Method for coating pipe inner surface of cast-iron pipe

Abstract

To provide a method for coating a pipe inner surface that can provide a cast-iron pipe that is further suppressed in generation of pin holes.SOLUTION: A method for coating a pipe inner surface of a cast-iron pipe that coats an inner surface of a cast-iron pipe with a powder coating material includes a first coating process of coating at least an inner surface of a cast-iron pipe with a powder coating material, and a second coating process. where the second coating process is the final coating process of coating an inner surface of a cast-iron pipe with a powder coating material and conducted so that a film thickness of a coating film that is formed by the second coating process becomes 2-3.5 times as thick as a film thickness of a coating film that is formed with a powder coating material before the second coating process.SELECTED DRAWING: None

Description

[Technical field]

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

[0002] Ductile iron pipes are used as piping materials for water supply systems, and their inner surfaces are coated with epoxy resin powder coating, which has excellent water hygiene and corrosion resistance. Epoxy resin powder coating also has excellent chemical resistance, so it is used for coating the inner surfaces of sewer systems.

[0003] Straight ductile iron pipes are manufactured by centrifugal casting, which tends to produce impurities and casting defects on the inner surface of the pipe. These impurities and casting defects on the inner surface of the pipe can cause defects such as pinholes in the coating when epoxy resin powder coating is applied. For this reason, a polishing process is usually carried out to remove them before epoxy resin powder coating.

[0004] Patent Document 1 discloses a technique for reducing pinhole defects by rotating a grindstone for polishing the inner surface of a tube in the opposite direction to a brush for cleaning the inner surface. Patent Document 2 discloses a technique for reducing pinhole defects in a coating method using silica sand by changing the coating speed in the forward and backward directions when coating the inner surface of a tube with powder paint in the axial direction. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2018-161680 A [Patent Document 2] Patent Publication No. 2021-30133 Summary of the Invention [Problem to be solved by the invention]

[0006] However, for example, in the case of ductile iron pipes with large nominal diameters or due to differences in manufacturing conditions, the condition of the inner surface of the pipe, such as impurities and casting defects, is unlikely to be constant, and when the condition of the inner surface is poor, pinhole defects may occur even if the above-mentioned polishing and cleaning process and the technology described in the patent documents are used.

[0007] Therefore, an object of the present invention is to provide a method for coating the inner surface of a pipe, which can provide a cast iron pipe in which the occurrence of pinholes is further suppressed. [Means for solving the problem]

[0008] As a result of intensive research into solving the above problems, the inventors have discovered that when painting the inner surface of a cast iron pipe with an epoxy resin powder paint, the occurrence of pinholes in the coating on the inner surface of a cast iron pipe can be suppressed by performing the coating process in the axial direction of the pipe at least twice and setting the thickness of the coating film in the final coating process to be 2 to 3.5 times the thickness of the coating film formed before the final coating process, and have completed the present invention.

[0009] That is, the present invention is a method for coating the inner surface of a cast iron pipe with a powder paint, comprising the steps of: The method includes a first coating step and a second coating step for coating an inner surface of a cast iron pipe with at least a powder coating material, The second coating process is a final coating process in which the powder paint is applied to the inner surface of the cast iron pipe, and is carried out so that the thickness of the coating film formed by the second coating process is 2 to 3.5 times the thickness of the coating film by the powder paint formed before the second coating process. This relates to a method for coating the inner surface of a pipe. Effect of the Invention

[0010] According to the present invention, a method for painting the inner surface of a pipe can be provided, which can produce a cast iron pipe in which the occurrence of pinholes is further suppressed in a cast iron pipe in which a powder paint is applied to the inner surface of the cast iron pipe. [Brief description of the drawings]

[0011] [Figure 1]FIG. 1 is a schematic diagram for explaining one embodiment of a method for coating the inner surface of a cast iron pipe according to the present invention. [Diagram 2] FIG. 2 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. [Diagram 3] FIG. 2 is a diagram for explaining the surface state of a coating film obtained by one embodiment of the cast iron pipe inner surface coating method of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The method for coating the inner surface of a cast iron pipe of the present invention will be described below with reference to Figures 1 and 2. Figure 1 is a schematic diagram for explaining one embodiment of the method for coating the inner surface of a cast iron pipe of the present invention. Figure 2 is a schematic diagram for explaining the process of forming a coating film obtained by the method for coating the inner surface of a cast iron pipe of the present invention.

