JPH03274300A - Electrodeposition coating method - Google Patents

Abstract

PURPOSE:To reduce the variance in the thickness of an electrodeposited film on a coated surface and to prevent defective coating by controlling the impressed voltage so that the current flowing between a work and an electrode is uniformized and then carrying out electrodeposition coating. CONSTITUTION:An electrode 1 is arranged on the left and right sides of an electrodeposition cell 2 and at its bottom, and connected to the plus side of a feeder circuit, a car body W is connected to the minus side, and a DC voltage is impressed to coat the body W. At this time, the impressed voltage is controlled by a control circuit 6 so that the current flowing between the body W and the electrode 1 is controlled to a set value. When the impressed voltage reaches the set value, coating is carried out at that voltage. Meanwhile, coating is carried out while rotating the body W around the longitudinal axis of the body W. Consequently, the variance in the thickness of the coating film at any part of the body W is drastically reduced, and the deposition of paint at the gap in the form of a bridge is prevented.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to an electrodeposition method in which a workpiece is immersed in an electrodeposition tank and a DC voltage is applied between the electrodes in the tank and the workpiece to form a coating film. Regarding the coating method.

(Prior Art) Conventionally, electrodeposition coating is performed by applying a constant voltage between a workpiece and an electrode (Japanese Patent Publication No. 5B-33320).

(Problems to be Solved by the Invention) In the conventional method described above, a large current flows as shown by the dashed line in FIG.
Because the paint adheres rapidly, the thickness of the electrodeposited film tends to vary between parts of the workpiece near and far from the electrode, and paint bridges occur in gaps such as between the car body and the door, as shown in Figure 7. This may cause paint defects that adhere to the surface.

The present invention has been made in view of such problems, and
The purpose is to provide an electrodeposition coating method that can prevent variations in coating thickness and coating defects in gaps.

(Means for Solving the Problems) In order to achieve the above object, in the present invention, a workpiece is immersed in an electrodeposition bath, and a DC voltage is applied between the electrodes in the bath and the workpiece to form a coating film. Electrodeposition coating equipment performs electrodeposition coating while controlling the applied voltage so that the current value flowing between the workpiece and the electrode reaches the set value, and after the applied voltage reaches the predetermined value, this voltage value I left it as is and applied electrodeposition coating.

(Function) Since electrodeposition coating is performed by controlling the applied voltage so that the current value flowing between the workpiece and the electrode is constant, the paint gradually adheres from the time the current is applied, causing the coating surface to deteriorate. Variations in the thickness of the electrodeposited film are reduced, and coating defects in the gap areas can also be prevented.

When the electrodeposited film becomes thick and the resistance value increases, the applied voltage for maintaining the set current value may exceed the coating breakdown voltage value.

In this case, if the set current value is set relatively low so that the applied voltage does not exceed the coating film breakdown voltage value, the coating time will increase and productivity will decrease. By performing electrodeposition coating at this voltage value after reaching the voltage value, the coating time will be shortened without causing damage to the coating film.

When the workpiece is a car body, it is relatively large, so the distance to the electrode placed on the side of the car body and the roof is large (this tends to cause variations in coating thickness, but it is difficult to move the car body in the longitudinal direction). By rotating around the axis, the distance to the electrodes on the side of the car body and roof becomes uniform over time, and in combination with the gradual deposition of paint as described above, the electrodeposition coating has a uniform thickness. You can get

(Example) Fig. 1 is a schematic explanatory diagram of an electrodeposition coating apparatus for carrying out the electrodeposition coating method of the present invention, and (1) is a diagram showing the electrodeposition coating apparatus arranged on both left and right sides and at the bottom of the electrodeposition tank (2). electrode, (3) is the power supply circuit,
(4> is a thyristor that constitutes the AC/DC converter installed in the power supply circuit (3), (5) is a smoothing circuit, and the positive side of the power supply circuit (3) is connected to the electrode (1) and the negative side. is connected to the vehicle body W which is the workpiece.

(6) is a control circuit that controls the voltage applied between the electrode (1) and the length of the vehicle body W, and the control circuit (6) is connected to the thyristor (4).
> a gate drive circuit (7) that outputs a control signal to the
A current detector (8) that detects the current of the power supply circuit (3), a voltage detector (9) that detects the voltage thereof, and a current setter (
a current error amplifier (11) that amplifies the difference between the signal from the current detector (8) and the signal from the voltage limit setter (12); A voltage error amplifier (13) that amplifies the difference between the voltage error amplifier (13) and the output signals from the gate width amplifiers (11) and (18) are inputted, and the signal with the lower level is selected to drive the gate drive circuit (
The comparison and selection circuit 14 outputs an operation signal to the circuit 7.

Output current from current detector (8). That is, a current feedback signal I' proportional to the current flowing between the electrode (1) and the vehicle body W is outputted, a voltage feedback signal V proportional to the output voltage Vo is outputted from the voltage detector (9), and the current The difference E1 between the setting signal 1 of the setting device (io) and the current feedback signal generator is amplified by a current error amplifier 11>, and a current operation signal U is output from the amplifier (H).

