JP2656607B2 - Painting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application field) The present invention relates to a coating method.

(Prior art and its problems) When coating an object to be coated, for example, the outer surface of an automobile body, a preparation step of removing dust adhering to the object to be coated,
The method includes a step of applying a paint to an object to be coated and a drying step of drying the applied paint. This drying step is performed in two steps of a setting step and a baking step, for example, in the case of a flame-hardening paint, and the setting step is performed before the baking step at a lower temperature than this baking step, for example, room temperature or pre-baking. As described above, the baking is performed in an atmosphere at a temperature of 40 ° C to 60 ° C (the baking temperature in the baking step is usually around 140 ° C).

The object to be coated usually undergoes the above-described preparation step, coating step, and drying step while being conveyed by a conveying means such as a cart, but the posture of the object to be coated is maintained at a predetermined position in each step. It is being done as it is.

By the way, as one criterion for evaluating the quality of a painted surface, there is smoothness (flatness). The greater the smoothness, the smaller the degree of unevenness of the painted surface, and the better the painted surface. It is already known that the smoothness of the painted surface can be improved by increasing the thickness of the coating film, that is, the thickness of the applied paint.

On the other hand, there is a "drip" of the paint as an obstacle to the quality of the painted surface. The term “drip of paint” means that a trace of 1 to 2 mm from the position where the paint is sprayed on the object to be coated is cured by a drying process, and can be visually confirmed on the painted surface. Therefore, the trace is at least
If it exceeds 2mm, it means sagging. Therefore, to have a thickness within the sagging limit means that
It is a thickness that does not cause sagging in the drying process even if it is left as it is after spraying the paint. Conversely, the term "thickness equal to or greater than the sagging limit" means a thickness that causes sagging at least in the drying step if left as it is after spraying the paint.

The dripping of the paint is caused by the flow of the paint applied downward due to gravity, so that the greater the thickness of the paint applied at one time, the more easily the dripping occurs. Is, due to the effect of gravity, that tends to occur on the surface of the object to be coated extending in the vertical direction, that is, the so-called vertical surface.

Therefore, the surface extending in the horizontal direction of the object to be coated on which the “sagging” of the paint is not a problem, that is, the so-called lateral surface,
It is possible to make the thickness of the applied paint larger than the vertical surface. Even if the thickness of the coating on the horizontal surface is the same as the thickness of the coating on the vertical surface, the unevenness is reduced by a slight flow of paint that does not cause dripping on the horizontal surface, and the vertical surface is smooth. The degree of smoothness better than the degree is obtained.

In view of the above, conventionally, in order to obtain a painted surface with as large a smoothness as possible while preventing "sagging" of the paint, painting has been performed using a paint with as low fluidity as possible. The so-called "sagging limit" of the coating on the vertical surface, that is, the so-called "sagging limit", was, for example, about 40 μm in the thickness of the coating film of a thermosetting coating material. More specifically, the “sag” of the paint is likely to occur in the setting step and the baking step, particularly in the baking step, and the thickness of the paint applied in the painting step is determined so that “sag” does not occur at this time. The maximum value of the determined thickness, that is, the sag limit value is about 40 μm. Therefore, in order to obtain a coated surface with even greater smoothness, in the conventional coating method, it is necessary to repeat a series of steps from the coating step to the drying step a plurality of times, for example, twice coating. was there. In this way, in the process of applying a paint, for example, in the case of electrostatic coating, only attention is paid to prevention of overspray, since the “drip” is prevented as much as possible from the thickness of the coating film. A coating gun is installed, and as a specific layout, when the object to be coated is focused on as an automobile body, as shown in FIG.
Guns 1, 2, and 3 for applying paint to the vertical surface of the body W in order from the lag side to the advancing side with respect to the traveling direction (left side to right side (arrow direction) in the figure) It has been general that a top gun 4 for applying a paint is arranged on a lateral surface of the body W.

By the way, regarding the above-mentioned problem of smoothness, it is considered effective to rotate the article to be coated about a substantially horizontal axis in the drying step. That is, by appropriately changing the direction of gravity acting on the paint applied to the object to be coated, if the fluidity of the paint is positively utilized, even if the thickness of the coating film is the same. Even so, a painted surface with even greater smoothness can be obtained. In order to sufficiently obtain such an effect, it is necessary to apply the paint beyond the "drip limit", contrary to the conventional technique.

