EP0505967B1 - Process for depositing an enamel layer - Google Patents

Description

The invention relates to a method for depositing an enamel layer according to the preamble of claim 1.

Electro-dip enamelling means the transport of charged particles in liquid emulsion through an applied electric field. In contrast to electrolysis, this involves the transport of much larger particles; furthermore, there is no chemical and physical change in the deposited substance. When in contact with water as a dispersion medium, the enamel particles become negatively charged and migrate to the workpiece when an electric field is applied, which is connected as an anode. At the same time, electroosmosis causes the opposite movement of the liquid to the cathode in the deposited enamel slip. The deposited enamel layer is thereby pre-dried, although the part remains in the aqueous medium.

Workpiece surfaces on which no enamel layer is to be deposited or formed could in principle be covered within the scope of the generic method. Thus, an essentially tight connection between a workpiece surface to be kept essentially free of an enamel application and a cover element prevents enamel particles from depositing on the workpiece surface to be kept free. The arrangement and the subsequent removal of such cover elements on workpieces is relatively time-consuming, so that productivity is reduced or production costs are increased by such method steps.

For the deposition of thicker enamel layers on a specific workpiece surface, it is also known to design the cathode to be adapted to the workpiece shape in such a way that there is a dense course of electrical current lines toward this workpiece surface, and thus sufficient deposition of enamel particles. The adaptation of the cathode shape to the workpiece shape is complex, and therefore uneconomical, in particular in the production of small series.

The invention has for its object to develop a generic method so that predeterminable workpiece surfaces can be covered in a simple and reliable manner only with an enamel layer of small thickness, and that predeterminable workpiece areas in a simple and reliable manner with an enamel layer of greater thickness or with one compared to others Workpiece surfaces of constant thickness can be covered.

The invention is further based on the object of creating a device for simple implementation of the method.

The object relating to the method is achieved according to the invention in a generic method by the characterizing features of claim 1.

According to the invention, to influence the thickness of an enamel layer which is deposited on a predefinable workpiece surface, a spacing element is assigned to the predefinable workpiece surface.

The thickness of the enamel layer deposited on a workpiece surface can thus be influenced in a simple and reliable manner. It can be ensured, for example for a back of an object to be enamelled that does not need to be enamelled, that only a thin layer of enamel is applied there. This saves email compared to keeping email completely free. However, the solution according to the invention enables a rapid process sequence and thus high productivity with sufficient savings in enamel compared to the known sealing cover.

Furthermore, the method according to the invention can also be used to ensure in a simple manner that a workpiece surface is formed with a relatively thick enamel layer or an enamel layer with a thickness that is constant compared to the other workpiece surfaces.

Thus, on the one hand, according to the invention, an enamel layer can be reduced to a relatively small thickness in a simple and inexpensive manner where it is not necessary for an enamel layer to be formed. On the other hand, according to the invention, a constant or an increased thickness of the Eamille layer can also be achieved where this is required, for example, for functional or optical reasons.

In a preferred exemplary embodiment of a method in which the workpiece is arranged on a carrier device before being immersed in the enamel slip bath and is immersed with the latter in the enamel slip bath, the shielding element is arranged on the carrier device according to the invention; the assignment to the predeterminable workpiece surface takes place by fastening the workpiece to the carrying device.

In a particularly advantageous manner, the shielding element can, at least in regions, have an insulating section on the side facing away from the workpiece, which is connected to the electrically conductive section. On the one hand, the insulating section prevents the creation of a large potential between the cathode and the workpiece, and at the same time it ensures that only small amounts of enamel particles are deposited on the surface of the insulating layer. This will make an otherwise necessary frequent cleaning of the shielding element avoided. The insulating layer can be formed with a size corresponding to the size of the conductive section and can be connected to the conductive section essentially over the entire surface. However, the insulating section can also be made smaller and connected to it covering only a region of the conductive section.

In a further preferred embodiment, the shielding element is a shielding plate which is arranged at a distance essentially parallel to the predeterminable workpiece surface. Both the conductive section and the insulating section can be plate-shaped and lie flat against one another.

