including a nozzle 11 that ejects paint 2 to apply paint 2 to a target coating surface 3, and applies paint 2 so that the film thickness t of a peripheral portion 25b of a wet coating film 25 applied to the coating surface 3 becomes thinner toward the peripheral edge 25c. Figure 3A shows a wet coating film 25 applied by the inkjet coating method according to one embodiment of the present invention, with the upper left diagram being a plan view, the lower left diagram being a cross-sectional view taken along line X-X (hereinafter also referred to as the X-X cross-sectional view), and the upper right diagram being a cross-sectional view taken along line Y-Y (hereinafter also referred to as the Y-Y cross-sectional view). In the two cross-sectional views, a wet coating film corresponding to one droplet 24 of paint 2 is schematically represented by a single rectangle.

[0019] 3A, the central region of the wet coating film 25 applied to the coating surface 3 is referred to as the central portion 25a, the surrounding area is referred to as the peripheral portion 25b, and the outermost end of the peripheral portion 25b is referred to as the peripheral edge 25c. In the inkjet coating method of this embodiment, the film thickness t of the peripheral portion 25b of the wet coating film 25 gradually decreases toward the peripheral edge 25c in both the X-X cross section and the Y-Y cross section of the coating surface 3. As a result, when the coating surface 3 is rectangular, the wet coating film 25 as a whole has the shape of a quadrangular pyramid.

[0020] The wet coating film 25 of the embodiment shown in Figure 3A is not particularly limited, but has a four-layer structure consisting of a first layer 251, a second layer 252, a third layer 253, and a fourth layer 254, and is formed by four coats. The thickness of each layer is set to a thickness equivalent to one droplet 24 of the coating material 2 ejected from the nozzle 11. No baking or drying process is performed between the coating processes of each layer, and the coating material 2 is applied wet-on-wet. After the coating process for all coating surfaces 3 is completed, the coating material 2 is baked and dried. When applying a primer coat or top coat to automotive parts such as the exterior and interior panels of an automobile body or bumpers, the coating is generally performed so that the dried coating film has a thickness of 10 µm to 50 µm, although this varies depending on the location and coating specifications. Therefore, the multi-layer structure and the number of coats are set accordingly in the inkjet coating method of this embodiment.

[0021] Next, the procedure for applying the wet coating film 25 shown in FIG. 3A will be described with reference to FIGS. 3B to 3E. FIGS. 3B to 3E are plan views, cross-sectional views along line X-X, and cross-sectional views along line Y-Y showing the movement trajectory of the nozzle 11 for forming the wet coating film 25 shown in FIG. 3A. FIG. 3B shows an example of the movement trajectory TR of the nozzle 11 when applying a first layer 251, a second layer 252, a third layer 253, and a fourth layer 254. In FIGS. 3B to 3E, the thick solid arrow extending in the X-axis direction indicates the movement trajectory TR of the nozzle 11, which is the trajectory along which droplets 24 of the coating material 2 are ejected. The thin dashed arrow extending in the Y-axis direction indicates the movement trajectory TR of the nozzle 11, which is the trajectory along which the ejection of the coating material 2 is stopped. The symbol ST indicates the start position of the movement of the nozzle 11, and the symbol EN indicates the end position of the movement of the nozzle 11. It should be noted that these movement trajectories TR are merely examples of the present invention and are not limited to these, and can be modified as appropriate.

[0022] The nozzles 11 are attached to, for example, a robot hand or the like and move along a pre-taught movement trajectory TR, which will be described below, while simultaneously controlling the ON / OFF of the coating material 2. First, with reference to FIG. 3B , the procedure for applying a first layer 251 to a coating surface 3 will be described. Since the coating range of this first layer 251 is the entire coating surface 3 as shown in FIG. 3B , for example, the upper left corner is set as the movement start position ST and the lower right corner is set as the movement end position EN. Between these movement start position ST and movement end position EN, a movement trajectory TR is set that moves back and forth in the left and right directions along the X axis, with a shift of two droplets in the Y axis direction. The movement start position ST is a position where droplets 24 dripping from the two nozzles 11, 11 contact the periphery 25 c of the upper side and the periphery 25 c of the left side of the coating surface 3. At this movement start position ST, the two nozzles 11, 11 are aligned side by side in the Y axis direction.

