EP0297520A2 - Method and apparatus for lacquering work pieces with an electrically insulating surface by electrostatic coating or by spray coating - Google Patents

Abstract

When painting workpieces with an insulating, non-conductive surface by means of electrostatic coating or spray coating, interference effects in the form of non-uniform paint layers frequently occur. The new method and new for painting workpieces with an electrically insulating, non-conductive surface by electrostatic coating or spray coating make possible uniform paint application by producing a uniform charge state on the surface of the workpiece prior to the application of the paint. Painting of undercoated vehicle bodies.

Description

The invention relates to a method and a device for painting metallic workpieces with an electrically insulating, non-conductive surface, in particular workpiece surfaces that have already been painted, or workpieces that consist entirely of an electrically non-conductive plastic, by electrostatic application or spray application.

It is known that when painting such insulating workpieces, in particular when painting metal workpieces that have already been pre-painted at least once, such as automobile bodies and their parts, uneven layers of paint often occur in a painting zone by electrostatic application or spray application and, moreover, interference effects in the form of so-called emerging Mountain ridges, valleys and flat table mountains. Uneven layers of paint and interfering effects - as just described - are also known from the field of plastic painting. These phenomena occur particularly in electrostatic painting or when spraying large workpieces such as Bodies that already have at least one layer of paint e.g. are provided with the primer. Workpieces of this type are conveyed through a painting zone by means of a transport system, such as a floor conveyor system, and from this zone are fed to further processing, such as baking, etc., with the aid of this conveyor system.

The undesirable phenomena - as described above - appear after the stoving of the lacquer layer produced in the lacquering zone (result in rejects) and have so far had to be remedied by grinding and re-lacquering.

The object of the present invention is to provide a method and a device for painting workpieces with an insulating surface, such as workpieces that have already been painted, by means of electrostatic techniques or spraying techniques, which lead to a uniform application of paint and thus to such a thing without interference effects.

Surprisingly, it has been shown that the previously unavoidable and previously described disruptive effects of the finished lacquer layer can be avoided if the workpieces to be lacquered with an insulating or non-conductive surface are passed through an ionization zone directly in front of the lacquering zone.

The method according to the invention is therefore characterized in that a uniform charge state is generated on the workpiece surface before the coating is applied.

Without leading the at least once previously painted workpieces with the resulting insulating or non-conductive surface through an ionization zone, the surface can have a wide variety of electrical charge states, which can be demonstrated by measurement and can also be intentionally brought about beforehand by spraying on charges in order to explain the interference effects can. The various electrical charge states - ranging from positive charging (polarity) to the charge-free state to negative charging (polarity) - have their causes in the treatment history. For example, by sanding out painting defects in the surface that has already been painted at least once, surface elements are generated that are charged by frictional electricity, the same thing happens with the so-called heads of the surface that has been painted at least once with sandpaper in order to flatten the surface. Charge states are also generated by triboelectricity when the initial coating leaves the paint dryer (baking oven) and shrinking forces in the paint layer become slow as it cools down. The vigorous blowing with air for the purpose of cooling the painted surface leads to electrical charges, the blowing speed for certain surface elements can be very different. Added to this are the various charging conditions by wiping the sanding dust with dry or damp dust-absorbing cloths, by blowing off sanding dust with compressed air and, if necessary, by passing the workpiece surface, which has already been painted at least once, through an electrostatic dust removal system, which generally still has residual charges on the surface leaves behind.

In the experimental elucidation of the interfering effects, it could be demonstrated in connection with the negative charging of the electrostatically applied paint droplets but also the paint droplets, which are also negatively charged due to the tearing processes in conventional spraying techniques, that flat and flat table mountains arise, for example, where a surface element of the surface to be painted is positively (pre-) charged. There are electrostatic attractions.

Mountain ridges and valleys arise where there are negative (pre) charges in narrower areas. Mountain ridges also tend to occur at the edge of negative (pre) charges.

In the method according to the invention, the targeted and completely uniform charge state can be brought about either by applying unipolar air ions to the electrically insulating surface or by already painting the workpiece surface. However, it is possible to achieve a state of charge neutrality which is already being used today to avoid electrostatically adhering dust, especially in automotive OEM painting, using ionization electrodes or devices fed by high-voltage alternating current.

A unipolar state of the workpiece surface can be achieved with the aid of ionization electrodes or devices which are fed with a high DC voltage, so that a unipolar charged ion wind, namely a either positively or negatively charged ion wind or air flow, is generated. The positive polarity is preferably used.

