JP2008093616A - Method for forming coating film on metallic light alloy wheel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for forming a coating film on a light alloy wheel capable of preventing decline of product yield by eliminating a defective appearance of a metallic layer, which is achieved by forming a sufficiently thick layer by static powder coating to eliminate an apparent unevenness of the skin of the metal casting. <P>SOLUTION: The method for the forming coating film on the metallic light alloy wheel is characterized by comprising the step of forming a layer by electrostatically coating at least a part of the surface side of the decorative face of the light alloy wheel body with a powder coating material by injecting the same from the backside of the decorative face of the light alloy wheel body and the step of forming a metallic layer over the electrostatically coated layer. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for forming a coating film of a light metal alloy wheel having an aluminum alloy as a base, which has been subjected to metallic coating, and in particular, to apply a metallic coating film to a cast surface portion. is there.

  In a light alloy wheel that is an automobile (for example, passenger car) part, a multi-layer coating is applied to the surface of a base body in order to exhibit a high-quality appearance. In recent years, in order to further enhance the high quality, a metal-like silver coating (metal-tone coating) that is reminiscent of a polished surface of metal has been manufactured. In this metallic silver coating, there is a demand for a metallic coating that gives a high-density feeling that does not give the gritty particles of the conventional silver metallic system, and a large shading feeling depending on the visual direction. Various proposals have been made to achieve this.

  As a general metal-like silver coating, Patent Document 1 discloses a coating-like structure of a plating-like aluminum wheel. On the surface of the aluminum wheel, a powder primer, a solvent or an aqueous color base coat, a solvent or an aqueous plating-like coating, A four-layer coating consisting of a solvent or powder topcoat (clear) is described. In addition, in Patent Document 2, a polyester, acrylic, epoxy-based powder resin coating film having a thickness of 20 to 200 [mu] m is provided on the surface of the coating material, and thermosetting using a polyester resin as a main material thereon. A four-layer coating material in which a 10-40 μm undercoat layer made of a functional resin is provided, a titanium alloy thin film is provided thereon, and a transparent resin coating film is provided thereon is used, and an aluminum wheel is used as an object to be coated It is described.

The metallic light alloy wheel is usually a metallic paint in which vapor-deposited aluminum flakes having a thickness of 0.1 μm or less are dispersed inside in order to give the appearance a metallic tone. However, this metallic paint is very expensive. Usually, the metallic paint is higher than the sum of the three paint costs of the other primer paint, the color base paint, and the clear paint. Therefore, when a defect in the appearance of the metal light alloy wheel is found after the metallic paint is formed, the metallic paint has to be applied again after reworking, so that the cost becomes very high.
Many appearance defects are caused by irregularities in the cast surface of the wheel base. By thickly applying the base layer, the surface of the coating film can be smoothed and the occurrence of poor appearance can be suppressed. In order to coat the underlayer thickly, electrostatic coating using a powder coating is used instead of a solvent coating. As for this electrostatic coating, there is one that sprays a powder paint with a spray gun and sprays it on a design surface as in Patent Document 3.
JP 2000-140756 A Japanese Patent Application Laid-Open No. 2002-219771 JP-A-11-179247

  However, when the front side of the design surface is a cast surface, even a slight unevenness causes a poor appearance, so it is difficult to completely remove the unevenness even if a ground layer is simply provided by powder electrostatic coating. As a result of examining these causes, when spraying powder paint with a spray gun, the powder paint that has already adhered to the front side of the design surface is blown off by the spray pressure of air from the spray gun, and is necessary on the front side of the design surface. It was found that the powder coating was not piled up to the film pressure, and a thin metallic layer of 10 μm or less was affected by this unevenness.

  Therefore, the present invention solves the above problems, sufficiently thicken the electrostatic powder coating layer to eliminate the irregularities of the casting surface, without causing poor appearance even if a thin metallic layer is formed, It aims at providing the coating-film formation method of the light alloy wheel which can prevent the fall of a yield.

  As a result of the study by the present inventor to achieve the above-mentioned object, it is more effective to blow from the back side of the design surface than to spray the powder paint from the front side of the design surface as described above. It has been found that since the film pressure of the body electrostatic coating film is increased and the unevenness of the underlying layer is eliminated, appearance defects on the uppermost surface of the design surface that are most noticeable are eliminated even when a thin metallic layer is applied.