[0023] 3B , the linear movement trajectory TR extending from the movement start position ST toward the right along the X axis is the trajectory along which the upper nozzle 11 of the two nozzles 11, 11 arranged side by side in the Y axis direction moves along a position one droplet's width below the periphery 25 c of the upper side of the surface 3 to be coated, and the thick solid arrow corresponds to the trajectory of the center positions of the two nozzles 11, 11 arranged side by side in the Y axis direction. The right end point of the first stage of the movement trajectory TR, which begins from the movement start position ST, is the position where the right end of the droplet 24 from the nozzle 11 touches the periphery 25 c of the right side of the surface 3 to be coated, and the droplet 24 of the paint 2 is dripped from the movement start position ST to this end point. Here, the discharge of the paint 2 is temporarily turned off, and the two nozzles 11, 11 arranged side by side in the Y axis direction move downward in the Y axis direction by the distance of two droplets of the droplet 24.

[0024] The subsequent second stage movement trajectory TR is a straight trajectory from the right end to the left end, where the discharge of paint 2 resumes at the right end and stops again at the left end. This operation is repeated up to the third, fourth, fifth, sixth, and seventh stages. The distance in the Y-axis direction between each stage is the same as the distance between the first and second stages, and is a distance equivalent to two droplets of droplets 24. The movement end position EN is the position where the droplets 24 dropping from the two nozzles 11, 11 contact the periphery 25c of the lower side and the periphery 25c of the right side of the coating surface 3. Through the above operations, the application of the first layer 251 of the wet coating film 25 is completed.

[0025] Next, the coating procedure for the second layer 252 will be described with reference to FIG. 3C . As shown in FIG. 3C , the coating range of the second layer 252 is a narrower area, extending from the periphery 25c of the coating surface 3 to a position one droplet 24 inward. Since the movement end position EN of the first layer 251 is the lower right corner of the coating surface 3, the lower right corner is set as the movement start position ST, and the upper right corner is set as the movement end position EN. The nozzle 11 is then moved along a movement trajectory TR, which moves back and forth in the left-right direction of the X axis while being shifted by two droplets in the Y axis direction. The movement start position ST is set as a position where the droplets 24 dropped from the two nozzles 11, 11 are in contact with a position one droplet 24 inward from the periphery 25c of the bottom edge of the coating surface 3 and a position one droplet 24 inward from the periphery 25c of the right edge, and the two nozzles 11, 11 are aligned side by side in the Y axis direction.

[0026] 3C , the linear movement trajectory TR extending from the movement start position ST toward the left along the X axis is the trajectory along which the lower nozzle 11 of the two nozzles 11, 11 arranged side by side in the Y axis direction moves along the upper side by two droplets from the periphery 25 c of the lower side of the surface 3 to be coated, and the thick solid arrow corresponds to the trajectory of the center positions of the two nozzles 11, 11 arranged side by side in the Y axis direction. The left end point of the first movement trajectory TR from the bottom, which starts from the movement start position ST, is the position where the left end of the droplet 24 from the nozzle 11 touches a position one droplet inward from the periphery 25 c of the left side of the surface 3 to be coated, and the droplet 24 of the paint 2 is dripped from the movement start position ST to this end point. Here, the discharge of the paint 2 is temporarily turned off, and the two nozzles 11, 11 arranged side by side in the Y axis direction move upward in the Y axis direction by the distance of two droplets of the droplet 24.

[0027] The following movement trajectory TR, the second tier from the bottom, is a straight trajectory from the left end to the right end, where the discharge of paint 2 resumes at the left end and stops again at the right end. This operation is repeated for the third tier from the bottom, the fourth tier from the bottom, the fifth tier from the bottom, and the sixth tier from the bottom. The distance in the Y-axis direction between each tier is the same as the distance between the first tier from the bottom and the second tier from the bottom, and corresponds to two droplets of droplets 24. The movement end position EN is the position where the droplets 24 dropped from the two nozzles 11, 11 contact a position one droplet inward from the periphery 25c of the upper edge of the coating surface 3 and a position one droplet inward from the periphery 25c of the right edge. The above operations complete the application of the second layer 252 of the wet coating film 25.

