DE69009336T2 - Spray coating and drying processes. - Google Patents

Description

The present invention relates to a painting process having the features mentioned in the preamble of claim 1.

A method of painting an external surface of a painting substrate such as a vehicle body generally comprises the steps of: the preparation step of removing dirt adhering to the painting substrate, the painting step of spraying the painting substrate with a paint, and the drying step of drying the paint sprayed onto the painting substrate. The drying step may generally be carried out in two stages: a setting step and a baking step. The setting step is usually carried out before the baking step at temperatures lower than those used in the baking step, e.g. in ambient air or at temperatures of 40 to 60°C, which is called provisional baking. The temperature in the baking step may usually be about 140°C.

Typically, the painting substrate goes through the preparation step, the painting step and the drying step while being transported on a transport device such as a trolley. The painting substrate is held in a predetermined position during each step in which it is treated.

A standard for evaluating the quality of a painted surface is a degree of flatness (a degree of smoothness). The greater the degree of flatness becomes, the smaller a degree of irregularities or the roughness on the painted surface, thereby producing a better painted surface. It is known that the thickness of a painted layer, ie the thickness of a painted paint, can be increased to improve the degree of flatness.

In contrast, "sagging" of a paint is a factor that adversely affects the quality of the surface of the painted substrate. Sagging occurs when the sprayed paint flows downward to a large extent due to gravity, with "sagging" becoming more likely as the thickness of the sprayed paint increases. The cause of "sagging" may be an influence of gravity, so that sagging occurs more easily on a surface of the painting substrate that runs in a vertical direction, i.e., a so-called vertical surface. For example, if the body of an automobile is considered as a painting substrate, a vertical fender easily causes sagging, while a transverse hood and a transverse trunk lid hardly cause sagging when painted with a paint.

Accordingly, it is possible to increase the thickness of the paint on a surface of the painting substrate extending in the horizontal direction, i.e. a so-called horizontal surface, which does not cause "sagging" problems as much as a vertical surface. Furthermore, if the thickness of the paint layer on the horizontal surface is equal to that on the vertical surface, the irregularities on the horizontal surface due to the flow of the paint sprayed thereon become smaller than on the vertical surface to such an extent that no sagging is caused, whereby a higher degree of flatness is achieved on the horizontal surface than on the vertical surface.

From the above viewpoint, painting is carried out using a paint having the smallest possible fluidity or viscosity in order to produce a paint surface with the greatest possible degree of flatness while preventing "sagging" of the sprayed paint. A so-called "sagging limit" is a limit of the thickness of the paint that causes sagging on the vertical surface and is about 40 µm, which is the maximum of the thickness of the paint layer in conventional heat curing. More specifically, "sagging" of such heat curing paints occurs in the initial stages of the setting and baking stages, particularly in the initial stage of the baking stage, so that the thickness of the paint to be sprayed in the painting step is determined so that "sagging" is not caused in this stage. The maximum thickness of the paint determined at this stage is the so-called sagging limit of 40 µm. In order to produce a paint surface with a significantly higher degree of flatness, conventional painting processes therefore require two coats of paint, etc., i.e. a series of steps that must be repeated several times, from the painting step to the baking step.

US-A 4874639 (corresponding to EP-A 261 644) and US-A 4919977 disclose a painting method according to the preamble of claim 1, which can produce a paint surface with a higher degree of flatness when the two film thicknesses of the paints are equal, overcoming the sagging limit of the paint, which may cause a problem when spraying in the manner described above. More specifically, the painting method comprises painting by spraying the paint so that a paint layer with a film thickness greater than its sagging limit is formed, and rotating the painted substrate about its substantially horizontal axis until the sprayed paint no longer experiences sagging. This painting process can produce a painted surface with a higher degree of flatness when the thicknesses of the two paints are equal, and the occurrence of paint sagging can be prevented by taking advantage of such high fluidity of the paint used.

However, in these painting methods, there is a risk that the sprayed paint may sag during the transition period from the painting step to the drying step. As long as the paint is sprayed with a film thickness greater than its sag limit at which it begins to sag, or is sprayed with such a film thickness in these methods while diluting the paint with a solvent or the like, the paint may cause sagging or dripping immediately after spraying. In this case, if the transportation of the painted substrate from the painting step to the drying step would require a certain period of time, the sagging of the paint should be prevented during the transition period when the painted substrate is transported from the painting step to the drying step.

On the other hand, the paint can of course be prevented from sagging or dripping by spraying the paint substrate with the paint at a film thickness thinner than its sagging limit at which it will experience sagging at least during the transition phase, or by diluting the paint with the solvent or the like to reduce the fluidity of the paint. However, these techniques are compromised by the disadvantages inherent in conventional painting methods and may reduce the advantages which have been achieved with great effort by painting processes as disclosed in the earlier patents described above and which have overcome the barrier set by a sagging limit of the paint to be sprayed on.

