Pipe internal coating method, pipe internal coating apparatus and pipe

Abstract

This invention provides a painting method, etc., for achieving a good painted surface on the inner surface of a socket without causing painting defects, when using water-based paint for coating the inner surface of the socket. [Solution] The pipe inner surface coating method is a method for coating the inner surface of a pipe (1) having a straight section (3) and a socket (2), and includes a powder coating step of coating the inner surfaces of the straight section (3) and the socket (2) with powder coating, a roughening step of roughening the powder coating film (C1) formed on the inner surface of the socket (2), and a coating step of coating the coating film (C1) which has been roughened by the roughening step with water-based paint.

Description

Technical Field

[0001] The present invention relates to a method for coating the inner surface of a pipe, an apparatus for coating the inner surface of a pipe, and a pipe.

Background Art

[0002] Various coatings for protecting the inner surface are applied to the inner peripheral surface of metal pipes used for water and sewage. In particular, cast iron pipes used for water pipes are often powder-coated on the inner surface of the straight section. Regarding powder coating on the inner surface of the socket, there is also a possibility that coating defects may occur due to unevenness existing on the inner surface of the socket. In order to prevent such coating defects from occurring, there is a technique disclosed in Patent Document 1.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In recent years, in order to reduce VOC (volatile organic compounds), it has been required to change the solvent-based paint used for coating the inner surface of the socket to an aqueous paint. However, the technique described in Patent Document 1 is to apply a solvent-based liquid paint on top of powder coating, and using an aqueous paint for coating the inner surface of the socket is not assumed.

[0005] One aspect of the present invention aims to provide a coating method and the like for achieving a good coating state on the inner surface of the socket without causing coating defects when an aqueous paint is used for coating the inner surface of the socket.

Means for Solving the Problems

[0006] To solve the above problems, a pipe inner surface coating method according to one aspect of the present invention is a pipe inner surface coating method for coating the inner surface of a pipe having a straight section and a socket, comprising: a powder coating step of coating the inner surfaces of the straight section and the socket with powder coating; a roughening step of roughening the coating film of the powder coating formed on the inner surface of the socket by the powder coating step; and a coating step of coating the coating film, which has irregularities formed by the roughening step, with water-based paint.

[0007] Furthermore, a pipe inner surface coating apparatus according to one aspect of the present invention is a pipe inner surface coating apparatus for coating the inner surface of a pipe having a straight section and a socket, comprising: a first coating mechanism for coating the inner surfaces of the straight section and the socket with powder coating; a roughening mechanism for roughening the coating film of the powder coating formed on the inner surface of the socket by the first coating mechanism; and a second coating mechanism for coating the coating film, which has been roughened by the roughening mechanism, with water-based paint.

[0008] Furthermore, a pipe according to one aspect of the present invention is a pipe having a straight section and a socket, wherein a powder coating having irregularities is formed on the inner surface of the socket, and a coating film of water-based paint is formed on the powder coating having irregularities. [Effects of the Invention]

[0009] According to one aspect of the present invention, when water-based paint is used to coat the inner surface of a socket, a painting method can be provided that will improve the painted condition of the inner surface of the socket without causing painting defects. [Brief explanation of the drawing]

[0010] [Figure 1] This is a cross-sectional view illustrating a pipe inner surface coating apparatus according to an embodiment of the present invention, which processes the coating of the inner surface of a pipe with powder coating. [Figure 2] This is a cross-sectional view illustrating a process in which a pipe internal coating apparatus roughens the surface of a powder coating film formed on the inner surface of a pipe. [Figure 3]This is a cross-sectional view illustrating a process in which a pipe internal coating apparatus applies a water-based paint to a coating film that has been roughened to create an uneven surface. [Figure 4] This figure shows the surface state of the test specimen in Example 1 of the present invention. [Figure 5] This figure shows the indentation formed in the coating film of the test specimen in Example 1 of the present invention. [Figure 6] This figure shows the relationship between laser output and arithmetic mean roughness Ra in Embodiment 1 of the present invention. [Figure 7] This figure shows the state in which the water-based paint is in close contact with the test piece after an adhesion test has been performed on the test piece in Example 2 of the present invention. [Figure 8] This figure shows the relationship between the arithmetic mean roughness Ra and the ten-point mean roughness Rz in Examples 1 and 2 of the present invention. [Figure 9] This figure shows the relationship between sand particle size and arithmetic mean roughness Ra in Example 2 of the present invention. [Modes for carrying out the invention]

[0011] Figure 1 is a cross-sectional view illustrating a process in which the pipe interior coating apparatus 100 according to an embodiment of the present invention coats the inner surface of a pipe 1 with powder coating. Figure 2 is a cross-sectional view illustrating a process in which the pipe interior coating apparatus 100 roughens the powder coating film formed on the inner surface of the pipe 1. Figure 3 is a cross-sectional view illustrating a process in which the pipe interior coating apparatus 100 coats the coating film, which has been roughened to create irregularities, with water-based paint.

