EP0297422B1 - Device for the electrostatic-coating of flat objects - Google Patents

Abstract

Apparatus for electrostatic spray coating of plate-like substrates, particularly printed circuit boards, has one or more endless conveyors or one or more conveyors of finite length with one or two pairs of strip-shaped holding elements for the edges of substrates. The holding elements engage a substantial part or the entire edge of the substrate which is transported through one or more spray coating stations. Each holding element is made of or contains an electrically conductive material, and the holding elements are grounded, at least during travel through the coating station or stations. The holding elements can engage the edges of substrates by friction or they are coated with an adhesive so that they adhere to the edges of the substrates.

Description

The invention relates to a device for the electrostatic spray coating of plate-shaped workpieces, with a conveyor which transports the workpieces at least through a coating zone and carries fastening elements which act on the workpiece and which simultaneously serve to ground the workpiece.

If workpieces are to be coated electrostatically, regardless of whether the atomization of the coating material, such as paint or paint, is supported by mechanical or pneumatic means, it must be ensured that the workpiece is not charged by the charge carriers hitting the spray particles. In the simplest case, the workpiece is grounded via the conveyor and its fastening elements, which carry the workpieces and are designed as hooks. As DE-PS 24 46 767 notes, the critical point here is the suspension of the workpieces on the conveyor. Because this suspension is also coated with spray material over time, so that the contact resistance increases and the workpieces are charged accordingly. For this reason, contactless discharge by ionization devices is provided.

The invention has for its object to provide a device of the type described above, which enables secure attachment with good grounding.

This object is achieved in that the fasteners have two retaining strips that receive an edge of each workpiece between them at least over most of its length and that the fasteners consist of an electrically conductive material.

Because of the large length of the holding strips, small clamping pressures are sufficient to hold the workpiece securely. There is therefore no risk of damage to the workpiece surface. The holding surface can be kept narrow, so that the non-coated edge area of the workpiece is very small. Since the holding surfaces are arranged on the inside of the holding strips and bear against the workpiece during coating, contamination of the holding surfaces is largely excluded. Because of the length of the holding surfaces, there are definitely sections over which a safe ground contact can be made. Another advantage is that there is surely a surface free of coating material at the edges of the workpieces. Should the workpiece therefore have to be gripped again by fastening elements in a later process section, it can be set up without difficulty that they attack the coating-free surface and are not soiled by coating material that is already present and softens during the treatment.

All of this is particularly advantageous when electrical printed circuit boards that are coated on one or both sides with a copper layer are to be coated with photoresist. Because the copper layer contacted at the edge is not damaged and results in a good grounding effect over the entire plate surface. Since the holding surface and the copper layer touch over a longer length, the contact resistance that is important for grounding is extremely low. It is irrelevant that the plate remains on a narrow edge without applying photoresist, because in the finished printed circuit board there is generally no conductor path along the edge anyway. And if a solder resist is to be applied later in a similar way, there are coating-free areas on the edge for attacking the fastening elements then required. In all of this, no places need to be left out in the middle of the circuit board pattern, where a fastening hook or the like. can attack.

The holding strips run particularly advantageously. They therefore extend over the entire length of the edge of the workpiece and beyond to the next workpiece. The holding surfaces can abut each other in the free space, so that there is no fear of contamination there either. The workpieces can have different lengths in the transport direction without having to make a change.

In one embodiment, endless tether strips are used that are in orbit. Repeated circulation is permitted because the holding surfaces are so protected that there is no risk of contamination. Of course, a cleaning zone can be run through for each cycle after leaving the working zones.

An alternative is to use finite holding strips that can be removed from a strip supply and can be fed to a collection point. This is particularly suitable for holding strips made of a material that is subsequently to be thrown away.