[0013] First, the structure of a pipe inner surface coating device (hereinafter, also simply referred to as a coating device) D used in the cast iron pipe inner surface coating method of this embodiment will be described with reference to Fig. 1. 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.

[0014] As shown in Fig. 1, a coating apparatus D of this embodiment includes a discharge device 1 that discharges powder coating material 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 section 1a, as described below. In this embodiment, the coating apparatus D also includes a movement mechanism 3 that moves the paint discharge section 1a along the pipe axial direction, as shown in Fig. 1.

[0015] In this embodiment, as shown in FIG. 1, the pipe body rotating device 2 includes a rotating roller 21 on which a preheated cast iron pipe (hereinafter, also simply referred to as a pipe body) P to be coated is placed and 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 the paint discharge part 1a moves along the pipe axial direction by the moving mechanism 3, so that the entire inner surface of the cast iron pipe P is coated. The shape of the cast iron pipe P is not particularly limited, but it can be, for example, a straight pipe with a nominal diameter of 50 to 2600, a receiving port 5 on one end side of the cast iron pipe P, and a spigot 6 on the other end side. The nominal diameter of the cast iron pipe P is not particularly limited, but is more preferably 900 to 2600. In the case of a cast iron pipe with a nominal diameter of 900 or more, the condition of the inner surface of the pipe before coating may be poor, and this is preferable because the effect of the pipe inner surface coating method of the present invention is easily exhibited.

[0016] The discharge device 1 is a device that discharges powder paint 4 and coats the inner surface of a cast iron pipe P. In this embodiment, as shown in Fig. 1, the discharge device 1 includes a paint supply mechanism 1c for supplying the powder paint 4 from a supply source of the powder paint 4 (not shown), a paint pipe 1b connected to the paint supply mechanism 1c and supplying the powder paint 4 to the paint discharge unit 1a, and the paint discharge unit 1a connected to the paint pipe 1b and discharging the powder paint 4.

[0017] The paint supply mechanism 1c is not particularly limited as long as it can supply the powder paint 4 to the paint discharge portion 1a.

[0018] The paint pipe 1b forms a passage for supplying the powder paint 4 to the paint discharge portion 1a. In this embodiment, the paint pipe 1b is provided between the paint supply mechanism 1c and the paint discharge portion 1a, and extends approximately parallel to the pipe axis of the cast iron pipe P.

[0019] The paint discharge part 1a is a part that discharges the powder paint 4. The structure of the paint discharge part 1a is not particularly limited as long as it is capable of discharging the powder paint 4, and a known nozzle used for discharging paint can be used.

[0020] The powder coating material 4 used in this embodiment can be any coating material that melts and hardens by heat after application to the cast iron pipe P. For example, in the case of a ductile iron pipe for water supply, an epoxy powder resin coating material that meets the standards is used. The gel time of the powder coating material 4 is preferably 80 to 120 seconds, more preferably 90 to 120 seconds, at 170°C.

[0021] The moving mechanism 3 is a mechanism for relatively moving the paint discharge part 1a and the cast iron pipe P in the axial direction of the cast iron pipe P. In this embodiment, the moving mechanism 3 is configured to move the paint discharge part 1a inside the cast iron pipe P along the axial direction, but the moving mechanism 3 may move the cast iron pipe P without moving the paint discharge part 1a, or may be configured to move the cast iron pipe P and the paint discharge part 1a. Although details will be described later, by relatively moving the cast iron pipe P and the paint discharge part 1a in the axial direction of the cast iron pipe P, the inner surface of the cast iron pipe P can be painted. In FIG. 1, the moving mechanism 3 is generally shown as a moving body moved by wheels, but the structure of the moving mechanism 3 is not particularly limited as long as it can move at least the paint discharge part 1a.

[0022] Next, one embodiment of the method for coating the inner surface of a cast iron pipe according to the present invention will be described with reference to Figures 1 and 2. In the following description, the above-mentioned coating device D will be used as an example, but coating devices of other structures may be used as long as they can achieve the same effect.

[0023] The method for coating the inner surface of a cast iron pipe P in this embodiment is a method for coating the inner surface of a cast iron pipe P with a powder paint 4, and includes at least a first coating process and a second coating process for coating the powder paint 4 onto the inner surface of the cast iron pipe, the second coating process being the final coating process for coating the powder paint 4 onto the inner surface of the cast iron pipe, and being carried out so that the thickness of the coating film formed by the second coating process is 2 to 3.5 times the thickness of the coating film made of powder paint 4 formed prior to the second coating process.