[0012] In Figures 1 to 3, the axial direction of pipe 1 is defined as the X-axis, the direction opposite to the direction of gravity is defined as the positive Z-axis, and the direction perpendicular to both the X-axis and Z-axis is defined as the Y-axis. Furthermore, it is assumed that the socket of pipe 1 (not shown) faces the positive X-axis, and the socket 2 of pipe 1 faces the negative X-axis. The following explanation mainly describes the processes performed by the pipe internal coating apparatus 100, but also includes an explanation of the pipe internal coating method for coating the inner surface of pipe 1.

[0013] <Configuration of Pipe 1> Pipe 1 has a substantially cylindrical shape and has a receiving port 2 and a straight portion 3 as shown in FIG. 1. Pipe 1 is, for example, a ductile cast iron pipe mainly used as a water pipe. For Pipe 1, it is necessary to paint the inner surface in order to prevent rust and the influence on water quality. The pipe inner surface coating device 100 is a device for coating the inner surface of Pipe 1.

[0014] On the negative X-axis side of the inner surface 21 of the receiving port 2, irregularities 211 for connecting Pipe 1 to other pipes are formed. The inner surface 21 has a portion where the irregularities 211 are formed and an intermediate portion R1. The intermediate portion R1 is a portion between the portion where the irregularities 211 are formed and the inner surface 31 of the straight portion 3. The inner surface 31 of the straight portion 3 is formed substantially flat. The inner surface of Pipe 1 is composed of the inner surface 21 and the inner surface 31.

[0015] <Process of Coating with Powder Coating> The pipe inner surface coating device 100 includes a first coating mechanism 11 and a pipe holding portion 4. The first coating mechanism 11 has a nozzle 12 and a robot arm 13. The nozzle 12 is provided at the tip of the robot arm 13 and discharges powder coating. A control unit (hereinafter simply referred to as the control unit), not shown, included in the pipe inner surface coating device 100 controls the operation of the robot arm 13 and can change the position of the nozzle 12 in the X-axis direction, Y-axis direction, and Z-axis direction.

[0016] The control unit arranges the nozzle 12 at a position surrounded by the inner surface 31 of the straight portion 3 by controlling the operation of the robot arm 13. The first coating mechanism 11 discharges powder coating from the nozzle 12 in a state where the nozzle 12 is arranged at a position surrounded by the inner surface 31 of the straight portion 3, thereby coating the inner surface 31 of the straight portion 3 and the inner surface 21 of the receiving port 2 with powder coating (powder coating process).

[0017] The tube holding part 4 holds the tube 1 so that the tube axis is horizontal and rotates the tube 1 by rotating around the X-axis direction. While the tube holding part 4 rotates the tube 1, the first coating mechanism 11 discharges powder paint from the nozzle 12. Also, while the control part moves the nozzle 12 by a predetermined distance in the X-axis direction, the first coating mechanism 11 discharges powder paint from the nozzle 12.

[0018] Here, when the nozzle 12 discharges powder paint, the powder paint adheres to the intermediate part R1 of the inner surface 21 that is close to the inner surface 31. As a result, not only the inner surface 31 but also the intermediate part R1 of the inner surface 21 is coated with the powder paint. Therefore, a paint film C1 of the powder paint is formed on the inner surface 31 and the intermediate part R1. The paint film C1 formed on the intermediate part R1 is thinner than the paint film C1 formed on the inner surface 31.

[0019] Note that without installing a masking body for closing the inside of the tube 1 and preventing paint from adhering to a part of the inner surface of the tube 1, the first coating mechanism 11 coats the inner surface 31 by powder coating. For this reason, the labor of installing the masking body can be reduced. After the coating by the first coating mechanism 11 is completed, the control part moves the nozzle 12 in the negative X-axis direction to retract the nozzle 12 from the tube 1.

