JPS6059027B2 - How to paint the inner surface of metal pipes - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a method for coating the inner surface of a metal tube with a powder coating.

There are several methods for applying powder coatings, such as fluidized dipping, thermal spraying, electrostatic powder coating, and compressed air spraying, but electrostatic powder coating is the most suitable method for painting the inner surface of metal pipes. method and compressed air blowing method.

To apply powder coating to the inner surface of a metal tube by air spraying, a long arm inserted into the metal tube is supported on a trolley, a powder paint spray nozzle is attached to the tip of this arm, and the metal tube to be coated is coated with powder. A method is used in which the powder coating is heated above its melting point, the metal tube is rotated, and the nozzle is moved into the tube from one end of the tube to the other by the running of a cart, and the nozzle is compressed.・One゛”〜π−＾1,L】upper★eightL
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It is fused due to the high temperature of the tube itself.

According to this air spraying method, when painting the inner surface of both ends of the tube, a relatively large amount of the powder paint sprayed from the nozzle escapes from the opening at the end of the tube, so that the thickness of the coating on the inner surface of both ends of the tube is the same as that of the inner surface of the middle part. There are disadvantages when the coating is thinner than the coating thickness of .

On the other hand, in order to apply powder coating to the inner surface of a metal tube using the electrostatic coating method, an electrostatic coating gun is attached to the tip of a long arm supported on a trolley in the same way as above, and the metal tube is heated and rotated in the same manner as above. , a method is used in which the gun is moved from one end of the tube to the other end of the tube by running the cart. With this method, the powder paint flies toward the inner surface of the tube along the electrostatic field and adheres to the inner surface of the tube, so there is no disadvantage of reducing the coating thickness on the inner surface of both ends of the tube as in the case of the above-mentioned spraying method. Rather, since the electrostatic field becomes stronger at both ends of the tube due to the edge effect of the tube, the coating thickness on the inner surfaces of both ends of the tube can be increased.

This is advantageous considering that corrosion protection at both ends of the pipe is particularly important during storage of the pipe. However, in the above-mentioned electrostatic painting, in order to achieve electrostatic atomization and ensure a specified electric field strength near the inner surface of the tube, depending on the type of paint and the diameter of the tube, a considerably high voltage (90,000 volts) is required. (above)) may be unavoidable, and if (a) is used, there is a great risk of sparks.

If a spark occurs in the middle of the pipe, the middle part of the pipe is closer to a closed room than both ends of the pipe, so there is a problem that the air shock when the spark occurs is severe and the painted surface may be damaged. As mentioned above, both electrostatic powder coating and air spray powder coating of the inner surface of a tube have advantages and disadvantages.

The method for coating the inner surface of a metal tube according to the present invention was invented in view of the above points, and the inner surface of both ends of the metal tube is coated by electrostatic powder coating, and then the inner surface of the metal tube is coated. This method is characterized by painting by spraying powder paint.

Hereinafter, the present invention will be explained in connection with coating the inner surface of a cast iron pipe.

FIG. 1 shows a cast iron pipe whose inner surface is coated according to the present invention.
The inner surface of the tube to be painted according to the present invention is the portion A excluding the inner surface of the socket.

In carrying out the present invention, the entire inner surface of the cast iron pipe is treated with a wire brush, then dry air is blown to remove dust, rust, etc. that are harmful to the coating, and after that, the cast iron pipe is processed. The heated cast iron pipe is heated to 270 to 300°C in a furnace and placed on rollers.

Next, the heated cast iron pipe is rotated by the drive of a roller, and powder is applied to the inner surface part B of the pipe adjacent to the socket and the inner surface part C of the socket (width of each part is 200 to 400 TWL) using an electrostatic pan 3 gun. Apply body paint, such as epoxy resin powder.
This electrostatic coating is carried out in a short time of 20 to 4 times. After electrostatically painting the specified inner surfaces of both ends of the tube in this way,
As shown in Fig. 2, the cart 1 of the powder coating spraying device S that has been arranged in advance is moved, or the cart P with rollers on which the cast iron pipe is placed is moved. Powder paint spray nozzle 2 at the end of the long arm is installed inside the heated cast iron pipe of
and spray paint from a nozzle, for example, epoxy resin 4F powder, onto the inner surface of the tube.

Coating by this spraying method is performed over the electrostatic coating film, and the time required for spraying is a short time of 1 to 4 minutes, depending on the air cooling rate of the cast iron pipe. After spray painting as described above, powder paint, such as epoxy resin powder, is applied to the inner surface of the socket using the electrostatic handgun, and then heated to a temperature of about 200°C in a heating furnace. A post-heat treatment is performed to sufficiently harden the coating film, thereby completing the coating work.

As described above, in the powder coating method for the inner surface of a metal tube according to the present invention, the inner surface of both ends of the tube is coated by electrostatic powder coating, and then the powder coating is applied to the inner surface of the tube over the inner surfaces of both ends. Since the coating is applied using a spraying method, even though the coating film on the inner surface of both ends of the tube is thinner due to the spraying method, the reduction in the thickness is
This can be compensated with an electrostatic coating film, and the entire inner surface of the tube can be coated with a sufficient coating thickness.

In addition, the coating on the inner surface of both ends of the pipe, which requires special protection against rusting and damage during storage, is done using electrostatic coating that meets these requirements, but the distance between the electrostatic gun and the painted surface The work is safe because it can be done with a spark-free hand gun that can generate a strong electric field even under relatively low voltage (60,000 volts or less).

Even if sparks occur, the interior of both ends of the tube is a completely open space with good ventilation, so severe air impact can be avoided and roughening of the coating film can be prevented.

Table 1 shows the results of pinholes in the coating film on the inner surface of the center part and the inner surface of the pipe end when the inner surface was coated according to the present invention, and Table 2 shows the pinhole results in the case of the conventional example (air spraying of powder coating). The same pinhole results as above are shown, and according to the present invention, pinholes were completely eliminated.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing a cast iron pipe coated by the inner surface coating method according to the present invention, and FIG. 2 is an explanatory diagram showing a coating apparatus used in the present invention.

Claims (1)

[Claims] 1. A method for painting the inner surface of a metal tube, which comprises painting the inner surface of both ends of the metal tube by electrostatic powder coating, and then painting the inner surface of the metal tube by spraying powder paint.