Therefore, the present invention has the object of providing a painting method designed to prevent the occurrence of sagging of the paint sprayed onto a painting substrate during a period in which the painted substrate is transported from the painting step to the drying step by making the paint sprayed onto the painting substrate physically less flowable without limiting the painting conditions under which the substrate is sprayed with the paint.

To achieve the above object, the present invention proposes a painting method according to claim 1.

This arrangement of the steps of the painting process allows the paint formed on the substrate to dry without causing sagging because the paint sprayed onto the substrate does not sag due to changes in the direction of gravity acting on the paint, which are produced by rotating the substrate about its substantially horizontal longitudinal axis; this axis is sometimes referred to simply as the "horizontal axis" or similar words.

This allows a film thickness of the paint to be formed in one painting process much thicker than in conventional painting processes, thus producing a paint surface with a degree of flatness that is higher than the level that would be considered a limit in conventional processes. It should also be noted that this painting process can produce a paint surface with a smaller degree of irregularity, ie a higher degree of flatness, than the conventional processes if the film thicknesses of the two paints are the same. It should also be noted that if a paint is to be produced whose surface has the same degree of flatness as, for example, one that can be achieved with conventional painting processes, this painting process can reduce the film thickness of the paint, whereby the amount of paint saved can be used elsewhere.

It is clear here that the paint can be sprayed by electrostatic painting. Sagging of the paint means movement of the paint to such an extent that movement can be detected by visual observation when the paint is left as sprayed (which is observed as a thread-like trail), the paint being determined to have sagged when the paint has generally flowed about 2 mm. Thus, spraying the paint in a film thickness greater than its sagging limit means that such film thickness of the paint allows the paint to flow at least about 2 mm when left as sprayed.

Thus, it is clear that with higher fluidity of the paint used, the sagging limit thickness of the paint to be sprayed decreases. To make the film thickness of the paint greater than its sagging limit thickness, the paint may be sprayed once (as in so-called "single-stage spraying") or sprayed in two or three or more devices ("multi-stage spraying") so as to achieve a final film thickness greater than its sagging limit thickness. It should also be noted that if the rotation of the painted substrate about its nearly horizontal axis is carried out to a sufficient extent so that the sprayed paint is sagging due to gravity, the painted substrate can be rotated continuously or intermittently in one direction or in alternating directions until the paint passes into a state of so little flow that it does not experience sagging, that is, for a period until the paint has dried. Furthermore, the angle through which the painted substrate is rotated about its horizontal axis is so high, at about 270º, that any area of the coating formed by spraying the paint with a film thickness greater than its sagging limit can be inverted with respect to the direction of gravity. The axis about which the painted substrate is rotated can be inclined or pivoted by about 30º with respect to its true horizontal axis.

It should also be noted that the terms "substantially up-down surface of the substrate" and related terms used herein are intended to mean a surface of the substrate that extends in an up-down or rising-and-falling direction and on which the sprayed paint flows downward due to gravity and begins to sag when the paint is not rotated.

The cooling step used in the coating method according to the present invention is designed so that the paint in the coating formed on the substrate increases its viscosity so that the paint becomes less fluid and flows less downwards, such that the paint of the coating formed thereon does not sag during the transition phase when the substrate is transported from the coating step to the drying step. In other words, the fluidity of the paint is reduced by cooling the paint of the coating formed on the substrate, thereby preventing the occurrence of sagging of the paint on the substantially up-down or substantially vertical surface of the substrate is prevented.

Other objects, features and advantages of the present invention will become apparent in the course of the following description of the preferred embodiments when taken in conjunction with the accompanying drawings.

Fig. 1 is a schematic representation of an overview of the painting process according to the present invention.

Fig. 2 is a schematic diagram showing changes in the posture of the vehicle body during rotation.

Fig. 3 is a graph showing the relationship between the rate of paint sagging and the setting and baking times.

Fig. 4 is a graph showing the relationship between the image sharpness levels and the final coating film thicknesses.

Fig. 5(a) is a partially sectional view showing the painting step and the cooling step of the painting line.

Fig. 5(b) is a partially sectional view showing the setting step of the painting line.

Fig. 5(c) is a partially sectional view showing the baking step of the painting line.

Fig. 6 is a characteristic curve showing the relationship between the viscosity of the paint and the temperature of the paint.

Fig. 7 is a graph showing the relationship between the paint sagging rate of a thermosetting high-strength melamine alkyd paint with the initial viscosity of 0.6 poise and the time for the cooling, setting and baking steps.

Fig. 8 is a graph showing the relationship between the paint sagging rate of a thermosetting high-strength melamine alkyd paint with the initial viscosity of 0.2 poise and the time for the cooling, setting and baking steps.