However, if this is performed in the same application sequence as in the prior art, sagging will occur on the vertical surface of the object before shifting to the drying step, because the vertical surface will be applied in advance, even if the object is coated in the drying step. Even if it is rotated, there is a risk that a situation may occur in which it is difficult to repair it.

Accordingly, an object of the present invention is to provide a coating method in which, when the paint is applied beyond the sagging limit, sagging is prevented as much as possible before shifting to the drying step.

(Means for Solving the Problems and Action) In order to achieve the technical object, the present invention provides a method of applying a paint to an object having a vertical surface extending vertically and a horizontal surface extending horizontally. An application step of applying, and a drying step of drying the coating applied to the object to be coated, on the premise of a coating method, provided in the coating step, the coating to a thickness of the sagging limit or more with respect to the lateral surface. Simultaneously with the application, or thereafter, by applying a paint to the vertical surface to a thickness equal to or more than the sag limit, the application step is completed, and in the drying step, the object to be coated is rotated about a substantially horizontal axis. It is configured to rotate.

According to the present invention, basically, even if the thickness of the coating film is the same as in the prior art, utilizing a fluidity of the paint, a material having less unevenness, that is, an excellent painted surface having a greater smoothness. can do. Of course, even if the fluidity of the paint is used, it is 1 to 1 from the adhesion position at the time of spraying.
It does not move paint much more than 2mm. More specifically, when the paint adheres to the object to be coated, it is inevitably affected by the surface tension when viewed in an enlarged manner.
And are in a continuous state. The high portion of the paint is intended to use the fluidity to fill the low portion in the immediate vicinity.

Furthermore, if it is intended to obtain a coated surface having the same smoothness, for example, the same smoothness as that obtained by a conventional coating method, the thickness of the paint to be applied can be made thinner than the conventional one, The amount of paint used can be reduced by the amount that can be made thinner.

Of course, paints that cause "sags" even with thin coatings
What is necessary is just to obtain from the conventional paint by reducing the content of the anti-sagging agent which inhibits the fluidity by a predetermined ratio, or by increasing the content of a thinner such as thinner.

On the other hand, for the problem that dripping is likely to occur before shifting to the drying step, the application of the vertical surface is finished later or at the same time as the application of the horizontal surface, so that the application from the vertical surface to the drying The transition time to the process can be shortened, thereby reducing the risk of sagging on the vertical surface before transition to the drying process.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

Overview of Overall FIG. 1 shows the overall steps in the case of coating an automobile body W as an object to be coated, and the steps are indicated by P1 to P4.

First, the body W, for which undercoating has been completed as is known by electrodeposition coating, is sent to the preparation process P1 while being held by the carriage D. In the preparation process P1, dust inside and outside the body W is removed by, for example, air blow or vacuum suction. Next, after the paint is sprayed on the body W in the step P2, the paint is dried in the setting step P3 and the baking step P4.

When the processes P1 to P4 are halfway, the body W is sent to the overcoating process after the process P4. In addition, steps P1 to P4
Is for overcoating, the body W is transported to the assembly line in a known manner.

Dust Removal The dust removal in the process P1 may be performed while rotating the body W around the horizontal axis l, as shown in FIG. That is, for example, first, after the rotation of the body W is stopped in the state shown in FIG. 2A to remove dust, the posture of the body W is changed to the state shown in FIG. It stops at the position and the dust is removed again. In this way,
As shown in (c), (d), (i) of FIG. 2, dust is removed while the body W is intermittently rotated.

As described above, by removing the dust while rotating the body W, the dust adhered to the inside of the closed section such as the inner corner of the roof panel or the side sill of the body W, that is, the body W must be rotated. It is possible to completely remove dust that does not fall.

The rotation range of the body W is 36 as shown in FIG.
The rotation may be 0 °, but may be adjusted to the rotation angle range of the body W in the drying process described later.

Spraying and Drying of Paint First, spraying of paint at P2, which will be described in detail later, is performed so that the thickness of the coating film is equal to or greater than the sag limit.
That is, in a thermosetting paint generally used in the past, the maximum thickness of the paint that does not cause “sag”, that is, the sag limit value is about 40 μm, but in the process P2, it is far more than this sag limit of 40 μm. The paint is sprayed to give a thick coating (for example, 65 μm).