In a method for a workpiece with a flat base body and side walls extending essentially perpendicularly therefrom, the shielding plate is advantageously arranged such that it extends essentially in one plane with end faces of the free ends of the side walls.

The shield can advantageously cover the predeterminable workpiece surface with an overhang. It can thus be ensured in a simple manner that a workpiece surface for which only a small enamel layer is to be deposited is provided essentially in its entirety with an enamel layer of the desired smaller thickness.

If the procedure is essentially the same, in a method in which a thicker enamel layer is to be deposited on a predefinable workpiece surface, according to the invention, in order to influence the electrical potential of the enamel slip bath compared to the predefinable workpiece surface, a shielding element is assigned to it which has at least one curved or a prism-shaped area. Due to the area which is curved or prismatic in cross section, for example, the course and the density of electrical current lines can be predetermined in certain areas in such a way that a thicker enamel layer is deposited in the desired manner.

Due to the shape and arrangement of the shielding element, for example, different distances from workpiece surfaces to the cathode can be compensated for in such a way that with a predeterminable voltage or current between the cathode and the workpiece as an anode, a predeterminable dwell time of the workpiece in the enamel slip bath is essentially uniform or at least a sufficient thickness of the enamel layer is obtained at each point with regard to the requirements.

In a further preferred embodiment for a method for a workpiece with a recess to be covered with an enamel layer, according to the invention, for an adequate deposition of enamel, an insulating plate with an electrically conductive section extending at least in regions thereof is assigned as a shielding element such that between A region with a high potential difference arises from the cathode and anode, from which the enamel particles in a recess via the conductive section Amount supplied that is sufficient for the deposition of an enamel layer of predeterminable thickness on the surfaces of the recess.

It can thus be ensured in a simple manner that an enamel layer which is constant or reinforced compared to the other workpiece surfaces is deposited specifically on surfaces of recesses on which the thickness of the enamel layer is normally reduced.

In a device for carrying out the method according to one or more of the above-mentioned features with an enamel slip bath with a cathode and a connection for the workpiece as an anode, a shielding element to be assigned to the workpiece is provided according to the invention, with an electrically conductive section to be connected as a secondary anode and a section attached to it , the electrically conductive section partially covering insulating section. With the shielding element according to the invention, the deposition of enamel on an assigned workpiece surface can be influenced in a simple and reliable manner.

In the practical application of electro-dip enamelling (ETE), the bath voltage is approximately 80 to 120 volts at a current density of 3 to 6 A / dm ² and the electrical conductivity of the slip to be separated is between 1500 and 3500 µS / cm. The conductance is brought to this value with sodium aluminate in order to achieve a wrapping around of the coating, ie enamel also deposits on the side facing away from the cathode. The disadvantage of the process lies in the insufficient edge coating, since the enamel "prefers" to wander the edges. After firing, there is a black, narrow stripe on the edges, which makes the goods sharp-edged.

It is therefore a further object of the invention to prevent edge deterioration by means of procedural measures. This can be done by the measure according to claims 11 to 15.

Fig. 1

einen Querschnitt durch ein Emailschlickerbad mit darin angeordnetem Werkstück gemäß einem ersten Ausführungsbeispiel,

Fig. 2

einen Querschnitt durch ein Emailschlickerbad mit darin angeordnetem Werkstück gemäß einem zweiten Ausführungsbeispiel,

Fig. 3

einen Querschnitt durch ein Emailschlickerbad mit darin angeordnetem Werkstück gemäß einem dritten Ausführungsbeispiel und

Fig. 4

eine unvollständige Vorderansicht zur Erläuterung des dritten Ausführungsbeispiels,

Fig. 5 bis Fig. 7

Darstellungen von Prizip-Anordnungen für eine Elektro-Tauchemaillierung mit Hilfskathoden.

