GB2136018A - Electroplating rack contacts - Google Patents

Abstract

A rack or jig for supporting workpieces during an electrolytic treatment comprising a horizontal flight bar and at least two vertical spline members 87, each supporting laterally extending support arms 86, is characterised in that each support arm is provided with at least one contact assembly comprising an outer protector sleeve 82, formed of resiliently compressible non- conductive material and an inner electrically conductive contact member 81 sealed in the sleeve, the sleeve being secured in liquid-tight relationship with the support arm with the inner end of the contact member out of electrical contact with the support arm and being adapted to yield sufficiently under axial pressure to bring said contact member into electrical contact with the support arm. <IMAGE>

Description

SPECIFICATION Improved contacts for electrolytic treatment apparatus The present invention relates to anodising aluminium and to other electrolytic processes in which a load of metallic workpieces is subjected to electrolytic treatment. In particularthe invention is related to contact members by means of which the workpieces are connected to the electrical supply.

The present invention is hereinafter discussed with particular reference to anodising aluminium, but similar difficulties arise in connection with other electrolytic processes and the apparatus provided by the present invention is applicable thereto, possibly with certain modifications of the materials employed therein as will be obvious to those skilled in the art.

Other electrolytic processes to which the apparatus ofthe present invention are applicable are the electrocolouring of anodised aluminium, electrophoretic painting of metallic panels and other workpieces and electroplating, particularly where heavy currents are employed.

It is already known in the artto employ stacker jigs or racks in which a load of elongated workpieces, such as aluminium extruded sections, are placed on horizontal support arms, carried by vertical flight bars and forming part ofthe electrical supply connection to the workpieces. It has been found necessary to subject such support arms to an oxide-stripping operation after each anodising operation to ensure a good electrical contact between them and the workpieces, which is essential for the avoidance of local overheating underthevery high current conditions employed in anodising operations.

In one known form of carrier employed in anodising lengths of extruded aluminium sections the carrier comprises an overhead horizontal member, usually known as the flight bar, and vertical members, usually referred to as splines, which are suspended from the flight bar and are electrically connected to the ends of the extruded sections. The two splines carry horizontal arms onto which the extruded sections are loaded by hand and then secured thereto by clamps.

The flight bar and the splines constitute the electrical conductors forthe whole currentthroughput ofthe cell and for economy in the use of electrical energy and to keep down the weight load imposed on the overhead structure from which the carrier is sup ported the flight bar and the splines are constructed of aluminium; in some instances the aluminium splines are provided with a thin layer of material which is conductive butsubstantially unaffected by the cell electrolyte. In conventional D.C. sulphuric acid anodising it is known to provide a titanium surface layer on the splines. The titanium layerforms a very thin barrier-layertype anodicfilm under D.C. anodising conditions, so that it does not act as a thief of the anodising current.The thus formed anodic layer on the titanium isso thin and friable that it breaks under the pressure applied when clamping an aluminium workpieceto it. In consequence it has been considered unecessaryto apply an oxide stripping step between successive anodising steps.

It is also well known to use uncoated aluminium splines from which the oxide coating is chemically or mechanically stripped after each anodising cycle to removethe aluminium oxide coating since the pre- sence of such an oxide coating adversely affects the contact between an arm and theworkpiece supported by it.

Some difficulty is experienced in anodising and similar processes as a result of decrease of the contact area attheworkpiece/supportarm interface. The decrease of contact area is created by undercutting by the anodic oxide around the margin ofthe contact area and may result in overheating at the contact area.

In orderto overcome this difficulty the support arm may be provided with one or more contact members, at least partly surrounded by a compressible nonconductive protector which blanketstheface ofthe workpiece around most or all of the contact area between the workpiece and the contact member, so thatthere is little or no formation of anodic oxide within an annularzone immediately surrounding the contact area. The faces of any contact members not in contact with a workpiece will undergo some anodisation and thus require stripping at longer or shorter intervals, depending upon the metal by which the contact members are faced.