Fig. 9 is a graph showing the relationship between the paint sagging rate of thermosetting high-strength melamine-alkyd paints with initial viscosities of 0.6 and 0.2 poise and the time for the cooling, setting and baking steps.

Fig. 10 is a graph showing the relationship between the rate of paint sagging of a two-component reactive paint with the initial viscosity of 0.6 poise and the time for the cooling, setting and baking steps.

Fig. 11 is a graph showing the relationship between the paint sagging rate of a two-component reactive paint with the initial viscosity of 0.2 poise and the time for the cooling, setting and baking steps.

The present invention will be described in more detail by way of example with reference to the accompanying drawings.

Overview of the painting process

Fig. 1 shows an overview of all steps of the painting method according to the present invention, in which a vehicle body is painted as a painting substrate, the steps being designated by steps P1 to P3, respectively.

The vehicle body is first primed by the conventional electrolytic deposition method and then transported to the painting step P1 while being carried by a cart. In the painting step P1, an outer surface of the body is completely sprayed with a paint of a desired color to form a paint finish. The body W is then transported to the cooling step P2. The paint finish formed on the body is then cooled in the cooling step P2 and then transported to the drying step P3, where the body is successively set and baked to dry the paint finish to a sufficient degree of dryness.

In the painting step P1, the substrate is sprayed with the paint to produce a paint with a film thickness greater than a film thickness that would cause sagging if the paint were left as sprayed. In the drying step P3, the substrate is rotated about its substantially horizontal axis in a manner as shown in Fig. 2 until the paint formed on the surface of the substrate is set and dried to a sufficient degree of dryness.

The rotational speed of the substrate such as the vehicle body W, etc. can be varied with the film thickness and viscosity of the sprayed paint. Basically, the substrate is rotated at a speed between an upper limit and a lower limit as defined below. The upper limit of the speed at which the substrate rotates is a minimum value of the rotational speed at which the painted substrate is rotated at least from its vertical position to its horizontal position before the paint on the surface of the paint flows down by its weight and sags due to gravity. On the other hand, the upper limit of the rotational speed is a maximum value thereof at which the paint does not sag due to centrifugal force. The painted substrate may preferably be rotated at a speed of 380 cm per second or less as measured at a radially outer tip portion of the substrate. When the painted substrate is rotated about its substantially horizontal axis, it should be noted that the axis of rotation may be inclined by approximately 30°, preferably by approximately 10° or less, with respect to its horizontal axis. Furthermore, the horizontal axis l about which the substrate is rotated may pass through the center of gravity of the substrate.

The period of time for which the painted substrate is rotated about its substantially horizontal axis in the drying step may be from the time before the sprayed paint begins to sag on its paint surface until it has dried to a sag-free state. In terms of equipment and other things, the painted substrate may be rotated throughout the entire length of the drying step. The rotation of the painted substrate may continuously or intermittently in one direction, alternately in one direction and then in the opposite direction, or intermittently to stop the rotation.

Specific examples of conditions for painting a vehicle body (1) Primer:

Cationic electrodeposition

Baking: 170ºC for 30 minutes

Film thickness: 20 ± 2 um

(2) Final painting 1) Paint with a viscosity of 0.6 poise:

a. Paint: heat-curing, high-strength melamine-alkyd paint (base resin component, average molecular weight 2,800; color: black)

b. Viscosity when spraying: 0.6 poise

c. non-volatile components: 48% by weight

d. Solvent: toluene, 25 parts by weight;

Solvesso 100, 25 parts by weight;

Solvesso 150, 50 parts by weight

e. Addition to prevent subsidence:

cross-linked acrylic resin powder, 3% by weight based on the weight of non-volatile components

f. Painting machine: Minibell (hopper size: 60 mm; Nippon Lundsberg, K. K.)

Minibell speed: 16,000 min&supmin;¹

Voltage: -90 kV

Processing air pressure: 3.0 Kg/cm²

Distance from the gun: 30 cm

Injections: two-stage at 5-minute intervals

g. Spray atmosphere 20ºC ± 2ºC

Air speed in the cell:

0.3 ± 0.1 m/s (pressure-suction-downflow)

h. Setting conditions: initial temperature, 20ºC ± 2ºC;

Set time, 7 minutes

i. Baking conditions: temperature, 140ºC /time, 25 minutes

Baking temperature increase rate 8 minutes (from 20ºC to 140ºC)

j. Shooting conditions:

Rotating the painted substrate about its horizontal axis, which is 75 cm from its central axis, so that its two mutually parallel side surfaces can rotate at a speed of 6 min-1.

