EP0292778B1 - Process and installation for electrostatic coating with a conductive coating product - Google Patents

Description

The invention relates to a method according to the preamble of claim 1 and a coating system for performing the method.

In conventional electrostatic coating systems, such as are used in particular for painting motor vehicle body shells, the spray head of rotary atomizers or the like is connected to high voltage in order to thereby generate the field which charges the sprayed coating particles between the spray head and the grounded object to be coated . The problem arises here that when using a coating material of relatively good conductivity, such as so-called water-based paints, the insulation resistance via the line connecting the spray head to the paint supply system is too low if the storage system is at earth potential.

To solve this problem, there is the possibility of isolating the entire storage system from earth, but this is particularly unsuitable if the storage system consists of a large number of storage containers because of the possibility of changing colors. Apart from the considerable amount of insulation required, a large storage system can have such a large capacity that the corresponding charging energy becomes too large with regard to the risk of explosive discharges at the spray head. Furthermore, containers lying at the high potential cannot be refilled without switching off the voltage unless complex additional devices such as intermediate containers or the like are provided for this (cf. DE-PS 2900660). In addition, some require known systems complex, that is uneconomical high-voltage sources of high power.

According to another known solution to the problems caused by the conductivity of the coating material, the entire paint supply system is grounded from the storage container to the spray head of the atomizer and the radially sprayed material is indirectly charged via the external electrodes surrounding the spray head (EP-OS 0171042). However, this solution is ruled out if the coating material is to be charged directly through the spray head.

In a coating system for electrically conductive materials known from DE-OS 3014221, a separate storage container is provided for each color, which is arranged insulated from earth and from the other containers and feeds the spray device at high voltage potential via a color changer and a connecting line. After the coating operation has ended, the connecting line is flushed with a given color and before changing to a different color with solvent (water) and dried with compressed air in order to maintain the required insulation for the container subsequently connected to the spraying device. Especially with a large number of selectable colors and corresponding containers, this system is structurally complex and bulky.

The invention has for its object to provide a method or a coating system for performing a method that requires significantly less construction than the last-mentioned known system, and in which there is the possibility to continue the normal coating operation even with a color change without significant interruption .

This object is achieved by the method characterized in claim 1 or by the coating system characterized in the further claims.

The invention has the essential advantage that the storage container can be made smaller and simpler than in the comparable known systems and can be used in common for all selectable colors. Under certain circumstances, a simple short line or hose piece with a defined volume is sufficient as a storage container. If you work with two such reservoirs in push-pull mode, a practically non-stop coating of any size series of workpieces is possible even with a color change. The effort required is low, especially with regard to the insulation between the grounded supply system and the spray device. Another advantage is that only minimal losses of coating material have to be accepted. The invention is suitable e.g. especially for the large-scale coating of automobile body shells.

According to a particularly expedient development of the invention, a storage container is used, the volume of which can be adjusted and, before the coating material is filled in, is adjusted in accordance with the amount required for coating (for example a single workpiece), as a result of which the cleaning effort can be reduced. A suitable storage container can consist of a metering cylinder with a displaceable piston.

• Fig.1 ein System zur zwangsdosierten Versorgung eines Rotationszerstäubers oder einer sonstigen elektrostatischen Sprühvorrichtung mit Wasserlack oder einem ähnlich leitfähigen Beschichtungsmaterial;
• Fig.2 ein der Fig. 1 entsprechendes, aber teilweise etwas abgewandeltes System; und
• Fig.3 eine zweckmässige Ausführungsform eines als Vorratsbehälter für das System nach Fig. 2 dienenden Dosierzylinders.

The invention is explained in more detail in a preferred exemplary embodiment. In the drawing show schematically

• 1 shows a system for the forced dosing of a rotary atomizer or other electrostatic spraying device with water-based paint or a similar conductive coating material;
• 2 shows a system corresponding to FIG. 1, but partially somewhat modified; and
• 3 shows an expedient embodiment of a metering cylinder serving as a storage container for the system according to FIG.

