JP7560971B2 - Powder coating manufacturing method and manufacturing equipment - Google Patents

Description

The present invention relates to a manufacturing method and manufacturing equipment for powder-coated products.

In recent years, from the perspective of environmental protection, the release of organic solvents into the atmosphere has become a major problem, and within the paint industry, expectations are growing for powder paints as an alternative to conventional organic solvent-based paints, as they are environmentally friendly paints that do not require exhaust or wastewater treatment and can be recovered and reused.

JP 2005-218998 A

If the powder coating is thermosetting, it hardens through a cross-linking reaction when heat is applied, so after the powder coating has been applied to the substrate, it must be left in an atmosphere of a certain temperature for a certain period of time to melt and harden, i.e., baked. For this purpose, a hot air circulating drying oven, in which extremely slow hot air is circulated inside the oven, as described in Patent Document 1, is used.
Since the baking temperature is relatively high, it takes time to heat up. Therefore, if multiple powder-coated objects are passed through a drying oven in succession, the oven length becomes longer, and the scale of the manufacturing equipment becomes larger. Also, if the temperature rise speed of the powder paint is slow, melting and hardening may occur partially, which may cause coating defects.
In response to this problem, it has been proposed to install an infrared ray irradiation furnace before the drying furnace and quickly raise the temperature of the powdered material to the baking temperature by irradiating it with infrared rays. However, if there are any shaded areas that are not hit by the infrared rays, those areas will not be heated enough.

The present invention has been made in response to the above-mentioned problems in the conventional art, and has as its object to provide a new and useful method for producing powder-coated objects which incorporates a heating method capable of quickly and evenly raising the temperature of a powder-coated object to which a powder paint has been applied up to the baking temperature.
Another object of the present invention is to provide a production facility for the above-mentioned production method.

The present invention has been made to achieve the above object, and the invention [1] is a method for manufacturing a powder-coated product, which includes a powder-adhering step in which powder paint is applied to a substrate using static electricity, a temporary welding step in which induction heating is applied after the powder-adhering step to melt the boundary side with the substrate to temporarily weld the powder paint, a heating step in which a hot air flux is blown at high speed to raise the temperature to a baking temperature after the temporary welding step, and a baking step in which the powder paint is baked to produce a powder-coated product after the heating step, and the temporarily welded powder paint is capable of withstanding the impact of the hot air flux blown at high speed in the heating step and is prevented from flying out.

The invention [2] is the method for producing a powder-coated article described in [1], characterized in that the thickness of the article to be coated is 0.5 mm or more.

The invention [3] is a method for producing a powder-coated article as described in [1] or [2], characterized in that in the temporary fixing and welding step, the surface of the powder paint applied to the article to be coated is heated to 55°C or higher and lower than 80°C.

The invention of [4] is a method for producing a powder-coated article according to any one of [1] to [3], characterized in that in the heating step, hot air having a temperature higher than the baking temperature is blown so that the air speed in the vicinity of the powder coating applied to the article is 5 m/sec or more.

The invention of [5] is a manufacturing facility for powder-coated objects for carrying out the method according to any one of [1] to [4], characterized in that a coating booth for carrying out the powder attachment process, an induction heating device for carrying out the temporary fixing and welding process, a jet ejection heating device for carrying out the temperature increase process, and a soaking furnace for carrying out the baking process are installed in a line, and the object to be coated is transported and subjected to each process in sequence.

The invention of [6] is a manufacturing equipment for powder-coated products described in [5], in which the induction heating device inductively heats the workpiece to which the powder coating has been applied by passing it between opposing work coils, and is characterized in that the manufacturing equipment is provided with, as a welding promotion mechanism, a distance changing means for changing the distance between the work coil and the workpiece and/or a spacing changing means for changing the vertical spacing between the multiple coil elements that make up the work coil.

The manufacturing method of the present invention incorporates a heating method that can quickly and evenly raise the temperature of the powder-attached material to the baking temperature, and this manufacturing facility can shorten the baking line installed downstream of the paint booth.

1 is a line configuration diagram of a manufacturing facility according to an embodiment of the present invention. FIG. 2 is a configuration diagram of a pair of work coils of the induction heating device of FIG. 1. FIG. 3 is a schematic diagram of the induction heating device of FIG. 2 . FIG. 3 is a schematic explanatory diagram of an induction heating device according to another embodiment of the present invention, different from that shown in FIG. 2 . FIG. 2 is an image explanatory diagram of the jet ejection type heating device of FIG. 1. FIG. 2 is a perspective view showing temperature measurement points of the powder deposit used in the examples. 1 is a graph showing temperature rise of powdery deposits obtained in the examples.

