Patent Application
20200171518
The present invention relates to a bell cup of a rotary atomization-type coating device.
A bell cup of a rotary atomization-type coating device is known, in which the cup inner surface has a coating material spreading surface that is constituted of a convex curved surface toward the axis of rotation (Patent Document 1: JP1998-52657A). It is said that the use of this bell cup allows the particle diameter distribution of a coating material to be sharp.
[Patent Document 1] JP1998-52657A
However, when evaluating the atomization performance (average particle diameter) of coating materials using the above bell cup of the convex curved surface, the present inventors have found that the atomization performance of a low-viscosity coating material is lower than that of a high-viscosity coating material even under the same conditions of the composition, discharge rate, and rotation speed. This may lead to a problem in that the coating conditions including the rotation speed of the bell cup have to be made different depending on the viscosity of the coating material.
A problem to be solved by the present invention is to provide a bell cup of a rotary atomization-type coating device with which uniform atomization can be achieved regardless of the viscosity of a coating material.
The present invention solves the above problem by providing a bell cup in which a predetermined region of the coating material spreading surface is constituted of a convex curved surface toward the axis of rotation and the outermost surface of at least part of the coating material spreading surface is covered with a diamond-like carbon film free from silicon at least on its outermost surface.
According to the present invention, water-repellent properties or oil-repellent properties of the diamond-like carbon film formed on the outermost surface of the bell cup suppress a waving phenomenon of the coating material on the coating material spreading surface. This can make the atomization uniform regardless of the coating material viscosity.
Hereinafter, one or more embodiments of the present invention will be described with reference to the drawings.
The rotary atomization-type coating device 1 illustrated in
The rotary atomization-type coating device 1 operates in such a manner that coating material particles having been charged by application of voltage from a high-voltage power supply 16 travel in the air along an electrostatic field formed between the device and an object to be coated and the object is coated with the coating material particles. Although not illustrated, the object to be coated is located on the left side of
The rotary atomization-type coating device 1 operates to discharge an air flow referred to as shaping air from air ejection ports 17 disposed on the back surface side of the bell cup main body 30 and deflect the coating material particles, which are atomized by the bell cup main body 30, in a direction toward the object located ahead of the bell cup main body 30. To this end, part of the housing 11 is formed with an air passage 19 connected to an air supply device 18, and the distal end of the housing 11 is formed with an annular air passage 20 communicating with the air passage 19. The air ejection ports 17, which communicate with the annular air passage 22, are formed at predetermined intervals along the distal end circumferential surface of the housing 11. By adjusting the flow rate and blowing angle of the shaping air blown from the air ejection ports 17, the traveling direction of the coating material particles released from the distal end of the bell cup main body 30 in the tangent direction, that is, the coating pattern, can be controlled. The coating material particles are given kinetic momentum caused by the shaping air in addition to the force caused by the above-described electrostatic field. The air ejection ports 17 for the shaping air illustrated in
The tip end of the feed tube 15 protrudes from the tip end of the hollow shaft 13 and extends toward the interior of the bell cup main body 30. The feed tube 15 is supplied with the coating material or cleaning thinner from the coating material supply device 14 and feeds the coating material or cleaning thinner to a coating material spreading surface 31 of the bell cup main body 30 from the tip end of the feed tube 15. The cleaning thinner is a cleaning liquid (in the case of an organic solvent-based coating material, an organic solvent, or in the case of an aqueous coating material, water) for cleaning the coating material spreading surface 31 of the bell cup main body 30 and the bell hub 40, which will be described later. When the rotary atomization-type coating device 1 of this example is applied to a top coat coating process or a middle coat coating process, which requires a color switching operation, the cleaning thinner is supplied for the purpose of cleaning when switching the color of the coating material. Accordingly, in coating processes in which color switching operations are not needed, such as a middle coat coating process involving the coating only with a single type of middle coat coating material, for example, the feed tube 15 may be supplied only with the coating material. Color switching operations are carried out using a color switching valve unit, such as a color change valve, not illustrated, which is included in the coating material supply device 14.