In the system according to FIG. 1, the main needle valve HNV of the atomizer Z placed in operation at high voltage potential, for example in the order of magnitude of 100 kV, is to be supplied with water paint stocks (not shown) of different colors via a color changer FW in a manner known per se. The color changer FW has color valves F1, F2, F3 ... Fn for a corresponding, largely any number of n selectable colors. Furthermore, the color changer has a valve V₀ for flushing liquid and a valve PL₀ for compressed air.

A distribution valve VV is connected to the color changer via a flushable dosing pump DP₀, which is driven by a stepping motor M or the like with an insulated or isolating shaft and has a bypass valve controlled by a bypass valve. Instead of the dosing pump DP₀, another dosing device controlled by a flow meter can also be provided. Via the distributor valve VV, the water-based paint coming from the color changer FW can be directed into one of two supply lines LVA or LVB under the control of two paint flow valves FV₀. The ink supply valves FV₀ are arranged parallel and symmetrically to each other, and the distributor valve VV also has two correspondingly arranged return valves RF₀.

Each of the supply lines LVA and LVB leads via a first flushing valve arrangement SP1 into a flushable, pressurizable storage container V, the output of which is connected to a changeover valve UV via a second flushing valve arrangement SP2 and a connecting line LZA or LZB.

The purge valve arrangement SP1 has two valves V₁ and V₁₂ for flushing liquid, two valves PL₁ and PL₁₂ for compressed air and a paint flow valve FV₁. The flushing device SP2 has a valve V₂ for flushing liquid, a valve PL₂ for compressed air, a color flow valve FV₂ and a return valve RF₂.

The circuit with the lines LVB and LZB also contains built-up and arranged flush valve arrangements with a flushable reservoir pressure container in between.

The switching valve UV connects these mutually parallel circles or branches as shown via a further flushable dosing pump DP d or the like, which correspond to the dosing pump DP₀, i.e. in particular can have a stepper motor with an insulating shaft and a bypass, with the main needle valve HNV of the atomizer Z. Das In addition to the main needle HNV, the main needle valve HNV contains valves V₄ for rinsing liquid and a return valve RF₄.

The two storage containers V shown preferably have only a capacity of the order of magnitude of the amount of lacquer required for coating a single workpiece. In the case of vehicle bodies, e.g. a content of approximately 0.8 l is sufficient. The pressurizable container V is filled by the dosing pump DP₀ under a predetermined pressure with a predetermined amount of paint. The required fill quantity is stored in the form of data in the higher-level control system of the system, which controls the metering pump accordingly and at the same time automatically opens the valve for the desired color in the color changer FW. In addition to the amount of lacquer required for the workpiece, this compulsory metering also takes into account the volume of the line sections which are also to be filled, which in the example mentioned can be of the order of 0.1 l. In cases in which other, in particular smaller, workpieces are to be coated, the forced metering can also be dimensioned for a plurality of workpieces.

The required lines such as LVA, LVB, LZA and LZB are hoses made of insulating and water-repellent material, preferably made of plastic, such as PTFE.

During operation, the color changer FW and usually also the dosing pump DP₀ and the distribution valve VV are constantly at ground potential, while the atomizer Z with its main needle valve HNV and usually the flushable dosing pump DP₄ (with the exception of its insulated drive motor) and the changeover valve UV are constantly at high voltage. In a modification of this example, there is also the possibility of realizing the cyclic insulation between the container V and the atomizer Z (lines LZA, LZB) in the line between the switching valve UV or the metering pump DPpumpe on the one hand and the atomizer Z on the other hand. The parallel branches connected between them with their respective storage containers V, on the other hand, constantly change their potential intermittently between the high and the low potential, depending on the electrical connection made by the conductive coating material to the grounded supply system and the atomizer.