Hereinafter, a method for producing a powder coated article according to an embodiment of the present invention will be described together with a production line implemented as a production facility.
The coating object of the present invention is made of a conductive material, mainly metal, because induction heating is used. Thin objects are 0.5 to 4.0 mm thick, and thick objects are thicker than that, but the coating object of the present invention is intended to be 0.5 mm or thicker. In other words, the present invention is intended to be used for both thin and thick objects.
Powder coatings can be used without any particular restrictions as long as they have been used for powder coating in the past. Powder coatings are solid powders made mainly of polymer resins and pigments, and do not contain organic solvents. However, they are broadly divided into thermoplastic and thermosetting types, and thermosetting types include epoxy-based powder coatings, polyester-based powder coatings, epoxy-polyester-based powder coatings, fluororesin-based powder coatings, and acrylic-based powder coatings. When using these types of powder coatings, they are usually baked after application to melt and harden, so currently, the present invention mainly assumes the use of these types.

Inside the manufacturing equipment, the object to be coated (W) is transported by being hung from a jig or attached to a belt conveyor; in the manufacturing equipment 1 shown in Figure 1, the object is hung from a jig in the attachment/detachment area and transported on the transport path (L). Note that when powder is attached, it becomes a powder-adhered object, when it is temporarily welded, it becomes a powder-welded object, and when baking is completed, it becomes a powder-coated object; however, in the following description, the terms object to be coated (W1), powder-adhered object (W2), powder-welded object (W3), and powder-coated object (W4) are used depending on the situation.

Before the powder is applied, a series of processes is carried out to remove oil and other foreign matter and to prepare the surface for easy adhesion of the paint, for example washing with hot water, degreasing (iron phosphate), water washing 1, 2, 3, draining, and cooling. These are conventional processing methods and equipment.
The workpiece (W1) that has been subjected to this pretreatment is loaded into the coating booth 3 from the loading area.

In the coating booth 3, a powder application process is carried out in which powder paint is applied to the workpiece (W1) using static electricity. In this embodiment, an electrostatic dry spraying method is used, in which powder paint is sprayed onto the surface of the workpiece (W1). According to this method, charged powder paint is sprayed from an electrostatic gun toward the workpiece (W1), and the powder paint adheres to the surface of the workpiece (W1) due to the attractive force between positive and negative ions. Another method that uses static electricity is frictional charging coating, and both of these methods can be used in the present invention.

Both coating methods have been widely adopted, and the coating booth 3 in which the electrostatic dry spraying method is performed is also of a conventional configuration. First, the powder paint is applied to the entire workpiece (W1) in the main spray section, and then fine adjustments are made in the sub-spray section to ensure uniform application of the powder paint.
The amount of coating is adjusted so that the thickness of the final coating film is about 30 to 150 μm, which is conventional for powder coating.

When the workpiece (W1) leaves the coating booth 3, the powder paint has adhered to the surface of the workpiece (W1) to form a powder deposit (W2). This powder deposit (W2) is sent to the induction heating device 5.
In the induction heating device 5, high-frequency induction heating is applied to melt the boundary between the workpiece (W1) and the attached powder paint, thereby carrying out a temporary welding process to temporarily weld the powder paint to the workpiece (W1).
Specifically, the device is configured with the device described in JP 2005-259575 A. As shown in Fig. 2 and Fig. 3, a pair of work coils 5a, 5a are connected by a flexible tube 5b through which a copper wire is passed and are erected in parallel, and the powder deposit (W2) passes between the pair of work coils 5a, 5a.

In the high-frequency induction heating method, the workpiece (W1) is heated, and the powder paint adhering to it receives heat from the workpiece (W1) and is heated accordingly, with melting of the powder paint starting from the boundary side with the workpiece (W1).
In the present invention, this mechanism is utilized to melt only the boundary side with the substrate (W1) rather than the entire powder paint, and the molten portion acts as an adhesive to weld the powder paint to the substrate (W1) and temporarily secure it. The strength of the temporary welding need only be sufficient to prevent the powder paint from flying off due to the impact of the high-velocity hot air in the next heating process, in which a beam of hot air is blown at high speed to quickly raise the temperature to the baking temperature, and the purpose can be sufficiently achieved even with thick substrates if the temporary attachment is performed.

Although it is possible to perform the baking process using only high-frequency induction heating, if the workpiece (W1) is thick and has various shapes and sizes, the heating is likely to be uneven if the workpiece is baked all at once. On the other hand, it is not realistic in terms of cost and work to change the work coil for each product type.
In contrast, the present invention is used in the preliminary stage of temperature rise as described above, and is therefore fully capable of handling thick objects in this stage as described above.