A recommended further development has a double conveyor, which has two holding strips for gripping each other on opposite parallel edges of the workpiece. This leads to an extremely stable holding of the workpiece during transport. When the workpiece is moved vertically in front of a coating device, the working distance is precisely defined. There is no danger that the workpiece will move laterally under the influence of the mechanical forces of spray particles or of atomizing or shaped air. It is therefore also possible to use coating devices which release particles with a relatively large kinetic energy. This also applies in particular when the vertically arranged workpiece passes through two coating zones in succession, in which it is coated from both sides.

The path of the double conveyor can also be designed so that the workpieces cross their inclination to the horizontal during transport change to conveying direction. The workpiece can therefore always be in such a position as is most favorable for the operation in question.

For example, the path of the double conveyor can be designed such that the workpieces are picked up in the horizontal position between the holding strips, guided in the vertical position at least through one coating zone and put down again in the horizontal position. The horizontal position is best for picking up stacked workpieces and for placing them in the stack, while the vertical position is recommended for coating, especially double-sided coating. Other zones for evaporation, drying or cooling can be run through vertically or horizontally.

In a preferred embodiment, an endless elastic conveyor belt has holding strips formed integrally therewith along one edge, which act elastically against one another. This results in a very simple combination of conveyor belt and holding strip. Since small clamping pressures are sufficient, the inherent elasticity of the holding strips is sufficient in most cases. Otherwise, spring elements extending in the longitudinal direction, preferably embedded, can be provided.

The conveyor belt expediently has such a profiling that the two retaining strips are moved apart when two pressure elements engage externally. In the area of the pressure elements, the gap between the holding strips opens, so that the plate-shaped workpiece can be inserted between the holding strips or removed between them.

In particular, the profiling can consist of a longitudinal cavity adjacent to the holding strip and the pressure elements can engage in the area of the longitudinal cavity. This is a very simple embodiment.

Furthermore, the conveyor belt should have an outer longitudinal profile on both sides, on which support rollers engage. Such support rollers can also deflect the conveyor belt.

Furthermore, it should be ensured that the conveyor belt has such a rigidity due to its material and / or cross section that it spans the coating zone in an unsupported manner in an upright position. There are then no guide parts within the coating zone that could become dirty there.

This can be achieved above all by the fact that the conveyor belt has reinforcing inserts in the middle of its elastic material.

In another embodiment it is ensured that the conveyor is formed by a tape provided at its edges with adhesive, which can be bent in front of the coating zone along its longitudinal center to form holding strips, thereby engaging around a guide rail and with its edges being glued to the workpiece and can be cut behind the coating zone along the longitudinal center, the two halves of the tape being removable. This tape can consist, for example, of paper, plastic film or the like, that is to say of a cheap disposable material. The adhesive can be chosen so that the tape halves are easy to peel off. Because of the gluing over the entire length of the edge, relatively low adhesive forces per unit length are sufficient. The guide rail runs through the coating zone, but is protected against contamination by the wrapping tape.

The tape can be peelable from a roll. Furthermore, the band halves can be wound on driven rollers. As a result, the belt acts as a conveyor.

Furthermore, profile rollers can be provided for bending the band and / or for supporting the guide rail. In most cases, it is not necessary to support the guide rail in the middle of the coating zone in this way.

The guide rail is preferably a round rod. This makes it easier to install and bend the tape.

• Fig. 1 eine schematische Draufsicht auf eine erfindungsgemäße Vorrichtung zur elektrostatischen Sprühbeschichtung,
• Fig. 2 eine Seitenansicht des Anfangsteils der Beschichtungsstrecke,
• Fig. 3 den Bahnverlauf eines Doppelförderers bei einer abgewandelten Ausführungsform,
• Fig. 4 in einem Schnitt längs der Linie A-A der Fig. 3 verschiedene Lagen des plattenförmigen Werkstücks,
• Fig. 5 einen Schnitt durch ein Förderband an einer Umlenkstelle,
• Fig. 6 das Förderband der Fig. 5 mit Andruckelementen,
• Fig. 7 das Förderband der Fig. 5 und 6 mit eingeklemmtem Werkstück,
• Fig. 8 eine schematische Seitendarstellung einer abgewandelten Ausführungsform,
• Fig. 9 einen Schnitt längs der Linie B-B der Fig. 8,
• Fig. 10 einen Schnitt längs der Linie C-C der Fig. 8,
• Fig. 11 einen Schnitt längs der Linie D-D der Fig. 8,
• Fig. 12 einen Schnitt längs der Linie E-E der Fig. 8 und
• Fig. 13 einen Schnitt längs der Linie F-F der Fig. 8.