In the following, the mode of operation is explained by the description of the different successive or in part also simultaneous operating phases:

First, one of the valves, e.g. F1 of the color changer FW filled by the downstream metering pump DP₀ the container V of the left branch in the drawing, namely via the color flow valve FV₀ of the distribution valve VV, the line LVA and the color flow valve FV₁ of the flushing valve arrangement SP₁. The filling extends up to the color flow valve FV₂ of the flushing valve arrangement SP₂.

After filling the container V, the ink supply valve FV₀ is closed and the color changer FW rinsed. For this purpose, solvent (which in the example under consideration can mainly consist of water) is fed into the color changer via the valve V₀ of the color changer. It washes also the dosing pump DP₀ and reaches the disposal device ES via the return valve RF₀ of the distribution valve VV, taking along existing paint residues via a disposal line LES. At the same time and / or subsequently, the valve PL₀ of the color changer, which is designed as a check valve, blows in air to dry the rinsed paths.

In particular, after the container V has been filled, the insulating section formed by the line LVA between the distributor valve VV and the flushing valve arrangement SP1 must also be flushed and dried. For this purpose, the valve V₁₂ of the purge valve arrangement SP1 for solvents and its air valve PL₁₂ are opened simultaneously or alternately. The solvent or the air, taking the color residues remaining in the LVA line, pass through the valves FV₀ and RF₀ of the distribution valve VV into the disposal line LES. After switching off the solvent by closing the valve V₁₂, the entire path leading from the air valve PL₁₂ through the distributor valve VV into the disposal line must be completely blown dry by air.

Now the paint can be supplied from the pressurized (or pressurized via the air valve PL 1) container V via the switching valve UV and the dosing pump DP₄ the atomizer. This is done via the color flow valve FV₂ of the flushing valve arrangement SP2, the line LZA, the color flow valve FV₃ of the changeover valve UV and the lines leading to the dosing pump DP₄ and the main needle valve HNV. The container V is under high voltage, but is isolated from the paint supply system due to the described emptying of the line LVA.

Preferably, the coating material from the container V is initially only up to the closed main needle valve of the atomizer Z "pressed", appropriately via the bypass of the dosing pump DP₄. This "pressure path" can lead up to or beyond the return valve RV₄ of the main needle valve HNV. Only then is the main needle valve opened in this preferred mode of operation and the paint is pumped by the dosing pump DP₄ into the atomizer Z for spraying. The pressure in container V can be in the order of 2.5 to 4 bar, for example.

The atomizer can then both inside, i.e. from the switching valve UV to the main needle valve HNV, as well as outside, i.e. rinsed on the bell plate or the like. Both happens via the air valve PL₃ and the solvent valve V₃ of the switching valve UV. The paint residues in the inner line system between the switching valve UV and the main needle valve HNV are discharged through the return valve RF₄ into the disposal device ES.

While the atomizer Z is supplied from the container V of the left branch in the drawing, the parallel right branch can be prepared for the coating of the next vehicle body in the manner described above. Depending on requirements, the same or a different color can be selected. The relevant, possibly other valve such as e.g. F2 of the color changer FW is opened and the coating material is conveyed into the right reservoir by the dosing pump DP₀, which is available again at this point in the coating of a body, via the paint supply valve on the right of the distributor valve VV in the drawing, the line LVB and the right flush valve arrangement.

The color changer is then rinsed again in the manner described, also during the coating.

While the atomizer Z is still working, the insulating section formed by the line LVB between the container on the right in the drawing and the distribution valve VV can then be rinsed and then blown completely dry. How this is done has already been explained for the LVA management.

Subsequently and after the coating of the body under consideration has ended, the coating material of the new paint can now be “pressed on” from the right container to the main needle valve HNV, the right container being placed under high voltage. Then the dosing pump DP₄ pumps this coating material into the atomizer Z, which sprays it onto the next body.

Appropriately during the time in which the coating material from the right-hand container reaches the main needle valve of the atomizer, the left-hand container V in the drawing, in which the first color was, can be rinsed. For this purpose, solvent is passed through the valve V 1 of the flush valve arrangement SP1 through the container V and via the return valve RF 2 of the flush valve arrangement SP2 into a line leading to the disposal device ES. Simultaneously or alternately, air can be blown through the container V via the valve PL 1.