[0028] Next, the coating procedure for the third layer 253 will be described with reference to FIG. 3D . As shown in FIG. 3D , the coating range of the third layer 253 is a narrower area extending inward from the periphery 25c of the coating surface 3 by two droplets 24. Since the movement end position EN of the second layer 252 is the upper right corner of the coating surface 3, for example, the upper right corner is set as the movement start position ST, and the lower left corner is set as the movement end position EN. The nozzle 11 is then moved along a movement trajectory TR that moves back and forth in the left-right direction of the X axis while being shifted by two droplets in the Y axis direction. The movement start position ST is set as a position where the droplets 24 dropped from the two nozzles 11, 11 are in contact with a position two droplets 24 inward from the periphery 25c of the upper edge of the coating surface 3 and a position two droplets 24 inward from the periphery 25c of the right edge, and the two nozzles 11, 11 are oriented side by side in the Y axis direction.

[0029] 3D , the linear movement trajectory TR extending from the movement start position ST toward the left along the X axis is the trajectory along which the upper nozzle 11 of the two nozzles 11, 11 arranged side by side in the Y axis direction moves along a distance of three droplets below the periphery 25 c of the upper side of the surface 3 to be coated, and the thick solid arrow corresponds to the trajectory of the center positions of the two nozzles 11, 11 arranged side by side in the Y axis direction. The left end point of the first movement trajectory TR, which begins from the movement start position ST, is a position where the left end of the droplet 24 from the nozzle 11 touches a position two droplets inward from the periphery 25 c of the left side of the surface 3 to be coated, and the droplet 24 of the paint 2 is dripped from the movement start position ST to this end point. Here, the discharge of the paint 2 is temporarily turned off, and the two nozzles 11, 11 arranged side by side in the Y axis direction move downward along the Y axis by the distance of two droplets of the droplet 24.

[0030] The following second stage movement trajectory TR is a straight trajectory from the left end to the right end, where the discharge of paint 2 resumes at the left end and stops again at the right end. This operation is repeated up to the third, fourth, and fifth stages. The distance in the Y-axis direction between each stage is the same as the distance between the first and second stages, which is a distance equivalent to two droplets of droplets 24. The movement end position EN is the position where the droplets 24 dropping from the two nozzles 11, 11 contact a position two droplets inward from the periphery 25c of the lower side of the coating surface 3 and a position two droplets inward from the periphery 25c of the left side. The above operation completes the application of the third layer 253 of the wet coating film 25.

[0031] Next, the coating procedure for the fourth layer 254, which is the top layer, will be described with reference to FIG. 3E . As shown in FIG. 3E , the coating range of the fourth layer 254 is a narrower area three droplets' worth of droplets 24 inward from the periphery 25c relative to the entire coating surface 3. Since the movement end position EN of the third layer 253 is the lower left corner of the coating surface 3, for example, the lower left corner is set as the movement start position ST, and the upper left corner is set as the movement end position EN. The nozzle 11 is then moved along a movement trajectory TR that moves back and forth in the left-right direction of the X axis while being shifted by two droplets' worth in the Y axis direction. The movement start position ST is set as a position where the droplets 24 dropped from the two nozzles 11, 11 touch a position three droplets' worth of droplets 24 inward from the periphery 25c of the bottom edge of the coating surface 3 and a position three droplets' worth of droplets 24 inward from the periphery 25c of the left edge, and the two nozzles 11, 11 are oriented side by side in the Y axis direction.

