Cast iron pipe internal coating method

Abstract

The objective is to provide a method for coating the inner surface of cast iron pipes that can further suppress the occurrence of blowholes. [Solution] A method for coating the inner surface of a cast iron pipe with powder coating, comprising at least two coating steps: a first coating step and a second coating step, wherein the first coating step is performed before the second coating step and forms the thinnest coating film among the coating films formed by the coating step performed before the second coating step, the second coating step forms the thickest coating film among the coating films formed by all coating steps and has a coating film thickness of 1.75 to 4.75 times the coating film thickness formed by the first coating step, and the final coating step is the forward coating step.

Description

Technical Field

[0001] The present invention relates to a method for coating the inner surface of a cast iron pipe.

Background Art

[0002] Ductile iron pipes have extremely high toughness compared to other pipes such as conventional ordinary cast iron pipes and high-grade cast iron pipes, and are widely used as water and sewage pipes. In order to use ductile iron pipes as water and sewage pipes, since high corrosion resistance is required for the inner surface of the pipes, a countermeasure of applying a powder coating using resin to the inner surface of the pipes is known.

[0003] Straight pipes of ductile iron pipes are generally manufactured by the centrifugal casting method. However, in casting by the centrifugal casting method, it often happens that impurities are mixed in or casting defects occur on the inner surface of the pipe. As a result, the mixed impurities and the generated casting defects present on the inner surface of the pipe after casting contribute to causing defects such as pinholes and blowholes in the powder coating film such as resin formed after casting.

[0004] So far, various countermeasures have been taken to reduce pinholes. For example, Patent Document 1 discloses an invention related to a method for treating the inner surface of a pipe body in which the rotation of a grindstone for inner surface grinding and a brush for grinding and cleaning on the inner surface of the pipe body is reversed. In addition, Patent Document 2 discloses an invention related to a method for coating the inner surface of a cast iron pipe in which silica sand is used and the coating speeds in the forward and return paths when reciprocally coating a powder coating in the pipe axis direction of the inner surface of the pipe are changed.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, neither Patent Document 1 nor Patent Document 2 mentions blowholes, and no method for reducing blowholes is disclosed. When blowholes occur, each one needs to be reworked, which increases the number of steps, time, and cost. Furthermore, as will be discussed later, pinholes and blowholes are different, and measures taken to reduce pinholes do not necessarily lead to a reduction in blowholes.

[0007] Therefore, the present invention aims to provide a method for coating the inner surface of cast iron pipes that can further suppress the occurrence of blowholes (i.e., reduce blowholes). [Means for solving the problem]

[0008] The inventors of the present invention have diligently studied and found that when coating the inner surface of a cast iron pipe with powder coating, the coating process is performed at least twice in the direction of the pipe axis, and the thickness of the thickest coating film formed in each coating process is 1.75 to 4.75 times the thickness of the thinnest coating film formed in a coating process performed prior to the coating process that forms the thickest coating film. Furthermore, by making the final coating process the "forward pass," the occurrence of blowholes in the inner coating film of the cast iron pipe can be suppressed, and thus the present invention has been completed.

[0009] In other words, the present invention is a method for coating the inner surface of a cast iron pipe with powder coating, The painting process includes at least two steps: a first painting step and a second painting step. The first painting step is performed before the second painting step, and forms the thinnest coating film among the coating films formed by the painting step performed before the second painting step. The second painting step forms a coating film that is the thickest among all the coating films formed by the painting steps, and has a film thickness of 1.75 to 4.75 times that of the coating film formed by the first painting step, and The final painting process is on the outbound journey. This invention provides a method for coating the inner surface of pipes (hereinafter sometimes simply referred to as "this coating method"). [Effects of the Invention]

[0010] According to the present invention, in the internal coating of cast iron pipes, in which powder coating is applied to the inner surface of the cast iron pipe, a coating method is provided that can further suppress the occurrence of blowholes (i.e., blowholes are reduced). [Brief explanation of the drawing]

[0011] [Figure 1] This is a schematic diagram illustrating one embodiment of the cast iron pipe internal coating method of the present invention. [Figure 2] This is a schematic diagram illustrating the process of forming a coating film obtained by one embodiment of the cast iron pipe inner surface coating method of the present invention. [Figure 3] This figure illustrates the surface state of the coating obtained by one embodiment of the cast iron pipe inner surface coating method of the present invention. [Modes for carrying out the invention]

[0012] In this specification, unless otherwise specified, when a numerical range is indicated using "~", that numerical range includes the numbers at both ends.

