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(11) | EP 0 170 429 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Internal electroplating of tubular or hollow workpieces |
| (57) A hollow workpiece such as a waveguide (2) is electroplated internally using an anode
(1) which is is inserted into the workpiece and follows and bends therein, without
coming into contact with the internal surfaces of the workpiece. The anode (1) consists
of a wire bent into a convoluted profile and provided with captive insulating elements
(3) at the crests and troughs of its profile, which prevents the anode wire from contacting
the internal surface of the workpiece. |
[0002] The invention is particularly, but not exclusively, applicable to the internal electroplating of waveguides.
[0003] It is clearly desirable to employ lightweight materials such as aluminium. and light alloys for waveguide construction, particularly for aeronautical and satellite applications. In order to avoid unacceptably high signal loss the internal surfaces of such waveguides are commonly electroplated with a conductive metal, usually silver. Such internal electroplating requires the positioning of an anode extending generally axially through the waveguide cavity. It is, of course, necessary to maintain this anode spaced from the internal surfaces of the waveguide, and for this purpose it is known to provide insulating spacers at intervals along such an electroplating anode.
[0004] Difficulties arise where it is desired to electroplate internally a waveguide or other tubular or hollow workpiece which is non-linear in shape: for example a waveguide may have bends in more than one plane, and may also exhibit a twisted shape. An earlier proposal for an electroplating anode comprises an anode wire would helically upon a flexible rod of plastics material, with generally annular insulating spacers supported upon the rod at intervals.
[0005] Such an anode is not in general satisfactory for electroplating the inside of a waveguide or pipe having a curved profile, since there is a strong possibility of the anode wire coming into contact with the internal surface of the workpiece, and thereby creating a short circuit.
[0006] The present invention affords an effective and simple solution to this problem by providing an anode for use in electroplating a tubular or hollow workpiece such as a waveguide internally, comprising a wire of convoluted shape, and insulating elements captive on the wire at each crest and trough thereof, so as to prevent the wire from coming into contact with the internal surface of the workpiece in use of the anode.
[0007] The convolutions of the wire preferably comprise undulations disposed in a single plane when the anode is flat. The anode wire can be introduced into a waveguide or other workpiece to be electroplated internally and can follow any bends or twists in the workpiece by virtue of its inherent flexibility, while the insulating elements at the crests and troughs of the wire ensure that the wire does not at any point come into contact wih the internal surface of the workpiece.
[0008] For the internal electroplating of a rectangular section waveguide a satisfactory electroplating anode according to the invention has undulations of substantially trapezoidal shape, the insulating elements comprising beads of insulating material threaded on the straight outermost portions of the undulatory wire. Each insulating element is preferably freely rotatable on the wire.
[0009] The invention also affords, according to another aspect, a method of electroplating the internal surface of a tubular or hollow workpiece such as a pipe or waveguide, in which an anode is located within and extends along the workpiece, the anode comprising a wire of convoluted profile which is prevented from coming into contact with the internal surface of the workpiece by insulating elements captive on the wire at the crests and troughs of its profile.
[0010] The invention will be further described, by way of example only, with reference to the accompanying purely diagrammatic drawing, which is a perspective view of part of a rectangular waveguide showing part of an electroplating anode according to one embodiment of the invention.
[0011] The drawing illustrates diagrammatically an electroplating anode 1 for use in electroplating internally a rectangular waveguide 2.
[0012] The anode 1 comprises a convoluted wire of, for example, stainless steel having, in this embodiment, an undulatory shape comprising generally trapezoidal indulations disposed in a single plane when the anode wire is flat as shown. Insulating elements comprising beads 3 of plastics material, for example polystyrene, are threaded on the anode wire and captive on each of the straight portions forming the crests and troughs of the undulatory profile, that is, the laterally outermost parts of the anode. Each insulating bead 3 is, in this embodiment, freely rotatable on the respective part of the anode wire upon which it is captive.
[0013] In an alternative embodiment of the invention, not shown, insulating elements may be provided on the crests and troughs of a convoluted or undulatory anode wire and may be non-rotatably captive thereof. The insulating elements may be in the form of short sleeves or sheaths of insulating material.
[0014] The undulatory anode wire 1 is flexible both in its own plane and in a direction perpendicular to its plane, thereby suiting it for insertion in a waveguide 2 having bends or twists along its length. The insulating elements 3 prevent the anode wire 1 from coming into contact with the internal surface of the waveguide 2 during the electroplating process, when the internal cavity of the waveguide 2 will be filled with electrolyte and the anode wire 1 and waveguide 2 respectively form the anode and cathode of an electroplating circuit.
[0015] For the successful deposition of a conductive metal film of, for example, silver on the internal surfaces of a waveguide or other tubular workpiece the electroplating process itself should be preceded by an initial electroless nickel plating process. This process is followed by heat treatment to increase the adhesion of the nickel coating by causing some diffusion of the nickel into the surface of the metal forming the waveguide, commonly aluminium. The anode 1 is then inserted into the workpiece to be electroplated and an initial test for short circuits between the anode wire and the internal surface of the workpiece is then carried out under "dry" conditions. Having established that there are no short circuits between the anode wire 1 and the internal surfaces to be electroplated the electroplating process is then carried out using, for example, a potassium sodium cyanide solution for electroplating with silver.
[0016] During the electroplating process the anode wire 1 should be removed at least once by a small amount so as to prevent the occurrence of voids in the elctroplated coating at the points of contact of the insulating elements 3 with the internal surfaces of the workpiece 2.
[0017] Although the insulating elements 3 as described are of plastics material, they may be made of any convenient insulating material, such as ceramics or glass, which is not attacked by the electrolyte.
[0018] Although described with particular reference to the internal electroplating of tubular workpieces such as waveguides, the invention is more widely applicable to the internal electroplating of through holes or blind holes in hollow conductive workpieces generally, for example, work-pieces having cavities of convoluted or curved shape.
1. A method of electroplating the internal surface of a tubular or hollow workpiece
(2) such as a pipe or waveguide, in which an anode (1) is located within and extends
along the workpiece
characterised in that
the anode comprises a wire (1) of convoluted profile which is prevented from coming into contact with the internal surface of the workpiece by insulating elements (3) captive on the wire (1) at the crests and troughs of its profile.
characterised in that
the anode comprises a wire (1) of convoluted profile which is prevented from coming into contact with the internal surface of the workpiece by insulating elements (3) captive on the wire (1) at the crests and troughs of its profile.
2. An anode for use in electroplating a tubular or hollow workpiece (2) such as a
waveguide internally, characterised in that it comprises a wire (1) of convoluted
shape, and insulating elements (3) captive on the wire (1) at each crest and trough
thereof, so as to prevent the wire from coming into contact with the internal surface
of the workpiece (2) in use of the anode.
3. An electroplating anode according to Claim 1, characterised in that the convolutions
of the wire comprise undulations disposed in a single plane when the anode is flat,
for insertion in a straight workpiece (2).
4. An electroplating anode according to Claim 2 or Claim 3, characterised in that
the wire (1) has a regular undulatory shape with a predetermined amplitude and wavelength
or pitch.
5. An electroplating anode according to Claim 4, characterised in that each undulation
of the wire (1) has a substantially trapezoidal shape, and the insulating elements
(3) comprise beads of insulating material threaded on the straight outermost portions
of the indulatory wire.
6. An electroplating anode according to Claim 5, in which each insulating bead element
(3) is freely rotatable on the wire (1).