[0032] 3E , the linear movement trajectory TR extending from the movement start position ST toward the right along the X axis is the trajectory along which the lower nozzle 11 of the two nozzles 11, 11 arranged side by side in the Y axis direction moves along the upper side by four droplets from the periphery 25 c of the lower side of the surface 3 to be coated, and the thick solid arrow corresponds to the trajectory of the center positions of the two nozzles 11, 11 arranged side by side in the Y axis direction. The right end point of the first movement trajectory TR from the bottom, which starts from the movement start position ST, is a position where the right end of the droplet 24 from the nozzle 11 touches a position three droplets inward from the periphery 25 c of the right side of the surface 3 to be coated, and the droplet 24 of the paint 2 is dripped from the movement start position ST to this end point. Here, the discharge of the paint 2 is temporarily turned off, and the two nozzles 11, 11 arranged side by side in the Y axis direction move upward in the Y axis direction by the distance of two droplets of the droplet 24.

[0033] The subsequent movement trajectory TR in the second row from the bottom is a straight line trajectory from the right end to the left end, where the discharge of paint 2 resumes at the right end and stops again at the left end. This operation is repeated for the third row from the bottom and the fourth row from the bottom. The distance in the Y-axis direction between each row is the same as the distance between the first row from the bottom and the second row from the bottom, which is a distance equivalent to two droplets of droplets 24. The movement end position EN is the position where the droplets 24 dropped from the two nozzles 11, 11 contact a position three droplets inward from the periphery 25c of the upper side of the coating surface 3 and a position three droplets inward from the periphery 25c of the left side. The above operation completes the application of the fourth layer 254 of the wet coating film 25, and thus the wet coating film 25 is formed by the inkjet coating method of this embodiment.

[0034] <<Mechanism of action for suppressing coffee ring phenomenon>> In the inkjet coating method of this embodiment, the occurrence of the coffee ring phenomenon is suppressed by applying the paint 2 so that the film thickness t of the peripheral portion 25b of the wet coating film 25 applied to the coating surface 3 becomes thinner as it approaches the peripheral edge 25c, and the mechanism of action therefor will now be explained.

[0035] FIG. 4A is a cross-sectional view of a coating film illustrating the mechanism by which the coffee ring phenomenon occurs. The left side of FIG. 4A shows a wet coating film 25, and the right side shows a dried coating film 26 obtained by baking and drying the wet coating film 25. The wet coating film 25 shown in the left side of FIG. 4A contains a volatile solvent, so the solvent of the paint 2 evaporates from the wet coating film 25. At this time, the peripheral portion 25b of the wet coating film 25 has a larger contact area with air than the central portion 25a, resulting in a relatively faster evaporation rate of the solvent. Therefore, a liquid flow occurs within the wet coating film 25 from the central portion 25a to the peripheral portion 25b, causing solute particles and other particles present in the central portion 25a to migrate to the peripheral portion 25b of the wet coating film 25. The solute particles that migrate to the peripheral portion 25b then accumulate at the peripheral portion 25b due to a pinning effect until the dryness is achieved by baking. As a result, the coffee ring phenomenon occurs in the dried coating film 26 shown in the right side of FIG. 4A, where the peripheral portion 25b bulges in a circular shape. The occurrence of this coffee ring phenomenon impairs the smoothness of the surface of the dried coating film 26, and is therefore a major problem when applying the inkjet coating device 1 to automobile painting.

[0036] Figure 4B is a cross-sectional view of a coating film illustrating the mechanism for suppressing the occurrence of the coffee ring phenomenon in the inkjet coating method of this embodiment. The left side of Figure 4B shows a wet coating film 25, and the right side of Figure 4B shows a dried coating film 26 obtained by baking and drying the wet coating film 25. Because the wet coating film 25 of this embodiment shown in the left side of Figure 4B contains a volatile solvent, the solvent of the paint 2 evaporates from the wet coating film 25. Furthermore, although the thickness of the peripheral portion 25b of the wet coating film 25 decreases toward the peripheral portion 25c, the contact area with air is larger than that of the central portion 25a, resulting in a relatively faster evaporation rate of the solvent. Therefore, a liquid flow occurs within the wet coating film 25 from the central portion 25a to the peripheral portion 25b, causing particles such as solutes present in the central portion 25a to move to the peripheral portion 25b of the wet coating film 25.