[0013] In this specification, "blowhole" refers to a hole in the coating caused by gas generated or entering during the casting of a cast iron pipe, which rises to the surface. In other words, in this specification, "blowhole" refers to a hole in the coating that does not penetrate to the metal surface. On the other hand, in this specification, "pinhole" refers to a hole that penetrates to the metal surface, caused by gas generated or entering during the casting of a cast iron pipe.

[0014] In this specification, if the maximum number of blowholes on the inner surface of a cast iron pipe measured according to the method described in the "Blowhole Inspection" section of the embodiments below is 10 or less, it is considered that "blowholes have been reduced."

[0015] Hereinafter, the method for coating the inner surface of a cast iron pipe of the present invention will be described with reference to FIGS. 1 and 2. FIG. 1 is a schematic diagram for explaining an embodiment of the method for coating the inner surface of a cast iron pipe of the present invention. FIG. 2 is a schematic diagram for explaining the formation process of a coating film obtained by the method for coating the inner surface of a cast iron pipe of the present invention.

[0016] First, referring to FIG. 1, the structure of an in-pipe coating device D (hereinafter, also simply referred to as a coating device) used in the method for coating the inner surface of a cast iron pipe of the present embodiment will be described. Note that the coating device D shown in FIG. 1 is merely an example and is not limited to the structure shown in FIG. 1.

[0017] As shown in FIG. 1, the coating device D of the present embodiment includes a discharge device 1 that discharges a powder coating 4 and a pipe body rotation device 2 that supports a cast iron pipe P and rotates the cast iron pipe P. The discharge device 1 includes a paint discharge portion 1a as will be described later. Further, in the present embodiment, as shown in FIG. 1, the coating device D includes a moving mechanism 3 that moves the paint discharge portion 1a along the pipe axis direction.

[0018] In the present embodiment, as shown in FIG. 1, the pipe body rotation device 2 includes a rotating roller 21 on which a cast iron pipe P (hereinafter, also simply referred to as a pipe body), which is a preheated coating object, is placed and which rotates the cast iron pipe P around its axis. The rotating roller 21 rotates the cast iron pipe P during coating by the discharge device 1, and as the paint discharge portion 1a moves along the pipe axis direction by the moving mechanism 3, the entire inner peripheral surface of the cast iron pipe P is coated. Further, the shape of the cast iron pipe P is not particularly limited, but for example, it can be a straight pipe having a nominal diameter of 50 mm to 2600 mm, an insertion port 5 on one end side of the cast iron pipe P, and a receiving port 6 on the other end side. The nominal diameter of the cast iron pipe P is not particularly limited, but a nominal diameter of 900 mm to 2600 mm is more preferable. For a cast iron pipe with a nominal diameter of 900 mm or more, the state before coating the inner surface of the pipe may be poor, and it is preferable because the effect of the method for coating the inner surface of the pipe of the present invention is easily exerted.

[0019] The discharging device 1 is a device that discharges the powder coating 4 and coats the inner surface of the cast iron pipe P. In this embodiment, as shown in FIG. 1, the discharging device 1 includes a paint supply mechanism 1c for supplying the powder coating 4 from a supply source (not shown) of the powder coating 4, a paint pipe 1b connected to the paint supply mechanism 1c for supplying the powder coating 4 to the paint discharging portion 1a, and a paint discharging portion 1a connected to the paint pipe 1b for discharging the powder coating 4.

[0020] The paint supply mechanism 1c is not particularly limited as long as it can supply the powder coating 4 to the paint discharging portion 1a.

[0021] The paint pipe 1b forms a passage for supplying the powder coating 4 to the paint discharging portion 1a. In this embodiment, the paint pipe 1b is provided between the paint supply mechanism 1c and the paint discharging portion 1a and extends substantially parallel to the pipe axis of the cast iron pipe P.