The application of paint is performed by spraying because it is possible to control the thickness of the coating film as desired. The concept of spraying is electrostatic coating (electrostatic atomizing coating). ). On the other hand, in the case where the paint is applied by dipping, when the object to be coated is lifted from the dipping tank, the above-described visual movement of the paint far exceeding 1 to 2 mm occurs. And
If the coating is applied more than the sagging limit and is rotated after the sagging has occurred once, even after the subsequent rotation, the painted surface of the part where the sagging occurs once is as smooth as other painted surfaces where no sagging occurs You can't get sex.

After P2, the process proceeds to the setting step of P3. In this setting step P3, before sagging occurs on the surface of the body W which has been coated in the coating step P2, FIG.
The body W is rotated in the horizontal direction while reversing, for example, in the maximum angle range of 360 ° shown in (i). That is,
The body W is rotated about a rotation axis l extending in the horizontal direction. In the embodiment, the rotation axis l extends in the front-rear direction of the body W.

The rotation speed of the body W changes depending on the thickness and viscosity of the sprayed paint, but is basically set to a rotation speed in the range between the lower limit and the upper limit as described below. In other words, the lower limit of the rotation speed is the minimum value of the rotation speeds at which the coating surface can be changed from at least the vertical state to the horizontal state before the paint on the coating surface moves due to gravity and sags. The upper limit value is a maximum value among rotation speeds at which no dripping occurs due to centrifugal force generated by rotation. When the body W is rotated about a substantially horizontal axis, the rotation axis may be inclined by about 30 ° with respect to the horizontal axis, but it is preferable that the inclination be within 10 °.

The period during which the body W is rotated about the substantially horizontal axis may be at least in a drying step from before the coating surface is dripped until it is cured until dripping does not occur. Of course, the object to be coated may be rotated over the entire drying process from the viewpoint of equipment and the like.

In addition, this rotation may be any of continuous rotation in one direction, forward / reverse rotation alternately performing forward rotation and reverse rotation, and intermittent rotation in the middle of a rotation stop period.

The temperature atmosphere in the setting step P3 is room temperature in the embodiment, but the temperature in the next baking step such as 40 ° C. to 60 ° C.
An appropriate temperature can be set in a temperature range lower than the temperature atmosphere in P4. Of course, this setting step P3 is for preliminarily evaporating the low-boiling components in the paint, thereby causing pinholes on the painted surface due to the rapid evaporation of the low-boiling components in the next baking step P4. Is prevented.

In the baking step P4, for example, the paint is baked in a 140 ° C. temperature atmosphere. Also at P4, the body W is rotated about a substantially horizontal axis while reversing, as in the setting step of P3.

Due to the horizontal rotation of the body W at P3 and P4 described above, the paint is dried without dripping even if the paint is sprayed to a thickness greater than the sag limit at P2. As a result, a high-quality coated surface having extremely high smoothness, which cannot be obtained by the conventional coating method, can be obtained. Further, if the reversal is performed, it is possible to prevent a situation in which only the painting process of the corner portion of the body W becomes extremely thick and swells.

The reason why the body W is rotated in both the steps P3 and P4 is that the thermosetting paint that causes dripping is used in both the steps P3 and P4. In the case of the powder paint, the baking step P4 is performed.
In the case of a two-component curable paint, it is sufficient to perform this rotation only in the setting step.

Here, when the rotation of the body W is accompanied by reversal, for example, the following may be performed. That is, FIG. 2 (a)
When one cycle of the rotation of the body W with reversal is considered using the position of (a) as a reference position, the body W is rotated from the state of (a) by a predetermined angle in the normal direction (clockwise direction in FIG. 2) and the reverse direction. Just do it. For example, when both of the predetermined angles are set to 90 °, the posture change of the body W is expressed in FIG.
(A) → (b) → (c) → (b) → (a) (i) →
One cycle is (h) → (g) → (h) → (i) (a). Of course, the inversion is performed in the states (c) and (g). Further, as another example, the predetermined angles are both set to 13
Assuming 5 ゜, one cycle is (a) → (b) → (c)
→ (d) → (c) → (b) → (a) (i) → (h) →
(G) → (f) → (g) → (h) → (i) (a). The inversion at this time is performed in the cases of (d) and (f).