2) Paint with a viscosity of 0.2 poise:

a. Varnish: the same as above

b. Viscosity when spraying: 0.2 poise

c. Non-volatile components: 35% by weight

d. Solvent: toluene, 35 parts by weight;

Solvesso 100, 25 parts by weight;

Solvesso 150, 50 parts by weight

e. Addition to prevent subsidence: same as above

f. Painting machine: the same as above

g. Spray atmosphere: the same as above

h. Settlement conditions: the same as above

i. Burn-in conditions: the same as above

j. Shooting conditions: the same as above.

Two-component paint

The intermediate and final coats are carried out with the same paint under the following conditions:

a. Varnish: polyester urethane varnish; white ("P-263u; Nippon Bee Chemical K. K.)

Base resin: Polyester-polyol

Curing agent hexamethylene diisocyanate

Mixing ratio: 4 (base resin) to 1 (hardener)

b. Painting machine: Pressure flow air spray gun (Iwata Tosoki K.K.; "Wider-W71")

c. Spray viscosity 0.6 poise and 0.2 poise

d. Sprayed paint quantity: 350 cm³ per minute

e. Processing air pressure: 4.0 Kg/cm²

f. Distance from the gun: 30 cm

g. Interval between two coats of paint: 5 minutes

h. Drying conditions: Set, 7 minutes (at room temperature)

90ºC for 25 minutes (increased from 20ºC to 90ºC in 5 minutes)

Thermosetting varnish:

(1) Intermediate coating:

a. Paint: Thermosetting, oil-free polyester urethane paint; black

b. Viscosity when spraying: 0.6 poise and 0.2 poise

c. Painting machine: Minibell (hopper size: 60 mm)

Speed: 22,000 min&supmin;¹

Voltage: -90 kV

Processing air pressure: 3.0 Kg/cm²

Distance from the gun: 30 cm

d. Drying conditions: Set 7 minutes room temperature), then 140ºC for 25 minutes

(2) Final painting

a. Lacquer: Thermosetting acrylic melamine lacquer; black

b. Viscosity when spraying: 0.6 poise and 0.2 poise

c. Content of non-volatile components:

42 wt.% (0.6 poise)

33 wt.% (0.2 poise)

d. Solvent:

(i) 0.6 poises:

Toluene: 50 wt.%

Solvesso 100: 50 wt.%

(ii) 0.2 poises:

Toluene: 55 wt.%

Solvesso 100: 45 wt.%

e. Addition to prevent subsidence:

cross-linked acrylic resin powder (6 wt.% of non-volatile components)

The other conditions such as the painting machines are the same as when painting with the thermosetting high-strength melamine alkyd paint, as described above.

Preferred paints for a vehicle body

The paints to be used particularly for painting vehicle bodies may be any paints containing a resin having an average molecular weight in the range of 2,000 to 20,000 as shown in Table 1 below.

The reason for the preference for paints containing the resin with an average molecular weight in the range of 2,000 to 20,000 is that those with an average molecular weight of less than 2,000 correspond to paints that can be cured by means of electron beams or ultraviolet rays and which, due to their high cross-linking density, are so brittle that they are less durable (two to three years), so that they are not suitable for the external surfaces of a motor vehicle, while a Latex polymer having an average molecular weight of more than 20,000 becomes very viscous immediately after injection molding and a degree of flatness can hardly be improved, so such a polymer is not preferred. TABLE 1 Paint Resin Type Medium Molecular Weight Solid Paint Metallic Base Coat Metallic Clear Coat Melamine Alkyd Melamine Acrylate Urethane Isocyanate Typically High Strength

Relationship between coating thickness and sagging limit, degree of smoothness and rotation around the horizontal axis

Fig. 3 shows the influences of the film thicknesses of a thermosetting paint coating on the sagging limit. Fig. 3 takes three different kinds of film thicknesses as examples, i.e. 40 µm, 53 µm and 65 µm. In any case, it can be found that the saggings have reached their peak values in the early stages of both the setting stage and the baking stage. The sagging limit of the paint is usually defined as the value at the time when the saggings occur at a rate in the range of 1 to 2 mm per minute. More specifically, the sagging limit of the paint is a limit of the film thickness at which in the drying step after drying a trace can be seen on the paint surface caused by a movement of the paint of 1 to 2 mm from the initial position at which the paint was sprayed. Using a conventional paint, the maximum film thickness ever achieved in an area below a sagging limit was about 40 µm thin.