[0022] The paint discharging portion 1a is a portion for discharging the powder coating 4. The structure of the paint discharging portion 1a is not particularly limited as long as it can discharge the powder coating 4, and a known nozzle used for discharging the paint can be used.

[0023] Also, the powder coating 4 used in this embodiment can be used without particular limitation as long as it melts by heat and then cures after being applied to the cast iron pipe P. For example, in the case of a ductile iron pipe for water supply, an epoxy powder resin coating conforming to the standard is used. The gel time of the powder coating 4 is preferably 80 to 120 seconds, more preferably 90 to 120 seconds, at 170°C.

[0024] The moving mechanism 3 is a mechanism that moves the paint discharge unit 1a and the cast iron pipe P relative to each other in the direction of the pipe axis of the cast iron pipe P. In this embodiment, the moving mechanism 3 is configured to move the paint discharge unit 1a along the pipe axis inside the cast iron pipe P, but the moving mechanism 3 may also be configured to move the cast iron pipe P without moving the paint discharge unit 1a, or to move the cast iron pipe P and the paint discharge unit 1a. As will be described in detail later, by moving the cast iron pipe P and the paint discharge unit 1a relative to each other in the direction of the pipe axis of the cast iron pipe P, it becomes possible to paint the inner surface of the cast iron pipe P. In Figure 1, the moving mechanism 3 is schematically shown as a moving body that moves by wheels, but the moving mechanism 3 only needs to be able to move at least the paint discharge unit 1a, and its structure is not particularly limited.

[0025] Next, an embodiment of the cast iron pipe internal coating method of the present invention will be described with reference to Figures 1 and 2. In the following description, the coating apparatus D described above will be used as an example, but other coating apparatuses with different structures may be used as long as they can achieve similar effects.

[0026] The method for coating the inner surface of a cast iron pipe P according to this embodiment is a method for coating the inner surface of a cast iron pipe P with powder coating 4, and the coating process for coating the inner surface of the cast iron pipe with powder coating 4 includes at least two steps: a first coating step and a second coating step, the first coating step is performed before the second coating step and forms the thinnest coating film among the coating films formed by the coating steps performed before the second coating step, the second coating step forms the thickest coating film and is performed such that the thickness of the coating film formed by the second coating step is 1.75 to 4.75 times the thickness of the coating film formed by the first coating step, and the final coating step for coating the inner surface of the cast iron pipe with powder coating 4 is performed on the forward pass.

[0027] Here, "forward path" refers to the painting direction in which the discharge unit 1a, which discharges the powder coating 4, is inserted into the interior of the cast iron pipe P from one end in the pipe axis direction and proceeds toward the other end while painting. Therefore, as shown in Figure 1, when the discharge unit 1a, which discharges the powder coating 4, is inserted into the interior of the cast iron pipe P from the receiving port 6 in the pipe axis direction, the "forward path" is in the direction of →A. Although not shown in the figure, when a dust collector is installed, which is generally used when powder coating is applied to the inner surface of a pipe, the dust collector is usually installed on the opposite end of the cast iron pipe P from the end in the pipe axis direction where the coating machine (discharge device 1) is located (the insertion port 5 side in Figure 1). Therefore, "forward path" can also be defined as "the painting direction in which the cast iron pipe P is painted along the pipe axis direction, from the end furthest from the dust collector to the other end closer to the dust collector." As mentioned above, of course, when painting the inner surface of the cast iron pipe in the "forward" direction, the relative position of the cast iron pipe P and the discharge section 1a can be moved not only by the moving mechanism 3 but also by moving the cast iron pipe P.

[0028] The cast iron pipe P to be painted on its inner surface according to this embodiment is not particularly limited by its casting method, and can be centrifugal cast by die casting, resin sand casting, or heavy coating casting. However, it is preferable to use a pipe manufactured by resin sand casting or heavy coating casting and cooled at room temperature, as this allows the effects of the present invention to be more fully realized. Furthermore, it is preferable to perform a treatment such as polishing on the inner surface of the cast iron pipe P prior to painting the inner surface. In addition, it is preferable to carry out the painting process after heating the pipe body temperature to 220°C to 250°C in a heating furnace.