The invention is explained below with reference to exemplary embodiments illustrated in the drawing. Show it:

• 1 is a schematic plan view of an apparatus for electrostatic spray coating according to the invention,
• 2 is a side view of the initial part of the coating section,
• 3 shows the path of a double conveyor in a modified embodiment,
• 4 in a section along the line AA of FIG. 3 different layers of the plate-shaped workpiece,
• 5 shows a section through a conveyor belt at a deflection point,
• 6 the conveyor belt of FIG. 5 with pressure elements,
• 7 shows the conveyor belt of FIGS. 5 and 6 with a clamped workpiece,
• 8 is a schematic side view of a modified embodiment,
• 9 shows a section along the line BB of FIG. 8,
• 10 shows a section along the line CC of FIG. 8,
• 11 is a sectional view taken along line DD of FIG. 8,
• Fig. 12 is a section along the line EE of Fig. 8 and
• 13 shows a section along the line FF of FIG. 8.

In the devices for electrostatic spray coating illustrated in FIGS. 1 and 2, plate-shaped workpieces 1 are coated with a liquid spray material. In particular, the workpieces are circuit board blanks, which carry a thin copper layer on both sides and are coated with a photoresist, so that after subsequent exposure and the etching away of the unexposed parts, the remaining parts of the copper layer are the conductors form tracks. A solder resist can be applied later in a similar device.

There is a conveyor 2 with two superimposed conveyor belts 3 and 4, which grasp the workpieces at opposite edges over their entire length. The conveyor belts 3 and 4 are deflected over a plurality of support rollers 5 to 10 and kept in circulation by a motor 11 driving the support roller 10. The following zones are run through:

In a loading zone 12, a transfer device 13 equipped with suction devices picks up the uppermost workpiece of a stack 14 and pivots it into a vertical position in which it is gripped by the two conveyor belts 3 and 4 directly or after using an intermediate carrier. An intermediate carrier moving in the conveying direction is required, for example, if the conveyor belts are driven continuously. A device 16 is schematically illustrated, with the aid of which the workpieces can be fastened to the conveyors, as is explained in more detail in connection with FIG. 6.

A pre-cleaning zone 17 is provided in order to clean the workpieces with ionized blown air.

A first coating zone 18 has a closed housing 19, in which a coating device 20 is arranged. This has a rotary atomizer 21 for mechanical atomization and a high-voltage electrode for ionizing these spray particles. This high-voltage electrode, which can also be formed by the rotary atomizer 21, is connected to a high-voltage source 22. The two conveyors 3 and 4 consist of an electrically conductive material and are grounded, which can be done, for example, by grounding a support roller 9. The workpieces are therefore coated on one side in the coating zone 18.

A second coating zone 23 follows, which in turn has a closed space 24 with a coating device 25. This has a rotary atomizer 26. The associated high-voltage electrode is in turn connected to the high-voltage generator 22. In this coating zone 23, the workpieces are coated on the second side.

An evaporation zone 27 follows, which gives the applied lacquer time to form a uniform film. Support rollers 28 are provided at the exit from the second coating zone. The conveyor belts 3 and 4 can have such a sufficient rigidity that no further support takes place between the support rollers 10 and the support rollers 28, so there is also no risk that such rotating operating parts become inoperative due to the coating material over the course of time.

A drying zone 29 now follows. Drying can take place, for example, using IR radiators. A controllable solvent suction device can also be connected.

A cooling zone 30 follows. Cooling can take place, for example, with filtered fresh air. A suction device for the resulting solvent can also be connected here.