In the induction heating device 5, the holding member 5c of the work coil 5a can be moved in the opening and closing direction by a ball screw mechanism 5d connected to the opening and closing motor, and the opening and closing motor and the ball screw mechanism 5d can be moved together by driving the center movement motor 5e. This makes it possible to change the distance between the work coil 5a and the powder deposit (W2) and to change the high-frequency output. The temporary welding state can be optimized according to the shape and size of the workpiece (W1) within the range of application of these changing means. In other words, these changing means are used as a welding promotion mechanism.

In the temporary welding process, the completion of the temporary welding is determined when the surface of the powder paint applied to the workpiece (W1) reaches a temperature of 55°C or higher and less than 80°C, for powder paints currently available on the market. Completion can be determined by time management based on temperature measurements using a non-contact thermometer or pre-accumulated temperature rise data.

The induction heating device 5 may be another example described in JP 2012-138175 A. In this device, not only can the high-frequency output be changed, but as shown in Fig. 4, the work coil 5a is composed of multiple coil elements 5f, 5f ..., which are held by holding members 5g, 5g ..., and the holding members 5g, 5g ... are moved in the vertical direction to change the opposing distance between the coil elements 5f, 5f ... in the vertical direction by utilizing the rotation of the arm member 5h, and by utilizing this changing means, the temporary welding state can be optimized according to the shape and size of the workpiece (W1) within the applicable range of this means.
It is technically possible to use a structure in which this is also used in conjunction with a distance changing means, and a combination of both change means may be used, but this is not necessarily required for the purpose of temporary welding.

The powder welded material (W3) coming out of the induction heating device 5 is sent to a tunnel-shaped furnace 7. A jet ejection type heating device 9 is arranged upstream of the furnace 7, and a heating process is first carried out, where the powder welded material (W3) is heated to a baking temperature.
The jet-type heating device 9 transmits heat by colliding a jet of hot air with the object to be heated, and is also called a jet oven, and can transmit thermal energy to the surface of the object to be heated with a high thermal conductivity. Taking advantage of this feature, the present invention aims to quickly raise the temperature to the baking temperature.
In this jet-ejection type heating device 9, as shown in Fig. 5, a number of nozzle groups 9a, 9a, ... are arranged vertically and front to back, and are arranged opposite each other on the left and right, and the nozzle outlets of each nozzle face each other on the left and right. The powder welded material (W3) passes between the left and right nozzle groups. Therefore, the powder welded material (W3) receives the impact of the high-speed hot air flux from both the left and right sides.

Since the baking temperature and baking time ranges are determined to realize the optimal baking state depending on the type of powder paint, the temperature and wind speed of the high-velocity hot air are optimally set to quickly bring the powder welded material (W3) up to the baking temperature, taking into account the temporary welding state and the equipment capacity.
It is recommended that the temperature be set to a temperature somewhat higher than the baking temperature of the powder paint, preferably about 20 to 50° C. higher, for example, about 200 to 250° C. The higher the temperature, the more heat can be applied to the powder deposit (W3), but if the temperature is too high, it will exceed the baking temperature and will overheat, so an upper limit is set.
It is recommended that the wind speed be set to 5 m/sec or more near the powder welded material (W3) in order to make the jet spray effective. The faster the wind speed, the stronger the force of the hot air flux becomes, but if the wind speed is too fast, the strength of the temporary welded joint may be defeated by the impact and the powder paint may fly out, so it is assumed that the upper limit is about 15 m/sec.

The temporarily welded powder coating material receives this high-velocity hot air flux from the surface side, and is melted evenly throughout, including the inside, until it reaches the baking temperature, after which it is transported downstream of the furnace 7.
The downstream side of the furnace 7 is a hot air circulation type, and the air heated by burner combustion is blown into the furnace 7 by the outlet fan using the uniform heating and circulation device 11, and the air circulates inside the furnace 7 and is collected outside the furnace 7 by the operation of the intake fan, where it is heated again.
Within the furnace 7, the wind flows at a very low speed, specifically, about 0.1 to 1 m/sec, and there is almost no wind.
The baking of the powder coating material proceeds by leaving it at this atmospheric temperature for a certain period of time, for example, about 20 minutes at 180°C.

According to the method of the present invention, the atmospheric temperature required for this baking is not used to raise the temperature to the baking temperature, but the induction heating device 5 and the jet ejection type heating device 9 are used to raise the temperature to the baking temperature in one go, making it possible to shorten the furnace length to about one-third compared to when the inside of the furnace 7 is maintained at the atmospheric temperature and these devices are also responsible for raising the temperature to the baking temperature of the powder paint. This reduction more than covers the additional installation of the induction heating device 5 and the jet ejection type heating device 9.
Furthermore, the rapid heating process avoids the mixture of melting and hardening that is typical of powders, resulting in good coating quality.