In the preferred application of the positively charged air ion flow, in addition to the improved paint condition - meaning a paint layer evenly distributed over the entire workpiece surface with a finely structured surface and good packing density, but without markings due to the avoided interference effects - there is also the avoidance or reduction of electrostatic adhesive forces Dusting. It is known that the majority of dusts that occur in paint shops are positively charged at 80% by weight. A targeted and completely uniform charge state with positive polarity of the insulating or already at least once painted workpiece surface then leads to the fact that the majority of the dusts do not affect this surface due to the electrostatic repulsive forces.

Should it e.g. B. come to a short-term dust infestation through the air duct, then no adhesive forces can form between the dust particles and the workpiece surface to be painted. The dust particles are then simply blown away and filtered off in the air recirculation system.

The method according to the invention makes it possible in particular to improve both the electrostatic application of paints, in particular metallic basecoats, and their applicator with conventional spraying devices. This makes it possible to achieve the same good effects in the electrostatic application of, for example, metallic basecoats as in the undisturbed spraying of such paints with normal compressed air guns by hand.

When applying metallic basecoats, there are often shifts in the color impression and in the effect formation due to irregular storage and orientation of the metallic pigments due to different electrostatic interactions in the area of the surface to be painted (change in the flip-flop).

According to the prior art, a good metal effect is achieved by the horizontal alignment of the platelet-shaped metal pigments being as uniform as possible.

The procedure according to the invention makes it possible to carry out electrostatic painting, since interference effects due to special charge states of the paint droplets and partially the insulating - for example previously coated - workpiece surfaces are eliminated, which ensure a more uniform orientation and storage of the metal pigments and thus a more uniform effect formation with a defined absorption -, reflection and scattering behavior.

The invention also relates to an apparatus for performing the described method. This device is characterized in that it has a conveyor system for the workpieces to be painted which passes through a painting zone which is preceded by an ionizing zone.

In the ionization zone of the painting device according to the invention, a targeted and completely uniform charge state is achieved on the workpiece surface. The subsequent painting process can therefore take place under clear electrostatic conditions. This eliminates electrostatic interference caused by different charge states on the workpiece surface to be painted.

The ionization zone has ionization electrodes or ionization devices. Known electrodes or devices such as are already in use, for example, for avoiding or reducing electrostatic dust adhesion forces can be involved.

These ionization electrodes or devices are either fed in with high direct voltages to achieve unipolarly charged ion winds or air ion currents, the positive sense of polarity preferably being used, or they can also be fed in with high-tension alternating currents, producing an air ion flow which is approximately in the contains negative and positive air ions at the same time. This air ion flow, consisting of both types of air ions, is fed to the workpieces with an insulating surface in the ionization zone, with undesired electrostatic precharging on the workpiece surfaces (for example the surface of automobile bodies). or be avoided in some areas by creating a charge-neutral state.

In order to considerably and variedly demand the desired and uniform charge state in the case of automobile body painting for the purpose of an improved paint level and in particular good effect formation in electrostatic metallic painting, it is favorable, according to a preferred embodiment of the invention, to use a special conveyor device (separate floor conveyor) which , - separated from the usual floor conveyor system - guides the bodies through the ionizing zone and the subsequent painting zone.

The special conveyor is designed to allow the targeted and completely uniform state of charge and the subsequent painting process to be carried out under clearly controllable and controllable electrical conditions with regard to the application process technology and the safety rules and regulations to be observed.

The special conveyor (separate floor conveyor) for the bodies to be painted is preferably designed in the device according to the invention in such a way that the mounting or support elements for the bodies consist of insulators made of highly insulating material, such as highly insulating plastic or ceramic. For example, a normal floor conveyor can be used as a special conveyor, which is separated from the rest of the otherwise usual floor conveyor system and has insulators as support devices. The attached FIGS. 1 and 2 (FIG. 1 shows the top view; FIG. 2 shows the associated side view of a schematic diagram consisting of both figures with the designation "schematic diagram, controllable charging of the bodies with ionized air") describe an example of a device according to the invention.

Fig. 1 (top view) shows the special conveyor and the incoming and outgoing strands of the usual floor conveyor system (1) for car bodies. The bodies are transported from right to left in the figure.

As can be seen in FIG. 2 (side view), the conventional floor conveyor system (1) lowers in front of the ionizing and painting zone. The bodies are taken over by the special conveyor / special floor conveyor (2). In general, such a special floor conveyor can be, for example, an endlessly rotating plate conveyor, see Fig. 2 (side view), the plates made of insulating material e.g. consist of plastic or ceramic. The arrangement of the plates is shown in Fig. 1 (supervision). The special floor conveyor conveys the bodies through the ionizing and painting zone. Then the usual floor conveyor system returns to its original level, takes over the bodies and runs to the other operations in the usual way.