Three implementation examples for the method according to the invention are explained in more detail with reference to the drawings. Each shows in a schematic view:

Fig. 1

3 shows a cross section through an enamel slip bath with a workpiece arranged therein according to a first exemplary embodiment,

Fig. 2

3 shows a cross section through an enamel slip bath with a workpiece arranged therein according to a second exemplary embodiment,

Fig. 3

a cross section through an enamel slip bath with a workpiece arranged therein according to a third embodiment and

Fig. 4

an incomplete front view to explain the third embodiment,

5 to 7

Representations of principle arrangements for electro-dip enamelling with auxiliary cathodes.

In Figure 1, a workpiece 1 is shown, which is to be provided on its outer surface 3 with an enamel layer. The workpiece 1 is shown in a state in which it is immersed in an enamel slip bath 5, not shown, which surrounds the workpiece on all sides. The workpiece 1 has a flat base body 7, with side walls 9 extending essentially perpendicularly therefrom. The workpiece 1 consisting of a conductive metal is connected in a manner not shown as an anode, while a cathode is also assigned to the enamel slip bath 5 in a manner not shown. A DC voltage acts between the cathode and the anode for depositing enamel particles 11 on the workpiece 1. Negatively charged enamel particles 11 are deposited on the positive workpiece surfaces of the workpiece 1 so that they adhere firmly. Such an enamel layer deposited on workpiece surfaces of the workpiece 1 adheres so well that only loosely adhering enamel slip particles 11 are rinsed off during a cleaning step following immersion in the enamel slip bath 5, and that in a subsequent process step the separated enamel layer can be baked in a kiln.

To explain the method according to the invention, it is assumed that the rear side 13 of the workpiece 1, which is formed both on the rear side of the side walls 9 and on the rear side of the flat base body 7, cannot be enamelled as a specifiable workpiece surface. This can be the case, for example, if the workpiece 1 is part of a double-walled domestic appliance door, such as for example, an oven door. It is only necessary that the outer surface 3 is enamelled.

In order to save enamel, which could be deposited on the back 13 of the workpiece 1 when the entire workpiece 1 is immersed in the enamel slip bath 5, a shielding element 15 is assigned to the workpiece 1 or its back 13. According to the shape of the base body 7, the shielding element is designed as a shielding plate. The shielding plate 15 consists of a likewise plate-shaped, electrically conductive section 17 and an insulating section 19 covering the entire surface of the latter and connected to it in an unillustrated manner, for example by gluing or screwing. The shielding plate 15 is arranged according to the invention in such a way that the insulating section 19 corresponds to this Workpiece 1 is facing away. The shielding plate 15 is also essentially parallel to the base body 7 and is arranged at a distance therefrom such that the shielding plate 15 extends essentially in one plane with end faces 21 of the free ends of the side walls 9. Between the side walls 9 shown in FIG. 1 and the shielding plate 15, a gap with a gap width is formed such that the shielding plate 15 can be assigned to the workpiece 1 in a simple manner. In a corresponding manner, the shielding plate 15 can be assigned to the upper and lower side walls, not shown.

In a manner not shown, the conductive section 17 of the shielding plate 15 is connected as a secondary anode, so that, as can be seen from FIG. 1, the surface of the conductive one Section 17 has the same polarity as the workpiece surfaces of the workpiece 1; the polarity is indicated by the "+" signs shown in FIG. 1.

In the space between the shielding plate 15 and the rear side 13 of the workpiece 1, the conductive section 17 connected as a secondary anode results in an essentially uniform potential. For enamel particles 11 'located there, the negative charge caused outside of this space by the cathode of the enamel slip bath 5 is therefore neutralized. There are essentially no attractive forces exerted on the enamel particles 11 'located there either by the rear side 13 or the conductive section 17; Enamel particles are thus deposited on these surfaces only to a small extent and with low adhesion. In this way, according to the invention, email can be saved in a simple manner because it effectively prevents email from being deposited to a significant extent on the rear side 13. This applies accordingly to the conductive section 17 of the shielding plate 15.

So that no enamel particles 11 are deposited on the negatively charged enamel particles 11 on the opposite side 23 of the conductive section 17, the insulating section 19 is arranged there, on which the enamel particles settle at most to a small extent and with low adhesion.