[0024] The method for coating the inner surface of a cast iron pipe P in this embodiment includes at least a first coating step and a second coating step for coating the inner surface of the cast iron pipe with the powder paint 4, and the second coating step is the final coating step for coating the inner surface of the cast iron pipe with the powder paint 4. In other words, additional coating steps may be included before or after the first coating step, and after all of these coating steps, the second coating step is carried out to complete the coating film on the inner surface of the pipe.

[0025] The cast iron pipe P to be coated on the inner surface according to this embodiment is not particularly limited by its casting method, and may be centrifugal cast by metal mold casting, resin sand casting, or heavy coating casting, but it is preferable to use a pipe manufactured by resin sand casting or heavy coating casting and cooled at room temperature, since the effect of the present invention can be more effectively achieved. It is preferable that the cast iron pipe P is subjected to a treatment such as polishing on the inner surface prior to coating the inner surface. It is also preferable that the coating steps 1 and 2 are performed after heating the pipe body to a temperature of 220 to 250°C in a heating furnace.

[0026] Each coating process is a process of powder coating the inner surface of the cast iron pipe P from one end side to the other end side of the cast iron pipe P. The coating direction is not particularly limited, and coating may be performed from the socket 5 side to the insertion port 6 side of the cast iron pipe P (direction →A in Figs. 1 and 2), or from the insertion port 6 side to the socket 5 side (direction →B in Fig. 1). In other words, the powder coating material 4 is applied to the inner surface of the cast iron pipe P along the pipe axial direction.

[0027] In this specification, the direction →A is also referred to as the "outgoing path" and means the coating direction in which the discharge part 1a that discharges the powder paint 4 is inserted into the inside of the cast iron pipe P from one end in the pipe axial direction and proceeds to the other end to paint. Specifically, in this embodiment, the paint pipe 1b is inserted into the cast iron pipe P to be coated from the opening on one end side (the receiving port 5 side) of the cast iron pipe P, and then the discharge of the powder paint 4 from the tip (paint discharge part 1a) of the paint pipe 1b is started just before the area R on the inner surface of the pipe that requires powder coating. In accordance with the discharge of the powder paint 4, the paint discharge part 1a is moved in the direction →A by the moving mechanism 3 over the area R that requires powder coating, so that the cast iron pipe P is painted while proceeding from one end side to the other end side. Also, when installing a dust collector (not shown) that is generally used when powder coating the inner surface of a pipe, it is usually installed on the other end side (the insertion port 6 side in FIG. 1) opposite to the one end side in the axial direction of the cast iron pipe P where the coater (discharge device 1) is placed, so the "outbound path" can also be said to be the direction toward the dust collector. As mentioned above, of course, in this case, the relative positional movement between the cast iron pipe P and the discharge part 1a can also be achieved by moving the cast iron pipe P instead of the moving mechanism 3.

[0028] On the other hand, in this specification, the direction of →B is also called the "returning path" and means a coating direction in which the discharge part 1a, which discharges the powder paint 4 that has passed through the inside of the pipe from one end in the pipe axis direction of the cast iron pipe P and moved to the outside of the pipe from the other end side in the pipe axis direction of the cast iron pipe P, returns to the inside of the pipe from the other end in the pipe axis direction of the cast iron pipe P and paints while retreating to one end. Specifically, in this embodiment, the powder paint 4 is discharged from the discharge part 1a from the opening on the other end side (insertion port 6 side) of the cast iron pipe P and pulled back into the pipe, and the powder coating required area R on the inner surface of the pipe is painted while the discharge part 1a is retreated. After that, the discharge of the paint 4 is stopped at a point where the paint discharge part 1a has passed the coating required area R (within the receiving port area), and the paint pipe 1b (including the paint discharge part 1a) is pulled out from the cast iron pipe P to be coated. As described above, in this case, the relative positional movement between the cast iron pipe P and the discharge part 1a can also be performed by moving the cast iron pipe P instead of the moving mechanism 3.

[0029] In this embodiment, the second coating step is carried out so that the thickness of the coating film formed by the second coating step is 2 to 3.5 times, and preferably 2.5 to 3.5 times, the thickness of the coating film by the powder paint 4 formed before the second coating step. By setting the thickness ratio at such a ratio, pinhole defects are reduced.