Of course, the predetermined angle may be determined so that the direction of gravity acting on the paint (particularly, the paint applied on the vertical surface) is changed, and the shape of the object to be coated, particularly the position of the corner, is taken into consideration. . Therefore, FIGS. 2 (a) to (b), (c),
(D) and then to (i) in order (after 360 [deg.] Rotation), and then reverse rotation, such as 360 [deg.] Rotation, can be set as appropriate.

FIG. 3 shows the effect of the coating film thickness on the sag limit when focusing on a thermosetting coating material.
In FIG. 3, the coating thickness is 40 μm, 53 μm, 65
The three cases of μm are shown. It is understood that, in any of these thicknesses, a "drip" peak occurs both at the beginning of the setting step and at the beginning of the printing step. Also, the sag limit is 1-2 mm in the drying process P3 or P4
The value when sagging occurs (the paint surface is bad if sagging more than 2 mm is visually observed), but the maximum coating thickness obtained within the sagging limit is the conventional coating thickness. so
It is about 40 μm.

On the other hand, FIG. 4 shows the influence on the smoothness when the body W is rotated in the horizontal direction and when it is not. A in FIG. 4 shows a state in which the body W is not rotated (conventional coating method). FIG. 4B shows a case where the body W is rotated by 90 ° and then reversed (forward and reverse rotation between FIGS. 2 (a) and (c)). FIG. 4C shows
The case where the body W is rotated by 135 ° and then reversely rotated (forward and reverse rotation between FIGS. 2 (a) and (d)) is shown. 4th
FIG. D shows a case in which the body W is rotated 180 ° and then reversed (forward and reverse rotation between FIGS. 2 (a) and (e)). FIG. 4E shows a case where the body W is continuously rotated in the same direction (FIGS. 2 (a), (b),
(C)... (I) are taken in this order, and the process returns to (a) again).

As is apparent from FIG. 4, if the thickness of the coating film is the same, it is better to rotate the body W (FIG. 4, B, C, D,
E) When the rotation is not performed (FIG. 4A), a greater smoothness can be obtained.

As is known, the IG at the mapping sharpness shown in FIG.
(Image gloss) is 100 with mirror surface (black glass)
It indicates the ratio of sharpness to that, and also PGD
Is a value at which the smoothness of the painted surface decreases as the degree of discrimination of the reflected image decreases from 1.0.

The test conditions of the data shown in FIG. 3 and FIG. 4 are as follows. These test conditions show the conditions when overcoating is performed at P2.

a. Paint: Melamine alkyd (black) Viscosity: 22 seconds / 20 ° C with Ford cup # 4 b. Coating machine: Minibell (16,000 rpm) Shaping air: 2, 0 kg / cm ₂ c. The first time: 100 cc / min The second time: 150-200 cc / min d. Setting time: 10 minutes × normal temperature e. Baking conditions: 140 ° C. × 25 minutes f. Substrate smoothness : 0.6 (PGD value) (intermediate coating, on PE tape) g.Rotating or reversing operation area: setting (10 minutes) to baking (10 minutes) h.Coating object: Painted on the side of a 30 cm square cylinder, center I. Rotation speed of the object to be coated: 6 rpm, 30 rpm, 60 rpm, but there was virtually no difference due to the difference in rotation speed. In the above-mentioned paint, setting step P3, baking step The cause of sagging at P4 is as follows. First, the sag in the setting step P3 is because the paint applied in the coating step P2 has fluidity in a wet state, so the viscosity of the paint is greater than the conventional film thickness that does not cause sag. This is caused by the fact that the paint moves in the direction of gravity over the gravity applied by the paint. On the other hand, the sag in the baking process P4 is caused by the temperature of the baking process P4, which causes the paint to have fluidity before the paint is hardened (usually called thermal flow), and the gravity applied to the paint from the viscosity of the paint. More specifically, the paint moves in the direction of gravity.