The bell cup main body 30 of this example is composed of a conductive material such as aluminum, an aluminum alloy, titanium, a titanium alloy, a stainless alloy, or other metal material. However, the bell cup main body 30 applied to the above-described rotary atomization-type electrostatic coating device of an external application type may be composed of a hard resin material. The bell cup main body 30 of this example is approximately cup shaped and has the coating material spreading surface 31 of the cup-shaped inner surface, a cup-shaped outer surface 32, and a distal end edge 33 located at the distal end of the inner surface, from which the coating material is released. The configuration of the coating material spreading surface 31 will be described later.
The bell hub 40 is attached to the center on the proximal end side of the bell cup main body 30 in the vicinity of the tip end of the feed tube 15. This bell hub 40 can be composed of an electrically conductive material such as metal or an electrically insulating material such as a resin, but may more preferably be composed of a resin material. The bell hub 40 of this example is fixed by fastening a screw part 46 illustrated in
As the bell cup main body 30 is circular centered on a rotation center axis CL (including an extension of the center line of the hollow shall 13 as a rotary shaft) in the front view, the bell hub 40 is also circular in the front view. The outer circumferential part of the bell hub 40 is formed with a plurality of through holes 41 at predetermined intervals, and the coating material or cleaning thinner fed from the tip end of the feed tube 15 passes through the through holes 41 of the bell hub 40 and is guided onto the coating material spreading surface 31 of the bell cup main body 30 and then sprayed from the entire circumference of the distal end edge 33.
The bell hub 40 of this example is fixed to the proximal end part of the bell cup main body 30 by screw fastening in a state in which the spacer 50 is interposed between the bell hub 40 and the bell cup main body 30. As illustrated in
Configurations of the coating material spreading surface 31 of the bell cup main body 30 and the bell hub 40 in this example will then be described.
In particular, the coating material spreading surface 31 of this example includes a first region 31A that extends to the proximal end part including the start point facing any of the through holes 41 and a second region 31B that merges into the first region 31A and extends to the distal end edge 33 of the bell cup main body 30. The first region 31A is constituted of a curved surface that forms an angle of more than 0° and less than 5° with the rotation center axis CL, while the second region 31B is constituted of a convex curved surface toward the rotation center axis CL. The coating material spreading surface within the first region 31A may also be referred to as a first coating material spreading surface 31A, and the coating material spreading surface within the second region 31B may also be referred to as a second coating material spreading surface 31B. As illustrated in
If the angle α formed between the rotation center axis CL and the straight line L1 passing through the first coating material spreading surface 31A is 0°, the coating material or cleaning thinner discharged onto the first coating material spreading surface 31A is less likely to flow to the second coating material spreading surface 31B even with the centrifugal force due to the rotation of the bell cup main body 30. If the angle α formed between the rotation center axis CL and the straight line L1 passing through the first coating material spreading surface 31A is less than 0°, that is, if the curved surface of the first coating material spreading surface 31A of the first region is in a side surface shape of an expanding circular truncated cone toward the proximal end side, the coating material or cleaning thinner discharged onto the first coating material spreading surface 31A is likely to flow adversely toward the proximal end part of the bell cup main body 30 with the centrifugal force due to the rotation of the bell cup main body 30. On the other hand, if the angle α formed between the rotation center axis CL and the straight line L1 passing through the first coating material spreading surface 31A is 5° or more, the coating material accumulation effect described below cannot readily be obtained. Accordingly, the angle α formed between the rotation center axis CL and the straight line L1 passing through the first coating material spreading surface 31A preferably satisfies 0°<α<5°.