In the following unloading zone 31, the workpieces are detached from the conveyor belts 3 and 4 by means of a schematically indicated device 32 and placed on a stack 33 with the aid of a transfer device 32 ′, optionally again using intermediate carriers.

On the return path, the conveyor 2 passes through a cleaning zone 34. The cleaning can be carried out chemically, mechanically, with ultrasound or in another known manner.

Any liquid residues are evaporated in a drying zone 35.

3 and 4, reference numerals increased by 100 are used for corresponding parts. The two conveyor belts 103 and 104, which hold the workpieces 101 between them, are guided with the aid of support rollers (not illustrated) on such a path that the workpieces are vertical in the coating zone Z1, but are arranged horizontally in the loading zone Z2 and the unloading zone Z3, as illustrated in Fig. 4 for the dashed workpiece 101 '. There are therefore respective transition zones Z4 and Z5 in which the conveyor belts 103 and 104 are rotated helically in such a way that the desired change in inclination for the workpieces 101 takes place during transport. Now the workpieces no longer need to be pivoted vertically during loading and unloading, which simplifies assembly.

5 to 7 show an embodiment of one of the conveyor belts, here the conveyor belt 3. The conveyor belt 3 has an outer longitudinal profile 36 and 37 on both sides, here in the form of a common bead 38. It consists of rubber-elastic material, which can be reinforced in the upper part by an elastic metal strip 39.

At the lower free end two holding strips 40 and 41 are provided, which are held against each other due to their inherent elasticity. An inner profile 42 in the form of a longitudinal cavity is provided near these holding strips. If pressure elements 43 and 44 engage in the form of rollers at the level of this profiling, the holding strips 40 and 41 diverge so that holding surfaces 45 and 46 are exposed. Now the plate-shaped workpiece 1 can be inserted between the holding strips 40, 41 without difficulty. If the pressure elements 43 and 44 are then removed, the holding strips 40 and 41 try to return to their rest position and clamp the workpiece 1 over its entire edge.

This results in a grounding of the workpiece with a very low contact resistance. Only a very narrow edge area is required for clamping. The holding surfaces 45 and 46 are well protected when passing through the coating zones because they are either on the workpiece 1 or abut each other. After the treatment, defined edge areas remain on the workpieces, which can be detected again in a later treatment, without coating material being at the point of contact, which could increase the contact resistance. The device is suitable for workpieces of different lengths without modification. If different widths are also to be treated, the distance between the conveyor belts 3 and 4 can be changed.

The material for the conveyor belts is selected so that it is resistant to the coating material at least on its surface and that it is made electrically conductive by itself or by the addition of additives or by a surface coating. A tape made of silicone rubber, which is coated with a layer of polytetrafluoroethylene, is particularly suitable. The electrical conductivity can be achieved by the inclusion of carbon particles.

In the embodiment according to FIGS. 8 to 13, reference numerals increased by 200 are used for corresponding parts. In this case, a belt 203 serves as the conveyor 202 and is unwound from a supply roll 47. It is provided with an adhesive strip 48 and 49 on each of its two edges. Both the tape 203 and the two adhesive strips 48 and 49 are made of an electrically conductive material. A guide rail 50 extends here over the entire working area, here in the form of a round bar, which is supported at least at its two ends via grounded supports 51 and 52. The web can be made of crepe paper, for example.

With the aid of a first profile roller 53, which has a circumferential groove 54 in the center, the band 203 is held against the guide rail 50 (FIG. 9). This is followed by two inclined guide rollers 55 and 56, which bend one half of the band 203 downward over the guide rail 50 to form two holding strips 57 and 58 (FIG. 10). Then come two pressure rollers 59 and 60, which press the adhesive strips 48 and 49 against the edge of the workpiece 201. A profile 61 serves to support the guide rail 50. The workpiece 201 can now be transported into the coating zone. If necessary, further profile rollers 62 and 63 can be provided which support the guide rail 50 with a circumferential groove 64 (FIG. 12).