Also while the ink is being supplied from the right-hand container to the main needle valve, the insulating section formed by the line LZA between the flushing valve arrangement SP2 and the changeover valve UV can be flushed and then blown dry. This is done via the valves PL₂ and V₂ of the flush valve assembly SP2 and the return valve RF₃ of the changeover valve. The existing paint residues are again supplied to the disposal device ES via a line connected to the valve RF₃.

As soon as the line LZA is dry, the first operating phase, ie the connection of the left tank V to the supply system, can be started again. It goes without saying that after the coating of the second body has been completed, the atomizer Z can be rinsed again, and during the coating of the next body the right container must be rinsed and the insulating section formed by the line LZB must be rinsed and dried.

All of these processes are repeated cyclically from car body to car body and can be easily controlled in a switching sequence that ensures perfect electrical isolation.

If a storage container with a given unchangeable volume is used in the method described above with reference to FIG. 1, the container must obviously be dimensioned so large that it is sufficient for the largest possible workpiece to be coated. In many cases, however, the coating system should be used for workpieces of different sizes, i.e. once for larger and at other times for smaller workpieces. A typical example of this is the series coating of various motor vehicle body shells. When used for smaller workpieces, the storage container only has to be partially filled with paint, while after the subsequent cyclical emptying, the entire volume is filled with solvents for cleaning the container. As a result, more solvent is consumed than is actually necessary, in particular during the cyclical filling and removal of a relatively small amount of coating material, which is undesirable because of the associated environmental impact (immission), and cleaning also takes longer due to the longer filling time. which is lost for the actual series coating operation.

The system shown in Fig. 2 largely corresponds to that of Fig. 1, but has the advantage that the effort in solvent and time for cleaning the reservoir can be reduced to a minimum.

In operation, the color changer FW and the distributor valve VV are always at ground potential, while the atomizer Z with its main needle valve HNV and the changeover valve UV are constantly at high voltage. The interposed parallel branches with a respective dosing cylinder DZ, on the other hand, constantly change their potential intermittently between the high and the low potential, depending on the electrical connection made by the conductive coating material with the grounded supply system and the atomizer. Due to the alternating filling and emptying of the supply and connection lines on the inlet and outlet sides of the dosing cylinders DZ, the paint changer and the spray device are kept constantly isolated from one another.

Before the start of the coating operation, the volume of the two metering cylinders should be set in accordance with the amount of paint required in each case, ie in accordance with the size of the body to be coated. For this purpose, the dosing cylinder DZ according to FIG. 3 consists of a cylinder vessel 1 and a piston 2 which is displaceable therein and which is arranged at the end of a piston rod 3 which is sealingly guided by an end wall of the vessel. In its other end wall facing away from the piston rod 3, the cylinder vessel 1 has an outlet opening 4 which leads into the flushing valve arrangement SP2 (FIG. 2). The inlet of the metering cylinder DZ connected to the first flushing valve arrangement SP1 is located in the channel 5 inside the piston rod 3, which is designed as a hollow tube. The channel 5 is inside the piston 2 into a connecting channel 6, which leads to an annular channel 7 which runs concentrically to its displacement axis in the vicinity of the peripheral surface of the piston 2. Outflow nozzles 8 fed by the ring channel 7 run in the direction shown generally towards the inner wall of the vessel with a slight inclination forward in the direction of displacement (towards the outlet opening 4) and open into the end face of the piston 2 close to the inner wall of the vessel to which they are directed. Instead of a plurality of nozzles 8, an annular gap can also be provided. According to the illustration, the piston 2 can be in two parts, one part being integrally connected to the piston rod 3 and the other part fastened to it being able to contain the channels 6, 7. In its circumferential surface, the piston has sealing rings 9, with which it slides sealingly on the inner wall of the cylinder vessel 1, which is straight in the direction of displacement. The space between the lower end face of the piston 2 in FIG. 2 and the outlet opening 4 forms the adjustable volume of ink quantity. A compressed air connection 10 opens into the space on the other side of the piston, the purpose of which will be explained below.