[0029] In this specification, the "area requiring powder coating R" means the area from the inlet end of the cast iron pipe P (which is also one end of the cast iron pipe P) to the socket end of the straight section. In other words, in this specification, the area requiring powder coating R means the entire straight section of the cast iron pipe P.

[0030] As described above, in this specification, "painting process" refers to the process of painting at least the area R on the inner surface of the cast iron pipe P from one end to the other, along the pipe axis, and the number of processes can be counted as one when at least the area R on the inner surface of the cast iron pipe P is painted once. For painting processes other than the final painting process of this painting method, the direction of painting along the pipe axis of the cast iron pipe is not particularly limited, and the inner surface of the cast iron pipe P may be painted from the end on the receiving end 6 side toward the end on the insertion end 5 side, or from the end on the insertion end 5 side toward the end on the receiving end 6 side.

[0031] In this specification, the direction →A in Figures 1 and 2 corresponds to the "forward path" as described above, and is the painting direction in which the discharge section 1a, which discharges the powder coating 4, is inserted into the interior of the cast iron pipe P from one end in the pipe axis direction and paints while advancing toward the other end. Specifically, in this embodiment, the paint pipe 1b is inserted into the cast iron pipe P to be painted from the opening on one end side (receiving port 6 side) of the cast iron pipe P, and then the discharge of the powder coating 4 from the tip of the paint pipe 1b (paint discharge section 1a) starts from just before the powder coating required area R on the inner surface of the pipe. In conjunction with the discharge of the powder coating 4, the paint discharge section 1a is moved in the direction →A by the moving mechanism 3, so that painting is performed while advancing from one end side to the other end side of the cast iron pipe P. As described above, of course, the relative position of the cast iron pipe P and the discharge section 1a can also be moved by moving the cast iron pipe P instead of the moving mechanism 3.

[0032] On the other hand, as shown in Figure 1, when the discharge unit 1a that discharges the powder coating 4 is inserted into the inside of the cast iron pipe P from the receiving port 6 in the axial direction of the pipe, the direction →B corresponds to the "return path" in relation to the "forward path". That is, the "return path" means the painting direction in which the discharge unit 1a, which is inserted from one end in the axial direction of the cast iron pipe P, passes through the inside of the pipe and moves out of the pipe from the other end, returns to the inside of the pipe from the other end in the axial direction of the cast iron pipe P, and paints while retracting toward the one end. Specifically, in this embodiment, the powder coating 4 is discharged from the discharge unit 1a from the opening on the other end side (insertion port 5 side) of the cast iron pipe P and pulled back into the pipe, and the area R on the inner surface of the pipe that requires powder coating is painted while the discharge unit 1a is retracted. Subsequently, the discharge of the powder coating 4 is stopped when the paint discharge section 1a has passed the area R where powder coating is required (within the receiving area), and the paint piping 1b (including the paint discharge section 1a) is withdrawn from the cast iron pipe P to be painted. As described above, even at this time, the relative position of the cast iron pipe P and the discharge section 1a can be moved by moving the cast iron pipe P rather than by moving the moving mechanism 3.

[0033] In this invention, the final painting process is performed on the "forward journey," but each of the other painting processes may be performed on either the "forward journey" or the "return journey," and is not particularly limited.

[0034] In this embodiment, the first painting step is performed before the second painting step and forms the thinnest coating film among the coating films formed by the painting steps performed before the second painting step. Because the first painting step is performed before the second painting step, the coating film formed by the first painting step is formed relatively closer to the cast iron pipe than the coating film formed by the second painting step. Because the first painting step, which forms the thinnest coating film, is performed before the second painting step, gas can easily escape even if it is generated from the cast iron pipe side after the first painting step. As a result, blowholes are intentionally generated at this stage, but these blowholes can be easily filled with powder coating in subsequent painting steps. Furthermore, generating blowholes at this stage tends to reduce the amount of gas generated after subsequent painting steps, thereby suppressing the generation of blowholes on the final coating surface. Therefore, in this painting method, it is more preferable that the first painting step is the first painting step.