[0037] However, in the wet coating film 25 of this embodiment, the thickness of the peripheral portion 25b becomes thinner toward the peripheral portion 25c, and the amount of liquid in the coating film at the peripheral portion 25b is small (the range of reduced liquid amount is shown by the two-dot chain line in Figure 4B), so even if solutes inside the wet coating film 25 move to the peripheral portion 25b as the solvent evaporates, the amount of liquid does not reach a level sufficient to cause coffee ring. As a result, the occurrence of the coffee ring phenomenon can be suppressed, as shown in the right diagram of Figure 4B.

[0038] FIG. 5 shows cross-sectional views of an example of a wet coating film 25 and a dried coating film 26 applied by the inkjet coating method according to the present invention, as well as a comparative example. The wet coating film 25 of the example is an example of a coating film in which the first layer is applied in 14 droplets (equivalent to droplets 24), the second layer in 10 droplets, the third layer in 6 droplets, and the fourth layer in 2 droplets (equivalent to droplets 24), and the thickness of the peripheral edge 25b gradually decreases toward the peripheral edge 25c. The cross-sectional profile of the dried coating film 26 obtained by baking and drying this wet coating film 25 is shown in the right figure. The peripheral edge of the dried coating film 26 is smoothly curved, and no coffee ring phenomenon is observed. In contrast, the comparative example uses the same paint, and the first through fourth layers are applied in 7 droplets (equivalent to droplets 24), with the peripheral edges 25c of each layer aligned vertically. The cross-sectional profile of the dried coating film 26 obtained by baking and drying this wet coating film 25 is shown in the right figure. A very sharp coffee ring phenomenon is observed.

[0039] The inkjet coating method of this embodiment can be modified within an appropriate range to suppress the occurrence of the coffee-ring phenomenon by the above-described mechanism of action. Modifications of this embodiment will be described with reference to Figures 6A to 6C.

[0040] For example, in the wet coating film 25 shown in FIG. 3A , the wet coating film 25 is coated so that the thickness of the wet coating film 25 is different at its peripheral portion 25 b and the height of adjacent peripheral portions, i.e., the height of each layer from the first layer 251 to the fourth layer 254, is equivalent to one droplet 24, but two or more droplets may be coated, or the layers may be coated so that the heights of each layer are different, or a combination of these may be used.

[0041] 3A , the wet coating film 25 is coated such that the width of the peripheral portion 25b of the wet coating film 25 where the film thickness is equal (the width of a step in the cross-sectional view), i.e., the length by which the lower layer of two adjacent layers protrudes relative to the upper layer, is equivalent to one droplet 24. However, the wet coating film 25 may be coated such that the width is two or more droplets. Fig. 6A is a cross-sectional view showing a wet coating film 25 coated by an inkjet coating method according to another embodiment of the present invention. In this wet coating film 25, the first layer 251 has a width equivalent to 14 droplets 24, the second layer 252 has a width equivalent to 10 droplets, the third layer 253 has a width equivalent to 6 droplets, and the fourth layer 254 has a width equivalent to 2 droplets.

[0042] In the embodiments shown in Figures 3A and 6A, the protruding width (length) of each layer is the same for all layers, i.e., one drop in Figure 3A and two drops in Figure 6A, but it may be different widths. Furthermore, in this case, the width of each layer may decrease toward the periphery 25c of the wet coating film 25. Figure 6B is a cross-sectional view showing a wet coating film 25 applied by an inkjet coating method according to yet another embodiment of the present invention. In this wet coating film 25, the first layer 251 has a width equivalent to 14 droplets 24, the second layer 252 has a width equivalent to 12 droplets, the third layer 253 has a width equivalent to 8 droplets, and the fourth layer 254 has a width equivalent to 2 droplets. Therefore, the difference in width between the first layer 251 and the second layer 252 is one drop on one side, the difference in width between the second layer 252 and the third layer 253 is two drops on one side, and the difference in width between the fourth layer 254 and the third layer 253 is three drops on one side, and the widths decrease as one approaches the peripheral edge 25c.