  That is, in the method for forming a coating film of a light metal alloy wheel of the present invention, the powder coating is blown from the back side of the design surface of the light alloy wheel base to form a base layer having a layer by electrostatic coating on the front side of the design surface. And a step of forming a metallic layer outside the underlayer. When the powder coating is blown from the back side of the design surface, it is preferable to carry out with the design surface facing upward. Moreover, it is preferable that the powder coating material is blown from the back side of the design surface while rotating the light alloy wheel.

  The coating film preferably forms a clear layer on the metallic layer. The underlayer is preferably composed of a powder primer layer formed on the surface of the substrate and a color base layer formed on the primer layer. The metallic layer preferably has a metallic layer thickness of 10 μm or less, and metal flakes having a thickness of 10 to 500 mm and an aspect ratio of 300 to 800 dispersed in the metallic layer. Further, the metal light alloy wheel is preferably one in which the cast skin portion is coated.

  According to the present invention, it is possible to easily increase the thickness of the foundation layer on the design surface by blowing the powder coating from the back side of the design surface of the light alloy wheel base. The appearance defect caused by the unevenness is reduced, and a thin metallic layer of 10 μm or less is not affected by the unevenness. Therefore, it is possible to suppress the appearance of defective products before the formation of the expensive metallic layer.

In the method of forming a coating film for a metal light alloy wheel according to the present invention, the powder coating layer 11 of the underlayer is thick, the surface of the underlayer is smoothed, and appearance defects due to the unevenness of the underlayer are less likely to occur. Become. The design surface formed by casting is provided with a draft in the vertical wall portion in order to improve releasability from the mold. The present invention is preferably applied to a light alloy wheel in which the vertical wall portion spreads from the front side to the back side of the design surface as shown in FIGS.
FIG. 4 schematically shows the AA cross section of the wheel base body of FIG. 2 and the flow of the powder coating and air in the coating film forming method of the present invention. The powder coating 18 and air sprayed from the back side are blown into the design surface front side through the window portion 16. Since a part of the sprayed air is blocked by the back surface of the spoke portion, the powder coating material 18 deposited on the front side of the design surface is not directly blown off.
On the other hand, as shown in FIG. 5, when the powder coating material 18 and air are sprayed from the front side of the design surface, the powder coating material 18 deposited on the front side of the design surface is blown off. As a result, the powder coating material 18 is sufficiently deposited. do not do. In particular, this tendency is prominent in the vertical wall portion 17 serving as a draft portion.
When the powder coating is blown from the back side of the design surface, it is preferable to perform with the design surface 15 facing upward, and the powder coating tends to be deposited on the upper surface side of the design surface 15 by gravity.
Moreover, it is preferable that the powder coating material is blown from the back side of the design surface while rotating the light alloy wheel. Since the vertical wall portion moves relative to the powder coating material, the floating powder coating material tends to adhere to the surface of the design surface. Moreover, a coating film having a uniform thickness can be formed in each part in the circumferential direction regardless of the number and arrangement of spray guns.
When the powder paint is ejected only from the back side, the membrane pressure tends to be small on the inner diameter side and outer diameter side of the window portion, which are vertical walls substantially parallel to the circumferential surface. Therefore, when there is a portion where such film pressure is insufficient, it is preferable to spray the powder coating from the back surface side and at the same time blast the portion from the front side.

  The underlayer may be composed of, for example, only a powder primer layer formed on the surface of the substrate, or may be a multilayer coating film composed of a color base layer formed on the powder primer layer. In order to give a metallic tone, the base layer is preferably black or dark blue. Therefore, it is preferable to use a deep-colored paint for the uppermost layer of the underlayer regardless of whether the underlayer is a single layer or a multilayer.

  Further, the metallic layer preferably contains metal flakes in an amount of more than 20% with respect to the coating film volume. The content of the metal flakes can be obtained from the cross-sectional area of the coating film and the cross-sectional area of the metal flakes obtained by taking a cross-sectional image of the coating film and moving to the section image. For example, a metallic layer having a thickness of 10 μm or less and metal flakes having a thickness of 10 to 500 mm and an aspect ratio of 300 to 800 in the metallic layer is applicable. Since these metal flakes are thinned by vapor deposition, this type of metallic paint is very expensive. The method of forming a coating film for a metal light alloy wheel according to the present invention has an effect that the unevenness of the underlayer is reduced, the generation rate of defective products is reduced, the repainting of expensive metallic paint is reduced, and the cost is reduced.

  For the powder primer layer, an epoxy powder paint or an epoxy polyester powder paint can be employed. By forming a primer layer with an epoxy polyester powder coating and forming an acrylic color coat layer thereon, a coating film having good weather resistance and adhesion can be formed.