Fig. 4 shows the effects of horizontal rotation of the vehicle body W on the degree of flatness. In Fig. 4, reference character A denotes a state of painting in which the vehicle body W is not rotated (in a conventional method). Reference character B in Fig. 4 denotes the state of painting achieved by rotating the vehicle body W by the angle of 90º clockwise and then reversing the direction and rotating the vehicle body back by 90º, namely, by rotating from the position of Fig. 2(a) through (b) to (c) and then reversing from the position (c) through (b) back to (a). The reference symbol C in Fig. 4 indicates the state of painting achieved by rotating the vehicle body W through an angle of 135º, then reversing the direction and stopping the rotation of the body in the initial position, namely, by rotating from the position of Fig. 2(a) through (b) and (c) to (d) and then returning from the position of Fig. 2(d) through (c) and (b) back to the initial position (a). The reference symbol D in Fig. 4 indicates the state of painting achieved by rotating the vehicle body W through an angle of 180º, namely, by rotating the body from the position of Fig. 2(a) through (b), (c) and (d) to (e) and then from (e) back through (d), (c) and (b) to the initial position of Fig. 2(a). The reference symbol E in Fig. 4 indicates the state of painting achieved when the vehicle body W is rotated one full revolution in one direction, namely from the starting position of Fig. 2(a) via (b), (c), (d), (e), (f), (g) and (h) back to the starting position of Fig. 2(i) or Fig. 2(a).

As can be seen from the results of Fig. 4, when the film thicknesses of two paints are equal, a higher degree of flatness in the paint is achieved when the vehicle body W is rotated (as shown by reference symbols B, C, D and E in Fig. 4) than when it is not rotated (as shown by reference symbol A in Fig. 4). Also, it is noted that in the examples in which the vehicle body W is rotated, the full rotation of the vehicle body in one direction by 360° is preferred to produce a paint with a higher degree of flatness. Furthermore, it should be noted that in the examples in which the vehicle body W is not rotated as in the conventional manner, the film thickness of the paint is limited to a certain value, resulting in a limited degree of flatness.

In comparison, a film thickness of 65 µm produced by rotating the vehicle body through 360º results in a painted surface that reaches a value of 87 on the I.G. (image sharpness) scale (the lower limit at a PGD value of 1.0). A film thickness of 40 µm reaches a value of 58 on the I.G. scale (the lower limit at a PGD value of 0.7) when produced without rotating the vehicle body, while it reaches a value of 68 on the I.G. scale (the lower limit at a PGD value of 0.8) when produced with a rotation of 360º.

As is known to those skilled in the art, it should be noted that an IG value (image sharpness) is a ratio of an image sharpness level with respect to a mirror surface on a black glass is 100, and PGD values represent a degree of recognition of a reflected image and are graded so that they decrease with decreasing degrees of flatness starting from 1.0.

The data shown in Figs. 3 and 4 were obtained under the following test condition under which the final coating was carried out in step P1:

a) Lacquer: Melamine-Alkyd (black)

Viscosity: 22 seconds / 20ºC (measured with Ford Cup #4)

b) Film coating machine: Minibell (16,000 min-1)

Processing air: 2.0 Kg/cm²

c) Spray quantities (twice)

first time: 100 cm³/minute

second time: 150 - 200 cm³/minute

d) Setting time/temperature: 10 minutes / room temperature

e) Baking temperature/time: 140ºC / 25 minutes

f) Degree of primer flatness: 0.6 (PGD value)

(Intermediate coating via PE tape)

g) Time for rotation and reversal:

10 minutes (for the riser)

10 minutes (for the burn-in stage)

h) Material to be painted: The side surfaces of a square hollow profile with a 30 cm side are painted and supported in such a way that the hollow profile can rotate.

i) Rotation speed of the material to be painted: 6, 30 and 60 min-1; practically no difference was observed.

Painting line:

Fig. 5(a) shows the painting step P1 for spraying the vehicle body W with the paint and the cooling step P2 for cooling the paint formed on the vehicle body, which is arranged immediately after the painting step P1.

As shown in Fig. 5(a), reference numeral 12 designates a carriage for transporting the vehicle body W in a painting line, the carriage 12 being arranged so that its wheels 14 mounted in its bottom portion run on a track 16. In a trench 18 disposed beneath the ground on which the track 16 is laid, a transport chain 20 is arranged so that it can be driven by a suitable drive device, the carriage 12 being moved due to engagement with a rod 22 fixed to the transport chain 20.

The carriage 12 has a pair of support pins 24 and 25 disposed at its front and rear portions, respectively, for rotatably supporting the vehicle body W and extending in its horizontal and longitudinal directions. The body support pin 24 is provided at its front end portion with a gear box 26 in which a bevel gear 28 is installed. The bevel gear 28 is in mesh with another bevel gear 32 which is fixed to an upper end portion of a vertical shaft 30 which is in turn rotatably supported in a position in front of the vehicle body W. A sprocket 34 is fixed to a lower end portion of the shaft 30 which in turn is in mesh with a circulating chain 36 disposed within the trench 18. The vertical shaft 30 is rotated in conjunction with the circulating chain 36, whereby via the bevel gears 28 and 32 of the body support pin 24 is rotated and possibly a rotation of the vehicle body W is brought about.