[0020] <Process for roughening the paint film C1> As shown in FIG. 2, the tube inner surface coating device 100 includes a roughening mechanism 14. The roughening mechanism 14 has an irradiation source 15 and a robot arm 16. The irradiation source 15 is provided at the tip of the robot arm 16 and irradiates laser light. The control part controls the operation of the robot arm 16 and can change the position of the irradiation source 15 in the X-axis direction, Y-axis direction, and Z-axis direction.

[0021] The control unit controls the movement of the robot arm 16 to position the irradiation source 15 in a location surrounded by the intermediate portion R1 of the inner surface 21. With the irradiation source 15 positioned in a location surrounded by the intermediate portion R1 of the inner surface 21, the roughening mechanism 14 irradiates laser light from the irradiation source 15. As a result, the roughening mechanism 14 roughens the surface of the powder coating film C1 formed on the intermediate portion R1 of the inner surface 21 by the first coating mechanism 11 (roughening process).

[0022] In other words, the surface roughening mechanism 14 roughens the coating film C1 by irradiating it with laser light. Furthermore, the surface roughening mechanism 14 irradiates the laser light from a direction perpendicular to the inner surface 21. This allows for efficient surface roughening.

[0023] The roughening mechanism 14 roughens the coating film C1, forming a coating film C1 (powder coating) with irregularities on the inner surface 21 of the receiving opening 2. With the above configuration, since the coating film C1 can be roughened without contact by using laser light, a degree of freedom is guaranteed in the design of the position where the laser light irradiation source 15 is installed in the manufacturing equipment.

[0024] Furthermore, while the pipe holder 4 rotates the pipe 1, the surface roughening mechanism 14 irradiates laser light from the irradiation source 15. Specifically, the pipe holder 4 moves the irradiation position of the laser light in the circumferential direction of the pipe 1 as the pipe 1 rotates. As a result, the surface roughening mechanism 14 roughens the coating film C1 in the circumferential direction. In addition, the control unit controls the movement of the robot arm 16 and moves the laser light irradiation source 15 by a predetermined distance in the X-axis direction, so that the surface roughening mechanism 14 roughens the coating film C1 in the X-axis direction.

[0025] According to the above configuration, roughening of the circumferential surface of the pipe 1 can be efficiently performed by rotating the pipe 1, and the axial width / position of the pipe 1 related to the roughening area can be easily adjusted by moving the laser beam irradiation source 15 in the axial direction of the pipe 1.

[0026] When the surface roughening mechanism 14 performs surface roughening, the control unit moves the irradiation source 15 in the X-axis direction within the region surrounded by the intermediate portion R1. This allows the surface roughening mechanism 14 to roughen the coating film C1 formed on the intermediate portion R1. The irradiation source 15 is, for example, an irradiation source using a fiber laser, and irradiates pulsed laser light. The laser light from the irradiation source 15 is, for example, infrared light. After the surface roughening by the surface roughening mechanism 14 is completed, the control unit moves the irradiation source 15 in the negative X-axis direction to retract the irradiation source 15 from the pipe 1.

[0027] <Processing involving painting with water-based paint> As shown in Figure 3, the pipe internal coating apparatus 100 includes a second coating mechanism 17. The second coating mechanism 17 has a nozzle 18 and a robot arm 19. The nozzle 18 is located at the tip of the robot arm 19 and dispenses liquid water-based paint. The control unit controls the movement of the robot arm 19 and can change the position of the nozzle 18 in the X-axis, Y-axis, and Z-axis directions.

[0028] The control unit controls the movement of the robot arm 19 to position the nozzle 18 within the inner surface 21 of the socket 2. With the nozzle 18 positioned within the inner surface 21 of the socket 2, the second painting mechanism 17 discharges water-based paint from the nozzle 18, thereby painting the inner surface 21 of the socket 2 with water-based paint. In other words, the second painting mechanism 17 paints the coating film C1, which has been roughened by the surface roughening mechanism 14, with water-based paint (painting process). As a result, a coating film C2 made of water-based paint is formed on a portion of the coating film C1 which has irregularities.

[0029] While the pipe holder 4 rotates the pipe 1, the second painting mechanism 17 discharges water-based paint from the nozzle 18. Furthermore, while the control unit moves the nozzle 18 in predetermined increments in the X-axis direction, the second painting mechanism 17 discharges water-based paint from the nozzle 18.