On the other hand, the difference E between the setting signal V of the voltage limit setter (12) and the voltage feedback signal V.

is amplified by a voltage error amplifier (13), and a voltage operation signal U is output from the amplifier (13).

Each of the operation signals U, U, is input to a comparison and selection circuit (14), and the one with the lower level is selected and output as the operation signal U to the gate drive circuit (7).

Therefore, as shown in the characteristic diagram shown in Figure 3, immediately after the start of energization,
Since the resistance,R,of the painted object is smaller than,V,/I,, the output is controlled,equal to the set current value, becoming 1,′,■,, and Ul.
is a relatively small value. At this time, the output voltage is smaller than the limit setting value (, V, > V, so Uv becomes a large value, U, > U-, and Ul is selected as the manipulated variable, and from now on, the output current is The output voltage is controlled to reach the set current value.

As the electrodeposited coating becomes thicker and the resistance R of the coated object increases, the output voltage increases and approaches the limit setting value, U decreases and eventually becomes equal to U, and as the resistance R increases further, the output voltage increases. Voltage Vo becomes equal to the limit setting value and the output current value begins to decrease. At this time, I becomes >1 and U increases rapidly, and U+ becomes >U and Uv is selected as the manipulated variable,
Thereafter, the output voltage v0 is controlled to the limit setting value.

A car body W was electrodeposited using the method of the present invention.

That is, NiniBlime 5008 manufactured by Nippon Paint Co., Ltd.
Using MP electrodeposition paint, current was applied for 170 seconds at a liquid temperature of 30° C. with a set current value of 50 OA and a limit voltage value of 5 oov.

Then, the coating film thickness of each part of the car body indicated by A-J in FIG. 4 was measured for the car body coated by the method of the present invention and the car body coated by the conventional method, that is, by applying electricity for 180 seconds at an applied voltage of 280 V. In this case, the current application time was adjusted so that the amount of coulombs was the same between the method of the present invention and the conventional method. Furthermore, J.
indicates the floor frame of the vehicle body W.

As a result, a good coating film was obtained without causing a coating defect in which the coating material adhered in the form of a bridge in the gap between the vehicle body W and the door, as shown in the second figure. Further, as shown in Figure 5, the method according to the present invention indicated by the solid line greatly reduced the variation in coating film thickness at various parts of the vehicle body compared to the method according to the conventional method indicated by the two-dot chain line.

Moreover, the difference between the film thickness of the car body to which the thickest electrodeposition was applied and the film thickness of the car body to which the car body was electrodeposited the thinnest, that is, the width of variation in the coating film thickness from car body to car body, was significantly reduced.

Further, electrodeposition coating was performed by the method of the present invention while rotating the vehicle body W around the axis in the longitudinal direction of the vehicle at one revolution per minute. As a result, an electrodeposited coating film with a substantially uniform thickness was obtained as shown by the solid line in FIG. Furthermore, the variation in coating film thickness from car body to car body was also significantly reduced compared to when the car body was rotated and electro-deposition was applied using the conventional method, as shown by the two-dot chain line in the figure.

(Effects of the Invention) As described above, according to the invention of claim 1, it is possible to significantly reduce the variation in the coating film thickness in each part of the workpiece, and also to prevent coating defects such as bridge-like coating of the paint in the gaps and damage to the coating film. According to the second aspect of the invention, an electrodeposited coating film having a substantially uniform thickness can be obtained.

[Brief explanation of drawings]

Figure 1 shows one of the electrodeposition coating apparatuses for carrying out the method of the present invention.
A schematic explanatory diagram including a power supply circuit showing an example, FIG. 2 is a diagram showing the coating state of the gap part of the car body, FIG. 3 is a characteristic diagram of current and voltage in electrodeposition coating by the method of the present invention, and FIG. Figure 5 is a developed view of the car body showing the locations where the film thickness was measured, and Figure 6 is a diagram showing the film thickness of each part when electrodeposition coating was performed using the method of the present invention and the conventional method without rotating the car body. Figure 7 is a diagram showing the film thickness of each part when electrodeposition coating is performed while rotating the car body, and Figure 7 is a diagram showing the state of painting of the gap parts of the car body using the conventional method.
FIG. 8 is a characteristic diagram of current and voltage in electrodeposition coating using a conventional method. W...Car body (work) (1)...Electrode (2)...Electrodeposition tank Patent application Attorney: Akira Kitamura, Honda Motor Co., Ltd. - (3 others Figure 4) Figure 3 Figure 8F Time Time Figure 5 Figure 6

Claims (1)

[Claims] 1. In an electrodeposition coating method in which a workpiece is immersed in an electrodeposition tank and a DC voltage is applied between the electrode in the tank and the workpiece to form a coating film, the connection between the workpiece and the electrode is Electrodeposition coating is performed while controlling the applied voltage so that the current value flowing between them is the set value,
An electrodeposition coating method characterized in that after the applied voltage reaches a predetermined value, electrodeposition coating is performed while maintaining this voltage value. 2. The electrodeposition coating method according to claim 1, wherein the workpiece is a vehicle body, and the electrodeposition coating is performed while rotating the vehicle body around an axis in the longitudinal direction of the vehicle.