It should also be noted that the vehicle body W can be rotated in the manner disclosed in U.S. Patents 4,874,639 and 4,919,977, which prior patents are to be considered incorporated by reference in this application.

According to the present invention, the vehicle body W is first primed with a paint in a manner known per se to those skilled in the art, such as by electrodeposition, followed by removing dirt from the vehicle body W by cleaning means such as blowing air. Thereafter, the vehicle body W is transported by the carriage 12 to the painting step P1, wherein the primed vehicle body W is sprayed with the paint as it passes through a painting line where the painting step P1 is carried out. In the painting line for the painting step P1, a plurality of painting machines 38, 38 are mounted at appropriate positions above the body W and on the left and right sides thereof. During transportation of the vehicle body W in the painting step P1, a paint is formed by continuously spraying the body W with the paint from its front portion through its middle portion to its rear portion by the painting machines 38, 38 in sequence. The spraying of the body W can be completed in 2 to 3 minutes.

For the painting process according to the present invention, a station for carrying out the cooling step P2 is provided in the painting line adjacent to the painting step P1, namely in a position adjacent to the exit area of the painting step P1 from which the vehicle body is removed after painting. The station for the cooling step P2 is equipped with a cooling casing 40 which has openings in its front and rear regions. The cooling casing 40 is provided at its upper region with a plurality of pipes 42 through which the casing 40 is supplied with cooled air. The pipe 42 has a plurality of outlets 42a with openings directed downwards into the interior of the casing 40 where the paint formed on the vehicle body W is cooled. The air supplied through the pipe 42 and its outlets 42a is cooled by a heat exchanger 43 which in turn cools open air, the cooled air being blown by a fan 44 through the pipe 42 and its outlets 42a into the cooling casing 40. After the painting step P1, the vehicle body W painted with the paint enters the cooling step P2, whereupon the paint formed on the vehicle body W is cooled with the air blown through the outlets 42a of the pipe 42 of the housing 40, thereby increasing the viscosity of the paint of the paint formed thereon and, as a result, preventing the paint from sagging or dripping during the period when the vehicle body W is transported to the drying step P3.

Fig. 6 shows the relationship between the viscosity of the paint, expressed in poise, and the temperature of the paint, expressed in degrees Celsius. As shown in Fig. 6, it is found that the viscosity of the paint is increased from 0.6 poise at 20°C to about 2 poise by cooling to 5°C. Although the temperature to which the paint formed on the vehicle body W is cooled is not limited to a particular temperature range, it is thus to be noted that the temperature may be in the range of about 15°C to 3°C, or preferably from 10ºC to 5ºC, with a temperature around 5ºC being preferred.

In a preferred aspect of the embodiment of the painting method according to the present invention, the vehicle body W is rotated about its substantially horizontal and longitudinal axis by the support pins 24 and 25 in such a state that the body W has been removed from the painting step P1 and almost completely transported to the cooling step P2. The rotation of the vehicle body W is carried out to cool the paint formed thereon in a uniform shape and to prevent the paint from sagging or dripping on a substantially up-and-down or substantially vertical surface, particularly when the paint has been sprayed onto the vehicle body W with a film thickness greater than a thickness at which the paint begins to sag. In the cooling step P2, the paint formed on the vehicle body W is uniformly cooled, followed by a uniform increase in the viscosity of the paint formed thereon.

As described above, immediately after the completion of the painting of the body W with the paint in the painting step P1, the vehicle body W is transported to the cooling step P2 and then cooled in the cooling step P2, whereby the viscosity of the paint in the painting is increased to a level where the paint does not sag or drip.

Fig. 5(b) shows the position of the vehicle body W taken in the setting stage of the rotation baking step P3, where the body W is rotated by the angle of 180º from the initial position shown in Fig. 5(a). In the setting stage, the solvents, contained in the paint sprayed on the vehicle body W are volatilized while the body W is rotated about its axis which is substantially horizontal and longitudinal. The vehicle body W passes through the setting stage in a period of about 7 minutes, although the period during which the body W is conveyed in the setting stage may be shortened or lengthened according to the type of paint, etc. The solvents in the paint of the paint sprayed on the vehicle body W can be evaporated in the setting stage at ambient temperature or at an elevated temperature depending on the type of paint used. In the setting stage, the paint of the paint formed on the vehicle body W can be hardened to such an extent that it no longer sags or drips on the vertical surface of the paint even if it is left without rotation.

Fig. 5(c) shows the state of the vehicle body W in which the vehicle body W is in the baking step P3 to be carried out after the setting step. The station for carrying out the baking step P3 has a drying oven 45 in a tunnel shape provided with a suitable heating device. After being transferred from the setting step to the baking step P3, the vehicle body W is baked in the baking step P3 at a temperature of, for example, 145ºC. As described above, the paint of the paint has already hardened and does not sag or flow, so that it is not necessary to rotate the vehicle body W in the baking step P3, but it is preferred to rotate the body W about its substantially horizontal and longitudinal axis in order to bake the paint in a uniform shape.

