JPS6028861A - Powder coating method - Google Patents

Abstract

PURPOSE:To perform glazing of powder paint and curing of coated film at optimum temp. by performing heating of material to be coated prior to glazing by induction heating at low frequency and performing at high frequency after glazing. CONSTITUTION:An induction heating apparatus 2 for preheating at 50-500 cycle is provided to the fore position of a booth 1 in a painting line, and an induction heating apparatus 3 for post heating at 500-1,000 cycle is provided to the hind position of the booth 1. And, the heating of the object to be heated prior to glazing is performed with the induction heating apparatus 2, and the heating after glazing is performed with the induction heating apparatus 3. In this way, the temp. is controlled optimally and curing of paint film is performed.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a one-piece powder coating method in which the temperature of the object to be coated is raised by induction heating and the paint is baked.

Conventionally, induction heating has been used as a heating means for an object to be coated when performing powder coating. When heating an object to be coated, such as a steel pipe, by induction heating, it is known that a temperature difference occurs between the front and back surfaces of the steel pipe due to skin action. Even with heating, there was no effect of temperature difference between the front and back surfaces of the steel pipe due to skin action. However, if the steel pipe is thick, there will be a noticeable temperature difference between the front and back surfaces of the steel pipe due to skin action, and if induction heating is performed at a relatively high frequency, the temperature on the outside surface of the steel pipe will rise excessively. This causes the inconvenience that the surface of the steel pipe, which has been previously rust-removed by shot blasting or the like, is oxidized.

On the other hand, when curing a coating film formed on the surface of a steel pipe by melting powder, it is necessary to efficiently raise the temperature on the surface side of the steel pipe on which the coating film is formed, so induction heating at a relatively high frequency is used. There is a need. However, even in this case, if the frequency is excessively high, there is a risk that the surface of the coating film will locally exceed the allowable temperature limit.

This invention has been made in view of the above-mentioned conventional circumstances, and is a powder coating method that allows baking of the powder coating and keying of the coating film at an optimum temperature regardless of the thickness of the object to be coated. The purpose is to provide

That is, the powder coating method of the present invention is a powder coating method in which the temperature of the object to be coated is raised by induction heating and the powder coating is baked, and the temperature of the object to be coated is 50 to 50℃ before baking the powder coating.
It is characterized in that the powder coating is baked by low frequency induction heating for 0 cycles, and the object to be coated is heated by high frequency induction heating for 500 to 1000 cycles.

This invention will be explained in more detail below.

FIG. 1 shows an example of a painting line used to carry out the present invention. In the painting line shown in the figure, the front position 1 of the painting booth 1 is
An induction heating device 2 with a frequency of 50 to 500 cycles for pre-heating is arranged at d, and an induction heating device 3 with a frequency of 500 to 1000 cycles for post-heating is arranged at the rear of the painting booth 1. A cooling device 4 is arranged behind the device 3.

The steel pipe 5, which has been previously rust-removed by spot blasting, passes through an induction heating device 2, a coating booth 1, an induction heating device 3, and a cooling device 4 in that order while being skew-rotated by a skew roller 6.

In the present invention, powder coating is performed in the following manner using the above-described coating line.

First, the steel pipe 5 to be coated is heated in the induction heating device 2 at 50°C.
The temperature is raised by low frequency induction heating for ~500 cycles. If the frequency is less than 50 cycles, the temperature of the steel pipe 5 cannot be raised to a temperature sufficient to melt the powder coating, and conversely, if the frequency exceeds 500 cycles, the outer surface of the steel pipe 5 may become hotter than necessary. It has the disadvantage that it oxidizes when the temperature rises. This point will be explained in more detail using a specific example. Figure 2 shows an outer diameter of 24 inches and a wall thickness of 0.7 inches.
The temperature increase curve is shown when a 5-inch steel pipe is heated to 230° C. by induction heating while being conveyed at 1.34 m/min. A-C in the figure shows the temperature rise curve of the outer surface of the steel pipe, and a
"-" c indicates the temperature rise curve of the inner surface of the steel pipe. Also,
In the figure, A and a indicate that the current frequency in induction heating is 10.
In the case of 00 cycles, B and b in the figure also indicate the case of 500 cycles, and C and C in the figure also indicate the case of 300 cycles.

As shown in the figure, in both cases, at the part where the steel pipe separates from the heating coil (after point 0 in the figure), there is no temperature difference between the outer surface of the steel pipe and the inner surface of the steel pipe, and equilibrium is reached at 9.230°C. . On the other hand, in the portion inside the heating coil of the steel pipe (before point 0 in the figure), a temperature difference occurs between the outer surface of the steel pipe and the inner surface of the steel pipe, and the temperature rise on the outer surface of the steel pipe becomes significant. However, when the induction heating frequency is 500 cycles and 300 cycles, the temperature on the outer surface of the steel pipe only reaches a maximum of 290°C and 280°C, as shown by the temperature rise curves B and C in the figure.
When the frequency of induction heating is 1000 cycles, the temperature on the outer surface of the steel pipe reaches a maximum of 420°C, as shown by the temperature rise curve A in the figure.
℃, and the surface begins to oxidize. From the above, when a steel pipe is induction heated, a temperature difference occurs between the outer surface of the steel pipe and the inner surface of the steel pipe, and this temperature difference becomes more pronounced as the frequency of induction heating increases.In that case, the temperature of the outer surface of the steel pipe becomes excessively high. I can see that it will happen.