[0043] Furthermore, in the inkjet coating method of this embodiment, the film thickness t of the peripheral portion 25b of the wet coating 25 is formed so as to become thinner toward the peripheral edge 25c, but the film thickness of the wet coating in a region of a predetermined width extending inward from the peripheral edge 25c may be set to a predetermined value or more. Figure 6C is a cross-sectional view showing a wet coating 25 applied by an inkjet coating method according to yet another embodiment of the present invention. In the embodiment shown in Figure 3A and the like, the film thickness of the peripheral edge 25c of the wet coating 25 is equivalent to one droplet 24, but in the embodiment shown in Figure 6C, the film thickness of the peripheral edge 25c of the wet coating 25 is equivalent to two droplets 24. In this way, the film thickness of the region of a predetermined width extending inward from the peripheral edge 25c may be equivalent to two or more droplets 24.

[0044] <<Connecting Two Coating Surfaces>> When the surface 3 to be coated is large, as in the case of an automobile body, it may be divided into several regions, and the paint 2 may be applied sequentially to the coating surfaces 31, 32, etc. of each region. Figure 7 shows a plan view and a cross-sectional view of a wet coating film 25 illustrating an example of a coating method in which coating of the second coating surface 32 begins after coating of the first coating surface 31 is completed and before baking and drying. The first coating surface 31 is shown by a dashed line on the left side of Figure 7, and the second coating surface 32 is shown by a solid line on the right side of Figure 7. Note that the cross-sectional view of the wet coating film 25 on the second coating surface 32 is shifted slightly to the right to make the cross-sectional structure easier to understand. Figures 8A to 8D are cross-sectional views illustrating the coating procedure for forming the wet coating film 25 on the second coating surface 32, continuing from the first coating surface 31 in Figure 7. 8A shows the application procedure for the first layer 251, FIG. 8B shows the application procedure for the second layer 252, FIG. 8C shows the application procedure for the third layer 253, and FIG. 8D shows the application procedure for the fourth layer 254.

[0045] In the inkjet coating method of this embodiment, as shown in the cross-sectional view at the bottom of Fig. 7 , the coating material 2 is applied to the second coating surface 32 adjacent to the first coating surface 31 so that the vertical cross section of the wet coating film 25 applied to the second coating surface 32 is upside down relative to the vertical cross section of the wet coating film 25 formed on the first coating surface 31. That is, since the wet coating film 25 applied to the first coating surface 31 shown on the left side of Fig. 7 has a quadrangular pyramid shape with 14 drops of the first layer 251, 12 drops of the second layer 252, 10 drops of the third layer 253, and 8 drops of the fourth layer 254, the coating material 2 is applied to the second coating surface 32 so that the wet coating film 25 has a vertical cross section that is upside down, i.e., an inverted quadrangular pyramid shape with 8 drops of the first layer 251, 10 drops of the second layer 252, 12 drops of the third layer 253, and 14 drops of the fourth layer 254.

[0046] 8A to 8D, the procedure for applying the first layer 251 to the fourth layer 254 to the second coating surface 32 will be described. First, as shown in FIG. 8A, eight drops of the first layer 251 are applied to the second coating surface 32, continuing from the right end of the first layer 251 of the wet coating film 25 formed on the first coating surface 31 on the left side.

[0047] Next, as shown in Fig. 8B , ten drops of the second layer 252 are applied to the second application surface 32, continuing from the right end of the second layer 252 of the wet coating film 25 formed on the left first application surface 31. Here, nine of the ten drops of the applied second layer 252 are applied on the first layer 251 of the wet coating film 25 formed on the left first application surface 31 and on the first layer 251 applied in Fig. 8A , but the one drop at the right end (indicated by reference symbol 252a) is applied to the right end of the first layer 251 because there is no first layer 251 below it.