[0030] When adjusting the film thickness in the above-mentioned second coating step to be 2 to 3.5 times the film thickness obtained in the previous coating step, this can be appropriately carried out by adjusting either the paint discharge amount or the coating speed, or both.

[0031] The coating speed in each coating step is preferably 0.6 to 10 m / min, more preferably 1.0 to 8.5 m / min. If the coating speed is less than 0.6 m / min, the temperature of the pipe drops during coating, causing poor adhesion of the coating film, and the target physical properties of the finished powder coating film (hardness, adhesion, finish state of the coating film surface, etc.) may not be obtained. In one embodiment, if the discharge amount is not changed, the second coating step is performed at a coating speed of 1 / 2 to 2 / 7 of the coating speed in the first coating step, preferably 2 / 5 to 2 / 7.

[0032] In one embodiment, as shown in FIG. 2, regardless of the coating direction of the first coating step, the second coating step is performed in the "forward pass". In this case, the second coating step, which is the final coating step, is performed in the forward pass, and coating is advanced in the direction of travel of the discharge portion 1a, thereby improving the dust-like appearance defect such as roughness on the inner surface of the pipe, and a cast iron pipe P with an improved appearance state without roughness can be obtained. In the case of a cast iron pipe P with a large nominal diameter where a large amount of powder coating material 4 is applied, for example, a cast iron pipe P with a nominal diameter of 900 or more, if "backward pass" coating is performed while coating the discharge portion 1a while retracting, a part of the applied powder coating material 4 tends to remain unmelted, which may cause the surface to become rough and cause dust-like appearance defects, so "forward pass" coating is more preferable.

[0033] On the other hand, in another embodiment, the first coating step is performed on the "outbound" route, and the second coating step is performed on the "return" route. In this case, the coating is completed in one round trip, which saves time and labor, and is preferable from the standpoint of cost.

[0034] In one embodiment, the final film thickness of the coating film made of the powder coating material 4 is preferably 300 to 1000 μm, and more preferably 450 to 850 μm. The amount of the powder coating material 4 applied can be appropriately set in consideration of the final film thickness and the coating speed. EXAMPLES

[0035] Examples 1 to 4 and Comparative Examples 1 to 5 The pipe to be coated was a ductile iron pipe with a nominal diameter of 1650 and a length of 4 m, manufactured by centrifugal casting of resin sand casting. The rust on the inner surface of the pipe to be coated was polished with a grindstone and cleaned with a wire brush, then it was heated in a gas furnace set at 270 to 330 °C, placed on a pipe body rotating device, and coating was started at 240 °C (± 10 °C) (rotation speed: 160 m / min (30 rpm)). The discharge device was placed on the receiving port side, and the first and second coating processes were carried out in the order shown in Table 1 in the coating direction. Here, the "forward pass" coating was performed by inserting the discharge part of the paint pipe from the receiving port side, starting to discharge within the receiving port area, and proceeding at a constant speed to the insertion port side until it passed through the insertion port, and the "return pass" coating was performed by once the discharge part left the insertion port side and went outside the pipe, then turning back in the discharged state, and retreating at a constant speed from the insertion port side to the receiving port side until it entered the receiving port area. Other coating conditions are as shown in Table 1.

[0036] The paint used was an epoxy powder resin paint (gel time: 90 seconds) that is used for painting the inside of ordinary ductile iron water pipes. An injector-type powder coating device was used to discharge the powder paint, and the coating thickness on the inside of the pipe was adjusted to 600 μm by painting in the first and second coating processes. The discharge amount in each example and comparative example was set at a constant value of about 4000 to 4500 g / 30 s.

[0037] [Table 1]

[0038] Test example 1: Pinhole inspection The inner surface of the coating film of the cast iron pipes (receptacle 5 portion) obtained in Examples 1 to 4 and Comparative Examples 1 to 5 was evaluated using a Holiday detector with a brass wire brush probe at a voltage of 1000V (JWWA G 112 Epoxy resin powder coating on the inner surface of ductile cast iron pipes for water supply) according to the following criteria. The target performance was indicated by ◎ or ○, and △ or × was deemed to be non-compliant. The results are shown in Table 1. ◎: No pinholes ○: 5 or less places that need to be repaired due to pinholes △: 19 or fewer places that need to be repaired due to pinholes ×: 20 or more places needing repair due to pinholes