A spindle drive SM (FIG. 2) connected to the piston rod 3 and having a stepper motor can be used to adjust the volume of ink quantity, which is fed by pulses which the electronic control system of the system generates before the start of coating based on the body size stored in the form of data. Instead of a spindle drive, a rack or other drive system can also be used.

The dosing cylinders DZ are filled, emptied and cleaned essentially in the manner described above. First, the dosing cylinder DZ of one of the two branches is filled via the color changer FW. Because simply the previously set volume of the dosing cylinder no dosing pump is required. After rinsing and drying the color changer FW and the relevant supply line LVA or LVB, the color is removed from the dosing cylinder DZ and fed to the atomizer Z.

Then the dosing cylinder DZ should be rinsed. For this purpose, it would be sufficient to pass solvent through the flush valve arrangement SP1 through the space set between the piston 2 (FIG. 3) and the outlet opening 4. Preferably, however, to further save solvent, it is sprayed from the outflow nozzles 8 against the inner wall of the cylinder vessel 1, while at the same time the piston 2 containing the nozzles is moved in the direction of the outlet opening 4. Paint adhering to the wall is stripped with the solvent from the peripheral surface of the piston 2 provided with the sealing rings 9 and conveyed in the direction of the outlet opening 4. The movement of the piston can take place up to the stop of the piston end face on the appropriately shaped end wall of the cylinder vessel 1. To accelerate this cleaning movement can be effected by compressed air, which acts on the drive-side end face of the piston 2 through the mentioned compressed air connection 10. Control valves DLV for the compressed air drive are shown in Fig. 2. The piston 2 is then moved back into the position specified by the control system. The change in the drive design for the piston rod 2 with the spindle drive SM required for this mode of operation is not shown and is not the subject of the invention.

There are also various options for venting the dosing cylinder when filling and cleaning. For example, the venting can take place via one of the valves of the flushing valve arrangements SP2 leading to the disposal or the changeover valve UV or possibly also via the atomizer itself.

Claims (22)

1. Method of serially electrostatically coating workpieces with an electrically conductive coating material in which a storage container insulated from earth is first filled with a coating material, the coating material is then fed from the storage container via a connecting line to a spraying device at a high voltage potential, the container being electrically connected to the spraying device through the conductive coating material in the connecting line and the container is electrically insulated from the spraying device after the coating process by emptying the connecting line, characterised in that the coating material is fed to the storage container from a storage system at a low potential or earth potential through a supply line whilst the connecting line leading to the spraying device is emptied and that the storage container containing the coating material is electrically insulated from the storage system by emptying the supply line.

2. Method as claimed in claim 1, characterised in that the coating material is passed from the storage system via a first supply line (LVA) into a first storage container whilst the connecting line (LZA) extending from this storage container to the spraying device is emptied, that the first storage container containing the coating material is electrically insulated from the storage system by emptying the first supply line (LVA), that coating material is passed from the storage system via a second supply line (LVB) into a second storage container connected in parallel with the first storage container whilst the second connecting line (LZB) extending from the second storage container to the spraying device is emptied, that the second storage container containing the coating arterial is electrically insulated from the storage system by emptying the second supply line (LVB) and that each storage container is filled via its supply line whilst the coating material is supplied from the other storage container to the spraying device and is sprayed.

3. Method as claimed in claim 1 or 2, characterised in that the storage container is filled with a precisely metered amount of the coating material which is substantially sufficient only for coating a single workpiece or a predetermined small number of workpieces.

4. Method as claimed in one of the preceding claims, characterised in that the storage container is completely emptied after the supply line has been emptied and the coating material delivered to the spraying device.

5. Method as claimed in one of claims 1 to 4, characterised in that two storage containers are provided connected in parallel which are alternately filled with coating material of the same colour and emptied.