The curved surface of the second coating material spreading surface 31B of the second region is formed as a convex curved surface toward the rotation center axis CL, that is, a curved surface on which the angle formed between the rotation center axis CL and the tangent line to the curved surface increases gradually toward the distal end edge 33 of the bell cup main body 30. Although not particularly limited, as illustrated in
On the other hand, as illustrated in
The central part of the bell hub 40 is formed with a plurality of cleaning holes 45. The cleaning holes 45 have respective openings at the inner surface of the bell hub 40 and merge into a single opening at the outer surface of the bell hub 40. That is, each cleaning hole 45 is a hole inclined toward the rotation center axis CL, in other words, a hole inclined in the diameter reducing direction toward the distal end of the bell cup 3. The cleaning holes 45 of this example are used when cleaning the bell cup main body 30 and the outer surface of the bell hub 40 with the cleaning thinner. When the cleaning thinner is fed from the feed tube 15 in a state in which the rotation speed of the bell cup 3 is set low, large centrifugal force does not act on the cleaning thinner discharged onto the inner surface of the bell hub 40. Accordingly, part of the cleaning thinner reaches the outer surface of the bell hub 40 through the cleaning holes 45 and can clean the outer surface of the bell hub 40. However, when the bell cup 3 is rotated at a high speed, such as during the coating with the coating material, the coating material discharged onto the inner surface of the bell hub 40 does not reach the outer surface of the bell hub 40 via the washing holes 45 by virtue of the centrifugal force and the reverse inclination of the washing holes 45.
The present inventors have found that, when the coating is performed using the bell cup 3 having the second coating material spreading surface 31B formed as that type of convex curved surface, the viscosity of the coating material to be used significantly affects the average particle diameter. That is, the obtained knowledge is that, when two types of clear coating materials having different coating material viscosities are atomized at the same discharge rate and the same rotation speed, the average particle diameters of the obtained atomized particles are different and, in particular, the higher-viscosity coating material exhibits higher atomization performance than that of the lower-viscosity coating material. This means that the higher-viscosity coating material is atomized with a smaller average particle diameter. Specifically, the mass-average particle diameter of the clear coating material having a kinematic viscosity of 100 mPa·s was 58 μm, while the mass-average particle diameter of the clear coating material having a kinematic viscosity of 80 mPa·s was 70 μm. The conventional common sense is that the lower-viscosity coating material has higher atomization performance, but in this knowledge, the higher-viscosity coating material has higher atomization performance, which is the opposite result to the conventional common sense.
This means that, when the coating is performed using the bell cup 3 of the convex curved surface, the difference in the viscosity causes the atomization performance to differ even under the same conditions of the composition, discharge rate, and rotation speed. If so, a problem arises in that the coating conditions including the rotation speed of the bell cup 3 have to be made different depending on the viscosity at the time of coating. For example, in the above-described specific example, to reduce the mass-average particle diameter from 70 μm to 58 μm, the coating with this lower-viscosity coating material has to be performed at a higher rotation speed than that for the higher-viscosity coating material by about 10,000 rpm. As will be understood, it is technically possible to control the rotation speed of the bell cup 3 in accordance with the coating material viscosity, but in this case the rotation speed of the bell cup 3 has to be controlled while detecting the coating material viscosity in real time and the control thus becomes complicated because the coating material viscosity varies depending on the temperature.
The reason that such a waving phenomenon occurs appears to be because the speed of the coating liquid is significantly different between the bottom part of the coating liquid at the interface with the bell cup surface and the surface part of the coating liquid. In the case of the higher-viscosity coating material, the difference in speed is less likely to occur in the coating liquid itself, so no waving phenomenon is observed, while in the case of the lower-viscosity coating material, the difference in speed is more likely to occur in the thickness direction of the coating liquid, so this is because the waving phenomenon is observed. The flow of the coating liquid film on the second coating material spreading surface 31B of the bell cup main body 30 is preferably a laminar flow. However, depending on the properties of the coating material, particularly in a lower-viscosity coating material, the speed difference occurs between the bottom part and the surface part of the coating material liquid, which causes the waving phenomenon on numerous sites of the second coating material spreading surface 31B. This waving phenomenon leads to variation in the amount of coating material supplied to the large number of grooves provided near the outermost circumference of the bell cup main body 30 and appears as a phenomenon that the tops of waves get across walls between the grooves and are released as a film-like liquid rather than a thread-like liquid. If the coating material is released as a film-like liquid from the distal end edge of the bell cup main body 30, the shaping air supplied from the back surface of the bell cup main body 30 is entrained as air bubbles in the coating material, which then adheres to the object to be coated and may readily generate coating film defects similar to the foaming phenomenon on the coating surface.