As indicated by streamlines 25, a high streamline shape density results only in relation to the outside 3 of the workpiece 1 to be provided with an enamel layer. The inventive shielding measure of the assignment of a shielding element 15 to the workpiece 1 thus advantageously results in the formation of streamlines in such a way that deposition of enamel particles 11 on the outside 3 of the workpiece 1 is promoted.

The shielding element or the shielding plate 15 according to the invention can advantageously be arranged on a support device, not shown, on which the workpiece 1 can be fastened. A corresponding assignment between the shielding element 15 and the workpiece 1 is thus obtained in a particularly simple manner simply by attaching the workpiece 1 to the carrying device, and thus without a further additional method step. The workpiece 1 can also be pretreated with the carrying device, for example cleaning before being immersed in the enamel slip bath 5. It can then be immersed together with the carrying device in the enamel slip bath 5 and then subjected to an aftertreatment, such as a rinsing process.

In contrast to the method described in connection with FIG. 1, in the method to be explained with reference to FIG. 2, no flat shielding element is arranged to cover predetermined workpiece surfaces. The shielding element 31 is rather prism-shaped, in the present case having an essentially circular cross section. The conductive section 33 connected as an auxiliary anode, not shown, has a circular cross section. An exposed area of the conductive section 33 is arranged facing a workpiece 35, which in the present case has a Z-shaped cross section. At the The side facing away from the workpiece 35 is covered by an insulating section 37 which is designed in the manner of a hollow cylindrical half-shell.

As can be seen from the flow lines 39 from FIG. 2, negatively charged enamel particles 11 are attracted to the positive charge of the conductive section 33. It is thereby achieved that the workpiece surface 41 opposite the shielding element 31 is provided with an enamel coating, the thickness of which is less than that of adjacent sections to which streamlines 43 of higher density result than is the case for the workpiece surface region 41.

The arrangement of a shielding element 31 can thus be ensured according to the invention in a simple manner that, on the one hand, an enamel layer of smaller thickness is deposited on the workpiece surface region 41 opposite the shielding element 31, while on the other hand, adjacent to this region, an enamel layer of greater thickness is formed. In this way, areas of the workpiece 35, such as the free end 47 and the edge area 49, which are adjacent to the workpiece surface area 41, can be provided with an enamel layer 45 of sufficient thickness in a simple manner without, for example, the adaptation of the cathode to the shape of the Workpiece 35 needs.

As already described in connection with FIG. 1, the shielding element 31 from FIG. 2 can also be arranged in a corresponding manner on a support device (not shown), so that the assignment between the shielding element 31 and the workpiece 35 can be carried out more easily, without any further measure being required Way results in that the workpiece 35 is attached to the support device.

With the inventive assignment of the shielding element 33 to the workpiece 35, it can further be ensured that the effects of different distances of individual areas of the workpiece 35 from the cathode, not shown, on the layer thickness of the enamel particles 11 deposited on the workpiece 35 are compensated. By arranging the conductive region 33 or the shielding element 31 connected as a secondary anode, for example at a distance of 10 to 20 mm from the workpiece surface region 41 as the predeterminable region of the workpiece 35, the potential difference of the cathode relative to the workpiece surface region 41 becomes namely in a predeterminable manner reduced. This ensures that with a relatively long immersion time or a relatively large voltage or current intensity, which are required so that a sufficiently thick enamel layer is deposited in the end region 47 or the edge region 49 of the workpiece 35, which is deposited on the workpiece surface region 41 Enamel layer is not excessively thick. This ensures that an essentially uniform enamel layer that meets the requirements is deposited on areas 47 and 49, on which enamel particles 11 normally separate relatively slowly, and on a workpiece surface area 41, on which enamel particles 11 normally deposit quite quickly. In particular, it is also avoided that areas of the workpiece 35 are coated with an excessively thick enamel layer, which could tend to flake off during or after the enamel has been burned in in a subsequent method step.

