Chemistry: electrical and wave energy – Apparatus – Electrolytic
Utility Patent
1998-10-13
2001-01-02
Gorgos, Kathryn (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S297080, C204S272000, C204SDIG007
Utility Patent
active
06168691
ABSTRACT:
The invention relates to a method and device for the electrochemical treatment of elongate articles, in particular for the electrochemical etching or plating of a preferably bar-shaped item to be treated.
It is known to treat elongate cylinders, for example vehicle shock absorbers, piston rods or disc valve stems electrochemically to improve the abrasion and corrosion properties. With these methods which are used in practice, the bars are only coated electrolytically with hard chrome on the surfaces stressed in a later operation. The remaining areas of the bars are supposed to remain uncoated or almost uncoated. In order to improve the adhesion of the chrome layer on the bars, the surface is electrochemically etched in advance. For both stages in the procedure, insoluble electrodes are preferably used. The bars are treated preferably in dip bath units, treatment solution adhering to the bars after treatment being rinsed off again. A transportation device takes care of the transportation of the bars from bath to bath.
Since the bars may only be chromium-plated in the central region, their ends must be masked, i.e. screened in such a manner that, in exactly prescribed areas of the bars, no metal is deposited. The boundaries between the areas to be coated and those not to be coated are prescribed, small tolerances being maintained. With shock absorber cylinders, the transition between the areas to be coated and those not to be coated must be kept for example within±3 millimetres. The hard chromium layer must be uniformly thick right up to these boundaries. An increase or decrease in the layer thickness at these boundaries must be avoided. Furthermore, the layer must be totally uniformly thick over the entire bar periphery so that subsequent processing by polishing can be avoided.
For electrochemical treatment in known dip bath units, the bars are inserted on mountings which in turn are secured on transportable goods carriers. The mountings are provided with individual masks so that both bar ends are not coated in the intended area. Since various shock absorber cylinders with variable bar lengths, diameters and coating areas are to be treated in the central area of the bar, masks and mountings must be supplied in adequate number. The mountings are loaded with the bars by hand since the large product spectrum occurring in practice is not easily accessible to automation.
The mounting technique is also disadvantageous on account of the two-part masks to be used and because of the current supplies. Using tube-shaped electrodes to surround the bar in the electrochemical cell is not possible with the known mountings since a bar, which is secured to the mounting on both sides normally, cannot be introduced into an electrode of this type. For this reason, the electrodes are separated and stand opposite the bars on only two sides. Consequently, a uniformly thick layer is not deposited on the bars. A thicker layer is deposited on the bars at the areas lying opposite the electrodes than in the other areas. For this reason, the non-uniformly coated bar must be subsequently ground cylindrically. When coating with hard chrome, this subsequent mechanical processing represents a considerable cost factor.
The known mountings for dip bath units lead to considerable carry-over of the electrolyte liquid and the rinsing water from one bath to the subsequent one. Because of the masks and fixings which are secured to the mountings, carry-over becomes particularly great. Many rinsing stages must therefore be provided to avoid the carry-over of chromic acid particularly when coating with a hard chrome deposit. Because of the aggressiveness of this acid, very high demands are made on the properties of the materials which come into contact with the treatment solutions with respect to corrosion resistance. Synthetic materials are best suited to this purpose. On the other hand, the mountings for supplying the bath current to the bars must be electrically conductive and therefore must preferably be made of metal. The metal must therefore be protected by a plastic coating. This is however subject to defects.
After treatment in the active baths, the workpieces and mountings must be rinsed very carefully especially when, because of the masks used for screening the areas on the workpiece which have not to be coated, high carry-over has to be taken into account. In order to avoid this problem, an electroplating mounting is proposed in the publication DE 25 24 315 A1 for receiving a plurality of bar-shaped preferably cylindrical objects, such for example as piston rods. The cylindrical rods contained in collar-shaped receiving parts should be coated at the periphery with as uniform a layer thickness as possible. Since the anodes lying opposite the bars on both sides cause an elliptical distribution of layer thickness, however, it is suggested that the bars be turned around their longitudinal axis during the electrochemical treatment. By means of protective shells, which are longitudinally adjustable and individually coordinated to the item to be treated, the metal layer is delimited at their ends in the longitudinal direction of the bars. The plating mounting is designed in such a way that it can be accommodated to the length of the bar by means of vertically adjustable side parts.
Even when subsequent mechanical processing can be dispensed with by virtue of these measures, the method is technically exceptionally demanding since it is necessary to replace each individual rod in the aggressive bath in rotation. Furthermore, because of the rotation technically demanding solutions are required for transferring the generally very high electroplating current onto the rod, for example by means of brushes.
Another solution to the sketched problem is described in the publication DE-AS 11 03 103. A device is presented there for partial electroplating of elongate cylindrical bodies (disc valve stems). These bodies are arranged concentrically in tube-shaped anodes and are secured at one end in a stepped metal core. The cathodic current is supplied via this metal core. The metal core is separated from the electrolytic cell by an insulating body. The metal core and the insulating body delimit the electroplating layer at the upper end of the item to be treated. The lower end is protected by a protective lacquer from unwanted electroplating.
Since every single body to be electroplated must be individually secured in this embodiment and in addition since the protective lacquer for masking the one end area of the item to be treated must be applied to accurat dimensions and be removed again after completion of the electroplating, the described method is technically demanding and thus likewise disadvantageous.
Therefore, the problem underlying the present invention is to avoid the disadvantages of the known methods and devices and to make possible in particular uniform electrochemical treatment of elongate articles, simple screening of the articles at the areas which have not to be treated, exact delimitation of the areas to be treated being achieved within narrow tolerances, a small degree of carry-over of the treatment liquids and rinsing liquids from one bath to the next and a reasonably priced electrochemical treatment of articles with various dimensions. In addition, it should be possible to process many articles at the same time.
According to the invention, elongate articles, preferably bars, especially round rods, which if necessary are tapered at the ends or are provided with a thread, are treated electrochemically using a method with the following essential procedural steps:
a) clamping of at least one article at one end by means of a preferably electrically conductive clamp;
b) axial transference in as concentric a manner as possible of the object held by the clamp into a hollow chamber with a longitudinal axis in an electrode which is arranged in a dip bath and which has a periphery parallel to the axis,
c) the article being thereby directed through at least one axially displaceable perforated screening mask in such
Kauper Rudolf
Krepelka Manfred
Lummer Thomas
Richter Wolfgang
Silberhorn Reinhard
Atotech Deutschland GmbH
Gorgos Kathryn
Nicolas Wesley A.
Paul & Paul
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