  It is preferable to apply the coating film forming method of the present invention to a metal light alloy wheel for coating a cast surface portion. Even if there are irregularities on the casting surface, if the metal light-alloy alloy wheel of the present invention, the intermediate layer has the effect of reducing the irregularities, and the effect of improving the yield and reducing the cost is sufficiently exhibited. The The wheel base body used for the metal light alloy wheel is preferably a low pressure casting or a one-piece casting.

  The surface of the base (light alloy material) of the light alloy wheel is pretreated before providing the primer layer. In general, the pretreatment is performed by degreasing and pickling, followed by a chemical conversion treatment. In general, chromate treatment using chromium chromate or phosphate is applied as the chemical conversion treatment solution, but from the viewpoint of environmental protection, non-chromium chemical treatment solution using Ti acid or Zr acid is used. The

The primer paint can be a polyester, acrylic, epoxy, or urethane resin. Use of an epoxy powder coating material (mainly composed of glycidyl ether type (for example, bisphenol A type), glycidyl ester type, glycidyl amine type, and cyclic oxirane type epoxy resin) having good adhesion to metal due to orientation of hydroxyl groups preferable. A crosslinking agent and, if necessary, a pigment, a curing agent, a surface conditioner and the like are used.
The epoxy-based powder coating is particularly composed of a glycidyl group / carboxyl group-crosslinked epoxy resin having an acid-terminated polyester resin as a curing agent from the viewpoint of cost and adhesion to the substrate, and has a high solid content concentration (40 to 40). 60 mass%) epoxy / polyester powder coating is preferred. By baking the epoxy polyester / hybrid powder coating material at a temperature of 150 to 180 ° C. for 10 to 30 minutes, a smooth and strong polymer cross-linked coating film is formed. As described above, the primer layer is formed to a thickness of about 40 to 200 μm when it is necessary to hide the unevenness of the cast surface of the light alloy wheel. Further, when a primer single layer is used as the underlayer, a deep color type is used.

  In the present invention, the color base paint refers to a colored paint containing a pigment. Examples of color base paints include those made of acrylic, polyester, epoxy resin, and the like and containing a solvent. When an epoxy-polyester hybrid powder coating is used for the primer layer, it is preferable to use a thermosetting acrylic resin-based colored coating. A thermosetting paint obtained by dissolving or dispersing an acrylic resin, a curing agent, a pigment or the like in an organic solvent is preferable. In order to protect the environment, a high solid type having a small proportion of the organic solvent is preferable. For example, an acrylic resin, a curing agent, a pigment, and the like are preferably in a vol% of 50% or more. As a method of applying to the light alloy wheel, it is preferable to apply by air spray or electrostatic coating. The film thickness is generally 10 to 40 μm. A pigment is used for the color base paint, and it is necessary to form a base layer of a dark color system close to black. Known pigments such as organic pigments, inorganic pigments, carbon pigments (for example, carbon black, graphite, etc.), metallic powders, and the like are appropriately used to form a black dark base layer. In addition, additives such as a rust preventive pigment, a curing catalyst, an ultraviolet absorber, an ultraviolet stabilizer, an antioxidant, a surface conditioner, and a crack inhibitor can be blended.

  The metallic paint includes all paints including metal foil (flakes) of an alloy having a metallic (plating) appearance. For example, a paint containing metal foil (flakes) of chrome, aluminum, nickel and alloys thereof can be used. The metal foil is preferably a pulverized metal ribbon. For example, flakes having an average thickness of 10 to 900 mm and an average diameter of 2 to 70 μm can be used. More preferably, it includes flakes having an average thickness of 10 to 500 mm and an average diameter of 3 to 50 μm. The light projected on the part with the metal foil is reflected by the metallic layer, and the light projected on the part without the metal foil is reflected by the color base layer through the clear layer below it, thus reflecting the color tone of the color base layer. It becomes the metal-tone coating film which was excellent in the external appearance. The color base layer is preferably a deep color, and can give a tint of steel. A metal foil having an aspect ratio (width / thickness of metal flakes) of 30 or more can be used, preferably more than 300 to 800, and more preferably 400 to 700.

  Many of flat (scale-like) flakes serving as a metallic pigment are unevenly distributed in the vicinity of the interface with the clear layer 5 in a state of being aligned (orientated) in parallel along the clear layer 5. Leafed aluminum flakes can also be used to facilitate flake orientation. Leafed aluminum flakes can be obtained by coating with saturated fatty acids (eg stearic acid). The aluminum flakes are oriented in parallel with the clear layer, whereby a coating film having a high brightness feeling is obtained by reflected light or interference light. The thickness of this metallic layer is 0.05 to 10 μm, preferably 0.1 to 3 μm. When the thickness of the metallic layer is less than 0.05 μm, the ultraviolet rays easily reach the clear layer, the weather resistance is lowered, and when the thickness exceeds 10 μm, the layer formation time becomes longer.