6. Method as claimed in one of claims 1 to 5, characterised in that two storage containers are provided connecting in parallel and at a colour change the one storage container is filled with coating material of the one colour and the second storage container is filled with coating material of the other colour.

7. Method as claimed in one of the preceding claims, characterised in that after being emptied the connecting line, the storage container and/or the supply line are each flushed with a solvent for the coating material.

8. Method as claimed in one of the preceding claims, characterised in that after being emptied the supply line, the storage container and/or the connecting line leading to the spraying device are blown dry.

9. Method as claimed in one of the preceding claims, characterised in that before filling with coating material the volume of the storage container is adjusted in accordance with the amount required for coating.

10. Method as claimed in claim 9, characterised in that during cleaning the volume of the storage container is reduced.

11. Coating installation for serially electrostatically coating workpieces with electrically conductive coating material including a spraying device (Z), a storage system at a low potential or earth potential for the coating material, a storage container (V) insulated from earth, a connecting line (LZA, LZB) connecting the storage container (V) to the spraying device (Z) in an insulating manner, a controlled valve arrangement (SP2, UV) which is connected to the connecting line (LZA, LZB) and to a source of a means for emptying and/or cleaning the connecting line (LZA, LZB) and a colour changer (FW) which includes a plurality of paint valves (F1-Fn) connected to sources of coating material of different colour included in the supply system and is connected to the storage container (V), characterised in that the colour changer (FW) is constantly at the low or earth potential of the storage system and is connected to it via an insulating supply line (LVA, LVB) common to all the colours for filling the storage container (V) and that a further controlled valve arrangement (SP1) is connected to this supply line (LVA, LVB) and to a source of a means for emptying and/or cleaning the supply line (LVA, LVB).

12. Coating installation as claimed in claim 11, characterised in that a metering device (DP₀) for filling the storage container (V) with a predetermined amount of the coating material is connected upstream of the supply line (LVA, LVB),

13. Coating installation as claimed in claim 11 or 12, characterised in that a metering device (DP₄) for emptying the storage container (DZ) is connected between the storage container and the spraying device (Z).

14. Coating installation as claimed in claim 12 or 13, characterised in that the metering device (DP₀, DP₄) has a drive motor (M) which is electrically insulated from it.

15. Coating installation as claimed in one of claims 11 to 14, characterised in that two storage containers (V) are provided to which a respective supply line (LVA, LVB) leads from the storage system and which are connected to the spraying device (Z) via a respective connecting line (LZA, LZB) and that a distributor valve (VV) is connected between the two supply lines (LVA, LVB) and the earthed supply system (colour changer FW).

16. Coating installation as claimed in claim 15, characterised in that a switchable valve (UV) is connected between the two storage containers (V) and the spraying device (Z).

17. Coating installation as claimed in one of claims 11 to 16, characterised in that a respective flushing valve arrangement (SP1, SP2) is arranged at the inlet and/or outlet of the storage container (V).

18. Coating installation as claimed in one of claims 11 to 17, characterised in that the supply and connecting lines (LVA, LVB; LZA, LZB) are hoses comprising at least in part insulating plastics material.

19. Coating installation as claimed in one of claims 11 to 18, characterised in that the storage container comprises a metering cylinder (DZ) with a movable piston (2).

20. Coating installation as claimed in claim 19, characterised in that a conduit passage (5, 6, 7) for paint and solvent passes through a moveable piston rod (3), which is situated on the one end face of the piston (2) and extends out of the metering cylinder (DZ), and through the piston (2) to at least one opening (8) on its other end face.

21. Coating installation as claimed in claim 20, characterised in that one or more outlet nozzles (8), which communicate with the conduit passage (5,6,7) and are arranged in the vicinity of the side wall of the piston (2), are directed towards the inner wall of the metering cylinder (DZ).

22. Coating installation as claimed in one of claims 19 to 21, characterised in that a pulse-controlled stepper motor is provided for moving the piston (2).

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