To overcome such problems, in the bell cup main body 30 of this example, the outermost surface of at least part of the coating material spreading surface 31 is covered with a diamond-like carbon film 50 free from silicon at least on its outermost surface. As indicated by crosses in
The diamond-like carbon film 50 of this example is composed of diamond-like carbon (DLC) that is an amorphous material having both the SP3 bond of diamond and the SP2 bond of graphite as the skeleton structures of carbon atoms. In particular, the diamond-like carbon film 50 of this example is preferably composed of (a) diamond-like carbon that is hydrogenated amorphous carbon containing hydrogen and in which carbon atoms on its surface are terminated with hydrogen atoms, (b) diamond-like carbon that is hydrogenated amorphous carbon containing hydrogen and in which carbon atoms on its surface are not terminated with hydrogen atoms, or (c) diamond-like carbon that is amorphous carbon containing fluorine and in which carbon atoms on its surface are not terminated with fluorine atoms. As will be described later, a diamond-like carbon film composed of amorphous carbon that is diamond-like carbon but contains silicon Si is not preferred because the effect of the present invention of absorbing the viscosity difference of coating materials may not be exhibited.
The diamond-like carbon film 50 of this example can be formed on the bell cup main body 30 by a chemical vapor deposition method (CVD method) in which the film is formed from plasma of a hydrocarbon-based gas such as CH4 or C2H2 or a physical vapor deposition method (PVD method) in which the film is formed from solid carbon using sputtering or cathodic arc discharge. The diamond-like carbon film 50 of this example contains hydrogen or fluorine as described in the above (a) to (c) and can therefore be readily formed by the CVD method. It suffices that the diamond-like carbon film 50 of this example has a film thickness that allows the film to exhibit water-repellent properties to an aqueous coating material as the coating material to be applied or oil-repellent properties to an organic solvent-based coating material as the coating material to be applied. Although not particularly limited, the film thickness is 0.2 μm to 2.0 μm.
It is to be noted that the diamond-like carbon film 50 cannot be directly formed on a general iron-based material. This is because the wettability with iron is low and it is difficult to form a carbide layer at the interface, thus the film may readily delaminate. Accordingly, when the bell cup main body 30 is composed of the above-described aluminum, aluminum alloy, titanium, titanium alloy, stainless steel alloy, or other metal material, it is preferred to form an electroless plating film of metal such as nickel, a metal oxide film, or a silicon-containing diamond-like carbon film as an intermediate layer on the surface of the bell cup main body 30 and then form the diamond-like carbon film 50 of this example on the surface of the intermediate layer.
As described above, in the bell cup 3 according to one or more embodiments of the present invention, the diamond-like carbon film 50 composed of any of (a) diamond-like carbon that is hydrogenated amorphous carbon containing hydrogen and in which carbon atoms on its surface are terminated with hydrogen atoms, (b) diamond-like carbon that is hydrogenated amorphous carbon containing hydrogen and in which carbon atoms on its surface are not terminated with hydrogen atoms, and (c) diamond-like carbon that is amorphous carbon containing fluorine and in which carbon atoms on its surface are not terminated with fluorine atoms is formed at least on the outermost surface of the second coating material spreading surface 31B or on the outermost surface of the coating material spreading surface 31 at which the acute angle θ formed between the tangent line to the coating material spreading surface 31 and the rotation center axis CL is 60° to 90° and, therefore, the water-repellent properties or oil-repellent properties are exhibited to the coating material spreading from the proximal end side to the distal end side of the coating material spreading surface 31. This reduces the speed difference between the bottom part and the surface part of the coating material, and the occurrence of the waving phenomenon as shown in
The surface of the coating material spreading surface 31 of the bell cup 3 illustrated in
In Example 1 above, the average particle diameters of the three types of clear coating materials at the time of coating were measured. The method of measuring the average particle diameters includes forming a so-called spray pattern ahead of the rotary atomization-type coating device 1, moving a prepared glass plate to traverse and cross the spray pattern, and performing image processing to measure the particle diameter of the coating material particles collected on the glass plate. The measured average particle diameters are listed in Table 1. The average particle diameter is represented by a mass-average particle diameter (D43). This mass-average particle diameter is a physical quantity indicative of how small, on average, the diameter of particles in the coating film is when the total amount of particle cloud of the spray pattern adheres to the object to be coated. The smaller the numerical value, the better the atomization state is.