Specific Example 1 of Application Step P2 (FIGS. 5 to 9) In this example, the application of the paint to the body W is performed by electrostatic coating, and the application gun is a vertical gun (side gun). ) 10 and a side gun (top gun) 12 are provided, and the side gun 10 is a low 10a, a middle 1
It consists of three types of guns, 0b and high 10c. These guns 10 (10a, 10b, 10c), 12 are arranged at intervals in the transport direction of the body W (the direction of the arrow shown in FIG. 5) in order to prevent a reduction in efficiency due to electrostatic repulsion. I have.
The side gun 10 and the top gun 12 are configured such that the top gun 12 is disposed on the delay side and the side gun 10 is disposed on the advance side in the transport direction of the body W. With progress, from top gun 12, sequentially
The paint is discharged from the side gun low 10a, the side gun middle 10b, and the side gun high 10c, and after completing the series of spraying of the paint, the body W is carried out to the process P3 (see FIG. 7). In the case of two stages in which the spraying of the paint is performed twice, the above steps may be repeated as shown in FIG. 8, or as shown in FIG. May be applied in advance on the vertical surface as in the prior art.

Specific Example 2 of Painting Step P2 (FIG. 10) This example shows the case of spray coating.

In this example, a horizontal nozzle 20 that discharges paint on a horizontal surface and a vertical nozzle 22 that discharges paint on a vertical surface with respect to a stationary body W are provided in the vehicle longitudinal direction. The horizontal nozzle 20 and the vertical nozzle 22 are arranged such that the horizontal nozzle 20 is disposed on the leading side in the moving direction, and the vertical nozzle 22 is disposed on the delayed side. It is arranged.
As a result, the spraying on the body W is performed by the side surface nozzle.
20 is completed first, and the vertical surface nozzle 22 is completed later. The vertical nozzle 22 and the horizontal nozzle 20
May be arranged on the same line so that the spraying of the nozzles 20 and 22 onto the body W is completed at the same time.

Variations of paint (FIGS. 11 and 12) The paint used in the present invention is preferably a coating process.
Immediately after the completion of P2 and before starting the rotation in the setting step P3, a film thickness that is equal to or larger than the sagging limit at which sagging does not occur is appropriate. The coating film thickness defined in this manner is determined by the viscosity of the coating material, the content of the anti-sagging agent, and the like, and is not a ratio.

FIG. 11 and FIG. 12 show examples of paints that cause sagging only in the setting step P3 and do not cause sagging in the baking step P4. FIG. 11 shows an example of a two-component curable paint, and FIG. 12 shows an example of a thermosetting paint.

The test conditions for the data shown in FIG. 11 are as follows.

a. Paint: Polyester urethane paint white (trade name: R-263, manufactured by Nippon Bee Chemical Co., Ltd.) Main resin: Polyester polyol white Curing agent: Hexamethylene diisocyanate Mixing ratio (weight ratio): Hardening agent for main resin 4 B. Coating machine: Pressurized air spray gun (Iwata Coating Machine Co., Ltd.)
Spray viscosity: 16 seconds / Ford cup # 4 d. Paint discharge amount: 350cc / min e. Atomization air pressure: 4.0kg / cm ² f. Spray distance: 30cm g. Number of superpositions: 2 (interval: 3 minutes) FIG. 12 is a view showing the sagging characteristics of the thermosetting paint when the thermosetting paint is used as the paint used in the process P2. The setting process should be performed at a temperature of 20 ° C.
10 minutes. Also, the baking process should be performed between 20 ℃ and 140 ℃
To 140 ° C for 8 minutes.
For a minute.

FIG. 12 shows a total of three test examples a to c, that is, examples of thermosetting paints that can be used in the present invention. The specific test conditions for obtaining such data are as follows. It is as follows. In the following description, Test Example a
Those that do not distinguish between c and c are common matters.

(1) Paint ・ Solvent diluted thermosetting melamine kid (hue black) ・ Diluted solvent Triol 4% (wt%) Solvesso 100 (manufactured by Esso) 3% (wt%) Solvent 200 (manufactured by Esso) 3% ( (% By weight)-Anti-sagging agent It is a cross-linked acrylic resin, and all numerical values indicating the amount of addition are shown in Table 1 together with the viscosity in terms of% by weight based on the nonvolatile content.

-Viscosity All numerical values are indicated by Ford Cup # 4/20 ° C and are shown in Table 1 together with the content of the anti-sagging agent.