In the preferred embodiment of the invention, according to which the special floor conveyor passing through the ionization zone and painting zone is separated from the otherwise usual floor conveyor system, if necessary, an intermediate transport system can also be switched on, which transfers each individual body from the usual floor conveyor system to the special floor conveyor.

On the special floor conveyor, the body to be painted, which is usually located on a transport sled, is completely separated from the usual conveyor system, for example a floor conveyor system, through the ionization zone to create the appropriate charge and then through the zone for the painting process.

On the outlet side of the special conveyor, the body is returned to the normal transport system. If necessary, an intermediate transport can also be activated here to help smoothly transfer the workpiece, in particular the body.

The metal masses of bodies to be painted with an insulating surface are always forcibly grounded in the area of the usual conveyor system. Due to the separation of the two transport systems, there is also a compulsory earthing via cable in the area of the special conveyor system for each individual body, but for safety reasons only in the event of malfunctions or when entering the system again in parallel in direct galvanic form.

As I said, the separation of the two transport systems also means, of course, the separation of, for example, bodies from the usual grounding, via the metal transport slides which are in direct contact with the ground, for example, with a floor conveyor system. The transport slides are usually connected directly to the metal mass of each individual body, for example by means of conical metal pins.

In the area of the special conveyor, it is then ensured that the metal mass of each individual workpiece in the ionization zone is always grounded by a special contact, for example on the slide or at another suitable location, possibly via a cable to be connected. This results in advantageous technical possibilities. On the one hand, these advantages lie in compliance with safety regulations. On the other hand, however, the separation of the special system from the rest of the system also makes it possible to measure the current flowing to earth via the metal mass of each workpiece, and thus also to control it. For the control engineering side, the measurement of the current flowing out of the metal mass to earth is of great importance, since it can be seen from this whether the corresponding excess free charges generated by the influence flow out of the metal mass too quickly or in the desired sense, so that the targeted and completely uniform charge state is actually maintained.

If, for example, there was an undesired rapid charge equalization for the sprayed-in air ions within the already existing pre-coating due to the high electrical conductivity of the same, the size of the current flowing out of the metal mass of the body could be a triggering element for a control system, which causes the sprayed on Charges via the air ion current are available to an increased extent (for example by increasing the supply voltage for the ionization electrodes or devices). This is equivalent to adapting to the characteristics of the existing pre-coating (characteristics = physical properties).

Another possibility of adapting to the characteristics of a pre-painting is that the size of the current flowing from the metal mass of the body to earth is also used as a triggering element for a control system, which causes actuators consisting of high-resistance resistors and / or semiconductors connect the earth current line to limit the current so that this type of earthing then at least still remains effective as electrostatic earthing.

Of course, the grounding of the metal mass of the body described so far remains in the next process step or in the next step of the device according to the invention, the painting zone, because the painting process is carried out electrostatically in this area in order in turn to comply with the relevant safety regulations.

In many cases, despite the grounding of the metal mass of the workpiece or a body with a pre-lacquered paint structure during the electrostatic painting process, the effect of the charges that the paint droplets carry with them cannot be clearly guaranteed. Charges are applied to a pre-painted surface, which in the sense of a functioning electrostatic coating should actually be discharged to the earth, but this cannot be done because the surface they hit (this has a layer thickness of approx. 70 to 80 µm) as a hardened lacquer layer), has a high insulation capacity and therefore has an insufficient surface conductivity.

The special conveyor device of the device according to the invention, as used in the method according to the invention, also allows a solution to this problem.

In the special conveyor area, in the electrostatic painting process according to one embodiment of the invention, an earthed electrode touching the body surface can be installed on the special conveyor in such a way that it is and remains effective at a suitable point on the body surface in the entire electrostatic painting area. This electrode can be designed in a foldable construction and can be folded away when it is no longer required for the workpiece in question. There can also be an automatic cleaning device which provides for an interim cleaning of the foldable electrode so that it is available again for the next body to be painted.

According to a further embodiment of the invention, the contacting electrode can be transported with the body to be painted via its own transport system, as long as it is required. For the next body, the electrode can be quickly returned, if necessary, by switching on an intermediate cleaning process.

Instead of a touching, grounded electrode, a contactless spray electrode, which is also grounded, can also be used according to a further embodiment of the invention.

These charge leads secured to the earth have the advantage that the charges applied to the body surface by the paint droplets can be measured as earth current and can thus also be controlled. The device according to the invention can therefore have corresponding measuring and control devices.