  As metal flakes, vapor deposited aluminum flakes are preferred. Vapor-deposited aluminum flakes are prepared by vapor-depositing aluminum on the surface of a plastic film substrate and then crushing and dissolving and removing the substrate. Since the thickness is very thin and the aspect ratio is more than 300, the layers are oriented in parallel along the clear layer. Further, in the present invention, it is possible to obtain a higher brightness feeling by adding vapor deposition aluminum flakes together with leafed aluminum flakes. Furthermore, in the present invention, it is possible to create another design feeling by adding ground aluminum flakes in addition to the leafed aluminum flakes and / or vapor-deposited aluminum flakes.

  Further, a black pigment having an average particle size of 300 nm or less can be added as a pigment for the metallic paint. When the average particle size is larger than 300 nm, visible light passing through the clear layer and the base layer is blocked by the pigments 7 and 8 and cannot reach the underlying base layer 2. In order to achieve a silvery tone with astringency and a high-quality titanium alloy skin tone, the dark base layer becomes a reflection wall for visible light incident from the outside and must be reflected back to the clear layer. Don't be. Therefore, it is necessary to reduce the particle size of the black pigment to a predetermined value.

As the black pigment contained in the metallic layer, for example, an inorganic pigment or an organic coloring pigment exhibiting a black to brown or blue hue can be used in order to improve the flip-flop property. These pigments include carbon black, lamp black, bone black, graphite, magnetite, hematite (brown), inorganic black pigments such as copper / chromium black, cobalt black, copper / manganese / iron black, ultramarine blue, bitumen, and cobalt blue. And inorganic blue pigments such as phthalocyanine blue.
In addition, yellow pigments may be used, and yellow lead, yellow iron oxide, yellow LiFeO ₂ yellow titanium oxide powder, and the like can be used as appropriate.

  The clear paint applied on the metallic layer may be transparent, but it can be formed using a solvent-based clear paint or a powder-based clear paint in which a polymer or oligomer is dissolved in an organic solvent. . As the clear paint, it is preferable to use an acrylic paint excellent in transparency, gloss, weather resistance, etc., for example, an acrylic solvent paint having a solid content concentration of about 40 to 50% by mass. For the acrylic paint, a thermosetting acrylic resin obtained by solution polymerization of an acrylic monomer and a functional monomer such as an epoxy group, a carboxyl group, or a hydroxyl group is used. In particular, a curable acrylic resin using melamine as a crosslinking agent is used, and it is preferable to use an acrylic solvent paint having a resin solid content concentration of about 40 to 50% by mass. The clear layer is formed on the surface of the metallic base layer and then baked at a temperature of 130 to 180 ° C., for example, to a thickness of 10 to 40 μm.

  When solvent paint is used in the color base layer, metallic layer, and clear layer, the paint is sprayed only from the front side. However, when these layers are electrostatic powder coating, a method of spraying a powder coating from the back side of the design surface may be adopted as in the electrostatic powder coating of the underlayer.