[0030] The control unit moves the nozzle 18 in the X-axis direction within the region enclosed by the area where the irregularities 211 are formed on the inner surface 21 and the intermediate portion R1 of the inner surface 21. As a result, the coating film C2 is formed over the area where the irregularities 211 are formed on the inner surface 21 and the intermediate portion R1 of the inner surface 21.

[0031] Alternatively, a solvent-based paint may be applied manually using a brush to the coating film C1 formed on the inner surface 31 side of the intermediate portion R1, that is, on the rising portion of the intermediate portion R1, thereby forming a coating film C3. After the painting by the second painting mechanism 17 is completed, the control unit moves the nozzle 18 in the negative X-axis direction to retract the nozzle 18 from the pipe 1.

[0032] As a result of the above, since the water-based paint is applied to the uneven surface formed on the powder coating film C1 by roughening, the adhesion between the powder coating film C1 and the water-based paint is improved, and the coating condition of the inner surface 21 of the socket 2 can be improved. In addition, by using water-based paint instead of solvent-based paint, VOCs can be reduced. Furthermore, a sufficient coating film C2 can be formed even on the parts of the inner surface 21 where the unevenness 211 is formed.

[0033] As mentioned above, the effect of reducing VOCs contributes to achieving, for example, Goal 12 of the United Nations' Sustainable Development Goals (SDGs), "Ensure sustainable consumption and production patterns."

[0034] <Example 1> The surface roughening mechanism 14 may roughen the coating C1 by sandblasting. In this case, for example, the surface roughening mechanism 14 has a robot arm 16 and a nozzle instead of an irradiation source 15. The nozzle is provided at the tip of the robot arm 16 and discharges sand as an abrasive. The control unit controls the movement of the robot arm 16 and can change the position of the nozzle in the X-axis, Y-axis, and Z-axis directions.

[0035] The roughening mechanism 14 has a nozzle positioned within the intermediate portion R1 of the inner surface 21, and discharges sand from the nozzle. As a result, the roughening mechanism 14 roughens the powder coating film C1 formed on the intermediate portion R1 of the inner surface 21 by the first coating mechanism 11 (roughening process).

[0036] Furthermore, while the pipe holder 4 rotates the pipe 1, the roughening mechanism 14 discharges sand from the nozzle. Specifically, the rotation of the pipe 1 causes the pipe holder 4 to move the sand spraying position in the circumferential direction of the pipe 1. As a result, the roughening mechanism 14 roughens the coating film C1 in the circumferential direction.

[0037] Furthermore, the control unit controls the movement of the robot arm 16 and moves the nozzle in the X-axis direction, causing the roughening mechanism 14 to roughen the coating film C1 in the X-axis direction. With the above configuration, roughening over a wide area can be performed in a short time by sandblasting.

[0038] The control unit moves the nozzle in the X-axis direction within the region surrounded by the intermediate portion R1. This allows the roughening mechanism 14 to roughen the coating film C1 formed on the intermediate portion R1.

[0039] Furthermore, the roughening mechanism 14 may roughen the coating film C1 by dry ice blasting, or it may roughen the coating film C1 by pressing an abrasive material against the coating film C1. In addition, the roughening mechanism 14 may roughen the coating film C1 using a wire brush, a paper brush, or chemicals. A paper brush is, for example, a brush equipped with sandpaper.

[0040] <Example 1> In Example 1, a 100mm x 100mm specimen was cut from the straight section of a pipe with an outer diameter of 300mm, and a laser device with a rated power of 50W was prepared as the irradiation source. The specimen had a coating formed by powder coating.

[0041] The test specimen was irradiated with laser light using an irradiation source. The laser irradiation conditions were as follows: the laser spot diameter was 80 μm, and the irradiation source was moved to ensure that the laser light irradiated the test specimen at intervals of 160 μm. The irradiation area of ​​the test specimen was defined as 50 mm × 30 mm.

[0042] Regarding the laser output of the irradiation source, the laser light was irradiated at 5%, and also in 10% increments within the range of 10% to 100%. The laser output is the output relative to the maximum output of the irradiation source from which the laser light was emitted. The results of irradiating the test specimens with laser light as described above are shown in Figures 4 to 6 and Tables 1 to 3.