Fig. 7 corresponds to Fig. 3 and shows the relationship between the rate of paint sagging (mm per minute) and the time (minutes) during which the paint is finished in the painting step P1, cooled in the cooling step P2, set and then baked in the baking step P2 when a thermosetting high-strength melamine alkyd paint having a viscosity of 0.6 poise is sprayed to produce finishes with film thicknesses of 65, 53 and 40 µm.

As is apparent from the results shown in Figs. 3 and 7, it is found that the rate of paint sagging does not increase even four or five minutes after the paint is sprayed because the paint formed on the body W has been cooled in the cooling step to increase its viscosity immediately after the painting step as shown in Fig. 7, while the paint formed thereon increases its viscosity within two or three minutes after the painting if the paint is not cooled after the painting step as shown in Fig. 3. Thus, it is found that the introduction of the cooling step P2 into the painting process can postpone the time at which the paint flows downward or sags, thereby providing a sufficient period of time for the vehicle body W with the paint formed thereon to be transported from the painting step P1 to the drying step P3 without causing the paint of the paint to sag or drip.

As described above, the paint finish is considered poor if, after drying, a trace or traces remain to such an extent that they can be observed visually, which is the case when there is movement or flow of more than 1 to 2 mm from the point where the paint was sprayed, the sagging limit of the paint is a maximum value at which such a trace or traces do not remain. Now, referring to Fig. 7, it is found that the paint having the film thickness of 40 µm exceeds its sagging limit after about 7 minutes, the paint having the film thickness of 53 µm exceeds its sagging limit after about 5 or 6 minutes, and the paint having the film thickness of 65 µm exceeds its sagging limit after about 4 or 5 minutes. Therefore, the sagging of the paint of the paint formed on the vehicle body W can be prevented if the paint formed on the vehicle body W begins to settle while it is rotated in a manner as described above before the time when the film thickness of the paint formed thereon reaches the sagging limit of the paint.

Fig. 8 shows the example where the thermosetting high-strength melamine alkyd paint with the viscosity of 0.2 poise is sprayed onto the vehicle body W.

Fig. 9 shows the results when the thermosetting high-strength melamine alkyd paint with the initial viscosity of 0.6 poise is sprayed to produce a coating with the film thickness of 65 µm as shown by the solid line, while the results for the sprayed paint with the initial viscosity of 0.2 poise to produce a coating with the film thickness of 40 µm is shown by a dot-dash line. In the drawing, the dotted line indicates the body temperature W.

The two-component paints achieved similar results to those obtained in Fig. 9. As shown in Fig. 10, the solid line represents the example where the two-component paint was sprayed with the initial viscosity of 0.6 poise to obtain a paint having the film thickness of 65 µm, while the dashed line represents the example of the paint having the initial viscosity of 0.6 poise which was sprayed to produce a paint having the film thickness of 40 µm. On the other hand, Fig. 11 shows the case where the paint having the initial viscosity of 0.2 poise was sprayed onto the vehicle body W. From the results shown in Figs. 10 and 11, the introduction of the cooling step P2 after the painting step P1 can have the advantage that the timing of sagging of the paint sprayed onto the vehicle body W can be delayed, thereby providing a sufficient time for transportation from the painting step to the setting stage without causing sagging or dripping of the paint during the period in which the painted body is being transported.

Table 2 shows the test results expressed in terms of IG (image definition) values and PGD values of the finishes with film thicknesses of 50 - 55 µm and 62 - 68 µm obtained by separately spraying a hood and a door with the paint and immediately cooling them after the painting step, and for comparison those obtained without immediately following cooling. From the results shown in Table 2, it was found that there is little difference in IG and PGD values between the two paint categories. This means that cooling the final paint immediately after painting does not ultimately adversely affect the paint surface. Table 3 shows the surface results obtained by separately spraying the hood and the door with different paints and different viscosities. TABLE 2 Film thickness around without cooling step with cooling step Hood Door TABLE 3 Paint Type Film Thickness um Viscosity Poise Hood Door Melamine-Alkyd Finish Acrylic-Melamine Finish Polyester-Melamine (Intermediate Coat) (Thermosetting Oil-Free Polyester) Polyester, Urethane (Final & Intermediate Coat) Melamine-Alkyd Acrylic-Melamine Polyester-Melamine (Thermosetting Oil-Free Polyester) Polyester, Urethane

Referring to Fig. 1, after the cooling step P2, as shown in the drawing by the dotted line, a carriage exchange step P2' may be inserted, in which the vehicle body W is transferred to a carriage 12 designed exclusively for use in the drying step P3. The adding of the step P2' to the painting line of the painting method according to the present invention can bring about the advantage of preventing the paint adhered to the carriage 12 used for the painting and cooling steps from sputtering and sticking to the vehicle body W in the drying step P3, particularly in the baking step where the paint formed on the body W is baked at a very high temperature. Cooling the paint allows the addition of step P2' to the cooling step P2, since the paint is cooled in the paint shop and the sagging of the paint is delayed for a certain period of time after the painting step P1.