(Example)
Details of the present invention will be described below with reference to the accompanying drawings.
FIG. 3 is a diagram schematically showing a cross section of the metallic coating film. In FIG. 3, 1 is a base of a light alloy wheel formed by casting a metal such as Al, Mg, Fe or an alloy thereof, and a primer layer 11 by electrostatic coating of powder resin on the surface thereof, and a color base A total of four coating layers comprising the layer 12, the metallic layer 13, and the clear layer 14 are formed. Usually, before forming this coating film, the surface of the substrate 30 is pretreated in order to improve the corrosion resistance. In the case where the substrate 30 is an aluminum alloy such as a casting, chemical conversion treatment represented by alkali degreasing and chromate treatment is performed. The chemical conversion film is formed to have an adhesion amount of 3 mg / m ² or more (preferably 5 to 20 mg / m ² ). The substrate 30 after the chemical conversion treatment is washed with water and dried.
A base body (integrated aluminum wheel by low pressure casting) made of an Al—Si—Mg alloy (equivalent to JIS AC4CH material) was subjected to alkali degreasing and chromate treatment, then washed with pure water, dried and pretreated. Thereafter, as the primer layer 11, an epoxy / polyester powder coating (manufactured by Nippon Paint Co., Ltd.) was applied to the casting surface of the light alloy wheel base 1.
As shown in FIG. 1, the powder coating was applied while rotating the light alloy wheel base 1 fixed to the turntable 2 with the design surface facing upward. The powder paint was discharged from the spray gun 4 so as to pass through the window 16 from the back side of the design surface to the front side of the design surface. The powder paint is weighed in the discharge pump 3, and 80 to 200 g is ejected to one wheel base. The rotation speed of the turntable was 10 to 30 rpm, and the rotation direction was reversed in the middle and rotated. Electrostatic coating was performed under these conditions, and baked at a temperature of 150 to 180 ° C. for 30 to 60 minutes. When the film thickness of this powder coating layer was measured, the film pressure of the upper surface 15 of the design surface was 150 μm or more, and the film pressure of the spoke lateral wall 17 was 50 μm or more.
Thereafter, as the color base layer 12, an acrylic black thermosetting resin (acrylic melamine resin-based paint (AS70 black base) manufactured by Nippon Paint Kogyo Co., Ltd.) is sprayed with a spray gun so that the film thickness becomes 20 μm. Bake at ~ 130 ° C for 10-30 minutes. As a result, an underlayer composed of the primer layer 11a and the color base layer 12 was formed.
Thereafter, 100 parts by mass of binder resin (polyester resin) and 100 parts by mass of xylene were kneaded, and 15 mass parts of vapor-deposited aluminum flakes (average thickness of 170 mm, average particle diameter of 14 μm) were added to this resin, and the average particle diameter of 200 mm. The carbon black was mixed to 10 parts by mass to obtain a metallic paint. It sprayed with the spray gun so that the film thickness might be set to 0.5 micrometer, and it baked for 20 minutes at 100-130 degreeC after that, and formed the 0.5-micrometer-thick metallic layer.
After that, acrylic high solid type transparent thermosetting resin (solvent type clear, manufactured by Nippon Paint Co., Ltd.) is sprayed with a spray gun so that the film thickness becomes 30 μm, and baked at 130-160 ° C. to form a clear layer. Formed.
As a comparison, as shown in FIG. 5, a powder coating layer was formed by spraying a powder coating only from the front side of the design surface, and when the film pressure was measured, particularly in the spoke side wall extending in the radial direction. The coating part was thin and the coating part which was less than 50 micrometers was seen.
Moreover, when the appearance rate of the appearance defect was compared, it was confirmed that the occurrence rate of the appearance failure in the present invention is smaller than that in which the powder coating material is sprayed from the front side of the conventional design surface.

It is a schematic diagram which shows a mode that the powder coating material in this invention is ejected. It is a figure which shows a 4 layer metal-tone coating film typically. It is an example of the design surface of a wheel. It is a schematic diagram explaining the powder coating in this invention and the flow of air. It is a schematic diagram explaining the conventional powder coating material and the flow of air.

Explanation of symbols

1: Light alloy wheel base for vehicle, 2: Turntable, 3: Discharge pump, 4: Spray gun, 11: Electrostatic powder primer layer, 12: Dark color base layer, 13: Metallic layer, 14: Clear Layer: 15: Design surface, 16: Window, 17: Vertical wall (drafted part), 18: Powder coating

Claims (7)

A method of forming a coating film of a metal-like light alloy wheel, the step of blowing a powder paint from the back side of the design surface of the light alloy wheel base to form a layer by electrostatic coating on at least a part of the surface side of the design surface; And a step of forming a metallic layer on the upper layer side of the electrostatic coating layer. The method for forming a coating film on a metal light alloy wheel according to claim 1, wherein the powder paint is blown from the back side of the design surface while rotating the light alloy wheel. The method for forming a coating film of a metal light alloy wheel according to claim 1 or 2, wherein the powder coating is performed with the design surface facing upward when the powder coating is blown from the back side of the design surface. The method for forming a coating film of a metal light alloy wheel according to any one of claims 1 to 3, further comprising a step of forming a clear layer on the metallic layer. 5. The metal lightening according to claim 1, wherein the underlayer includes a powder primer layer formed on a surface of a substrate and a color base layer formed on the primer layer. A method for forming a coating film on an alloy wheel. The metallic layer has a thickness of 10 μm or less, and metal flakes having a thickness of 10 to 500 mm and an aspect ratio of 300 to 800 are dispersed in the metallic layer. The coating method of the metal light alloy wheel in any one of thru | or 5. The method of forming a coating film on a metal light alloy wheel according to any one of claims 1 to 6, wherein the metal light alloy wheel has a cast surface coated.

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