(2) Coating conditions Table 2 shows the results of two-stage spraying.

The setting and baking are as described above.

(3) Test plate ・ Cold rolled steel sheet treated with zinc phosphate ・ Undercoat Cathodic electrodeposition 170 ° C × 25 minutes baking thickness 25μm ・ Intermediate coating 140 ° C × 25 minutes baking thickness 40μm ・ Intermediate water sharpening (4) Evaluation method With the test plate of (3) set vertically, the topcoat paint was applied with various changes in the paint viscosity and the content of the anti-sagging agent under the above-mentioned coating conditions. Paints that cause sagging in the subsequent setting step and do not cause sagging in the baking step are shown in FIG. 15 as Test Examples a to c. In addition,
For Test Examples a to c, the test plate was rotated around the horizontal axis by 10 rp until sagging did not occur during the setting process.
The PGD value when rotated at a speed of m was measured with a resolution gloss meter. Only the test example c obtained a good PGD value of "0.9". An extremely excellent result with a PGD value of "1.0" was obtained.

Here, as the paint used for painting the automobile body W, as shown in Table 3 below, the paint resin preferably has a number average molecular weight in the range of 2,000 to 20,000.

The reason why the number average molecular weight is preferably in the range of 2,000 to 20,000 as a paint for automobiles is that if it is less than 2,000, it is a paint that cures with an electron beam or ultraviolet light, and this paint has a high crosslinking density and is durable because of its brittleness (2 to 3 years), which is not very suitable for use in automobile outer panels. Also, 20000
If the viscosity exceeds the above, a large amount of solvent is required because the viscosity becomes high, and it is not preferable because a large amount of solvent is discharged.Moreover, for latex polymers having a number average molecular weight of more than 20,000, the viscosity becomes high immediately after spraying, so that the It is not preferable because it is difficult to improve the properties.

Although the embodiment of the present invention has been described above, the present invention is not limited to this, but includes the following modified examples.

The thickness of the coating may be the same as the conventional one.

In order to increase the fluidity of the paint, the amount of a thinner such as thinner may be increased instead of the thickness of the coating, or the amount of the anti-sagging agent may be reduced.

If the pinhole problem does not occur, the setting step may be omitted.

(Effect of the Invention) As is clear from the above description. ADVANTAGE OF THE INVENTION According to this invention, even if it is the same film thickness compared with the past, the coating surface provided with the outstanding smoothness can be obtained by utilizing the fluidity of a coating material positively. In addition, since the coating on the vertical surface where dripping is likely to occur when shifting from the coating process to the drying process is performed with a delay compared to the conventional method, the coating method shifts to the drying process despite the method of using the fluidity of the coating. It is possible to reduce the risk of sagging before.

[Brief description of the drawings]

FIG. 1 is an overall process diagram showing one embodiment of the present invention. FIG. 2 is a view showing a state of a posture change accompanying rotation of an automobile body as an object to be coated. FIG. 3 and FIG. 4 are graphs showing the relationship among paint thickness, sag, smoothness of the painted surface, and rotation. FIG. 5 and FIG. 6 are views showing an arrangement relationship of a coating gun in a coating process. FIG. 7 to FIG. 9 are block diagrams showing the application order of the vertical and horizontal surfaces of the object to be coated. FIG. 10 is a layout diagram of a paint spraying nozzle in the case of spray painting. FIG. 11 and FIG. 12 are diagrams showing dripping characteristics of a paint suitable for use in the present invention. FIG. 13 is a view showing the arrangement relationship of a conventional coating gun. P2: Coating process W: Body (substrate) D: Dolly l: Rotation axis 10: Side gun (Vertical gun) 12: Top gun (Horizontal gun) 20: Horizontal nozzle 22: Vertical surface nozzle

Claims (1)

(57) [Claims] A coating step of applying a paint to an object having a vertical surface extending vertically and a horizontal surface extending horizontally, and a drying step of drying the paint applied to the object. In the coating method, in the applying step, the paint is applied to the horizontal surface at a thickness equal to or greater than the sag limit, or thereafter, the paint is applied to the vertical surface at a thickness equal to or greater than the sag limit. The coating method is thus completed, and in the drying step, the object to be coated is rotated around a substantially horizontal axis.