[0035] Furthermore, the thickness of the coating film formed by the first coating process is preferably 50 μm to 250 μm, more preferably 60 μm to 225 μm, even more preferably 70 μm to 175 μm, and particularly preferably 80 μm to 125 μm, from the viewpoint of reducing blowholes.

[0036] In this embodiment, the film thickness of the coating formed by the second coating step is the thickest coating thickness among all coating steps formed in all coating steps. Here, "all coating steps" means "all coating steps included in this coating method." Therefore, if this coating method includes n coating steps (where n is an integer of 2 or more), the coating formed by the second coating step is the thickest coating thickness among all coating steps formed in each of the n coating steps. The second coating step is performed after the first coating step and is carried out to form a coating with a film thickness of 1.75 to 4.75 times the film thickness of the coating made by the powder coating 4 formed in the first coating step. By forming a coating with such a relative thickness on top of the coating formed in the first coating step, it is possible to suppress the formation of blowholes, pinholes, etc. More specifically, the film thickness of the coating formed in the second coating step is preferably 2.0 times or more, more preferably 2.5 times or more, and even more preferably 3.0 times or more, than the film thickness of the coating formed in the first coating step, from the viewpoint of suppressing blowholes and pinholes. Furthermore, from the viewpoint of cost, the film thickness of the coating formed in the second coating step is preferably 4.5 times or less, more preferably 4.0 times or less, and even more preferably 3.5 times or less, than the film thickness of the coating formed in the first coating step.

[0037] The thickness of the coating film to be formed can be set as appropriate using either the paint discharge rate or the painting speed, or both.

[0038] The painting speed in each painting process is preferably 0.6 m / min to 10 m / min, and more preferably 1.0 m / min to 8.5 m / min. If the painting speed is less than 0.6 m / min, the temperature of the pipe will drop during painting, resulting in poor adhesion of the coating film, and it may not be possible to obtain the target physical properties (hardness, adhesion, surface finish of the coating film, etc.) of the finished powder coating film. In one embodiment, if the discharge amount is not changed, the second painting process is carried out at a speed of 0.22 to 0.5 times the painting speed of the first painting process, preferably at a speed of 0.33 to 0.45 times.

[0039] In the cast iron pipe internal coating method of the present invention, as shown in Figure 2, regardless of the coating direction of each coating step, the final coating step is performed in the "forward direction". By performing the final coating step in the forward direction and advancing the coating in the direction of the discharge section 1a, dust-like appearance defects such as roughness of the inner surface of the pipe can be improved, and a cast iron pipe P with an improved appearance without roughness can be obtained. For cast iron pipes P with a large nominal diameter where the amount of powder coating 4 applied is large, for example, a cast iron pipe P with a nominal diameter of 900 mm or more, if the final coating step is performed in the "return direction" coating, where the coating is applied while the discharge section 1a is retracted, there is a tendency for some of the applied powder coating 4 to remain unmelted, which can result in a rough surface and dust-like appearance defects. Therefore, it is more preferable to perform the final coating step in the "forward direction".

[0040] Furthermore, in one embodiment, the painting process can be carried out by only two steps: a first painting step and a second painting step. That is, the painting method may consist of two steps: a first painting step and a second painting step. In this case, the first painting step can be performed on either the "forward journey" or the "return journey," but since the second painting step is the final painting step, the second painting step will be performed on the "forward journey." The interval time between the first painting step and the second painting step (meaning the time from the end of discharge in the first painting step to the start of discharge in the second painting step) is preferably longer than in painting methods that include three or more painting steps, from the viewpoint of reducing blowholes. Specifically, it is preferably 20 to 40 seconds, and more preferably 25 to 35 seconds. Note that the "interval time" between painting steps means the time from the end of discharge in the immediately preceding painting step to the start of discharge in the next painting step.

[0041] In another embodiment, from the viewpoint of reducing blowholes, the painting process is preferably three or more steps, and more preferably five or more steps. In other words, the painting method preferably includes at least three steps, and more preferably at least five steps. Thus, when the painting method includes at least three steps, from the viewpoint of reducing blowholes, for example, the third painting step is preferably included between the first and second painting steps.