Although the provision of a titanium facing would appear two avoid the necessity of periodical stripping of the contact pin face, it has been found in practice in many instances that the use of a titanium protective coating leads to unsatisfactory results. Local "burning" of the workpiece can occur at the contact area, particularly where a high current is carried through the point of contact.

The difficulty has now been overcome according to the present invention by employing contact assemblies, each comprising an outer protective sleeve, formed of resiliently compressible non-conductive material, and an inner electrically conductive contact member sealed in said sleeve, said sleeve being secured in liquid-tight relationship with the support arm with the inner end of said contact member out of electrical contact with the support arm and being adapted to yield sufficiently under axial pressure to bring said contact member into electrical contact with the support arm. Atthe outer end of the contact memberthe sleeve extends up to and possibly slightly beyond the end face of the contact member.

When a contact assembly is subjected to axial pressure by means of a workpiece, the inner end face of the contact member is brought into electrical contactwith the support arm while the outer end face is brought into contact with the workpiece while sealed off from the surrounding electrolyte by the longitudinally compressed end portion cf the sleeve, with the result that there is little, if any, growth of anodic oxidefilm in the area immediately surrounding the outer end face of the contact memberto undercut the area of contact between the contact member and the workpiece.On the other hand, in the event of the outer end face of the contact member being exposed to electrolyte during an anodising operation by reason of the absence of a workpiece, there will be no growth of anodic oxide on it, because the inner end of the contact member will remain out of contact with the support arm.

In consequence the contact member may be formed of aluminium, although such a contact member would require periodic replacement by reason of chemical or mechanical erosion. Alternatively the outerface of the contact member may be coated with a relatively incorrodiblemetal,suchasTiorZr. Howeverinmost instances the Al contact members are preferred.

The contact member is preferably in the form of a circular pin, anchored in the sleeve and having its side surfaces in sealed relation with the sleeve, which is preferablyformedofa silicone rubberorthe like.The sleeve is also preferably circular in section and may be anchored against longitudinal movement in a recess in the support arm. The outer end of the contact member is preferably somewhat recessed within the sleeve so that contact is not made with the pin until the sleeve has undergone substantial compression. At the inner end the contact member is conventiently flush with or extends th rough the inner end of the sleeve.

The outersu rface ofthe sleeve atthat end is preferably slightly reduced in diameter to provide a plug portion to fit into and be sealed in a corresponding blind hole ofgreateraxial length in the support arm.

In order to ensure the exclusion of electrolyte, mastic sealing compounds may be used between the surfaces ofthe pin and the sleeve, and between the sleeve and its seating in the spline. Adhesive bonding compounds could be usedforthis purpose but their usewill make replacement ofthe components less easy; non-setting sealants are therefore preferred.

When the contact assembly is subjected to axial compression the contact member is brought into contactwith the bottom end surface of the blind hole.

While there is some diminution of the volume of the gas space in the hole, the increase in pressure is insufficient to rupture the seal between the sleeve and the contact member and the sleeve and the surface of the blind hole. In consequence there is no tendency for gas to be expelled from such space and no consequent tendency to suck in electrolyte.

In most instances the contact memberwill be a round pin in an essentially cylindrical sleeve. However the contact member may be elongate in section; such as a metal strip. The sleeve may be correspondingly shaped for clamping in a slot in the support arm. Many othervariations of cross section and shape are possible.

One form ofcontactassemblyin accordance with the invention is illustrated in the accompanying drawings in which Figure 1 shows the contact assembly arranged in a support arm and Figure 2 shows the same contact assembly under axial compression between a workpiece and a support arm.

Figures 3-7 show alternative forms of contact assembly in accordance with the invention. Figure 8 shows a further alternative form of contact assembly mounted on a support arm.

In Figure 1 the contact assembly comprises an axial contact pin 1 sealed into a sleeve 2, having a portion 3 of reduced diameter; the sleeve may be of silicone rubber or other resilient material which is resistant to the process liquids. The sleeve 2 is adapted to be plugged into a bore 4 in a support arm 5 of an anodising jig.

The axial length ofthe sleeve portion 3 is substantially less than that ofthe bore 4 so thatthe inner end face of the contact pin 1 is held out of contact with the bottom face ofthe bore 4 until the sleeve is subjected to axial compression as illustrated in Figure 2.