[0043] Figure 4 shows the surface state of the test specimens in Example 1 of the present invention. The numerical values ​​indicated by reference numerals 401 to 403 in Figure 4 represent the laser output when laser light is irradiated onto each of the multiple test specimens.

[0044] In Figure 4, reference numeral 401 indicates the result of laser irradiation under the condition of a single pass with a laser output of 5% to 50%. A single pass means that the laser beam was irradiated once to a specific location on the test specimen. In Figure 4, reference numeral 402 indicates the result of laser irradiation under the condition of a single pass with a laser output of 60% to 100%. In Figure 4, reference numeral 403 indicates the result of laser irradiation under the condition of five passes with a laser output of 30% to 80%. A five pass means that the laser beam was irradiated five times to a specific location on the test specimen.

[0045] As shown by reference numeral 401 in Figure 4, when the laser output was between 5% and 50%, a change in the color of the test specimen was observed, but no surface irregularities could be felt when the specimen was touched. When the laser output was between 60% and 100%, surface irregularities could be felt when the specimen was touched.

[0046] In Figure 4, reference numerals 404 to 406 indicate the surface condition of the test specimen when the laser output is 100%. In Figure 4, reference numerals 404 to 406 indicate the length of the line segment and the numerical length shown in the lower right, which is the length of the line segment in the figure. When the laser output is 80% or higher, for example, as shown in reference numeral 406 in Figure 4, multiple holes (shown in black in the figure) were observed in addition to the irregularities formed in the coating film.

[0047] Figure 5 shows the indentations formed in the coating of the test specimen in Example 1 of the present invention. Reference numerals 501 to 505 in Figure 5 indicate the results of laser irradiation under conditions where the laser output was 20, 40, 60, 80, and 100%, respectively. In addition, reference numerals 501 to 505 in Figure 5 indicate that laser irradiation was performed under a single-pass condition.

[0048] As shown in Figure 5, reference numerals 501 to 505 indicate, the greater the laser output, the larger the indentation formed by thermal deformation of the coating. In Figure 5, reference numerals 501 to 505 show two mutually orthogonal intersecting line segments and their lengths in the lower right, indicating that the length of one of the line segments in the figure is the length indicated by the numerical value.

[0049] In Figure 5, reference numeral 506 indicates the result of laser irradiation under the conditions of 80% laser power and 5 passes. As shown in reference numeral 506 in Figure 5, it was confirmed that irregularities were formed on the coating even with a laser power of 80% under the 5-pass condition.

[0050] Figure 6 shows the relationship between laser output and arithmetic mean roughness Ra in Example 1 of the present invention. In Figure 6, the horizontal axis represents laser output [%], and the vertical axis represents the arithmetic mean roughness Ra [μm] of the coating film on the test specimen after irradiation with laser light. The 3 passes shown in Figure 6 indicate that the laser light was irradiated three times on a specific location on the test specimen. The relationship between laser output and arithmetic mean roughness Ra is also shown in Tables 1 and 2 below. [Table 1] [Table 2]

[0051] Table 2 shows the arithmetic mean roughness Ra of the coating on the specimen before irradiation with laser light, and the arithmetic mean roughness Ra of the coating on the specimen after irradiation with laser light. Table 2 also shows the increment of the arithmetic mean roughness Ra after irradiation from the initial arithmetic mean roughness Ra. The number of passes shown in Table 2 refers to the number of times the laser light was irradiated to a specific location on the specimen.

[0052] As shown in Table 2, the increment increases as the number of passes increases. This suggests that the arithmetic mean roughness Ra could be increased by irradiating specific areas of the specimen with laser light while heat from the previous irradiation remained.

[0053] Figure 6 plots data showing the relationship between laser output and arithmetic mean roughness Ra on a coordinate plane. The region indicated by the dotted line in Figure 6 is the region where the arithmetic mean roughness Ra is between approximately 1.0 μm and approximately 3.0 μm. This region generally includes the data showing the relationship between laser output and arithmetic mean roughness Ra when sandblasting is performed in Example 2 described later.

[0054] As shown in Figure 6, under the single-pass condition, no increase in the arithmetic mean roughness Ra was observed compared to sandblasting. However, under the three-pass or five-pass conditions, i.e., when the laser beam was irradiated multiple times consecutively, the arithmetic mean roughness Ra could be made equal to or greater than that of sandblasting. In particular, when the laser output was 50% or higher and under the three-pass or five-pass conditions, the results for the arithmetic mean roughness Ra were good.