[0042] Furthermore, from the viewpoint of reducing blowholes, it is preferable that the second painting process and the final painting process are the same process. In other words, if the second painting process and the final painting process are the same process, the first painting process will be the process that forms the thinnest coating film among all the painting processes included in this painting method. Moreover, from the viewpoint of reducing blowholes, it is preferable that the first painting process is performed first. In other words, in this painting method, it is preferable that the first painting process is the first painting process.

[0043] In one embodiment, the final film thickness of the coating film made by the powder coating 4 is preferably 300 μm to 1000 μm, and preferably 450 μm to 850 μm. Here, "final film thickness" refers to "the film thickness of the coating film formed by this coating method," and can also be said to be "the total film thickness of the coating film formed by all coating steps included in this coating method." The amount of powder coating 4 applied can be appropriately set considering the final film thickness and coating speed. [Examples]

[0044] Examples 1-7 and Comparative Examples 1-11 As the pipe to be painted, a ductile iron pipe with a nominal diameter of 900 mm and a length of 4 m, manufactured by centrifugal casting of resin sand casting, was used. The rust on the inner surface of the pipe to be painted was polished using a grinding wheel and then scrubbed with a wire brush. After that, it was heated in a gas furnace set to 270-330°C, placed on a pipe rotating device, and painting was started at 240°C (±10°C) (rotation speed: 180 m / min (60 rpm)). The discharge device was positioned on the receiving end side, and painting processes a to e were carried out in the order shown in Table 1, in the direction of painting. Here, the "forward" painting was carried out by inserting the discharge part of the paint pipe from the receiving end side, starting discharge within the receiving end area, and proceeding at a constant speed toward the insertion end side until it passed through the insertion end. The "return" painting was carried out by having the discharge part exit the pipe from the insertion end side, then reversing while still discharging, and moving backward at a constant speed from the insertion end side toward the receiving end side until it entered the receiving end area. Other painting conditions are shown in Table 1.

[0045] The paint used was an epoxy powder resin paint (gel time: 90 seconds) commonly used for internal coating of ductile iron pipes for water supply. An injector-type powder coating apparatus was used to dispense the powder paint. Through each coating process, the final coating thickness on the inside of the pipe was adjusted to 400 μm to 850 μm. The discharge rate in each example and comparative example was set to a constant value of approximately 4000 g / 30s to 4500 g / 30s.

[0046] [Table 1]

[0047] Test Example 1: Blowhole Inspection Visual inspection was performed on all surfaces of the powder coating required area R of the cast iron pipe inner coating obtained in Examples 1-7 and Comparative Examples 1-11 (n=3 samples each). Conditions in the coating where abnormalities due to gas defects were confirmed were identified as blowholes, and their number was measured. The results are shown in Table 1, representing the range from the minimum to the maximum number of blowholes measured in each example and comparative example. The target performance was a maximum of 10 blowholes in the entire powder coating required area R; any number exceeding 10 was considered non-compliant.

[0048] Test Example 2: Pinhole Inspection In Examples 1-7 and Comparative Examples 1-11, all surfaces of the powder coating required region R of the cast iron pipe inner surface coating were evaluated using a wire brush-type Holiday detector with a brass probe at a voltage of 1000V (JWWA G 112 Epoxy Resin Powder Coating for Ductile Iron Pipes for Water Supply) according to the following criteria. The target performance was ◎ or ○, and △ or × was considered non-compliant. The results are shown in Table 1. ◎: No pinholes ○: There are 5 or fewer areas requiring repair due to pinholes. △: There are 6 to 10 areas that require repair due to pinholes. ×: There are 11 or more areas that require repair due to pinholes.

[0049] Test Example 3: Evaluation of Appearance The entire surface of the powder coating required area R of the cast iron pipe inner coating obtained in Examples 1-7 and Comparative Examples 1-11 was visually evaluated according to the following criteria. The results are shown in Table 1. Figure 3 shows photographs of the surface condition of the cast iron pipe inner coating corresponding to these evaluations. ○ (Acceptable): Smooth and free of roughness. × (Not acceptable): Causes a dusty, gritty texture.