[0048] Next, as shown in Fig. 8C , 12 drops of the third layer 253 are applied to the second coating surface 32, continuing from the right end of the third layer 253 of the wet coating film 25 formed on the first coating surface 31 on the left. Here, 10 of the 12 drops of the applied third layer 253 are applied onto the second layer 252 of the wet coating film 25 formed on the first coating surface 31 on the left and onto the second layer 252 applied in Fig. 8B , but the two rightmost drops (indicated by reference symbol 253a) are applied to the right ends of the first layer 251 and the second layer 252, respectively, because there is no first layer 251 or second layer 252 below them.

[0049] Finally, as shown in Fig. 8D , 14 drops of the fourth layer 254 are applied to the second coating surface 32, continuing from the right end of the fourth layer 254 of the wet coating film 25 formed on the left first coating surface 31. Here, 11 of the 14 drops of the applied fourth layer 254 are applied onto the third layer 253 of the wet coating film 25 formed on the left first coating surface 31 and onto the third layer 253 applied in Fig. 8C , but the rightmost three drops (indicated by reference symbol 254a) are applied to the right end of the first layer 251, the right end of the second layer 252, and the right end of the third layer 253, respectively, because there is no first layer 251, second layer 252, or third layer 253 below them.

[0050] As a result of the above, a wet coating film 25 can be formed on the second coating surface 32 continuously from the wet coating film 25 formed on the first coating surface 31. However, since the connecting portions of the two wet coating films 25, 25 have vertical cross-sectional shapes that are inverted from each other, the thickness of the wet coating film 25 after connecting is the same, resulting in good surface smoothness.

[0051] <<Actions and Effects of the Embodiment>> As described above, according to the inkjet coating method of the present embodiment, in which an inkjet coating device 1 including a nozzle 11 that ejects paint 2 is used to apply the paint 2 to a target coating surface 3, the paint 2 is applied so that the film thickness t of the peripheral portion 25b of the wet coating film 25 applied to the coating surface 3 becomes thinner toward the peripheral edge 25c. Therefore, due to the mechanism of action described with reference to Figure 4B, it is possible to suppress the occurrence of coffee ring even if there is an interval such as a setting time between the end of coating and the start of baking and drying, as in the painting process of an automobile.

[0052] Furthermore, according to the inkjet coating method of this embodiment, the paint 2 is applied so that the thickness of the wet coating film 25 is different at the peripheral portion 25b of the wet coating film 25 and the height of adjacent peripheral portions is a predetermined number of droplets when converted into droplets 24 of the paint 2 ejected from the nozzle 11, so that the inclination angle of the peripheral portion 25b of the wet coating film 25 can be set to a desired angle.

[0053] Furthermore, according to the inkjet coating method of this embodiment, the paint 2 is applied so that the width of the peripheral portion 25b of the wet coating film 25 where the film thickness of the wet coating film 25 is equal to a predetermined number of droplets 24 of the paint 2 ejected from the nozzle 11, and therefore the inclination angle of the peripheral portion 25b of the wet coating film 25 can be set to a desired angle.

[0054] Furthermore, according to the inkjet coating method of this embodiment, the paint 2 is applied so that the width of the peripheral portion 25b of the wet coating film 25 where the film thickness of the wet coating film 25 is the same results in a different number of droplets when converted into droplets 24 of the paint 2 ejected from the nozzle 11, so that the inclination angle of the peripheral portion 25b of the wet coating film 25 can be set to a desired angle.

[0055] Furthermore, according to the inkjet coating method of this embodiment, the paint 2 is applied so that the width of the peripheral portion where the film thickness of the wet coating film 25 is uniform decreases as the number of droplets converted into droplets 24 of the paint 2 ejected from the nozzle 11 decreases toward the peripheral edge 25c. Therefore, the inclination angle of the peripheral edge 25b of the wet coating film 25 can be set to a small angle, further suppressing the occurrence of the coffee ring phenomenon.

[0056] Furthermore, according to the inkjet coating method of this embodiment, the paint 2 is applied to the peripheral portion 25b of the wet coating film 25 in an area of ​​a predetermined width from the peripheral edge 25c so that the thickness of the wet coating film 25 is equal to or greater than a predetermined value, thereby increasing the strength of the peripheral portion 25b of the wet coating film 25.