By assigning the shielding element 31 to the workpiece 35 according to the invention, the prerequisite is also created for a relatively high voltage between the cathode and the workpiece to act as an anode and for a sufficient immersion time to be predetermined, so that it does not result in a Overcoating, for example with enamel, is avoided. It can thus be ensured in a simple manner that an enamel layer with an essentially constant thickness is deposited over the entire workpiece 35, which can be, for example, in the range from 200 to 250 micrometers.

Both in the method explained with reference to FIG. 1 and with the method explained with reference to FIG. 2, the conductive section 23 or 33 connected as an auxiliary anode is partially provided with an insulating section 19 or 37, through which an undesired application of enamel on the conductive sections 23 or 33 is at least largely reduced.

The method described with reference to FIGS. 3 and 4 differs from that of FIGS. 1 and 2 essentially in that the conductive section connected as a secondary electrode is not arranged to run parallel to the insulating section.

As can be seen from FIG. 3, the conductive section 51 is formed by a leg 55, which is connected to individual holding legs 53 arranged at a distance from one another. The legs 55 and the holding legs 53 are substantially perpendicular to one another. The leg 55, which runs essentially perpendicular to the workpiece 57, for example a domestic appliance door, is connected to the plate-shaped insulating section 59 via the holding leg 53. The insulating section 59 runs in the same way as the holding limbs 53 essentially parallel to the workpiece 57.

The insulating section 59 arranged in the enamel slip bath 5 between the workpiece 57 and the cathode (not shown), which, like the workpiece 57, is shown incompletely, extends beyond the holding limbs 53. The shielding element formed by the insulating section 59, the holding legs 53 and the legs 55, designated overall by 61, is arranged in such a way that the leg 55 of the conductive section 51 projects into a recess 63 formed on the workpiece 57 or is directed toward the latter.

As can be seen from Figure 4, the leg 55 of the conductive portion 51 of the recess 63 is formed accordingly, so that over the entire course of the circumferential recess 63, the conductive portion 51 is associated via leg 55. Over its entire course, the conductive section 51 is connected to the insulating section 59 via a plurality of the holding legs 53; only a single connecting leg is shown in the figures.

As can be seen from FIG. 3, the assignment of the conductive section 51 to the recess 63 causes enamel particles to reach the surfaces of the recess 63 in sufficient quantity. This ensures that an enamel layer is also deposited on the wall areas of the recess 63, the thickness of which is sufficient to meet requirements with regard to the function of the enamel layer or its visual impression.

As described in connection with the first and second exemplary embodiments, the shielding element 61 can be arranged on a support device (not shown).

According to the third exemplary embodiment, the insulating section 59, which is connected to the shielding element only in regions via its holding leg 53, acts as a shield against the cathode, as a result of which the streamlines or the flow of enamel particles to workpiece surfaces of the workpiece 57 are influenced in a relatively global manner in a similar manner as described in connection with the first and second embodiments. With the conductive section 51 or its leg 55 associated with the workpiece 57 or its recess 63, the transfer of enamel particles to predeterminable workpiece surfaces or their deposition on these surfaces is specifically influenced.

FIGS. 5 and 6 show the basic structures of arrangements for coating workpieces with an enamel slip in the electro-immersion enamelling method, additional cathodes being used. In an enamel basin 71, the slip bath 72 is connected to DC voltage U1 via the main cathode 73, which is essentially arranged on one side, and the workpiece 74, which forms the anode, which is arranged on the opposite side and forms the anode. Due to the current flow from the cathode 73 to the workpiece 74, the enamel slip is transported in the direction of the workpiece 74 and deposited there, preferably on the side facing the cathode 73. In order to avoid neglecting the coating on the edges of the workpiece 74, the one that is deliberately easy to enamel is in the vicinity An auxiliary cathode 75 or 75 'is arranged in the edges of the slip bath 72 and, according to FIG. 5, DC voltage U1 is also applied to workpiece 74 and individual DC voltage U2 according to FIG. 6. 7, the auxiliary cathode 75 "is not subjected to direct voltage from the outside. Here, the voltage difference between the workpiece 74 and the insulated electrical conductor piece is used, which thus serves as a cathode. By changing the cathode-workpiece distance and by changing the applied DC voltage is taken into account the requirements for the coating.