[0050] From the above, it can be seen that in Examples 1 to 7, where the film thickness of the coating film formed by the second coating process in this painting method, which forms the thickest coating film among all coating processes, is within the range of 1.75 to 4.75 times the film thickness of the coating film formed by the first coating process in this painting method, which forms the thinnest coating film among the coating processes performed before the said coating process, the occurrence of blowholes is suppressed. Furthermore, it can be seen that in Examples 1 to 5, which have three or more coating processes, the occurrence of blowholes is suppressed even further, and in Examples 1 to 3, which have five coating processes, the occurrence of blowholes is suppressed even more.

[0051] (summary) (1) A method for coating the inner surface of a cast iron pipe by applying powder coating to the inner surface of the cast iron pipe, The painting process includes at least two steps: a first painting step and a second painting step. The first painting step is performed before the second painting step, and forms the thinnest coating film among the coating films formed by the painting step performed before the second painting step. The second painting step forms a coating film that is the thickest among all the coating films formed by the painting steps, and has a film thickness of 1.75 to 4.75 times that of the coating film formed by the first painting step, and The final painting process is on the outbound journey. Method for coating the inside surface of cast iron pipes.

[0052] According to the cast iron pipe inner surface coating method described in (1) above, it is possible to provide a cast iron pipe inner surface coating method that further suppresses the occurrence of blowholes.

[0053] (2) In the method for painting the inner surface of a cast iron pipe described in (1) above, it is preferable to include at least three painting steps.

[0054] According to the cast iron pipe inner surface coating method described in (2) above, it is possible to provide a cast iron pipe inner surface coating method that further suppresses the occurrence of blowholes.

[0055] (3) In the method for painting the inner surface of a cast iron pipe described in (2) above, it is preferable to include at least five painting steps.

[0056] According to the cast iron pipe inner surface coating method described in (3) above, it is possible to provide a cast iron pipe inner surface coating method that further suppresses the occurrence of blowholes.

[0057] (4) In the cast iron pipe inner surface coating method described in (1) to (3) above, it is preferable to include a third coating step between the first coating step and the second coating step.

[0058] According to the cast iron pipe inner surface coating method described in (4) above, the occurrence of blowholes can be suppressed even more effectively.

[0059] (5) In the cast iron pipe inner surface coating method described in any of (1) to (4) above, it is preferable that the second coating step and the final coating step are the same step.

[0060] According to the cast iron pipe inner surface coating method described in (5) above, the appearance can be further improved.

[0061] (6) In the cast iron pipe inner surface coating method described in any of (1) to (5) above, it is preferable that the first coating step is the first coating step. [Explanation of symbols]

[0062] D Painting equipment A Outbound journey B Return trip R Powder coating required area 1 Discharge device 1a Paint discharge part 1b Paint piping 1c Paint supply mechanism 2. Pipe Rotation Device 21 Rotating Rollers 3 Moving mechanism P Cast Iron Pipe 4 Powder coating 5 sockets 6 socket

Claims

1. A method for coating the inner surface of a cast iron pipe, comprising applying powder coating to the inner surface of the cast iron pipe, The painting process includes at least two steps: a first painting step and a second painting step. The first painting step is performed before the second painting step, and forms the thinnest coating film among the coating films formed by the painting step performed before the second painting step. The second painting step forms a coating film that is the thickest among all the coating films formed by the painting steps, and has a film thickness of 1.75 to 4.75 times that of the coating film formed by the first painting step, and The final painting process is on the outbound journey. Method for coating the inside surface of cast iron pipes.

2. A method for painting the inner surface of a cast iron pipe according to claim 1, comprising at least three painting steps.

3. A method for painting the inner surface of a cast iron pipe according to claim 1, comprising at least five painting steps.

4. The method for painting the inner surface of a cast iron pipe according to claim 1, further comprising a third painting step between the first painting step and the second painting step.

5. The method for painting the inner surface of a cast iron pipe according to claim 1 or 4, wherein the second painting step and the final painting step are the same step.

6. The method for painting the inner surface of a cast iron pipe according to claim 1 or 4, wherein the first painting step is the initial painting step.

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