At the end of the treatment zones, a cutting device 65 is provided, which cuts the band 203 along its longitudinal center. The two halves serving as holding strips 57 and 58 are pulled off to the side and wound up on driven rollers 66 and 67. This also results in the drive movement of the conveyor 202. The tape material can then be thrown away. The guide rail 50 is covered by the tape 203 during the entire coating process, so that it cannot become dirty.

The devices described can be operated continuously or step by step. In particular, fully automatic operation is possible.

Claims (18)

1. Apparatus for electrostatically spray coating laminar workpieces, comprising a conveyor which transports the workpieces at least through one coating zone and carries fastening elements engaging the workpiece which also serve to earth the workpiece, characterized in that the fastening elements have two holding strips (40, 41; 57, 58) which take up one edge of each workpiece (1; 101; 201) at least over the greatest part of its length between them and in that the fastening elements are composed of an electrically conductive material.

2. Apparatus according to Claim 1, characterized in that the holding strips (40, 41; 57, 58) extend continuously.

3. Apparatus according to Claim 2, characterized by endless holding strips (40, 41) which are conveyed on a circulating track.

4. Apparatus according to Claim 2, characterized by endless holding strips (57, 58) which can be drawn from a strip feed (roller 47) and can be fed to a collecting point (rollers 66, 67).

5. Apparatus according to one of Claims 1 to 4, characterized by a double conveyor (2; 102) which has, in each case, two holding strips for acting on mutually opposite, parallel edges of the workpiece.

6. Apparatus according to Claim 5, characterized in that the track of the double conveyor (102) is so designed that the workpieces (101) change their inclination to the horizontal transversely to the conveying direction during transport.

7. Apparatus according to Claim 6, characterized in that the track of the double conveyor (101) is so designed that the workpieces (101) are taken up in the horizontal position between the holding strips, conveyed in the vertical position at least through one coating zone and set down again in the horizontal position.

8. Apparatus according to one of Claims 1 to 7, characterized in that and endless elastic conveyor belt (3, 4) has, along one edge and formed integrally with it, holding strips (40, 41) which act elastically towards each other.

9. Apparatus according to Claim 8, characterized in that the conveyor belt (3, 4) has a profiling (42) such that, when acted upon externally by two pressure elements (43, 44), the two holding strips (40, 41) are separated.

10. Apparatus according to Claim 9, characterized in that the profiling (42) is a longitudinal cavity adjacent to the holding strips (40, 41) and the pressure elements (43, 44) act in the region of the longitudinal cavity.

11. Apparatus according to one of Claims 8 to 10, characterized in that the conveyor belt (3, 4) has, on both sides, an external longitudinal profiling (36, 37) on which the supporting rollers (10) act.

12. Apparatus according to one of Claims 8 to 10, characterized in that, owing to its material and/or cross-section, the conveyor belt (3, 4) has a rigidity such that it spans, in a self-supporting manner, the coating zone (18, 23) in an upright position.

13. Apparatus according to Claim 12, characterized in that the conveyor belt (3, 4) has reinforcing inserts (39) surrounded by its elastic material.

14. Apparatus according to one of Claims 1 to 7, characterized in that the conveyor (202) is formed by a belt (203) which is provided with adhesive (48, 49) at its edges and which can be bent over along its longitudinal centre to form holding strips (57, 58) upstream of the coating zone, embraces a guide rail (50) in doing so and can be bonded by means of its edges to the workpiece (201) and can also be cut along the longitudinal centre downstream of the coating zone, in which process the two halves of the belt can be peeled off separately.

15. Apparatus according to Claim 14, characterized in that the belt (203) can be pulled off a roller (47).

16. Apparatus according to Claim 14 or 15, characterized in that the belt halves can be wound onto driven rollers (66, 67).

17. Apparatus according to one of Claims 14 to 16, characterized in that profiled rollers (53, 55, 56, 59, 60, 62, 63) are provided for bending the belt (203) over and/or for supporting the guide rail (50).

18. Apparatus according to one of Claims 14 to 17, characterized in that the guide rail (50) is a round rod.

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