Although the embodiments of the present invention have been described in detail above, the specific configuration is not limited to the above-mentioned embodiments, and the invention also includes design changes that do not deviate from the spirit of the present invention.

Using the induction heating device 5 (FIG. 2) according to the above embodiment and a jet ejection type heating device 9 installed in a furnace 7, a temporary fixing, welding and temperature rise test for the powdery adhesion (W2) was carried out.
The powder deposit (W2) was prepared by depositing an epoxy resin powder coating on a steel pipe having a wall thickness of 2 mm, a diameter of 32 mm and a length of 30 cm as shown in FIG. 6 to a coating thickness of 100 μm.
When all of the measurement positions (No. 1 to No. 4) in the induction heating device 5 reached 70°C or higher, the material was moved to the jet ejection heating device 9 (220°C, wind speed 12 m/sec) and heated to the baking temperature of 180°C. As shown in the temperature rise graph in Figure 7, it took about 5 minutes to put the material into the induction heating device 5, take it out, and then put it back into the jet ejection heating device 9 to finally raise the temperature to the baking temperature. Thereafter, the material was baked at 180°C by the uniform heating and circulation device 11, and the baking was completed in 10 minutes. In other words, it took 15 minutes to complete baking of the powder deposit (W2).

On the other hand, when the temperature of the powder-attached material (W2) was raised to 180° C. by the uniform heating and circulation device 11 and baking was then continued, the time required was 30 minutes. In other words, the time required to complete baking of the powder-attached material (W2) was 30 minutes.
In other words, according to the method of the present invention, the time until baking can be significantly shortened, so that the furnace length in a manufacturing facility in which multiple powder deposits (W2) are baked in succession can be shortened, thereby making the scale of the manufacturing facility more compact.
Furthermore, in the method of the present invention, even if the powder paint is subjected to an impact during the heating process, the powder paint is prevented from flying out, and the mixture of hardened and molten paint is suppressed, so that the coating film of the powder-coated product (W4) is densified, thereby improving the quality.

Reference Signs List 1... Manufacturing equipment 3... Paint booth 5... Induction heating device 5a... Work coil 5b... Tube 5c... Holding member 5d... Ball screw mechanism 5e... Center movement motor 5f... Coil element 5g... Holding member 5h... Arm member 7... Furnace 9... Jet ejection type heating device 9a... Nozzle 11... Uniform heating and circulation device W1... Coating object W2... Powder-adhered object W3... Powder-welded object W4... Powder-coated object

Claims (5)

The method includes a powder adhesion step in which powder paint is adhered to the substrate by utilizing static electricity, a temporary welding step in which induction heating is applied after the powder adhesion step to melt the boundary side with the substrate to temporarily weld the powder paint, a heating step in which a hot air flux is blown at high speed after the temporary welding step to raise the temperature to a baking temperature, and a baking step in which the powder paint is baked to produce a powder coated product after the heating step.
In the heating step, hot air having a temperature higher than the baking temperature is blown onto the powder coating material applied to the workpiece at a wind speed of 5 m/sec or more in the vicinity of the powder coating material applied to the workpiece,
A method for manufacturing powder coated articles, characterized in that the powder paint that has been temporarily welded is capable of withstanding the impact of a hot air flux being blown at high speed during the temperature raising process, thereby preventing it from flying out. In the method for producing a powder coated article according to claim 1,
A manufacturing method characterized in that the thickness of the substrate is 0.5 mm or more. In the method for producing a powder coated article according to claim 1 or 2,
This manufacturing method is characterized in that in the temporary fixing and welding process, the surface of the powder paint applied to the substrate is heated to a temperature of 55°C or higher and lower than 80°C. A powder-coated object manufacturing facility for carrying out the method according to any one of claims 1 to 3, comprising:
The painting booth for the powder application process, the induction heating device for the temporary fixing and welding process, the jet ejection heating device for the temperature increase process, and the soaking furnace for the baking process are all installed in a line.
A manufacturing facility characterized in that the workpieces are transported and subjected to each process in sequence . In the powder coating manufacturing equipment according to claim 4,
The induction heating device heats the workpiece with the powder coating by passing it between two opposing work coils.
A manufacturing equipment characterized in that, as a welding promotion mechanism, it is equipped with a distance changing means that can change the distance between the work coil and the workpiece, and/or a spacing changing means that changes the vertical spacing between multiple coil elements that constitute the work coil.

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