With the described methods according to the invention and the devices according to the invention described in connection therewith, the quality of the enamel application on workpiece surfaces can be increased in a simple and quick manner. This applies both with regard to the formation of the layer thickness of the enamel coating and with regard to its adherence. The quality of enamelled workpieces, such as, for example, household appliance components, can thus be improved in a simple and cost-effective manner without the need for complex process steps or device-side measures.

Claims (13)

1. Method for precipitating an enamel layer, in which method a workpiece with workpiece surfaces to be enamelled is dipped into an enamel slip bath, wherein a direct voltage is applied between a cathode associated with the enamel slip bath and the workpiece as anode in order to precipitate an enamel layer onto workpiece surfaces, characterised thereby, that for a reduction in the thickness of an enamel layer to be precipitated onto a given workpiece part surface (13, 41), a screening element (15, 31, 61), which comprises a portion (17, 33, 55) which is constructed to be electrically conductive and connected as auxiliary anode, is associated at a spacing with the given workpiece part surface (13, 41).
2. Method according to claim 1, in which the workpiece is arranged on a carrying device before being dipped into the enamel slip bath and is dipped together therewith into the enamel slip bath, characterised thereby, that the screening element (15, 31, 61) is arranged on the carrying device and the association with the predeterminable workpiece surface (13, 41) takes place by fastening the workpiece (1, 35, 57) on the carrying device.
3. Method according to one of the claims 1 and 2, characterised thereby, that the screening element (15, 31, 61) comprises an insulating portion (19, 37, 59), which is connected with the electrically conductive portion (17, 33, 35), at least in regions on the side remote from the workpiece (1, 35, 57).
4. Method according to one of the claims 1 to 3, characterised thereby, that the screening element is a screening plate (15), which is arranged at a spacing from and substantially parallelly to the predeterminable workpiece surface (13).
5. Method according to claim 4 for a workpiece with an areal basic body and side walls extending substantially perpendicularly therefrom, characterised thereby, that the screening plate (15) extends substantially in the same plane as end faces of the free ends (21) of the side walls (9).
6. Method according to one of the claims 1 to 5, characterised thereby, that the screening covers the predeterminable workpiece surface with an excess portion projecting beyond.
7. Method according to one of the claims 1 to 4
      for a predeterminable workpiece surface, at which a thicker enamel layer is to be precipitated at least in regions thereof, characterised thereby, that for the influencing of the electrical potential of the enamel slip bath relative to the predeterminable workpiece surface (41), this is associated with a screening element (31), which comprises at least one curved or one prismatic region.
8. Method according to one of the claims 1 and 2
      for a workpiece with a recess to be coated by an enamel layer, characterised thereby, that for an adequate precipitation of enamel, the recess (63) is so associated with an insulating plate (59) with an electrically conductive portion (51) as screening element (61) that a region of high current density arises between cathode and anode, from which region outwards enamel particles are fed to the recess (63) by way of the conductive portion (51), which extends as a limb (55) from the insulating portion (59) in at least a region thereof, in a quantity which suffices for the precipitation of an enamel layer of predeterminable thickness at the surfaces of the recess (63).
9. Method according to claim 1, characterised thereby, that apart from the main cathode, additional auxiliary cathodes are arranged in the enamel slip bath in the proximity of the workpiece.
10. Device according to claim 9, characterised thereby, that the auxiliary cathode is acted on relative to the workpiece by the direct voltage applied at the main cathode.
11. Method according to claim 9, characterised thereby, that the auxiliary cathode is acted on relative to the workpiece by an individual direct voltage.
12. Method according to claim 9, characterised thereby, that the auxiliary cathode is arranged to be insulated in the enamel slip bath.
13. Method according to one of the claims 9 to 13, characterised thereby, that the spacing between the cathodes and the workpiece is settable to be variable.

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