[0039] Test Example 2: Appearance Evaluation The surfaces of the coatings on the inner surfaces of the cast iron pipes (the portions of the receiving ports 5) obtained in Examples 1 to 4 and Comparative Examples 1 to 5 were visually evaluated according to the following criteria. The results are shown in Table 1. Photographs of the surface conditions of the coatings on the inner surfaces of the cast iron pipes corresponding to these evaluations are shown in FIG. 3. ○: No roughness and smooth ×: Dusty roughness occurs

[0040] From the above, it can be seen that the occurrence of pinholes is suppressed in Examples 1 to 4, in which the thickness of the coating film obtained by the second coating process is 2 to 3 times that of the coating process prior to the second coating process. Also, it can be seen that roughness is suppressed in Examples 2 and 4, in which the second coating process is the forward path, and a ductile iron pipe having a powder coating film on the inner surface of the pipe with a good appearance is obtained.

[0041] (summary) (1) A method for coating the inner surface of a cast iron pipe with a powder paint, comprising the steps of: The method includes a first coating step and a second coating step for coating an inner surface of a cast iron pipe with at least a powder coating material, The second coating process is a final coating process in which the powder paint is applied to the inner surface of the cast iron pipe, and is carried out so that the thickness of the coating film formed by the second coating process is 2 to 3.5 times the thickness of the coating film by the powder paint formed before the second coating process. Pipe inner surface painting method.

[0042] According to the above-mentioned pipe inner surface coating method (1), it is possible to provide a method for coating the inner surface of a pipe that can provide a cast iron pipe in which the occurrence of pinholes is further suppressed.

[0043] (2) In the pipe inner surface painting method described above in (1), the second painting step is preferably carried out by inserting an ejection part for ejecting the powder paint into the inside of the cast iron pipe from one end in the pipe axial direction and proceeding to the other end to paint.

[0044] According to the above-mentioned pipe inner surface coating method (2), it is possible to obtain a cast iron pipe having a good appearance with a coated surface free from dusty roughness.

[0045] (3) In the above-mentioned pipe inner surface coating method (1) or (2), the second coating step is preferably carried out so that the thickness of the coating film formed by the second coating step is 2.5 to 3.5 times the thickness of the coating film of the powder paint formed before the second coating step.

[0046] According to the above-mentioned pipe inner surface coating method (3), the occurrence of pinholes can be suppressed more effectively.

[0047] (4) In the pipe inner surface coating method according to any one of the above (1) to (3), the second coating step is preferably carried out at a coating speed that is 1 / 2 to 2 / 7 of the coating speed of the first coating step.

[0048] According to the pipe inner surface coating method (4) above, there is no need to change the discharge amount between the first and second coating steps, and the film thickness in each coating step can be easily changed. [Explanation of symbols]

[0049] D. Painting equipment A Outbound B Return journey 1 Discharge device 1a Paint discharge part 1b Paint piping 1c Paint supply mechanism 2. Tube Rotation Device 21 Rotating roller 3 Moving mechanism P Cast Iron Pipe 4 Paint 5 socket 6 Insertion port

Claims

1. A method for coating the inner surface of a cast iron pipe with a powder paint, comprising the steps of: The method includes a first coating step and a second coating step of coating an inner surface of a cast iron pipe with at least a powder coating material, The second coating process is a final coating process in which the powder paint is applied to the inner surface of the cast iron pipe, and is carried out so that the thickness of the coating film formed by the second coating process is 2 to 3.5 times the thickness of the coating film by the powder paint formed before the second coating process. Pipe inner surface painting method.

2. 2. The pipe inner surface painting method according to claim 1, wherein the second painting step is carried out by inserting an ejection section for ejecting the powder paint into the inside of the pipe from one end in the pipe axial direction of the cast iron pipe and proceeding to the other end while painting.

3. The pipe inner surface coating method according to claim 1 or 2, wherein the second coating process is carried out so that the thickness of the coating film formed by the second coating process is 2.5 to 3.5 times the thickness of the coating film of the powder paint formed before the second coating process.

4. 3. The method for coating the inner surface of a pipe according to claim 1, wherein the second coating step is carried out at a coating speed which is 1 / 2 to 2 / 7 of the coating speed of the first coating step.

5. 4. The method for coating the inner surface of a pipe according to claim 3, wherein the second coating step is carried out at a coating speed which is 1 / 2 to 2 / 7 of the coating speed of the first coating step.

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