[0055] Furthermore, in Example 1, no dust or other debris was visible to the naked eye on the test specimen, indicating that a function to collect dust generated by roughening is unnecessary, and that simply blowing air onto the inner surface of the tube is sufficient.

[0056] <Example 2> In Example 2, a steel plate measuring 150 mm x 70 mm and with a thickness of 2.0 mm was prepared as a test specimen of a portion of the pipe. The test specimen was coated with powder coating using an electrostatic gun or an air spray gun. The material of the test specimen in Example 2 was the same as the material of the test specimen in Example 1.

[0057] Furthermore, the specimens coated with powder coating were roughened by sandblasting using a blast gun as a surface roughening device. The sand used to blast the specimens included sand with a particle size of 150 μm, sand with a particle size of 250 μm, and sand with a particle size of 300 μm to 360 μm.

[0058] Furthermore, the roughened specimens were painted with water-based paint. Two sets of specimens were then prepared: one before immersion in tap water, and another after immersion in tap water for four weeks. Adhesion tests (cross-cut method) were performed on these specimens in accordance with JIS K 5600-5-6.

[0059] As described above, the results of the sandblasting treatment are shown in Table 3 below. In Table 3, air pressure [MPa] is the air pressure used when blasting the sand, and sand particle size [μm] is the particle size of the sand blasted onto the specimen. Gun movement speed [mm / s] is the speed at which the roughening equipment is moved when blasting the specimen with sand, and arithmetic mean roughness Ra [μm] is the arithmetic mean roughness of the coating on the specimen after sandblasting.

[0060] Furthermore, the ten-point average roughness Rz [μm] is the ten-point average roughness of the coating on the test specimen after sandblasting. For the adhesion shown in Table 3, "○" indicates that the adhesion test was passed, and "×" indicates that the adhesion test was failed. [Table 3]

[0061] In the sandblasting treatment, as shown in Table 3, except for the condition where sand with a particle size of 150 μm was blasted onto the specimen at an air pressure of 0.3 MPa, the specimens passed the adhesion test not only before immersion in tap water, but also after immersion in tap water for 4 weeks. Furthermore, even if the arithmetic mean roughness Ra was the same, differences in the ten-point mean roughness Rz resulted in differences in the pass / fail status of the adhesion test. In addition, a tendency was observed for the ten-point mean roughness Rz to increase as the air pressure and sand particle size increased.

[0062] Figure 7 shows the state in which the water-based paint is in close contact with the test specimen after an adhesion test has been performed on the test specimen in Example 2 of the present invention. Reference numeral 701 in Figure 7 indicates the state in which the water-based paint is in close contact with the test specimen when the arithmetic mean roughness Ra is 1.10 μm and the ten-point mean roughness Rz is 7.02 μm, according to Table 3.

[0063] In Figure 7, symbol 702 indicates the state in which the water-based paint adheres to the test specimen when the arithmetic mean roughness Ra is 1.19 μm and the ten-point mean roughness Rz is 8.42 μm, according to Table 3. As shown by symbols 701 and 702 in Figure 7, the larger the arithmetic mean roughness Ra and ten-point mean roughness Rz, the better the adhesion of the water-based paint to the test specimen.

[0064] Figure 8 shows the relationship between the arithmetic mean roughness Ra and the ten-point mean roughness Rz in Examples 1 and 2 of the present invention. In Figure 8, "○" indicates data for specimens that passed the adhesion test after sandblasting, and "□" indicates data for specimens that passed the adhesion test after laser irradiation. "△" indicates data for specimens that failed the adhesion test after sandblasting, and "×" indicates data for specimens that did not undergo roughening and failed the adhesion test.

[0065] Furthermore, in Figure 8, the relationship between the arithmetic mean roughness Ra and the ten-point mean roughness Rz is plotted on the coordinate plane using the data shown in Tables 4 to 6 below. The content of the first row of Tables 4 to 6 is the same as the content of the first row of Tables 2 and 3. [Table 4] [Table 5] [Table 6]

[0066] As shown in Figure 8, based on the data plotted on the coordinate plane, it is possible to determine the region A1 in which the adhesion test passes and the region A2 in which the adhesion test fails. For the arithmetic mean roughness Ra, the minimum line for passing the adhesion test is 1.1 μm, and for the ten-point mean roughness Rz, the minimum line for passing the adhesion test is 8.5 μm. Also, as shown in Table 5, when the sand particle size is small in sandblasting, it is necessary to increase the air pressure in order to pass the adhesion test on the test specimen.