Of course, the foregoing text and the drawings refer to embodiments of the present invention which have been given by way of example, but not by way of limitation. Within the scope of the present invention, which is defined by the appended claims, various other embodiments and variants are possible.

Claims (22)

1. Painting method with a painting stage (P1) in which paint is sprayed onto a substrate (W) and with a drying stage (P3) in which the paint sprayed onto the substrate is dried, in which the substrate has at least one largely vertically aligned surface and the paint is sprayed onto this surface in such a way that a layer is formed with a thickness which exceeds a thickness at which sagging of the paint begins, and in which further in the drying stage (P3) the substrate is rotated about a longitudinal axis (l) thereof, which extends largely horizontally or inclined by up to 30º to the horizontal, for a period of time from the start of the sagging of the layer on said surface to the point at which the layer assumes a largely sag-free state, wherein the rotation is carried out at a speed which is sufficiently high to remove said surface of the substrate from a vertical position into a horizontal position before the layer thereon sags significantly under the effect of gravity, but is sufficiently low so that no sagging is caused under the influence of centrifugal force, characterized in that the layer is cooled in a cooling stage (P2) immediately after the painting stage (P1) but before the drying stage (P3). 2. Painting method according to claim 1, characterized in that the drying stage (P3) comprises a setting process and a baking process, wherein the baking process is carried out at a temperature which is higher than the temperature at which the setting process is carried out. 3. Painting process according to claims 1 or 2, characterized in that the paint contains a volatile component. 4. Painting process according to claim 1 or 2, characterized in that the paint is of the type of a reaction paint consisting of two liquids, which contains a resin and a hardener as the main component. 5. Painting process according to claim 1 or 2, characterized in that the paint contains a resin with a molecular weight (number average) in the range from 2,000 to 20,000. 6. Painting method according to one of claims 1 to 5, characterized in that the paint is sprayed with an initial viscosity thereof of 0.6 poise. 7. Painting method according to one of claims 1 to 5, characterized in that the paint is sprayed with an initial viscosity thereof of 0.2 poise. 8. Coating method according to one of claims 2 to 7, characterized in that the paint is of a type that tends to sag both in the setting process and in the baking process, and that the rotation of the substrate takes place both in the setting process and in the baking process. 9. A coating method according to any one of claims 2 to 7, characterized in that the coating is of a type that tends to sag both in the setting process and in the baking process, and that the rotation of the substrate takes place at least in the setting process. 10. Painting method according to one of claims 1 to 9, characterized in that the substrate is a vehicle body and that the vehicle body is sprayed with paint in the painting stage (P1) and cooled in the cooling stage (P2) while it is being transported. 11. A painting method according to any one of claims 1 to 9, characterized in that the substrate is a vehicle body (W) and that the vehicle body, when transported through the series of stages from the painting stage (P1) to the drying stage (P3), is treated in each stage while being transported. 12. Painting process according to one of claims 1 to 11, characterized in that the layer on the substrate is cooled at a temperature in the range of 5ºC to 10ºC. 13. Painting process according to one of claims 1 to 12, characterized in that the layer is cooled by blowing cool air onto the substrate. 14. Painting method according to one of claims 1 to 13, characterized in that the substrate is arranged on a carriage (12) during its treatment in each of the stages during the period from the painting stage (P1) to the drying stage (P3). 15. Painting method according to claim 14, characterized in that the carriage is exchanged in an exchange process (P2') between the cooling stage (P2) and the drying stage (P3). 16. Painting process according to one of claims 1 to 15, characterized in that that the longitudinal axis (l) around which the substrate is rotated passes through the center of gravity of the substrate. 17. Painting method according to one of claims 1 to 16, characterized in that the substrate is sprayed twice with the paint in order to form a layer with a predetermined layer thickness. 18. Painting method according to one of claims 1 to 17, characterized in that the substrate is rotated in one direction. 19. Painting method according to one of claims 1 to 18, characterized in that the substrate is continuously rotated. 20. Painting method according to one of claims 1 to 18, characterized in that the substrate is rotated intermittently. 21. Painting method according to one of claims 1 to 17, characterized in that the substrate is rotated in one direction and then rotated back in the opposite direction. 22. Painting method according to one of claims 1 to 21, characterized in that the substrate is rotated at a speed of 380 cm/sec. or slower, measured at a radially outer tip part of the substrate.