When the contact assembly is brought into the position shown in Figure 2 (which is essential to complete the electrical circuit) the area of contact between the pin 1 and the workpiece W is sealed off from the surrounding electrolyte bythe compressed upper portion of sleeve 2 to prevent any undercutting ofthe area of contact by growth of anodic oxideor attack on the metal contact member bathe pracess liquids.

With this arrangementthe whole ofthe outer surface of the support arm 5 may be sheathed with p.v.c. or other material for protecting itfrom chemicai attack.

Though only one contact assembly is shown, each support arm will commonly be provided with two or more contact assemblies arranged for operation with workpieces of different sizes. Depending on the workload, not all the contact members may be in use fora given batch of work; in these circumstances the benefits ofthe present invention are particularly marked.

In the construction shown in Figure 3the contact pin 31 has a reduced waist 32 and an inner contact portion 33. It is anchored against longitudinal movement in a sleeve 34 which fits tightly into a cylindrical bore 35 in support arm 36. The outer end ofthe contact pin 31 is somewhat recessed within sleeve 34, which in turn projects from the surface ofthe support arm 36, with the result that the outer end of sleeve 34 must undergo substantial compression and effect a tight seal against the wall of bore 35 before a workpiece can come into electrical contact with the outer end ofthe pin 31 and the inner contact portion can be brought into electrical contactwith the support arm 36.

The construction of Figure 4 is designed to operate in a similar mannertothe construction of Figure 3. In this case the pin 41 is provided with an enlarged contact head 42 and a tapering inner contact portion 43. In this arrangementthe sleeve 44 is provided with a sealing shoulder 45 and there is little recessing ofthe pin 41 into the sleeve. The taper ofthe contact portion 43is matched bythecontourofthecorresponding portion of bore 46 in support arm 47 to assist in forming a good contact between pin 41 and support arm 47.

In the construction of Figure 5 contact pin 51 has an enlarged head 52 to hold it against movement inwardly in asleeve53whensubjectedtoaxial compression. In this case the head 52 projects slightly outwardly from sleeve 53, whilst its inner end isflush with the inner end of sleeve 53. In this case bore 54 in support arm 55 is provided with a lip 56, engaging in the sleeve 53to hold the sleeve against movement in the axial direction and to provide a seal with the sleeve, also held against inward axial travel by a shoulder 57 in bore 54.

The construction of Figure 6 is essentially identical in operation with that of Figure 3, exceptthat in this case the sleeve 62 for contact pin 61 is provided with a flange 63, clamped between separate support arm parts 64 and 65.

In the construction of Figure 7 contact pins 71 are held in a slight recessed location in sealed relationship with a common sheath-like sleeve member72, which covers the upper surface of support arm 73.

In the construction of Figure 8the contact assembly comprises a flanged pin 81 moulded into a silicone rubber pad 82,which has an externally reduced portion 83.

The pad 82 is mounted in a block 84 of polypropylene or like insulating material, which is secured to a metallic support arm 85 by metallic contact screws 86.

The support arm 85 is totally enclosed in a plastic coating 85 and is in turn supported by a vertical plastic-coated aluminium spline member 87.

When subjected to pressure the pad 82 yields axially to allow the pin 81 to contact screws 86. Alternatively the block84 may be a bare block of aluminium which is secured to the plastic-coated support arm by a contact screw. The contact screws 86 or the alternative solid aluminium block should be considered as being electrically part of the support arm.

Claims (1)

1. CLAIM

   1. Arackorjigforsupporting workpiecesduring an electrolytic treatment comprising a horizontal flight bar and at least two vertical spline members, each supporting laterally extending support arms characterised in that each support arm is provided with at least one contact assembly comprising an outer protector sleeve, formed of resiliently compressible non-conductive material and an inner electrically conductive contact member sealed in said sleeve, said sleeve being secured in liquid-tight relationship with the support arm with the inner end of the contact member out of electrical contact with the support arm and being adapted to yield sufficiently under axial pressurn to bring said contact member into electrical contact with the support arm.