The above types of secured charge dissipation during the painting process benefit from the fact that the paint for contact charging has to have a certain conductivity (about 10⁻⁸ ohm⁻⁸¹ x cm⁻¹ ± a power of ten). Since the electrostatic painting process represents a re-painting and re-painting of the surface elements - starting from the surface area of a body, for example, where the electrodes (direct contact electrode or contactless syringe electrode) are supposed to be effective - the charge dissipation in question here becomes complete painting at first the body itself from the freshly applied paint - seen over the entire body surface - adopted.

The whole is much easier to implement if the above technical possibilities in the area of the special conveyor according to the invention take place. You would otherwise have to equip each individual transport sled accordingly to ensure that loads are discharged during the painting process of every body that is transported using the usual floor conveyor system.

The attached Figures 1 and 2 represent an example of a device for painting workpieces with an insulating, non-conductive surface according to the invention. This device is particularly suitable for painting car bodies.

1 denotes a conventional floor conveyor system, where 1a represents the incoming line and 1b the outgoing line. Between 1a and 1b, a special conveyor 2 is designed as a separate floor conveyor. At 3 the transfer takes place on a plate conveyor (e.g. made of plastic), whereby an intermediate transport can take place if necessary. the special conveyor passes through the ionizing zone and the painting zone (E-statics). At 4, the transfer takes place to the running line of the floor conveyor system, possibly via an intermediate transport. Earthing elements 5 are provided in the special conveyor system depending on the division or arrangement of the plates. 6 shows a derivative that can be regulated, for example, via semiconductors. The triggering element for the control is the current flowing out of the metal mass to earth (this is determined via a current measuring element, not shown). A positive earthing 7 is provided, which is important in the event of a fault or when entering the system; positive earthing can be done, for example, in galvanic form. 8 shows a positive earthing of the workpieces or bodies upon leaving the system, for example in galvanic form. FIG. 2 clearly shows the lowering of the incoming line of the conventional floor conveyor system 1a, the renewed appearance of the outgoing line 1b in its original level, and the Special floor conveyor 2, which has, for example, plates made of plastic or ceramic.

Claims (18)

1. Process for painting workpieces with an electrically insulating, non-conductive surface, by electrostatic application or spray application,
characterized by
that a uniform charge state is generated on the workpiece surface before the paint is applied. 2. The method according to claim 1, characterized in that the uniform state of charge on the insulating, non-conductive workpiece surface is generated by ionization. 3. The method according to claim 2, characterized in that the ionization is generated by a positive and / or negative air ion flow. 4. The method according to any one of the preceding claims, characterized in that such a workpiece made of insulating, non-conductive plastic, or one with an insulating, non-conductive plastic coating is used. 5. The method according to any one of the preceding claims, characterized in that such a workpiece is used with a surface painted at least once. 6. The method according to claim 5, characterized in that an at least once pre-painted motor vehicle body or a component thereof is used as the workpiece. 7. Device for painting workpieces with an insulating, non-conductive surface, by electrostatic application or spray application, with a conveyor system for the workpiece to be painted, which passes through a painting zone,
characterized by
that the painting zone is preceded by an ionizing zone. 8. The device according to claim 7, characterized in that ionization devices for generating an ion wind or air ion current are present in the ionization zone. 9. Device according to one of claims 7 or 8, characterized in that in the region of the painting zone and the upstream ionization zone there is an additional conveyor device (special conveyor device) separate from the remaining conveyor device. 10. Device according to one of claims 7, 8 or 9, characterized in that the conveying device passing through the ionizing zone and painting zone has carrier or holding devices for the workpiece made of insulating material. 11. The device according to claim 9, that the ionizing and painting zone passing special conveyor is provided with insulating support or holding devices. 12. The device according to one of claims 7 to 11, characterized in that it is designed for the serial painting of motor vehicle bodies. 13. The device according to one of claims 7 to 12, characterized in that it has a hall conveyor system for automotive or body and Kakrossteile serial coatings with separate floor conveyor (special floor conveyor) with insulating supports for the ionizing and painting zone. 14. Device according to one of claims 9 to 13, characterized in that a device for permanent grounding of the metal mass of each individual workpiece to be painted is provided in the area of the special conveyor. 15. The apparatus according to claim 14, characterized in that a measuring device for measuring the current flowing through the permanent ground is present. 16. The apparatus according to claim 15, characterized in that the measuring device is coupled to a control device which is suitable for controlling the charge strength of the air ion flow in the ionization zone on the basis of the measured values supplied by the measuring device. 17. The apparatus according to claim 16, characterized in that it in the area of the special conveyor in the earth circuit can be switched on and off, optionally automatically switching on and off current-limiting actuators. 18. Device according to one of claims 14 to 17, characterized in that they have contacting or contactless grounding electrodes for the workpiece to be painted in the painting zone in the region of the special conveyor, which are effective during the entire painting process and can then be cleaned.

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