[0067] Figure 9 shows the relationship between sand particle size and arithmetic mean roughness Ra in Example 2 of the present invention. In Figure 9, "○" indicates data for specimens after sandblasting under conditions of an air pressure of 0.5 MPa and a gun movement speed of 10 mm / s. "△" indicates data for specimens after sandblasting under conditions of an air pressure of 0.3 MPa and a gun movement speed of 10 mm / s. "◇" indicates data for specimens that have not undergone roughening, and "□" indicates data for specimens that have been roughened using sandpaper.

[0068] As shown in Figure 9, a positive correlation was observed between the arithmetic mean roughness Ra and the grain size of the sand. However, when using sand with a large grain size, the arithmetic mean roughness Ra varied greatly depending on the air pressure, so it is necessary to set the air pressure appropriately. In addition, the arithmetic mean roughness Ra of the specimens subjected to sandblasting was greater than that of the specimens roughened with sandpaper.

[0069] <Modification 2> The pipe internal coating apparatus 100 can also be implemented using a support member instead of a robot arm. Specifically, the pipe internal coating apparatus 100 may be equipped with a first support member instead of a robot arm 13. In this case, the nozzle 12 is provided at the tip of the first support member. The control unit controls the operation of a first movement mechanism that can move the first support member in the X-axis direction, and can change the position of the nozzle 12 in the X-axis direction.

[0070] Furthermore, the pipe internal coating apparatus 100 may be equipped with a second support member instead of the robot arm 16. In this case, the irradiation source 15 is provided at the tip of the second support member. The control unit controls the operation of a second movement mechanism that allows the second support member to move in the X-axis direction, and can change the position of the irradiation source 15 in the X-axis direction.

[0071] Furthermore, the pipe internal coating apparatus 100 may be equipped with a third support member instead of the robot arm 19. In this case, the nozzle 18 is provided at the tip of the third support member. The control unit controls the operation of a third movement mechanism that can move the third support member in the X-axis direction, and can change the position of the nozzle 18 in the X-axis direction. The first movement mechanism, the second movement mechanism, and the third movement mechanism are provided in the pipe internal coating apparatus 100.

[0072] <Additional Notes> The present invention is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the multiple technical means disclosed in the embodiments are also included in the technical scope of the present invention. [Explanation of Symbols]

[0073] 1 tube 2 socket 21, 31 Inner self 3 Straight section 11. First Painting Mechanism 14. Roughening mechanism 15 Irradiation source 17. Second Painting Mechanism 100 Pipe Inner Surface Coating Apparatus C1, C2 coating film

Claims

1. A method for painting the inner surface of a pipe having a straight section and a socket, A powder coating step in which the straight portion and the inner surface of the socket are coated with powder coating, A roughening step is performed to roughen the coating film of the powder coating formed on the inner surface of the socket by the powder coating step, A painting step in which the coating film, which has been given an uneven surface by the roughening step, is painted with a water-based paint, A method for coating the inside surface of a pipe, including the coating method itself.

2. The method for coating the inner surface of a pipe according to claim 1, wherein the coating film is roughened by irradiating it with laser light in the roughening step.

3. In the surface roughening process, the irradiation position of the laser beam is moved in the circumferential direction of the tube by rotating the tube, thereby roughening the coating film in the circumferential direction, The method for coating the inner surface of a pipe according to claim 2, wherein the irradiation source of the laser light is moved in the axial direction of the pipe to roughen the coating film in the axial direction.

4. The method for coating the inner surface of a pipe according to claim 1, wherein the coating film is roughened by sandblasting in the roughening step.

5. A pipe inner surface coating apparatus for coating the inner surface of a pipe having a straight section and a socket, A first coating mechanism that coats the inner surfaces of the straight section and the receiving section with powder coating, A roughening mechanism for roughening the powder coating film formed on the inner surface of the socket by the first coating mechanism, A second painting mechanism paints the coating film, which has been roughened by the aforementioned surface roughening mechanism, with a water-based paint. A pipe internal coating apparatus equipped with the following features.

6. A tube having a straight section and a socket, A powder coating having irregularities is formed on the inner surface of the aforementioned socket. A pipe in which a coating film of water-based paint is formed on the powder coating film having the aforementioned irregularities.

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