FIG. 3 shows the temperature of a steel pipe similar to that shown in FIG. 2 being raised in the same manner, with various induction heating frequencies set, and the maximum temperature of the outer surface of the steel pipe measured for each. As shown in the figure, when the frequency exceeds 600 cycles, the surface temperature suddenly increases and begins to exceed 300° C., which can cause the material of the steel pipe to change or oxidize. Therefore, as is clear from FIG. 3, it is preferable that the frequency of induction heating for the object to be coated before painting is 500 cycles or less.

Next, in this invention, the object to be coated after baking the powder coating as described above is subjected to induction heating at a high frequency of 500 to 1000 cycles. If the number of cycles is less than 500, the temperature rise on the outer surface of the steel pipe is slow, as is clear from Fig. 3, and therefore it is impossible to efficiently raise the temperature on the outer surface of the steel pipe and to key the coating film. Can not.

However, on the contrary, the frequency of the current that performs induction heating is 1000
If the cycle is exceeded, temperature control becomes difficult as the temperature on the outer surface of the steel sheet rises rapidly and the coating film becomes overheated and begins to deteriorate.

Next, examples of this invention will be described.

Example: A steel pipe with a diameter of 24 inches and a wall thickness of 0.75 inches was coated on the coating line shown in FIG. The induction heating frequency before painting was 300 cycles, and the induction heating frequency after painting was 1000 cycles.

Comparative Example Coating was carried out in the same manner as in the example, with a pre-coating heating frequency of 1000 cycles and a post-coating heating frequency of 300 cycles.

FIG. 4 shows temperature rise curves on the outer surface of the steel pipe in Examples and Comparative Examples. The curve indicated by A in the figure is for the example, and the curve indicated by B in the figure is for the comparative example. Point (a) in the figure shows the temperature when the steel pipe leaves the preheating furnace, point (b) in the figure is the temperature at the start of painting, and point (c) is the temperature at the start of post-heating.
The dots indicate the temperature at the exit side of the post-heating furnace. As shown in the figure, in the case of the comparative example, the temperature rise at point (a) was excessive and the outer surface of the steel pipe was actually oxidized and discolored to a light blue color.

In addition, the coating films of the steel pipes obtained in the Examples and Comparative Examples were subjected to a cathodic peel test (A8TM G-8), a boiling water cycle test (DIN 53151), an impact test, and a bending test (DIN 30671) to examine their performance. .

The results are shown in Table 1.

Table 1 Performance test results Cathode peel test
60 with a voltage of 1.5 V applied in NaC6 solution.
The sample was left to stand for several days, and the peeling length of the coating film around the drill hole was measured.

Boiling water cycle test: A test piece with diagonal slits cut into the coating surface was immersed in boiling water for 20 hours and then left at room temperature for 4 hours.The process was repeated 20 times, and a peel test was performed to remove the squares where the coating remained. I counted the numbers.

Flexibility test: The limit angle α at which cracks occur on the coating surface was determined.

As shown in Table 1, in all tests, the results of the Examples were better than those of the Comparative Examples, and especially in the cathode peeling test, the results of the Examples were much better. It has become.

As described above, according to the present invention, powder coating by induction heating can be performed by optimizing the heating temperature before and after painting of the object to be coated, and oxidation of the outer surface of the object to be coated can be prevented.
A paint film with good mechanical and chemical properties can be obtained.

[BRIEF DESCRIPTION OF THE DRAWINGS] Figure 1 shows an example of a painting line used to carry out the present invention, Figure 2 shows a temperature rise curve when steel pipes are heated by induction, and Figure 3 shows an example of a painting line used to carry out the present invention. FIG. 4 is a diagram showing the relationship between the heating frequency and the temperature reached on the outer surface of the steel pipe, and FIG. 4 is a diagram showing the temperature increase curve of the outer surface of the steel pipe in Examples and Comparative Examples. 1...Painting booth, 2...Induction heating device, 3...
induction heating device, 4... cooling device, 5... steel pipe, 6.
...Ski-rolla. Applicant: Kawasaki Steel Co., Ltd. Agent: Taketo Toyota, patent attorney (and one other person)

Claims (1)

[Claims] In a powder coating method in which the temperature of the object to be coated is raised by induction heating and the powder coating is baked, the temperature of the object to be coated before baking the powder coating is 50 to 500? The powder coating is baked using low-frequency induction heating, and the subsequent heating of the object to be coated is performed at
A powder coating method characterized by performing 1000 cycles of high-frequency induction heating.