[0057] Furthermore, according to the inkjet coating method of this embodiment, the coating material 2 is ejected from the nozzle 11 to form the wet coating film 25 of the first layer 251 on the coating surface 3, and then the wet coating film 25 of the second layer 252 is formed on the wet coating film 25 of the first layer 251. This process of coating is repeated sequentially up to the top layer, so that natural drying (evaporation of the volatile solvent) progresses from the bottom up. As a result, the occurrence of the coffee ring phenomenon can be further suppressed.

[0058] Furthermore, according to the inkjet coating method of this embodiment, the paint 2 is applied to another coating surface 32 adjacent to the coating surface 31 so that the vertical cross section of the wet coating film 25 is an inverted vertical cross section, so that the film thickness of the wet coating film 25 after connection is uniform and the surface smoothness is good.

[0059] Furthermore, according to the inkjet coating method of this embodiment, the paint 2 is applied to an automobile body or an automobile part, so the efficiency of using the paint 2 is significantly improved and the scattering of paint dust is also significantly reduced, resulting in a significant improvement in the working environment.

[0060] REFERENCE SIGNS LIST 1... Inkjet coating device 11... Nozzle 111... Introduction section 112... Paint chamber 113... Discharge section 114... Housing 12... Needle 121... Tip section 122... Base section 123... Sealing member 13... Actuator 14... Pressure sensor 15... Control section 2... Paint 21... Paint tank 22... Paint piping 23... Paint pump 24... Droplet 25... Wet coating film 25a... Center section 25b... Peripheral section 25c... Periphery 251... First layer 252... Second layer 253... Third layer 254... Fourth layer 26... Dry coating film 3... Coating surface 31... First coating surface 32... Second coating surface

Claims

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10. An inkjet coating method for applying a coating material to a target surface using an inkjet coating device including a nozzle for ejecting the coating material, comprising: The coating material is applied to the coating surface so that the thickness of the wet coating film at the periphery becomes thinner as it approaches the periphery; An inkjet coating method in which the paint is applied so that the width of the peripheral portion of the wet coating film where the wet coating film has an equal film thickness is equivalent to a predetermined number of droplets of paint ejected from the nozzle.

11. An inkjet coating method for applying a coating material to a target surface using an inkjet coating device including a nozzle for ejecting the coating material, comprising: The coating material is applied to the coating surface so that the thickness of the wet coating film at the periphery becomes thinner as it approaches the periphery; An inkjet coating method in which the paint is applied so that the width of the peripheral portion of the wet coating film where the wet coating film has the same film thickness is converted into the number of droplets of paint ejected from the nozzle, which differs.

12. The inkjet coating method according to claim 11, wherein the coating material is applied so that the width of the peripheral portion where the wet coating film has a uniform thickness decreases in number of droplets of the coating material ejected from the nozzle as the width approaches the peripheral portion.

13. The inkjet coating method according to any one of claims 10 to 12, wherein the coating material is applied so that the thickness of the wet coating material is different at the peripheral portions of the wet coating material and the height of adjacent peripheral portions is a predetermined number of droplets converted into droplets of the coating material ejected from the nozzle.

14. The inkjet coating method according to any one of claims 10 to 13, wherein the paint is applied so that a region of a predetermined width from the periphery of the wet coating film has a thickness of the wet coating film equal to or greater than a predetermined value.

15. After the coating material is discharged from the nozzle to form a first layer of wet coating film on the coating surface, forming a second layer of wet coating film on the first layer of wet coating film; The inkjet coating method according to any one of claims 10 to 14, wherein the above-described coating is repeated until the top layer is reached.

16. The inkjet coating method according to any one of claims 10 to 14, wherein the coating material is applied to another coating surface adjacent to the coating surface such that the vertical cross section of the wet coating film is an upside-down vertical cross section.

17. A method for painting an automobile, comprising applying the paint to an automobile body or an automobile part by the inkjet coating method according to any one of claims 10 to 16.