Coating was performed under the same condition as in Example 1 except that the diamond-like carbon film 50 was composed of (b) diamond-like carbon that is hydrogenated amorphous carbon containing hydrogen and in which carbon atoms on its surface are not terminated with hydrogen atoms. The average particle diameters (mass-average particle diameters, D43) of the three types of clear coating materials at the time of coating are listed in Table 1.
Coating was performed under the same condition as in Example 1 except that the diamond-like carbon film 50 was composed of (c) diamond-like carbon that is amorphous carbon containing fluorine and in which carbon atoms on its surface are not terminated with fluorine atoms. The average particle diameters (mass-average particle diameters, D43) of the three types of clear coating materials at the time of coating are listed in Table 1.
Coating was performed under the same condition as in Example 1 except that an electroless nickel plating film (Ni) was formed on the surface of the coating material spreading surface 31 of the bell cup 3 as substitute for the diamond-like carbon film 50. The average particle diameters (mass-average particle diameters, D43) of the three types of clear coating materials at the time of coating are listed in Table 1.
Coating was performed under the same condition as in Example 1 except that a chromium nitride film (CrN) was formed on the surface of the coating material spreading surface 31 of the bell cup 3 as substitute for the diamond-like carbon film 50. The average particle diameters (mass-average particle diameters, D43) of the three types of clear coating materials at the time of coating are listed in Table 1.
Coating was performed under the same condition as in Example 1 except that a diamond-like carbon film composed of diamond-like carbon that is amorphous carbon containing silicon and in which silicon atoms are exposed on its surface was formed on the surface of the coating material spreading surface 31 of the bell cup 3 as substitute for the diamond-like carbon film 50. The average particle diameters (mass-average particle diameters, D43) of the three types of clear coating materials at the time of coating are listed in Table 1.
From the results of Table 1, it has been confirmed that, in Examples 1 to 3, the average particle diameter difference when performing the coating under the same condition is only 3 to 4 μm even with different kinematic viscosities of 80 to 120 mPa·s whereas, with regard to the bell cup in Comparative Examples 1 to 3, the average particle diameter difference is 11 to 14 μm, which is not negligible.
For the organic solvent-based clear coating material having a kinematic viscosity of 100 mPa·s and organic solvent-based clear coating material having a kinematic viscosity of 80 mPa·s in Example 1 and the organic solvent-based clear coating material having a kinematic viscosity of 100 mPa·s and organic solvent-based clear coating material having a kinematic viscosity of 80 mPa·s in Comparative Example 1, the average particle diameters (mass-average particle diameters, D43) when the rotation speed of the bell cup main body 30 was 25,000 rpm, 35,000 rpm, and 45,000 rpm were measured. The results are illustrated in
From the results of
Coating was performed under the same condition using the same bell cups of Examples 1 to 3 and Comparative Examples 1 to 3 except that the coating material was an organic solvent-based middle coat coating material (ORGA OP-61M Sealer available from NIPPON PAINT AUTOMOTIVE COATINGS CO., LTD.) as substitute for the clear coating material, the three types of kinematic viscosities were 135 mPa·s, 121 mPa·s, and 110 mPa·s, the discharge rate of the middle coat coating material was 400 ml/min, and the rotation speed of the bell cup main body 30 was 20,000 rpm, and the average particle diameters at the time of coating were measured. The results are listed in Table 2.
Coating was performed under the same condition using the same bell cups of Examples 1 to 3 and Comparative Examples 1 to 3 except that the coating material was an aqueous middle coat coating material (PROBLOCK N available from BASF Japan Ltd.) as substitute for the clear coating material, the three types of kinematic viscosities were 132 mPa·s, 117 mPa·s, and 101 mPa·s, the discharge rate of the middle coat coating material was 350 ml/min, and the rotation speed of the bell cup main body 30 was 20,000 rpm, and the average particle diameters at the time of coating were measured. The results are listed in Table 3.
From the results of Table 2 and Table 3, it has been confirmed that coating materials for which the bell cup according to one or more embodiments of the present invention is preferably used include clear coating materials as well as middle coat coating materials (organic solvent-based and aqueous ones) that are coating materials free from bright pigments.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2017/018487 | 5/17/2017 | WO | 00 |
